Microbial responses to elevated temperature: Evaluating bentonite mineralogy and copper canister corrosion within the long-term stability of deep geological repositories of nuclear waste

Deep Geological Repositories (DGRs) consist of radioactive waste contained in corrosion-resistant canisters, surrounded by compacted bentonite clay, and buried few hundred meters in a stable geological formation. The effects of bentonite microbial communities on the long-term stability of the repository should be assessed. This study explores the impact of harsh conditions (60 °C, highly-compacted bentonite, low water activity), and acetate:lactate:sulfate addition, on the evolution of microbial communities, and their effect on the bentonite mineralogy, and corrosion of copper material under anoxic conditions. No bentonite illitization was observed in the treatments, confirming its mineralogical stability as an effective barrier for future DGR. Anoxic incubation at 60 °C reduced the microbial diversity, with Pseudomonas as the dominant genus. Culture-dependent methods showed survival and viability at 60 °C of moderate-thermophilic aerobic bacterial isolates (e.g., Aeribacillus). Despite the low presence of sulfate-reducing bacteria in the bentonite blocks, we proved their survival at 30 °C but not at 60 °C. Copper disk's surface remained visually unaltered. However, in the acetate:lactate:sulfate-treated samples, sulfide/sulfate signals were detected, along with microbial-related compounds. These findings offer new insights into the impact of high temperatures (60 °C) on the biogeochemical processes at the compacted bentonite/Cu canister interface post-repository closure.

Saline groundwaters counteract up-flow of contaminants- implications for radionuclide repositories?

The high-level nuclear waste, HLW, from Swedish and Finnish reactors will be deposited in crystalline rock at depths around 500 m. The waste is enclosed in steel canisters protected against corrosion by a 5 cm thick copper shell, which ensures a lifetime far longer than 100 000 years. Should some canister be breached any leaking nuclides will have decayed to so low activity that even if they reached the biosphere, they would cause minimal risk to humans. The cost of the copper is significant. The dismantling of the nuclear reactors, with induced activity must also be disposed of and this waste volume is much larger than that of the HLW, which makes it impossible to protect it in the same way. This paper explores if by locating the waste at larger depth where the ground water is more saline, and where the hydraulic conductivity of the rock is lower up-flow of contaminated water can be ensured to be negligible because the denser water at larger depth counteracts up-flow due to negative buoyancy. Several processes that could cause local up-flow are addressed, such as infiltration of meteoric water, impact of surface topology, heat production of the waste, geothermal gradient, salinity gradient, hydraulic conductivity heterogeneities and salt migration between seeping water and salt in matrix pore water. Flow and transport simulations using data from extensive field investigations over more than ten years with scores of km deep boreholes suggest that a HLW repository at around one km depth may be sufficient to hinder up-flow to the biosphere.

Impact of dry density and incomplete saturation on microbial growth in bentonite clay for nuclear waste storage

AIMS

Many countries are in the process of designing a deep geological repository (DGR) for long-term storage of used nuclear fuel. For several designs, used fuel containers will be placed belowground, with emplacement tunnels being backfilled using a combination of highly compacted powdered bentonite clay buffer boxes surrounded by a granulated "gapfill" bentonite. To limit the potential for microbiologically influenced corrosion of used fuel containers, identifying conditions that suppress microbial growth is critical for sustainable DGR design. This study investigated microbial communities in powdered and gapfill bentonite clay incubated in oxic pressure vessels at dry densities between 1.1 g cm-3 (i.e. below repository target) and 1.6 g cm-3 (i.e. at or above repository target) as a 1-year time series.

RESULTS

Our results showed an initial (i.e. 1 month) increase in the abundance of culturable heterotrophs associated with all dry densities <1.6 g cm-3, which reveals growth during transient low-pressure conditions associated with the bentonite saturation process. Following saturation, culturable heterotroph abundances decreased to those of starting material by the 6-month time point for all 1.4 and 1.6 g cm-3 pressure vessels, and the most probable numbers of culturable sulfate-reducing bacteria (SRB) remained constant for all vessels and time points. The 16S rRNA gene sequencing results showed a change in microbial community composition from the starting material to the 1-month time point, after which time most samples were dominated by sequences associated with Pseudomonas, Bacillus, Cupriavidus, and Streptomyces. Similar taxa were identified as dominant members of the culture-based community composition, demonstrating that the dominant members of the clay microbial communities are viable. Members of the spore-forming Desulfosporosinus genus were the dominant SRB for both clay and culture profiles.

CONCLUSIONS

After initial microbial growth while bentonite was below target pressure in the early phases of saturation, microbial growth in pressure vessels with dry densities of at least 1.4 g cm-3 was eventually suppressed as bentonite neared saturation.

Predicting the distribution coefficient of cesium in solid phase groups using machine learning

The migration and retention of radioactive contaminants such as 137Cesium (137Cs) in various environmental media pose significant long-term storage challenges for nuclear waste. The distribution coefficient (Kd) is a critical parameter for assessing the mobility of radioactive contaminants and is influenced by various environmental conditions. This study presents machine-learning models based on the Japan Atomic Energy Agency Sorption Database (JAEA-SDB) to predict the Kd values for Cs in solid phase groups. We used three different machine learning models: random forest (RF), artificial neural network (ANN), and convolutional neural network (CNN). The models were trained on 14 input variables from the JAEA-SDB, including factors such as the Cs concentration, solid-phase properties, and solution conditions, which were preprocessed by normalization and log-transformation. The performances of the models were evaluated using the coefficient of determination (R2) and root mean squared error (RMSE). The RF, ANN, and CNN models achieved R2 values greater than 0.97, 0.86, and 0.88, respectively. We also analyzed the variable importance of RF using an out-of-bag (OOB) and a CNN with an attention module. Our results showed that the environmental media, initial radionuclide concentration, solid phase properties, and solution conditions were significant variables for Kd prediction. Our models accurately predict Kd values for different environmental conditions and can assess the environmental risk by analyzing the behavior of radionuclides in solid phase groups. The results of this study can improve safety analyses and long-term risk assessments related to waste disposal and prevent potential hazards and sources of contamination in the surrounding environment.

Unveiling fungal diversity in uranium and glycerol-2-phosphate-amended bentonite microcosms: Implications for radionuclide immobilization within the Deep Geological Repository system

Uranium (U) represents the preeminent hazardous radionuclide within the context of nuclear waste repositories. Indigenous microorganisms in bentonite can influence radionuclide speciation and migration in Deep Geological Repositories (DGRs) for nuclear waste storage. While bacterial communities in bentonite samples have been extensively studied, the impact of fungi has been somewhat overlooked. Here, we investigate the geomicrobiological processes in bentonite microcosms amended with uranyl nitrate and glycerol-2-phosphate (G2P) for six-month incubation. ITS sequencing revealed that the fungal community was mainly composed of Ascomycota (96.6 %). The presence of U in microcosms enriched specific fungal taxa, such as Penicillium and Fusarium, potentially associated with uranium immobilization mechanisms. Conversely, the amendment of U into G2P-suplemented samples exhibited minimal impact, resulting in a fungal community akin to the control group. Several fungal strains were isolated from bentonite microcosms to explore their potential in the U biomineralization, including Fusarium oxysporum, Aspergillus sp., Penicillium spp., among others. High Annular Angle Dark-Field Scanning Transmission Electron Microscopy (HAADF) analyses showed the capacity of F. oxysporum B1 to form U-phosphate mineral phases, likely mediated by phosphatase activity. Therefore, our study emphasizes the need to take into account indigenous bentonite fungi in the overall assessment of the impact of microbial processes in the immobilization of U within DGRs environments.

Radioactive waste microbiology: predicting microbial survival and activity in changing extreme environments

The potential for microbial activity to occur within the engineered barrier system (EBS) of a geological disposal facility (GDF) for radioactive waste is acknowledged by waste management organizations as it could affect many aspects of the safety functions of a GDF. Microorganisms within an EBS will be exposed to changing temperature, pH, radiation, salinity, saturation, and availability of nutrient and energy sources, which can limit microbial survival and activity. Some of the limiting conditions are incorporated into GDF designs for safety reasons, including the high pH of cementitious repositories, the limited pore space of bentonite-based repositories, or the high salinity of GDFs in evaporitic geologies. Other environmental conditions such as elevated radiation, temperature, and desiccation, arise as a result of the presence of high heat generating waste (HHGW). Here, we present a comprehensive review of how environmental conditions in the EBS may limit microbial activity, covering HHGW and lower heat generating waste (LHGW) in a range of geological environments. We present data from the literature on the currently recognized limits to life for each of the environmental conditions described above, and nutrient availability to establish the potential for life in these environments. Using examples where each variable has been modelled for a particular GDF, we outline the times and locations when that variable can be expected to limit microbial activity. Finally, we show how this information for multiple variables can be used to improve our understanding of the potential for microbial activity to occur within the EBS of a GDF and, more broadly, to understand microbial life in changing environments exposed to multiple extreme conditions.

