Doses and risks from the ingestion of Dounreay fuel fragments

Characterization of radioactive particles from the Dounreay nuclear reprocessing facility

Radioactive particles originating from nuclear fuel reprocessing at the United Kingdom Atomic Energy Authority's Dounreay Facility were inadvertently released to the environment in the late 1950s to 1970s and have subsequently been found on site grounds and local beaches. Previous assessments of risk associated with encountering a particle have been based on conservative assumptions related to particle composition and speciation. To reduce uncertainties associated with environmental impact assessments from Dounreay particles, further characterization is relevant. Results of particles available for this study showed variation between Dounreay Fast Reactor (DFR) and Materials Test Reactor (MTR) particles, reflecting differences in fuel design, release scenarios, and subsequent environmental influence. Analyses of DFR particles showed they are small (100-300 μm) and contain spatially correlated U and Nb. Molybdenum, part of the DFR fuel, was identified at atomic concentrations below 1%. Based on SR-based micrometer-scale X-ray Absorption Near Edge Structure spectroscopy (μ-XANES), U may be present as U(IV), and, based on a measured Nb/U atom ratio of ~2, stoichiometric considerations are commensurable with the presence of UNb₂O₇. The MTR particles were larger (740-2000 μm) and contained U and Al inhomogeneously distributed. Neodymium (Nd) was identified in atomic concentrations of around 1-2%, suggesting it was part of the fuel design. The presence of U(IV) in MTR particles, as indicated by μ-XANES analysis, may be related to oxidation of particle surfaces, as could be expected due to corrosion of UAl_x fuel particles in air. High ²³⁵U/²³⁸U atom ratios in individual DFR (3.2 ± 0.8) and MTR (2.6 ± 0.4) particles reflected the presence of highly enriched uranium. The DFR particles featured lower ¹³⁷Cs activity levels (2.00-9.58 kBq/particle) than the MTR (43.2-641 kBq ¹³⁷Cs/particle) particles. The activities of the dose contributing radionuclides ⁹⁰Sr/⁹⁰Y were proportional to ¹³⁷Cs (⁹⁰Sr/¹³⁷Cs activity ratio ≈ 0.8) and particle activities were roughly proportional to the size. Based on direct beta measurements, gamma spectrometry, and the VARSKIN6 model, contact dose rates were calculated to be approximately 74 mGy/h for the highest activity MTR particle, in agreement with previously published estimates.

Characterising radium-226 particles from legacy contamination to support radiation dose assessments

Radioactive particles are physically discrete sources of radioactivity that have been released into the environment as a result of past emergencies, events and practices. As the release of radioactive particles is often unplanned, the source term has not been characterised, and the potential radiation doses have not been prospectively assessed. If a plausible exposure pathway exists, radioactive particles in the environment may present a hazard to the public depending on their radiological, physical and chemical characteristics. Given their physically discrete nature, standard assessment approaches such as dispersion and transfer modelling of liquid and gaseous radioactive releases, are not appropriate for radioactive particles. The challenge for national regulatory authorities is to calculate potential radiation doses from unplanned releases of radioactive particles into the environment, assess whether the doses are relevant to radiological protection and decide whether actions are required to reduce potential doses. To address this challenge, this paper presents the approach being adopted to radiologically, physically and chemically characterise Ra-226 particles from a contaminated legacy site using gamma spectrometry, optical macroscopy and SEM-EDS. The use of particle characterisation data to support radiation dose assessments is discussed and consideration is given to radioactive particles in the context of radiological protection.

Linking heterogeneous distribution of radiocaesium in soils and pond sediments in the Fukushima Daiichi exclusion zone to mobility and potential bioavailability