Review of intermediate-scale field tests in support of disposal of waste forms

Nuclear waste has been generated from commercial nuclear reactors and from past nuclear weapons production activities. The safe disposal of this waste generally is planned to involve emplacement of packaged spent nuclear fuel (SNF) into the subsurface or reprocessing the used nuclear fuel and producing a sparingly soluble mineral or glass. The high-level waste form(s) would then be packaged and sent to a geologic repository. High-level waste (HLW) is expected to be sent for deep geological disposal while the low-level waste (LLW) is to be stored in near-surface facilities. In order to design and manage a secure disposal site, the LLW and HLW waste forms must limit the release of radioactive materials to the surrounding environment for very long time periods. This stability is dependent on the waste form itself as well as the physical and chemical characteristics imposed by the surrounding engineered systems and geology. When studying the chemical durability of the waste form in a laboratory setting, it is not possible to capture all of the complex and coupled processes that the waste form would be subjected to in the disposal system. Intermediate-scale tests can improve understanding and close knowledge gaps that may arise when applying laboratory experiments to a larger scale. The present paper gives an overview of the literature available on the interactions of waste forms and the surrounding environment (engineered barriers and geological materials), as well as suggestions on overcoming existing uncertainties that may aid in more robust performance assessment models.

Survivability and proliferation of microorganisms in bentonite with implication to radioactive waste geological disposal: strong effect of temperature and negligible effect of pressure

As bentonite hosts a diverse spectrum of indigenous microorganisms with the potential to influence the long-term stability of deep geological repositories, it is essential to understand the factors influencing microbial activity under repository conditions. Here, we focus on two factors, i.e., temperature and swelling pressure, using a suspension of Cerny Vrch bentonite to boost microbial activity and evaluate microbial response. Suspensions were exposed either to different pressures (10, 12 and 15 MPa; to simulate the effect of swelling pressure) or elevated temperatures (60, 70, 80 and 90 °C; to simulate the effect of cannister heating) for four weeks. Each treatment was followed by a period of anaerobic incubation at atmospheric pressure/laboratory temperature to assess microbial recovery after treatment. Microbial load and community structure were then estimated using molecular-genetic methods, with presence of living cells confirmed through microscopic analysis. Our study demonstrated that discrete application of pressure did not influence on overall microbial activity or proliferation, implying that pressure evolution during bentonite swelling is not the critical factor responsible for microbial suppression in saturated bentonites. However, pressure treatment caused significant shifts in microbial community structure. We also demonstrated that microbial activity decreased with increasing temperature, and that heat treatment strongly influenced bentonite microbial community structure, with several thermophilic taxa identified. A temperature of 90 °C proved to be limiting for microbial activity and proliferation in all bentonite suspensions. Our study emphasizes the crucial role of a deep understanding of microbial activity under repository-relevant conditions in identifying possible strategies to mitigate the microbial potential within the deep geological repository and increase its long-term stability and safety.

Factors influencing acceptability of final disposal of incinerated ash and decontaminated soil from TEPCO's Fukushima Daiichi nuclear power plant accident

TEPCO's Fukushima Daiichi nuclear power plant accident prompted extensive decontamination work. The decontaminated soil and incinerated ash generated by the process are scheduled for final disposal by March 2045 outside Fukushima Prefecture. The final disposal is unprecedented worldwide. Clarifying their acceptability will contribute to the final disposal of decontaminated soil and incinerated ash, as well as add knowledge about the perceived risk of low-concentration radioactive waste. A questionnaire survey was conducted to assess the psychological factors influencing final disposal acceptability. The results of the structural equation modeling demonstrated stable results, with risk perception decreasing acceptability, social benefits increasing acceptability, and personal benefits having limited impact. The initiative for the final disposal of decontaminated soil and incinerated ash can facilitate the reconstruction of Fukushima Prefecture after the disaster. Trust and intergenerational expectations are critical factors influencing the acceptability of this disposal. The responses were classified based on the relevance of moral norms using cluster analysis and moral foundations. The influence of each element on acceptability varied depending on the cluster. Trust was identified as the most influential factor in acceptability, regardless of the level of importance placed on moral norms.

Further development and application of a method for assessing radionuclide surface activity distribution and source location based on measurements of ambient dose equivalent rate. Examples for Andreeva Bay, Chernobyl NPP and Istiklol

A method for reconstructing surface activity density (SAD) maps based on the solution of the Fredholm equation has been developed and applied. The construction of SAD maps was carried out for the site of the temporary storage (STS) of spent fuel and radioactive waste (RW) in Andreeva Bay using the results of measuring campaign in 2001-2002 and for the sheltering construction of the solid RW using the results of measurements in 2021. The Fredholm equation was solved in two versions: under conditions of a barrier-free environment and taking into account buildings and structures located on the industrial site of the STS Andreeva Bay. Lorenz curves were generated to assess the compactness of the distributions of SAD and ambient dose equivalent rate (ADER) for the industrial site and the sheltering construction at STS Andreeva Bay, the area of the IV stage uranium tailing site near the city of Istiklol in the Republic of Tajikistan, and for roofs of the Chernobyl nuclear power plant. The nature of impact of the resolution (fragmentation) of the raster, the value of the radius of mutual influence of points (contamination sites), the height of the radiation detector above the scanned surface and the angular aperture of the radiation detector on the accuracy of the SAD reconstruction is shown. The method developed allows more accurate planning of decontamination work when only ADER measurements data is available. The proposed method can be applied to support the process of decontamination of radioactively contaminated territories, in particular during the remediation of the STS Andreeva Bay.

Extremophilic microbial metabolism and radioactive waste disposal

Decades of nuclear activities have left a legacy of hazardous radioactive waste, which must be isolated from the biosphere for over 100,000 years. The preferred option for safe waste disposal is a deep subsurface geological disposal facility (GDF). Due to the very long geological timescales required, and the complexity of materials to be disposed of (including a wide range of nutrients and electron donors/acceptors) microbial activity will likely play a pivotal role in the safe operation of these mega-facilities. A GDF environment provides many metabolic challenges to microbes that may inhabit the facility, including high temperature, pressure, radiation, alkalinity, and salinity, depending on the specific disposal concept employed. However, as our understanding of the boundaries of life is continuously challenged and expanded by the discovery of novel extremophiles in Earth's most inhospitable environments, it is becoming clear that microorganisms must be considered in GDF safety cases to ensure accurate predictions of long-term performance. This review explores extremophilic adaptations and how this knowledge can be applied to challenge our current assumptions on microbial activity in GDF environments. We conclude that regardless of concept, a GDF will consist of multiple extremes and it is of high importance to understand the limits of polyextremophiles under realistic environmental conditions.

Progress with the regulation of radiation safety during recovery and removal of spent nuclear fuel from the site for temporary storage at Andreeva Bay on the Kola Peninsula

In the 1960s, a shore technical base (STB) was established at Andreeva Bay on the Kola Peninsula, in northwest Russia. The STB maintained nuclear submarines and the nuclear icebreaker fleet, receiving and storing fresh and spent nuclear fuel (SNF) as well as solid and liquid radioactive waste (RW). It was subsequently re-designated as a site for temporary storage (STS) for SNF and RW. Over time, the SNF storage facilities partly lost their containment functions, leading to radioactive contamination of workshops and the site above permitted values. The technological and engineering infrastructure at the site was also significantly degraded as well as the condition of the stored SNF. At present, the STS Andreeva Bay facility is under decommissioning. This paper describes progress with the creation of safe working measures for workers involved in site remediation and SNF recovery operations, including the determination of safe shift times in high radiation areas, as part of overall optimization of safety. Results are presented for the successful application of these measures in the period 2019-2021, during which time significant SNF recovery and removal operations were completed without incident. Significant important experience has been gained to support safe removal of remaining SNF, including the most hazardous degraded fuel, as well as recovery of other higher level RW and decommissioning of the old storage buildings and structures.

Health risks from radioactive particles on Cumbrian beaches near the Sellafield nuclear site

A monitoring programme, in place since 2006, continues to recover radioactive particles (<2 mm diameter) and larger objects from the beaches of West Cumbria. The potential risks to members of the public using the beaches are mainly related to prolonged skin contact with or the inadvertent ingestion of small particles. Most particles are classified as either 'beta-rich' or 'alpha-rich' and are detected as a result of their caesium-137 or americium-241 content. Beta-rich particles generally also contain strontium-90, with90Sr:137Cs ratios of up to about 1:1, but typically <0.1:1. Alpha-rich particles contain plutonium isotopes, with Pu:241Amαratios usually around 0.5-0.6:1. 'Beta-rich' particles have the greatest potential to cause localised skin damage if held in stationary contact with the skin for prolonged periods. However, it is concluded that only particles of >106Bq of137Cs, with high90Sr:137Cs ratios, would pose a significant risk of causing acute skin ulceration. No particles of this level of activity have been found. Inadvertent ingestion of a particle will result in the absorption to blood of a small proportion of the radionuclide content of the particle. The subsequent retention of radionuclides in body organs and tissues presents a potential risk of the development of cancer. For 'beta-rich' particles with typical activities (mean 2 × 104Bq137Cs, Sr:Cs ratio of 0.1:1), the estimated committed effective doses are about 30µSv for adults and about 40µSv for 1 year old infants, with lower values for 'alpha-rich' particles of typical activities. The corresponding estimates of lifetime cancer incidence following ingestion for both particle types are of the order of 10-6for adults and up to 10-5for infants. These estimates are subject to substantial uncertainties but provide an indication of the low risks to members of the public.

Comparison of Doses from Disposals of Technologically Enhanced Naturally Occurring Radioactive Materials in Kentucky and Oregon

ABSTRACT

Oil and natural gas fracking waste contains technologically enhanced naturally occurring radioactive material (TENORM) and has increasingly been disposed of in unpermitted landfills, causing concern among regulators and the public about potential exposures. There are numerous issues with TENORM waste, including the lack of Federal regulations on its disposal and the lack of permitted landfills capable of accepting these waste streams. This paper examines two situations in which TENORM was placed in unpermitted landfills, one in Kentucky and one in Oregon. The same modeling and dose calculation methods were used in both cases, allowing for a comparison between the two sites. Site-specific differences, source terms, and doses from the disposals and potential remediation options are discussed and compared. Predicted groundwater concentrations are shown and compared against the relevant regulations for each site. Despite the differences in site and TENORM waste characteristics, it was more protective of the community and the environment to leave the waste in place at both sites. Radiation doses to landfill workers on site and to members of the public were low, both during the original disposal and for the remediation alternatives evaluated. Removal of the TENORM material in either case presents significant non-radiological risks that outweigh any benefit from the long-term dose reduction.

Radiological assessment of the disposal of bulk oil NORM waste: Case study from Brazil

The oil and gas industries are the largest producers of NORM wastes that are continuously generated during production and extraction activities. In addition, an increasing trend is observed in waste production worldwide due to the decommissioning of oil platforms. The problem is that most of these wastes are in activity concentration levels above the exemption and the clearance limits and are being accumulated in storage sites because no repositories exist in Brazil for NORM wastes generated by oil industries. There are regulations for radioactive wastes and for the licensing of repositories for managing wastes with low and intermediate levels of radiation but the current regulations apply only to packaged wastes. Therefore an initial radiological assessment was carried out with the RESRAD-OFFSITE code to show that bulk NORM wastes (not packaged wastes) could be disposed of in repositories near the surface without causing additional risk to the public above the criteria used. The results can also support decision-making by the Regulatory Authority to change the current regulations and allow for the disposal of wastes in bulk form.

SAFETY IMPROVEMENT OF THE RADIOACTIVE WASTE STORAGE FACILITY IN ALBANIA

The Albanian National Radioactive Waste Storage Facility (NRWSF) constructed in 1999 consists of an interim waste storage facility for very low level waste, low level waste, intermediate level waste and disused sealed radioactive sources (DSRS) coming from research institutions, agriculture and industrial activities and from applications in nuclear medicine in Albania. The safety assessment of this facility is performed considering its impact to workers, public and environment. In 2018, Albania started a new National project Alb 9010 titled 'Upgrading the Radioactive Waste Storage Building According to International Standards'. The two fundamental objectives are to ensure the protection of the public, workers and the environment and to ensure retrievability of waste packages for final disposal. The project focuses on ensuring the safety of the treatment, conditioning and storage of radioactive waste and DSRS. Under this project has been done the improvement of the safety of the RWSF and upgrading of the existing building infrastructure. In the end of the project the objective is to improve the safety of the personnel in the RWSF, protection of public and environment, Upgrade the Equipment's used in the RW Management Activities, Upgrade the existing Infrastructure of the RWSF Building and also ensure retrievability of waste packages for final disposal.

DEVELOPMENT OF SAFETY ASSESSMENT CODE FOR DECOMMISSIONING WASTE RECYCLING AND DISPOSAL

In Korea, 10 units of nuclear power plants are projected for decommissioning by 2030, which will consequently generate a large amount of decommissioning waste. It could bring about a lack in the capacity of the radioactive waste disposal facility. In this paper, a computational code, REcycling and DIsposal Safety Assessment has been developed for the safety assessment regarding the recycling and disposal of the decommissioning waste. It is composed of two modules: dose assessment module and maximum allowance concentration estimation module. The dose assessment module is intended to evaluate the exposure doses from radioactive nuclides in the wastes for each recycling or disposal scenario. The maximum allowance concentration estimation module has the function to translate the nuclides concentrations in the wastes from the exposure doses received by the end user for each scenario. The concentration of each nuclide provides the basis for the development of the technical criteria in accordance with the annual dose limit.

Design of an IoT-based indoor tracking and condition monitoring system for the safe and transparent management of drums storing low- and intermediate-level radioactive waste

Regulatory agencies and civil society organizations concerned with radioactive waste management are demanding maximized safety management of radioactive-waste-containing drums using advanced technology for more efficient and safe management. In the present paper, we propose a sensor system that can track and monitor drums containing low- and intermediate-level radioactive waste using advanced technology based on the Internet of Things (IoT). The proposed system consists of drum nodes, pallet nodes, and gateways. A drum node and magnetic sensor installed on a drum can be used to remotely check whether the drum lid is open or closed, and the location of the drum can be tracked using a near-field communication reader and an infrared module installed on the pallet node. Considering that radioactive waste is stored for a long time period, the proposed sensors are designed for low power consumption. Moreover, a pilot test involving 48 drums and 12 pallets was conducted to verify the system prototype. Accordingly, from the results of the pilot test, drawbacks were noted and solutions were proposed to improve the system in future work. Implications: We designed a test bed by fabricating a sensor-system prototype and used it in a simulation experiment. The results of this study will be used as basic data for establishing safety measures for radioactive waste management in the future through computer simulation of radioactive waste anomalies in a digital-twin system.

Identification of areas of interest for radiological impact assessments in an evolving landscape context

Regulations concerning potential future health impacts of the final disposal of radioactive wastes in geological disposal facilities are written in terms of annual dose to individuals who are representative of small groups living in the landscape in the vicinity of the repository site. As disposal programmes in Sweden and Finland have progressed towards licensing and construction, so too has detail describing the state and evolution of surface biosphere and the landscape around the proposed disposal sites increased. Simple and generic biosphere dose assessment models in early iterations have grown in complexity with increasing site-specific detail that aims to capture the radiologically significant features of the landscape into which future releases of radionuclides might credibly occur. Current dose assessment models used in support of license applications for disposal programmes in Sweden and Finland are highly complex and their application consequently lacks transparency. An alternative simpler approach to characterising landscape objects for dose assessment models would be beneficial in that it would offer an additional line of reasoning and would add clarity, thereby supporting the decision-making process of the regulatory authorities. In the context of coastal Fennoscandia, landscape change is relatively rapid and dramatic with post-glacial landrise transforming areas of the coastal seabed into terrestrial ecosystems over a period of a few thousand years, global sea level rise notwithstanding. The locations of the geosphere-biosphere interfaces for deep geologic disposal can be estimated with some precision but the nature of the receiving ecosystems at the time of the release is less certain. The approach described here provides a statistical quantification of key morphological characteristics of areas in the landscape where doses could arise, so as to better express uncertainties in dose modelling. The proposed method assumes that the variation in the morphology of potential release locations can be described by the variation in landscape objects seen in the landscape on a wider scale, providing a statistical description of the possible landscape objects, so allowing a more comprehensive range of potential future evolutions to be addressed. Our understanding of the evolution of the landscape, based on the kinds of terrain and ecosystem development models used by POSIVA in Finland and SKB in Sweden, suggests that objects identified in present-day maps can be used as analogues for a statistical characterisation of objects in the future landscape; objects identified in the observed topography and bathymetry can therefore serve as the basis for the statistical description of landscape dose objects over the period during which doses are likely to arise. Using digital elevation models around a disposal site in Finland, we show that the statistical descriptions of landscape dose objects at three times over a period of 10 kyear of the evolved landscape are sufficiently similar to establish the suitability of the approach. The aim of this statistical analysis is to supplement current methods for defining radiological assessment models so as to provide additional numerical support to both the simpler and more complex methods employed by implementors and regulators. The method has been developed in the context of the Swedish and Finnish regulatory review process and is referred to in the IAEA's revised BIOMASS methodology. We briefly address how the method might be applied in other landscape contexts.

Recent Development on Determination of Low-Level 90Sr in Environmental and Biological Samples: A Review

In the context of the rapid development of the world's nuclear power industry, it is vital to establish reliable and efficient radioanalytical methods to support sound environment and food radioactivity monitoring programs and a cost-effective waste management strategy. As one of the most import fission products generated during human nuclear activities, ⁹⁰Sr has been widely determined based on different analytical techniques for routine radioactivity monitoring, emergency preparedness and radioactive waste management. Herein, we summarize and critically review analytical methods developed over the last few decades for the determination of ⁹⁰Sr in environmental and biological samples. Approaches applied in different steps of the analysis including sample preparation, chemical separation and detection are systematically discussed. The recent development of modern materials for ⁹⁰Sr concentration and advanced instruments for rapid ⁹⁰Sr measurement are also addressed.