During the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011 significant amounts of radiocaesium were released into the atmosphere from the reactor units 1, 2 and 3. This caused a non-uniform deposition, in composition and direction, of ¹³⁴Cs and ¹³⁷Cs in the near field (<30 km) from the reactors. In this work, we elucidate the influence of speciation, including radioactive particles, on mobility and potential bioavailability of radiocaesium in soils and sediments from sites located in different directions and distances from the FDNPP. Samples collected in September 2016 were characterized and subjected to sequential chemical extractions and simulated gastrointestinal fluid leaching, and the ¹³⁷Cs and ¹³⁴Cs activities were determined in bulk, grain-size and extracted fractions. The results show that radiocaesium was mainly irreversibly bound and in an inert form. Combined, the two forms contained >90% of the activity present in soils and ~84% in sediments. Digital autoradiography revealed that the inert fraction was predominantly associated with heterogeneities, an indication of radioactive particles. The frequency of heterogeneities was correlated with ¹³⁷Cs activity concentrations, and both were in agreement with the ambient equivalent air doses measured in situ during sampling. Moreover, in situ gamma spectrometry measurements were used in the InSiCal software tool to derive ¹³⁴Cs and ¹³⁷Cs surface contamination. Soil activity concentrations and contamination density estimations, decay-corrected to the day of the FDNPP accident, resulted in ¹³⁴Cs/¹³⁷Cs ratios that match the reported release and deposition plumes from the reactor units. Overall, these results demonstrate the persistence of the particle contamination in the Fukushima near field and highlight the importance of including radioactive particles in environmental impact assessments.

Retention of radioactive particles and associated effects in the filter-feeding marine mollusc Mytilus edulis

Radioactive particles are aggregates of radioactive atoms that may contain significant activity concentrations. They have been released into the environment from nuclear weapons tests, and from accidents and effluents associated with the nuclear fuel cycle. Aquatic filter-feeders can capture and potentially retain radioactive particles, which could then provide concentrated doses to nearby tissues. This study experimentally investigated the retention and effects of radioactive particles in the blue mussel, Mytilus edulis. Spent fuel particles originating from the Dounreay nuclear establishment, and collected in the field, comprised a U and Al alloy containing fission products such as (137)Cs and (90)Sr/(90)Y. Particles were introduced into mussels in suspension with plankton-food or through implantation in the extrapallial cavity. Of the particles introduced with food, 37% were retained for 70 h, and were found on the siphon or gills, with the notable exception of one particle that was ingested and found in the stomach. Particles not retained seemed to have been actively rejected and expelled by the mussels. The largest and most radioactive particle (estimated dose rate 3.18 ± 0.06 Gyh(-1)) induced a significant increase in Comet tail-DNA %. In one case this particle caused a large white mark (suggesting necrosis) in the mantle tissue with a simultaneous increase in micronucleus frequency observed in the haemolymph collected from the muscle, implying that non-targeted effects of radiation were induced by radiation from the retained particle. White marks found in the tissue were attributed to ionising radiation and physical irritation. The results indicate that current methods used for risk assessment, based upon the absorbed dose equivalent limit and estimating the "no-effect dose" are inadequate for radioactive particle exposures. Knowledge is lacking about the ecological implications of radioactive particles released into the environment, for example potential recycling within a population, or trophic transfer in the food chain.

Micro-analytical characterisation of radioactive heterogeneities in samples from Central Asian TENORM sites

The present work focuses on the use of micro-analytical techniques to demonstrate the heterogeneous distribution of radionuclides and metals in soils collected at Former Soviet Union mining sites in Central Asia. Based on digital autoradiography, radionuclides were heterogeneously distributed in soil samples collected at the abandoned uranium mining sites Kurday, Kazakhstan, Kadji Sai, Kyrgyzstan and Taboshar, Tajikistan. Using electron microscopy interfaced with X-ray microanalysis submicron - mm-sized radioactive particles and rock fragments with U, As, Se and toxic metals on the surfaces were identified in Kurday and Kadji Sai samples. Employing scanning and tomographic (3D) synchrotron radiation based micro-X-ray fluorescence (μ-SRXRF) and synchrotron radiation based micro-X-ray diffraction (μ-SRXRD) allowed us to observe the inner structure of the particles without physical sectioning. The distribution of elements in virtual crosssections demonstrated that U and a series of toxic elements were rather heterogeneously distributed also within individual radioactive TENORM particles. Compared to archived data, U in Kadji Sai particles was present as uraninite (U4O9+y or UO2+x) or Na-zippeite ((Na4(UO2)6[(OH)10(SO4)3]·4H2O), i.e. U minerals with very low solubility. The results suggested that TENORM particles can carry substantial amount of radioactivity, which can be subject to re-suspension, atmospheric transport and water transport. Thus, the potential radioecological and radioanalytical impact of radioactive particles at NORM and TENORM sites worldwide should be taken into account. The present work also demonstrates that radioecological studies should benefit from the use of advanced methods such as synchrotron radiation based techniques.