Contaminant release, mixing and microbial fluctuations initiated by infiltrating water within a replica field-scale legacy radioactive waste trench

Numerous legacy near-surface radioactive waste sites dating from the mid 20th century have yet to be remediated and present a global contamination concern. Typically, there is insufficient understanding of contaminant release and redistribution, with invasive investigations often impractical due to the risk of disturbing the often significantly radiotoxic contaminants. Consequently, a replica waste trench (~5.4 m³), constructed adjacent to a legacy radioactive waste site (Little Forest Legacy Site, LFLS), was used to assist our understanding of the release and mixing processes of neodymium (Nd) - a chemical analogue for plutonium(III) and americium(III), two significant radionuclides in many contaminated environments. In order to clarify the behaviour of contaminants released from buried objects such as waste containers, a steel drum, representative of the hundreds of buried drums within the LFLS, was placed within the trench. Dissolved neodymium nitrate was introduced as a point-source contaminant to the base of the trench, outside the steel drum. Hydrologic conditions were manipulated to simulate natural rainfall intensities with dissolved lithium bromide added as a tracer. Neodymium was primarily retained both at its point of release at the bottom of the trench (>97 %) as well as at a steel container corrosion point, simulated through the emplacement of steel wool. However, over the 8-month field experiment, advective mixing initiated by surface water intrusions rapidly redistributed a small proportion of Nd to shallower waters (~1.5-1.7 %), as well as throughout the buried steel drum. Suspended particulate forms of Nd (>0.2 μm) were measured at all depths in the suboxic trench and were persistent across the entire study. Analyses of the microbial communities showed that their relative abundances and metabolic functions were strongly influenced by the prevailing geochemical conditions as a result of fluctuating water depths associated with rainfall events. The site representing steel corrosion exhibited divergent biogeochemical results with anomalous changes (sharp decrease) observed in both dissolved contaminant concentration as well as microbial diversity and functionality. This research demonstrates that experimental trenches provide a safe and unique method for simulating the behaviour of subsurface radioactive contaminants with results demonstrating the initial retention, partial shallow water redistribution, and stability of particulate form(s) of this radioactive analogue. These results have relevance for appropriate management and remediation strategies for the adjacent legacy site as well as for similar sites across the globe.

On the inclusion of forest exposure pathways into a stylized lake-farm scenario in a geological repository safety analysis

Geological disposal of radioactive waste has been recognized as the 'reference solution' to ensure the safety required for the present and future society and environment. To study the possible exposure pathways from groundwater to humans, radioactive transport modelling is used. One of the ecosystems that may play a significant role when assessing the dose conversion factor (i.e. the dose resulting from a nominal release of 1 Bq/year of each radionuclide) for humans is forest. In this paper we have developed a model of a lake-farm system with a forest component. The biosphere system used in this study represents a typical agricultural scenario in Finland, amended with a typical forest. A lake is assumed to form due to post-glacial land uplift. The main features of this future lake have been obtained from our probabilistic shoreline displacement model. Both deterministic calculations and sensitivity analysis were carried out to simulate the model. The deterministic simulation demonstrates the behaviour of the studied radionuclides (36Cl, 135Cs, 129I, 237Np, 90Sr, 99Tc and 238U) and the proportions of different exposure pathways to humans. Particularly for 135Cs and 129I, forest pathways make a notable contribution to the dose conversion factor. The sensitivity analysis was done using two methods: EFAST and Sobol'. With both methods, the parameters related to the farm contribute the most to the variance of the dose conversion factor for humans. The study demonstrates that the exposure pathways related to forest products may make a considerable contribution to the dose conversion factor in a lake-farm-forest system. It is also confirmed that an advanced sensitivity analysis for a radionuclide transport and dose assessment model on such a landscape scale is feasible even with moderate computational efforts.

A review of current inventory for major industries involving naturally occurring radioactive materials in Canada

Earth materials contain radionuclides of natural origin in varying concentrations. Exposure to natural sources dominates the occupational and public exposure to ionizing radiation. Canada has one of the largest and most diverse supplies of natural resources in the world. A large quantity of material production is associated with a large quantity of waste releases to the environment. Releases from industries involving naturally occurring radioactive material (NORM) can concentrate trace amounts of natural radionuclides and are often poorly characterized. This raises concerns for occupational and public health. Currently, there are data and knowledge gaps in radiological characteristics of NORM products and waste or releases to the environment. For the evaluation of occupational and public exposure to natural sources, a review of current inventory for major industries involving NORM in Canada was conducted with the objective to provide basic information on major issues, help set the priority for research needs to filling these data and knowledge gaps.

Adsorption characteristics of strontium by bentonite colloids acting on claystone of candidate high-level radioactive waste geological disposal sites

The bentonite colloid produced in the deep geological repository of high-level radioactive waste can directly affect the migration of radionuclide strontium when it acts on claystone. The adsorption characteristics of strontium were investigated on claystone with the presence or absence of bentonite colloids from the Suhongtu area of China. The effects of different influencing factors, such as pH and solid content, on the adsorption process were investigated by batch adsorption experiments, and spectroscopic techniques were used to characterize the samples before and after adsorption of strontium. The results show that the presence of bentonite colloids can promote the adsorption of strontium on claystone under alkaline conditions. and the general order kinetic model provided the best fit to the experimental data. Strontium is adsorbed on the surface of claystone and bentonite colloid by ion exchange and surface complexation. Most of the Sr²⁺ formed SrCO₃ with CO₃^2- after ion exchange with Ca²⁺ and Mg²⁺ in plagioclase and dolomite, and a small amount of Sr²⁺ was adsorbed by complexation with -OH, Al-O and Si-O. These results provide a scientific basis for predicting the migration of strontium in subsurface porous media and the siting of high-level radioactive waste repositories.

Effects of flow on uranium speciation in soils impacted by acidic waste fluids

Radioactive acidic liquid waste is a common byproduct of uranium (U) and plutonium (Pu) enrichment and recycling processes whose accidental and planned release has led to a significant input of U into soils and sediments across the world, including at the U.S. DOE's Hanford site (WA, USA). Because of the particularly hazardous nature of U, it is important to predict its speciation when introduced into soils and sediments by acidic waste fluids. Of fundamental importance are the coupled effects of acid-driven mineral transformation and reactive transport on U speciation. To evaluate the effect of waste-fluid residence time and co-associated dissolved phosphate concentrations on U speciation in impacted soils and sediments, uncontaminated surface materials (from the Hanford Site) were reacted with U-containing synthetic acidic waste fluids (pH 2) amended with dissolved phosphate concentrations in both batch (no flow) and flow-through column systems for 7-365 days. By comparing dissolved U behavior and solid phase speciation as a function of flow regimen, we found that the availability of proton-promoted dissolution products (such as Si) to sequester U into uranyl silicates was dependent on waste fluid-sediment contact time as uranyl silicates were not detected in short contact time flow-through systems but were detected in no-flow, long contact time, reactors. Moreover, the dominance of uranyl phosphate as neoprecipitate U scavenger (principally in the form of meta-ankoleite) in phosphate amended systems confirmed the importance of phosphate amendments for an efficient sequestration of U in the soils and sediments. Overall, our experiments suggest that the formation of uranyl silicates in soils impacted by acidic waste fluids is likely to be limited unless reaction products are allowed to accumulate in soil pores, highlighting the importance of investigating soil U speciation in flow-through, transport-driven systems as opposed to no-flow, batch systems. This study provides insights into uranium speciation and its potential changes under acidic conditions for better prediction of risks and subsequent development of efficient remediation strategies.

Radiobiology of Select Radionuclides in Hanford Site Tank Waste

There are several important radionuclides involved in the "clean-up" or environmental isolation of nuclear waste contained in US Department of Energy Hanford Site underground waste tanks that drive many of the decisions associated with this activity. To make proper human health risk analyses and ensure that the most appropriate decisions are made, it is important to understand the radiation biology and the human health risk associated with these radionuclides. This manuscript provides some basic radiological science, in particular radiation biology, for some of these radionuclides, i.e., 3 H, 90 Sr, 137 Cs, 99 Tc, 129 I, and the alpha emitters 239, 240 Pu, 233,234,235,238 U, and 241 Am. These radionuclides were selected based on their designation as "constituents of potential concern," historical significance, or potential impact on human health risk. In addition to the radiobiology of these select radionuclides, this manuscript provides brief discussions of the estimated cost of planned management of Hanford tank waste and a comparison with releases into the Techa River from activities associated with the Mayak Production Association. A set of summary conclusions of the potential human health risks associated with these radionuclides is given.

Practical learning through radiation physics problem solving

INTRODUCTION

The identification of unknown radionuclides is an authentic practical activity for students that provides the foundations for clinical problem solving, especially in the storage and management of radioactive waste. As different radionuclides have different half-lives, some of which are quite long, the storage of waste material has to accommodate the longest of these. Cross contamination requires a method of identifying the radionuclide samples in a mixed sample to safely and appropriately manage disposal. Similarly, identifying a single unknown sample of a radionuclide allows correct handling and disposal.

METHODS

Performing a systematic investigation of the physical properties of unknown radioactive samples is a rich learning opportunity to instil understanding of important physics principles among students in nuclear medicine.

RESULTS

This manuscript outlines an investigation developed that would allow students to identify single unknown radionuclides based on physical properties and identify the constituent radionuclides of a mixed sample using some additional mathematical curve stripping.

CONCLUSION

The processes and solutions are provided with real data and this practical activity can be replicated by students generating their own data.

IMPLICATIONS FOR PRACTICE

This paper provides a template and analysis/interpretation guideline for educators and clinicians to deepen understanding of foundation physics. Enhanced and deeper understanding are a vehicle for improved problem solving in clinical and research practice.

Rare earth elements and yttrium as tracers of waste/rock-groundwater interactions

Increasing concentrations of Rare Earth Elements (REE) plus yttrium (REY) are entering the environment due to human activities. The similar chemical behaviour across the whole REY, i.e. the lanthanide series (lanthanum to lutetium) and yttrium, allows their use as tracers, fingerprinting rock-forming processes and fluid-rock interactions in earth science systems. However, their use in fingerprinting waste and particularly low-level radioactive waste has not received much attention, despite the direct use of REE in the nuclear industry and the traditional use of REE as proxies to understand the environmental mobility of the actinide series (actinium to lawrencium). The highly instrumented low-level radioactive waste site at Little Forest (Australia) allows a detailed REY study, investigating interactions with local strata, neighbouring waste forms and shallow groundwater flows. Groundwater samples and solids from cored materials were recovered from 2007 to 2012 from the study site and regional baseline sites in the same geological materials. The REY in water samples were analysed by automated chelation pre-concentration (SeaFast, ESI) followed by ICP-MS determination, while solid samples were analysed using Neutron Activation Analysis (NAA) and X-ray fluorescence scanning (ITRAX). Solid rocks showed no REY departed from typical Upper Crust compositions in either Little Forest or regional background sites. Shallow groundwater from ~4-5 m, at or slightly below waste trench levels, showed water-waste interaction as a marked enrichment, relative to shale-normalised patterns, in samarium, europium and gadolinium, with depleted yttrium. Leachate samples from the neighbouring urban landfill show different REY normalised patterns. REY distribution changes with depth through increased interaction with shales and sandstones. Variations in pH and redox conditions lead to widespread precipitation of Fe-hydroxides, which scavenge REY with differential uptake by precipitating solids, resulting in increases in Y and higher Y/Ho ratio in the groundwater along the flow path. Our study revealed that the Little Forest low-level radioactive waste has a REY fingerprint different to that of groundwater in surrounding land uses. REY can be used to fingerprint diverse waste sources, assess the mobility of lanthanides inferring the mobility of selected actinides, and to trace the fate of REY during groundwater recharge. The approach presented can refine source allocation and trace pollutant mobility in current and legacy urban, mixed and radioactive waste sites around the world.

Exploring the governing transport mechanisms of corrosive agents in a Canadian deep geological repository

All nuclear energy producing nations face a common challenge associated with the long-term solution for their used nuclear fuel. After decades of research, many nuclear safety agencies worldwide agree that deep geological repositories (DGRs) are appropriate long-term solutions to protect the biosphere. The Canadian DGR is planned in either stable crystalline or sedimentary host rock (depending on the final site location) to house the used nuclear fuel in copper-coated used fuel containers (UFCs) surrounded by highly compacted bentonite. The copper-coating and bentonite provide robust protection against many corrosion processes anticipated in the DGR. However, it is possible that bisulfide (HS^-) produced near the host rock-bentonite interface may transport through the bentonite and corrode the UFCs during the DGR design life (i.e., one million years); although container performance assessments typically account for this process, while maintaining container integrity. Because the DGR design life far exceeds those of practical experimentation, there is a need for robust numerical models to forecast HS^- transport. In this paper we present the development of a coupled 3D thermal-hydraulic-chemical model to explore the impact of key coupled physics on HS^- transport in the proposed Canadian DGR. These simulations reveal that, although saturation delayed and heating accelerated HS^- transport over the first 100s and 10,000s of years, respectively, these times of influence were small compared to the long DGR design life. Consequently, the influence from heating only increased total projected HS^- corrosion by <20% and the influence from saturation had a negligible impact (<1%). By comparing the corrosion rate results with a simplified model, it was shown that nearly-steady DGR design parameters governed most of the projected HS^- corrosion. Therefore, those parameters need to be carefully resolved to reliably forecast the extent of HS^- corrosion.

Approaches to the definition of potentially exposed groups and potentially exposed populations of biota in the context of solid radioactive waste

A methodology for addressing the biosphere in safety assessments for solid radioactive waste disposal was developed through theme 1 of the IAEA coordinated research project on BIOsphere Modelling and ASSessment (BIOMASS) that ran from 1996 to 2001. This methodology provided guidance on how the biosphere can be addressed in safety assessments for disposal of solid radioactive waste. Since the methodology was developed, it has proven useful and has been widely referenced in assessments in a diversity of contexts encompassing both near-surface and deep geological disposal of solid radioactive waste. The principles that could be adopted for defining potentially exposed groups (PEGs) were an important aspect in the original BIOMASS methodology as the endpoint of an assessment usually includes the evaluation of individual dose or risk to human health. Identification of PEGs and definition of their characteristics are usually made to be consistent with the biosphere system description being developed, acknowledging that due to inherent uncertainties in projecting future human behaviour, the biosphere models adopted for assessing safety of a disposal system can only be illustrative. Since the publication of the original BIOMASS methodology, consideration has been extended to include potentially exposed populations of biota (PEPs), in the context of dose assessment and protection of the environment. Considering the need for the development of transfer pathways from a source term to an end point (for either PEGs or PEPs), the exposure modes that may occur and those to be assessed quantitatively should be identified. Within an expert working group (WG6) of the second phase of the IAEA coordinated project Modelling and Data for Radiological Impact Assessments (MODARIA II), the experience of participating organisations has been collected on topics associated with the definition of PEGs and PEPs using a questionnaire. The objective of the questionnaire was to review the current status and on-going discussions on the handling of issues related to definitions of PEGs and PEPs as an input to the development of biosphere models for assessing radiological impacts on human health and the environment. The answers received to the questionnaire provided a clear overview of the progress that has been made since the original BIOMASS methodology was published, together with the lessons learned from the application of that methodology in the development of safety cases. This paper summarises the questionnaire responses in five subject areas: (1) environment of the PEGs and its evolution; (2) linking the choice of PEGs to these environments; (3) food habits and consumption rates; (4) populations of non-human biota (PEPs) and (5) national and international regulations and guidance. We illustrate how the results of the questionnaire have been used to enhance the original BIOMASS methodology (IAEA Enhanced BIOMASS Methodology Report in press).

Using molybdenum carbiding to induce digestion of carbon in H2O2: A sustainable approach to eliminate radioactivity for hazardous graphite waste inherited from nuclear enterprise

To properly manage nuclear wastes is critical to sustainable utilization of nuclear power and environment health. Here, we show an innovative carbiding strategy for sustainable management of radioactive graphite through digestion of carbon in H₂O₂. The combined action of intermolecular oxidation of graphite by MoO₃ and molybdenum carbiding demonstrates success in gasifying graphite and sequestrating uranium for a simulated uranium-contaminated graphite waste. The carbiding process plays a triple role: (1) converting graphite into atomic carbon digestible in H₂O₂, (2) generating oxalic ligands in the presence of H₂O₂ to favor U-precipitation, and (3) delivering oxalic ligands to coordinate to Mo^VI-oxo anionic species to improve sample batching capacity. We demonstrate > 99% of uranium to be sequestrated for the simulated waste with graphite matrix completely gasifying while no detectable U-migration occurred during operation. This method has further been extended to removal of surface carbon layers for graphite monolith and thus can be used to decontaminate monolithic graphite waste with emission of a minimal amount of secondary waste. We believe this work not only provides a sustainable approach to tackle the managing issue of heavily metal contaminated graphite waste, but also indicates a promising methodology toward surface decontamination for irradiated graphite in general.