Solid state speciation and potential bioavailability of depleted uranium particles from Kosovo and Kuwait

A combination of synchrotron radiation based X-ray microscopic techniques (mu-XRF, mu-XANES, mu-XRD) applied on single depleted uranium (DU) particles and semi-bulk leaching experiments has been employed to link the potential bioavailability of DU particles to site-specific particle characteristics. The oxidation states and crystallographic forms of U in DU particles have been determined for individual particles isolated from selected samples collected at different sites in Kosovo and Kuwait that were contaminated by DU ammunition during the 1999 Balkan conflict and the 1991 Gulf war. Furthermore, small soil or sand samples heavily contaminated with DU particles were subjected to simulated gastrointestinal fluid (0.16 M HCl) extractions. Characteristics of DU particles in Kosovo soils collected in 2000 and in Kuwait soils collected in 2002 varied significantly depending on the release scenario and to some extent on weathering conditions. Oxidized U (+6) was determined in large, fragile and bright yellow DU particles released during fire at a DU ammunition storage facility and crystalline phases such as schoepite (UO(3).2.25H(2)O), dehydrated schoepite (UO(3).0.75H(2)O) and metaschoepite (UO(3).2.0H(2)O) were identified. As expected, these DU particles were rapidly dissolved in 0.16 M HCl (84 +/- 3% extracted after 2 h) indicating a high degree of potential mobility and bioavailability. In contrast, the 2 h extraction of samples contaminated with DU particles originating either from corrosion of unspent DU penetrators or from impacted DU ammunition appeared to be much slower (20-30%) as uranium was less oxidized (+4 to +6). Crystalline phases such as UO(2), UC and metallic U or U-Ti alloy were determined in impacted DU particles from Kosovo and Kuwait, while the UO(2,34) phase, only determined in particles from Kosovo, could reflect a more corrosive environment. Although the results are based on a limited number of DU particles, they indicate that the structure and extractability of DU particles released from similar sources (metallic U penetrators) will depend on the release scenarios (fire, impact) and to some extent environmental conditions. However, most of the DU particles (73-96%) in all investigated samples were dissolved in 0.16 M HCl after one week indicating that a majority of the DU material is bioaccessible.

SAF values for internal photon emitters calculated for the RPI-P pregnant-female models using Monte Carlo methods

Estimates of radiation absorbed doses from radionuclides internally deposited in a pregnant woman and her fetus are very important due to elevated fetal radiosensitivity. This paper reports a set of specific absorbed fractions (SAFs) for use with the dosimetry schema developed by the Society of Nuclear Medicine's Medical Internal Radiation Dose (MIRD) Committee. The calculations were based on three newly constructed pregnant female anatomic models, called RPI-P3, RPI-P6, and RPI-P9, that represent adult females at 3-, 6-, and 9-month gestational periods, respectively. Advanced Boundary REPresentation (BREP) surface-geometry modeling methods were used to create anatomically realistic geometries and organ volumes that were carefully adjusted to agree with the latest ICRP reference values. A Monte Carlo user code, EGS4-VLSI, was used to simulate internal photon emitters ranging from 10 keV to 4 MeV. SAF values were calculated and compared with previous data derived from stylized models of simplified geometries and with a model of a 7.5-month pregnant female developed previously from partial-body CT images. The results show considerable differences between these models for low energy photons, but generally good agreement at higher energies. These differences are caused mainly by different organ shapes and positions. Other factors, such as the organ mass, the source-to-target-organ centroid distance, and the Monte Carlo code used in each study, played lesser roles in the observed differences in these. Since the SAF values reported in this study are based on models that are anatomically more realistic than previous models, these data are recommended for future applications as standard reference values in internal dosimetry involving pregnant females.

Changes produced by external radiation in parameters influencing intestinal permeability and microparticle uptake in vitro

PURPOSE

To determine the interaction between X-irradiation and in vitro intestinal microparticle uptake through Caco-2 epithelial cells.

METHODS

Caco-2 cells were cultured on 3 microm porous membranes for 21 days, X-irradiated with 2 Gy or sham-irradiated, then incubated for 5 or 30 min and exposed apically for 30 min to 2 microm latex microparticles. Measurements included cell dimensions, from confocal microscope 'optical slices'; transepithelial resistance (TER) for tight junction (TJ) permeability; particle aggregation; and particle numbers on (adsorbed), in (intraepithelial) and through (submembranous) the epithelium.

RESULTS

Irradiation alone reduced TJ permeability more than sham-treatment, more so 5 min than 30 min after treatment. Irradiated epithelia were more permeable to particles than the equivalent sham-irradiated or previously untreated (particle only) groups: the latter two were similar. Irradiation altered adsorbed particle numbers and increased submembranous counts: particle uptake correlated best with cell height.

CONCLUSIONS

2 Gy X-irradiation increased particle uptake and translocation through the epithelium. This correlated well with the TJ opening seen after particle exposure in irradiated samples and changes in cell morphology. New data on cell dimensions underlined the similarity in particle uptake between this in vitro epithelium and that in an in vivo model, highlighting the translational significance of the work.

Particle data management--turning data into accessible information

The UKAEA has assigned significant resources to the monitoring and retrieval of particles from the vicinity of its site at Dounreay in the north of Scotland. The monitoring, retrieval and analytical processes each generate information that is required for interpretative and record purposes. As significant resources are made available for studying and retrieving particles, the information collected must be managed in a logical, stable and accessible manner to protect the investment in information. If suitable data management procedures are not in place there is a high risk of data loss and duplication, and stakeholders may be unable to discover what information is already available or be unable to access the existing information. UKAEA Dounreay operates a geographic information system (GIS) that interfaces with a data management system known as IMAGES. These systems are used to securely store and access a wide range of environmental data including those relating to particles. A case study is presented illustrating the processes and systems involved with the collection, storage, analysis and distribution of particle data. The advantages gained through use of the GIS system are considered in relation to alternative types of record management system such as paper, and electronic non-spatial systems. It is considered that the GIS system offers very significant benefits in terms of standardisation of data capture, security of storage and increased accessibility of data compared with other systems.

Hot particle dosimetry and radiobiology--past and present

Small high-activity radioactive particles of nominal diameter ranging from approximately 1 mm down to several microm have been a radiological concern over the last 30 years in and around European and American nuclear reactor facilities. These particles have often been referred to as 'hot particles'. The 'hot particle problem' came into prominent concern in the late 1960s. The potential carcinogenic effects in lungs as the result of irradiation by discrete small particles containing alpha-emitting radionuclides, particularly (239)Pu, were claimed by some to be several orders of magnitude greater than those produced by uniform irradiation to the same mean dose. The phrase 'hot particle problem' was subsequently used to refer to the difficulty of predicting health effects for all microscopic radioactive sources. The difficulty arose because of the paucity of comparative human, animal or cell studies using radioactive particles, and the lack of validated measurement or calculational techniques for dose estimation for non-uniform exposures. Experience was largely restricted to uniform, large-area/volume exposures. The concern regarding cancer induction was extended to deterministic effects when the ICRP in 1977 failed to give adequate dose limits for dealing with 'hot particle' exposures of the skin. Since 1980, considerable efforts have been made to clarify and solve the dosimetric and radiobiological issues related to the health effects of 'hot particle' exposures. The general recommendations of the ICRP in 1991 used the latest radiobiological data to provide skin dose limits which are applicable to 'hot particle' exposures. More recently the NCRP has extended considerations to other organs. This progress is reviewed and applied to the specific case of the recent evaluation of potential health effects of Dounreay fuel fragments commissioned by the Scottish Environment Protection Agency (SEPA). Analyses of possible doses and risks in this case indicate that the principal concern following skin contact, ingestion or inhalation is the possibility of localised ulceration of skin or of the mucosal lining of the colon or extra-thoracic airways.