Discharges of Nuclear Medicine Radioisotopes: The Impact of an Abatement System

ABSTRACT

Clinical uses of radiopharmaceuticals imply the administration of radioactive substances that are mainly excreted through urine. The Nuclear Medicine Department at the Instituto Nacional de Cancerología (INC-COL) in Bogota, Colombia, administers radiopharmaceuticals for diagnostics and treatment to many patients, resulting in tens of cubic meters of radioactive waste water (WW) every day. As Colombian regulatory limits for liquid radioactive discharges to the sewer system are lower than in other countries, longer WW decay times are required, even when an in-house waste water treatment plant (WWTP) is used. To fulfill the requirements for controlled disposal of radioactive discharges, a complementary abatement system was implemented to retain WW for periods as long as 360 d, and was connected to the hospital´s WWTP. These holding times can cause major changes in the WW physicochemical parameters, reaching levels higher than acceptable. In this study, we evaluate the decontamination and decay efficiency of the retention system using water quality parameters and the amount of radioactivity in the effluents stored in the tanks and the WWTP. According to the results, to maintain the physicochemical parameters below acceptable levels, biological and chemical treatment of decayed WW is necessary before discharging it into urban waste water. Using the principles of dilution, retention, and decay, an integral radioactive WW management system was implemented favoring the quality of discharges and activity levels to the sewer system, with efficiencies close to 100% for WW from discharges in diagnostic procedures ranging from 98% (131I) to 100% (177Lu) for WW from discharges in therapeutic procedures. Activity concentration assessment in medically-derived radionuclides using an in-house waste water treatment plant (WWTP) and a complementary abatement system; an in-house WWTP could be used as an abatement system for short-lived radionuclides; and a tank-based abatement system attached to the in-house WWTP showed higher efficiencies for long-lived radionuclides and adequate physicochemical parameters for the discharge to the city sewage system.

A research and development roadmap to support applications of the enhanced BIOMASS methodology

The International Atomic Energy Agency has coordinated an international project addressing enhancements of methods for modelling the biosphere in post-closure safety assessments of solid radioactive waste disposal. This has resulted in the enhanced BIOMASS methodology that is described elsewhere in this special issue. To a large degree, the enhancements to the BIOMASS methodology arose from experience gained in applying the original methodology, both in the context of other international projects and in assessments of existing or proposed disposal facilities for solid radioactive wastes. Here, this experience is used, together with information on the status of solid radioactive waste disposal programmes worldwide, to identify opportunities for applying the enhanced methodology and for learning from those applications. This provides a basis for identifying research and development to support application of the enhanced methodology in a variety of environmental settings. These research and development requirements include aspects related to climate change under a variety of forcing scenarios, landform development in climatic regimes ranging from cold, polar to arid, tropical, modelling of groundwater flow and contaminant transport in surface-water catchments where both fractured rock and porous sediments are present, and studies of the transport of key radioisotopes of elements central to major biogeochemical cycles, such as those of carbon, chlorine, sulphur and iodine. In addition, some remarks are made on aspects of the application of the enhanced methodology that could imply review and updating of regulations and regulatory guidance, e.g. in relation to the definition of representative persons or groups to be considered in assessments and in respect of approaches to the assessment of radiological impacts on non-human biota. Furthermore, consideration is given as to how the scientific and technical experience that has been gained and methods that have been developed in the context of solid radioactive disposal facilities could support management of contaminated sites and legacy facilities that are likely to require long-term management and stewardship.

Carbon-14 release and speciation during corrosion of irradiated steel under radioactive waste disposal conditions

Carbon-14 is a key radionuclide in the safety assessment of deep geological repositories (DGR) for low- and intermediate-level radioactive waste (L/ILW). Irradiated metallic wastes generated during the decommissioning of nuclear power plants are an important source of ¹⁴C after their disposal in a DGR. The chemical form of ¹⁴C released from the irradiated metallic wastes determines the pathway of migration from the DGR into the environment. In a long-term corrosion experiment with irradiated steel simulating the hyper-alkaline, anoxic conditions of a cement-based DGR, total inorganic (TI¹⁴C²) and organic ¹⁴C contents (TO¹⁴C) in the liquid and gas phases (TG¹⁴C), as well as individual ¹⁴C-bearing carbon compounds by compound-specific radiocarbon analysis (CSRA), were quantified using accelerator mass spectrometry (AMS). The AMS-based quantification allows the determination of ¹⁴C in the pico- to femtomolar concentration range. An initial increase in TO¹⁴C was observed, which could be attributed partially to the release of ¹⁴C-bearing oxygenated carbon compounds. In the long term, TO¹⁴C and the TI¹⁴C remain constant, while TG¹⁴C increases over time according to a corrosion rate of steel of 1 nm/yr. In solution, ¹⁴C-bearing carboxylic acids (CAs) contribute ~40% to TO¹⁴C, and they are the main ¹⁴C carriers along with ¹⁴C-bearing carbonate (¹⁴CO₃^2-). The remaining fraction of TO¹⁴C (~ 60%) is likely due to the presence of as yet non-identified polymeric or colloidal organic material. In the gas phase, ¹⁴CH₄ accounts for more than 80% of the TG¹⁴C, while only trace amounts of ¹⁴CO, and other small ¹⁴C-bearing hydrocarbons have been detected. In a DGR, the release of ¹⁴C will be mainly in gaseous form and migrate via the gas pathway from the repository near field to the surrounding host rock and eventually to the environment.

Strengthened erosion resistance of compacted bentonite by layered double hydroxide: A new electrostatic interaction-based approach

For the geological repository of high-level radioactive waste (HLW) built in granitic host rock，the control of buffer material (compacted bentonite) erosion and subsequent loss caused by groundwater in granite fissures is an unresolved problem of major concern. We propose here new insight into enhancing the erosion resistance of compacted bentonite by means of its electrostatic interaction with oppositely-charged layered double hydroxide (LDH). The interaction between bentonite and LDH was studied by dropwise addition of colloidal LDH into colloidal bentonite suspension, during which the variation in electrical conductivity, zeta potential and particle size proved a strong interaction between these two materials. Interestingly, in addition to their aggregation, intercalated structures of LDH and montmorillonite were found in the composite (BEN@LDH) by a combined characterization of X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM), and were confirmed by density functional theory (DFT) calculation. Colloid generation of compacted BEN@LDH under ultrasonic conditions is negligible comparing with that of compacted bentonite, indicating a significantly higher erosion resistance. Besides, a small amount of LDH by mechanically mixing with bentonite (mass ratio 1:99) can also effectively improve the erosion resistance of compacted bentonite. Moreover, BEN@LDH displayed stronger retention performance towards U(VI) and Se(IV) than bentonite under near-neutral/weakly alkaline conditions. Our results indicate that LDH is a promising additive in compacted bentonite, and this approach may be extended to common geotechnical structures built with clays and soils.

Environmental radiation and health risk assessment in the neighborhood of a radioactive waste management facility

The Radioactive Waste Management Center (RWMC) of the Ghana Atomic Energy Commission (GAEC) operates a licensed radioactive waste management facility known as the Centralized Radioactive Waste Management Facility (CRWMF). The Center undertakes environmental radiation monitoring in which indoor dose rates at various microenvironments, and nearby ambient environments of the facility are measured. A 2-year radiation dose data (i.e., 2017 and 2018) obtained from the monitoring exercise was used to determine whole-body exposure and cancer risk analysis for adult and child age groups. With the exception of the high dose area of the facility, observed doses in all microenvironments of the facility as well as the ambient environment were below the regulatory dose limits of 1 mSv/y and 20 mSv/y, set for radiation workers and the general public, respectively. Dose rate variation for the 2017 and 2018 datasets were not significant (p > 0.05) at 95% confidence interval (CI). Cancer risks due to exposure to alpha, neutron, and gamma radiation sources for both adult and child age groups were above the global average value of 2.90 × 10^-4 reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Gamma sources recorded the highest cancer risk followed by neutron and alpha sources with risk values of 3.95 × 10^-1 and 3.92 × 10^-2; 4.06 × 10^-2 and 4.03 × 10^-3; and 7.96 × 10^-4 and 7.91 × 10^-5 for the adult and child age groups, respectively. Radium (²²⁶Ra) recorded the highest activity concentration (9.62 × 10¹⁰ Bq) with 4 quantities in the inventory while plutonium-beryllium (as alloyed source) recorded the lowest activity concentration (9.82 × 10⁰¹) with 12 quantities in the inventory.

State-of-the-art progress for the selective crystallization of actinides, synthesis of actinide compounds and their functionalization

Crystallization and immobilization of actinides to form actinide compounds are of significant importance for the extraction and reutilization of nuclear waste in the nuclear industry. In this paper, the state-of-art progress in the crystallization of actinides are summarized, as well as the main functionalization of the actinide compounds, i.e., as adsorbents for heavy metal ions and organic pollutant in waste management, as (photo)catalysts for organic degradation and conversion, including degradation of organic dyes and antibiotics, dehydrogenation of N-heterocycles, CO₂ cycloaddition, selective alcohol oxidation and selective oxidation of sulfides. This review will give a comprehensive summary about the synthesis and application exploration of solid actinide crystalline salts and actinide-based metal organic frameworks in the past decades. Finally, the future perspectives and challenges are proposed in the end to give a promising direction for future investigation.

Safety assessments undertaken using the BIOMASS methodology: lessons learnt and methodological enhancements

The International Atomic Energy Agency has coordinated an international project addressing enhancements of methods for modelling in post-closure safety assessments of solid radioactive waste disposal. The project used earlier published work from the IAEA biosphere modelling and assessment (BIOMASS) project to further develop methods and techniques. The task was supported by a parallel on-going project within the BIOPROTA forum. The output from the project is described in detail in a forthcoming IAEA report. Here an overview of the work is given to provide researchers in the broader fields of radioecology and radioactive waste disposal with a summarised review of the enhanced BIOMASS methodology and the work that has been undertaken during the project. It is hoped that such dissemination will support and promote integrated understanding and coherent treatment of the biosphere component within the overall assessment process. The key activities undertaken in the project were: review and identification of those parts of the original BIOMASS methodology that needed enhancement, discussions on lessons learned from applying the BIOMASS method, using real examples to assess the methodology and its usefulness, and writing of those parts of the methodology that were considered could benefit from refinement or for which new guidance was required to take account of scientific developments. The work has shown that the overall approach in the original BIOMASS methodology has proven sound. However, the enhanced version clarifies the need for an iterative and holistic approach with system understanding central to the approach. Specifically, experience, especially in site-specific contexts, has emphasised that adequate system understanding is essential in underpinning safety assessments for radioactive waste disposal. The integral role of the biosphere within safety assessment is also emphasised in the enhanced methodology.

Assessment of Radiological Hazard of Radioactive Waste Using Effective or Organ Doses: How This May Affect Final Waste Disposal

ABSTRACT

The radiological hazard of spent nuclear fuel and radioactive waste slows down further development of nuclear energy systems. The authors evaluate timescales required to reduce the radiological hazard of accumulated waste to the reference level of natural uranium that had been consumed by the nuclear energy system. The estimate of this time scale depends on the radiological hazard metric used in the calculations. In this study, two metrics are compared: (1) the committed effective dose based on ICRP Publication 72 and (2) the lifetime radiation risk calculated with use of organ doses and recent radiation risk models recommended by ICRP. The effective dose of the waste reaches the reference level 300 y after the accumulation of waste, while lifetime attributable risk of waste converges to natural uranium in 100 y. Thus, the lifetime attributable risk (LAR) metric is more appropriate to estimate the time requirements for radioactive waste storage and disposal. The effective dose metric significantly overestimates this timescale as it is not intended for quantifying radiation-related risks.

External dose reconstruction at the shore of the Metlinsky Pond in the former village of Metlino (Techa River, Russia) based on environmental surveys, luminescence measurements and radiation transport modelling

The cohorts of people formerly living at the Techa River shoreline in the Southern Urals, Russia, are widely studied cohorts for the investigation of low-dose radiation effects to human health. The nuclear facilities of the Mayak Production Association (PA) discharged their radioactive effluents into the nearby Techa River, especially in the first years of operation. Health status of cohort member data is constantly being improved and updated. Consequently, there is a need to also improve and verify the underlying dosimetry, which gives information about the dose of cohort members. For the Techa River population, the dosimetry is handled in the Techa River Dosimetry System (TRDS). The present work shows results of a feasibility study to validate the TRDS at the location of the village of Metlino, a village just 7 km downstream from the Mayak PA. For this settlement there were two sources of external exposure, the contaminated banks of the Techa River and the contaminated shoreline of the nearby Metlinsky Pond. In the present study the north-western wall of a granary was used as a dose archive to validate dose estimates. Measurements of doses in brick accumulated over many decades and measurements of the current dose rate in bricks were combined with dose rate measurements in air above ground in front of the granary, historical contamination data and Monte-Carlo simulations. Air kerma estimates for 1949-1956 significantly different from zero could not be reconstructed for the Metlinsky Pond shoreline near the granary, but an upper dose limit could be estimated. Implications for TRDS-2016 are discussed.

Preparation and related properties of geopolymer solidified uranium tailings bodies with various fibers and fiber content

Uranium tailing ponds are a potential major source of radioactive pollution. Solidification treatment can control the diffusion and migration of radioactive elements in uranium tailings to safeguard the surrounding ecological environment. A literature review and field investigation were conducted in this study prior to fabricating 11 solidified uranium tailing samples with different proportions of PVA fiber, basalt fiber, metakaolin, and fly ash, and the weight percentage of uranium tailings in the solidified body is 61.11%. The pore structure, volume resistivity, compressive strength, radon exhalation rate variations, and U(VI) leaching performance of the samples were analyzed. The pore size of the solidified samples is mainly between 1 and 50 nm, the pore volume is between 2.461 and 5.852 × 10^-2 cm³/g, the volume resistivity is between 1020.00 and 1937.33 Ω·m, and the compressive strength is between 20.61 and 36.91 MPa. The radon exhalation rate is between 0.0397 and 0.0853 Bq·m^-2·s^-1. The cumulative leaching fraction of U(VI) is between 2.095 and 2.869 × 10^-2 cm, and the uranium immobilization rate is between 83.46 and 85.97%. Based on a comprehensive analysis of the physical and mechanical properties, radon exhalation rates, and U(VI) leaching performance of the solidified samples, the basalt fiber is found to outperform PVA fiber overall. The solidification effect is optimal when 0.6% basalt fiber is added.

The effect of low-pH concrete on microbial community development in bentonite suspensions as a model for microbial activity prediction in future nuclear waste repository

Concrete as an important component of an engineered barrier system in deep geological repositories (DGR) for radioactive waste may come into contact with bentonite, or other clays, rich in indigenous microorganisms, with potentially harmful impacts on barrier integrity. Our study aimed to assess the effect of a concrete environment on indigenous bentonite and groundwater microbial communities as these particular conditions will select for the potentially harmful microorganisms to the concrete in the future DGR. The two-month experiment under anoxic conditions consisted of crushed, aged, low-pH concrete, Czech Ca-Mg bentonite, and anoxic groundwater, with control samples without concrete or with sterile groundwater. The microbial diversity and proliferation were estimated by qPCR and 16S rRNA gene amplicon sequencing. The presence of concrete had a strong effect on microbial diversity and reduced the increase in total microbial biomass, though low-pH concrete harbored indigenous bacteria. The growth of sulfate reducers was also limited in concrete samples. Several genera, such as Massilia, Citrifermentans, and Lacunisphaera, dominant in bentonite controls, were suppressed in concrete-containing samples. In contrast, genera such as Bacillus, Dethiobacter and Anaerosolibacter specifically proliferated in the presence of concrete. Genera such as Thermincola or Pseudomonas exhibited high versatility and proliferated well under both conditions. Because several of the detected bacterial genera are known to affect concrete integrity, further long-term studies are needed to estimate the effect of bentonite and groundwater microorganisms on concrete stability in future DGR.

Radionuclide geochemistry evolution in the Long-term In-situ Test (LIT) at Grimsel Test Site (Switzerland)

The Long-term In-situ Test (LIT) of the Colloid Formation and Migration project (CFM) at the Grimsel Test Site, investigates the generation of bentonite colloids and, hence, radionuclide mobilization within a well-defined and controlled shear zone in a crystalline rock. In this context, the determination of radionuclide aqueous speciation is essential to understand whether radionuclides are easily transported or immobilized by precipitation or uptake processes in the bentonite barrier included in a repository concept for nuclear waste, and mimic in the LIT experiment. The objective of this work is to determine the aqueous speciation of seven radionuclides (i.e. ⁷⁵Se(VI), ⁹⁹Tc(VII),²³³U(VI), ²³⁷Np(V), ²⁴¹Am(III), Th(IV) and ²⁴²Pu(IV)) by thermodynamic calculations in different water compositions representing the geochemical evolution through the LIT. A comparison of the results obtained from two different modelling groups allows the identification of the geochemical key parameters affecting radionuclide mobility in this context and the corresponding numerical and conceptual uncertainties. Particularly, silicate complexes of trivalent actinides and uranium(VI) carbonato complexes (i.e. Ca_nUO₂(CO₃)₃^(4-2n) n = 1 or 2) seem to be crucial in these environments, even at reducing conditions. Conceptual uncertainties like inclusion/exclusion of tetravalent actinide-bearing colloids formation and polyselenides have clearly been identified.

Methodological developments and practice in characterisation of unconventional and legacy waste

Radioactive waste management requires planned and systematic actions to provide confidence that the entire system, processes and final products will satisfy given requirements for quality. The characterisation process is dependent on setting clear characterisation objectives and gathering the right information to underpin the decisions that need to be taken to manage the waste safely. This paper reviews experience of characterisation of waste generated from past nuclear activities that were not conducted in compliance consistent with current criteria, or from unexpected situations that were not planned for. This experience shows that the development of a reliable and efficient characterisation and categorisation methodology is a common challenge for such wastes, referred to here as unconventional and legacy (UL) waste. Through the activites of the Nuclear Energy Agency Expert Group on the Characterisation of Unconventional and Legacy Waste, consideration has been given to widely used waste stream characterisation procedures and methods that were originally developed primarily for application in conventional decommissioning work. Although they provide a substantial basis for characterisation, there are various additional factors that commonly need to be taken into account in the case of UL waste. By analysing the challenges and lessons learned from a variety of case studies and other international experience, it has been possible to identify opportunities for adaptations and enhancements to these characterisation methologies, and these are set out and explained. The need for integration of waste characterisation with other aspects of strategic planning for UL waste management is discussed, including characterisation to address any non-radiological hazards. The analysed case studies have also highlighted the importance of developing a robust legislative and regulatory framework in parallel with an appropriate waste infrastructure to treat, store and dispose of UL waste. Finally, the basic features of a UL waste characterisation roadmap are presented, including the interactions within a wider UL waste management programme and key areas for further consideration and possible development. It is anticipated such work can be supported by continued international cooperation.

Experience in radiological risk assessment of a surface waste disposal facility in Chişinău, Moldova

The long-term condition and potential radiological consequences of legacy radioactive waste stored in a RADON-type of near-surface disposal facility outside the city of Chişinău is of concern to the central government and health protection authorities of the Republic of Moldova. A 'zero alternative scenario' risk assessment has been undertaken in order to evaluate the potential radiological impact on humans and the environment of the facility, were it to be left in its current state with no remediation. The results have been used as a basis for regulatory decision making regarding remediation and decommissioning of the legacy radioactive waste facility. The aim of this study was two-fold: first to demonstrate a complete radiological risk assessment of a real site using a combination of methodologies developed by the IAEA (ISAM and BIOMASS), the second to illustrate the current state-of-the-art in respect of extracting site-specific information from site-descriptive material. We illustrate the practicality of employing geographic information systems techniques on site-specific topographic data to identify relevant biosphere dose objects, thereby allowing customisation of the generic ISAM model framework to site-specific conditions. As a result, a simple method is suggested to bound activity concentrations in well water based on an understanding of water balance in the local catchment area in which the biosphere dose object is embedded. With conservative assumptions, estimated doses from the calculation cases of the design scenario remain lower than the IAEA's dose criteria and environmental screening values. However, the results also indicate that human intrusion activities after the institutional control period could lead to radiological exposures above the IAEA's criteria for a period up to 100 000 years. The long-lived radionuclide²³⁹Pu dominates doses for the on-site residence scenario. Remediation measures should be implemented were the waste to remain at its present place of disposal.

Decommissioning procedure and induced activation levels, calculations and measurements in an 18 MeV medical cyclotron

The present article describes the decommissioning of a self-shielded 18 MeV medical cyclotron IBA Cyclone 18/9 after 14 years of operation. A Monte Carlo simulation of the possible nuclear reactions was performed in order to plan the decommissioning activities. During the cyclotron dismantling, the activities of the cyclotron components, concrete wall and floor samples were measured. Residual activities were analysed by means of an HPGe detector and liquid scintillation counting, and compared with simulation data. Dosimetry of the staff involved in the decommissioning procedure was monitored by individual TL dosimeters and/or digital dosimeter. The cyclotron component analysis confirmed the presence of gamma and pure beta emitters,²²Na,⁵⁴Mn,⁶⁰Co,⁶⁵Zn,²⁰⁷Bi,⁵⁵Fe,⁶³Ni at different values of specific activity, depending on the positioning of the sample point and on the alloy of the sampled part. In these components the presence of gamma and pure beta emitters was measured 5 years after the shutdown at levels far above clearance limits as defined by the 'Recommended radiological protection criteria for the recycling of metals from the dismantling of nuclear installations' (RP89) guidelines. The simulation, carried out by FLUKA Code (version 2020.0.5) on the cyclotron components, provided good agreement with measurements, with a maximum discrepancy of the same order as the uncertainties. Four engineers of the cyclotron maintenance staff were involved in the dismounting of the hottest components and rigging of the cyclotron in the deposit 6 months after shutdown and two engineers were involved during the drilling phase 3.5 years after shutdown. The measured dose from external exposure of the involved staff was lower than 100μSv person^-1during the first phase and lower than 20μSv person^-1during the final drilling phase. Measured doses from intake were negligible. In conclusion, the decommissioning of the 18 MeV cyclotron does not represent a risk for the involved staff, but, due to the presence of long-lived radioisotopes, the cyclotron components are to be treated as low level radioactive waste, and stored in an authorised storage area for at least 25 years after shutdown.

Sorption of beryllium in cementitious systems relevant for nuclear waste disposal: Quantitative description and mechanistic understanding

Beryllium has applications in fission and fusion reactors, and it is present in specific streams of radioactive waste. Accordingly, the environmental mobility of beryllium needs to be assessed in the context of repositories for nuclear waste. Although cement is widely used in these facilities, Be(II) uptake by cementitious materials was not previously investigated and was hence assumed negligible. Sorption experiments were performed under Ar-atmosphere. Ordinary Portland cement, low pH cement, calcium silicate hydrated (C-S-H) phases and the model system TiO₂ were investigated. Sorption kinetics, sorption isotherms and distribution ratios (R_d, in kg⋅L^-1) were determined for these systems. Molecular dynamics were used to characterize the surface processes driving Be(II) uptake. A strong uptake (5 ≤ log R_d ≤ 7) is quantified for all investigated cementitious systems. Linear sorption isotherms are observed over three orders of magnitude in [Be(II)]_aq, confirming that the uptake is controlled by sorption processes and that solubility phenomena is not relevant within the investigated conditions. The analogous behaviour observed for cement and C-S-H support that the latter are the main sink of beryllium. The two step sorption kinetics is explained by a fast surface complexation process, followed by the slow incorporation of Be(II) in C-S-H. Molecular dynamics indicate that Be(OH)₃^- and Be(OH)₄^2- are sorbed to the C-S-H surface through Ca-bridges. This work provides a comprehensive quantitative and mechanistic description of Be(II) uptake by cementitious materials, whose retention properties can be now reliably assessed for a wide range of boundary conditions of relevance in nuclear waste disposal.

Quantification of Raman-Interfering Polyoxoanions for Process Analysis: Comparison of Different Chemometric Models and a Demonstration on Real Hanford Waste

The Hanford site represents a complicated environmental remediation challenge, remaining from the production of nuclear weapons. Over 100 million gallons of liquid radioactive waste of unknown composition will be chemically processed and vitrified, but the varying chemical composition and highly radioactive nature of the waste preclude the implementation of more developed, offline technologies to determine the composition. The only practical approach to waste treatment will require the significant utilization of real-time, chemometric modeling approaches. Although chemometric approaches have been applied to the analysis of Hanford waste, the models developed were highly tank-specialized, and limited discussion was provided on how models fared with interfering signals. As the tank waste is largely composed of oxoanions, which tend to have interfering Raman spectra, the general question was posed as to what chemometric approach is best suited to accurately quantify analytes in the presence of interfering signals. This was carried out by examining the ability of classical least square (CLS), principal component regression (PCR), partial least square (PLS), and locally weighted regression (LWR) to quantify NO₃^- and CO₃^2- using their bands around 1050 cm^-1. For all samples, the PLS-based model was found to be the most efficient approach from a model building and application perspective.

NORM survey within the second census on pollution sources in China

The Second Census of Pollution Source in China was conducted from 2017 to 2020, and the radioactive target in this census was Naturally Occurring Radioactive Material (NORM).The census plan for the radioactive component was prepared by the Nuclear and Radiation Safety Centre of the Ministry of Ecology and Environment in accord with the work requirement of the overall census. The work steps involved in performing the census included establishing the organization structure, document formulation, conducting a pilot survey, relevant training, screening of preliminary survey data, quality assurance, detailed data collection and analysis, data verification and reporting, and final summarizing. The survey mainly involved 15 kinds of NORM industries, which include the rare earth, niobium/tantalum, zircon, zirconia and tin industries, etc. Almost 30,000 enterprises were investigated in the screening phase, and nearly 2000 enterprises were determined to satisfy the screening criteriafor the presence of NORM. A total of 3500 samples including discharge water, raw ore, milling ore and solid waste were obtained and measured resulting in about20,000 individual datum. The Nuclear and Radiation Safety Centre of the Ministry of Ecology and Environment had responsibility for the NORM census and has completed a comprehensive statistical analysis of the data including analysis of the characteristics from different perspectives.

The status and trends in radioactive contamination of groundwater at a LLW-ILW storage facility site near Sosnovy Bor (Leningrad region, Russia)

The article presents results of field studies at a site of radioactive waste storage and disposal facilities (the so-called LD RosRAO site, Sosnovy Bor, Leningrad region, Russia). The objective of the study is to overview the history and occurrence of groundwater contamination to answer the question whether the radioactive plume is historical (formed due to accidents, which happened at the beginning of the operations with the radioactive waste) or the release of radioactive solutions to groundwater is still occurring. The main method used to study the evolution of radioactive contamination is to analyze long-term observations of tritium, total alpha, and total beta activities in groundwater samples. The role of the hydrogeological conditions of the site, such as flow parameters, groundwater flow pattern, inter-aquifer downward groundwater leakage, and water-table fluctuations, in the evolution of contamination plumes has also been analyzed. In the field investigations, the integrity of the storage buildings to potential leaks was confirmed by different indirect methods. It was concluded that there is currently no significant release of radioactive components into groundwater at the LD RosRAO site and that the present conditions correspond to the stage of rehabilitation of the groundwater environment.