WEATHERING OF CAESIUM-137 IN SOIL

LARCalc, a tool to estimate sex- and age-specific lifetime attributable risk in populations after nuclear power plant fallout

A tool called LARCalc, for calculating the radiological consequences of accidental large scale nuclear power plant releases based on estimates of 137Cs ground deposition, is presented. LARCalc is based on a previously developed models that has been further developed and packaged into an easy-to-use decision support tool for training of decision makers. The software visualises the radiological impact of accidental nuclear power plant releases and the effects of various protective measures. It is thus intended as a rapid alternative for planning protective measures in emergency preparedness management. The tool predicts projected cumulative effective dose, projected lifetime attributable cancer risk, and residual dose for some default accidental release scenarios. Furthermore, it can predict the residual dose and avertable cumulative lifetime attributable risk (LAR) resulting from various protective measures such as evacuation and decontamination. It can also be used to predict the avertable collective dose and the increase in cancer incidence within the specified population. This study presents the theoretical models and updates to the previous models, and examples of different nuclear fallout scenarios and subsequent protective actions to illustrate the potential use of LARCalc.

Assessment of doses in contaminated urban areas: modelling exercise based on Fukushima data

State-of-the-art dose assessment models were applied to estimate doses to the population in urban areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. Assessment results were compared among five models, and comparisons of model predictions with actual measurements were also made. Assessments were performed using both probabilistic and deterministic approaches. Predicted dose distributions for indoor and outdoor workers from a probabilistic approach were in good agreement with the actual measurements. In addition, when the models were applied to assess the doses to the representative person, based on a concept recommended by the International Commission on Radiological Protection and in the International Atomic Energy Agency Safety Standards, it was evident that doses to the representative person obtained with a deterministic approach were always higher than those obtained with a probabilistic approach using the same model.

Temporal Attenuation of Gamma Dose Rate in Air Due to Radiocesium Downward Mobility in Soil

ABSTRACT

The Chernobyl and Fukushima Dai-ichi Nuclear Power Plant accidents have demonstrated that radiocesium deposited on the ground was one of most important pathway contributions to the air dose rate. Cesium-134 contributes more significantly in the first period of 2-3 y. However, 137Cs external exposure may remain relevant for decades. The contribution to the air dose rate attributable to these radionuclides is maximum at the deposition time and then usually decreases over time. The dose rate temporal reduction is a consequence of both the radionuclide physical decay and the radionuclide downward mobility in soil. In this investigation, this decreasing behavior of the air dose rate is approached using an empirical attenuation function, and its coefficients are computed in terms of the effective diffusion coefficient and downward migration rates of radiocesium in soil. The methodology is tested for different hypothetical scenarios and in real situations, including areas affected by the two major accidents at nuclear power plants.

Summary of temporal changes in air dose rates and radionuclide deposition densities in the 80 km zone over five years after the Fukushima Nuclear Power Plant accident

We summarized temporal changes in air dose rates and radionuclide deposition densities over five years in the 80 km zone based on large-scale environmental monitoring data obtained continuously after the Fukushima Nuclear Power Plant (NPP) accident, including those already reported in the present and previous special issues. After the accident, multiple radionuclides deposited on the ground were detected over a wide area; radiocesium was found to be predominantly important from the viewpoint of long-term exposure. The relatively short physical half-life of ¹³⁴Cs (2.06 y) has led to considerable reductions in air dose rates. The reduction in air dose rates owing to the radioactive decay of radiocesium was more than 60% over five years. Furthermore, the air dose rates in environments associated with human lives decreased at a considerably faster rate than expected for radioactive decay. The average air dose rate originating from the radiocesium deposited in the 80 km zone was lower than that predicted from radioactive decay by a factor of 2-3 at five years after the accident. Vertical penetration of radiocesium into the ground contributed greatly to the reduction in air dose rate because of an increase in the shielding of gamma rays; the estimated average reduction in air dose rate was approximately 25% with penetration compared to that without penetration. The average air dose rate measured in undisturbed fields in the 80 km zone was estimated to be reduced owing to decontamination by approximately 20% compared to that without decontamination. The average deposition density of radiocesium in undisturbed fields has decreased owing to radioactive decay, indicating that the migration of radiocesium in the horizontal direction has generally been slow. Nevertheless, in human living environments, horizontal radiocesium movement is considered to contribute significantly to the reduction in air dose rate. The contribution of horizontal radiocesium movement to the decrease in air dose rate was estimated to vary by up to 30% on average. Massive amounts of environmental data were used in extended analyses, such as the development of a predictive model or integrated air dose rate maps according to different measurement results, which facilitated clearer characterization of the contamination conditions. Ecological half-lives were evaluated in several studies by using a bi-exponential model. Short-term ecological half-lives were shorter than one year in most cases, while long-term ecological half-lives were different across the studies. Even though the general tendency of decrease in air dose rates and deposition densities in the 80 km zone were elucidated as summarized above, their trend was found to vary significantly according to location. Therefore, site-specific analysis is an important task in the future.

Decreasing trend of ambient dose equivalent rates over a wide area in eastern Japan until 2016 evaluated by car-borne surveys using KURAMA systems

As part of the investigation of the distribution of ambient dose equivalent rates around the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), car-borne surveys using Kyoto University RAdiation MApping (KURAMA) systems have been conducted over a wide area in eastern Japan since 2011. The enormous volume of measurement data collected until 2016, including those until 2012 which were reported in the previous paper, was analyzed, and dependencies of the decreasing trend of the dose rates in regions within 80 km of the FDNPP on land-use categories, evacuation order areas and magnitude of the dose rates were examined. The air dose rates within 80 km of the FDNPP tended to decrease considerably with respect to the physical decay of radiocaesium. The decrease of the dose rate in the "forest" was slower than its decrease in other regions, while that in "urban area" was the fastest. The decrease in the air dose rate from 2011 was the fastest outside the evacuation order area until 2015, and it was the slowest in the "difficult-to-return zone". However, the decreasing trend starting from 2013 showed that the decrease in the "zone in preparation for the lifting of the evacuation order" and in the "residence restriction area" was the fastest. It was found that the air dose rates decreased depending on the magnitude of the dose rates and elapsed time from the FDNPP accident, i.e. the decrease in air dose rates in areas with relatively low dose ranges (such as 0.2-0.5 μSv/h) was the largest during a period relatively early after the accident, and the decreasing rate in the dose rate ranges of 1.9-3.8 and 3.8-9.5 μSv/h were the fastest after 2013. The averaged ratios were analyzed to obtain the ecological half-lives of the fast and slow decay components, and those in whole area within 80 km of FDNPP were estimated to be 0.44 ± 0.05 y and 6.7 ± 1 y, respectively. The ecological half-lives with respect to the land use categories, evacuation order areas and magnitude of the dose rates were also evaluated. The decrease in the dose rates obtained by the car-borne survey was larger than that obtained on flat ground with few disturbances using the NaI(Tl) survey meter during approximately 1.5 y after the FDNPP accident. Thereafter, the difference of decreasing tendencies in the air dose rates between both the measurements was negligibly small, with the ratio of dose rates by the car-borne survey to those by the fixed-point measurement of 0.72-0.77.

Measurement of ambient dose equivalent rates by walk survey around Fukushima Dai-ichi Nuclear Power Plant using KURAMA-II until 2016

Ambient dose equivalent rates in various environments related to human lives were measured by walk surveys using the KURAMA-II systems from 2013 to 2016 within an 80-km radius of the Fukushima Dai-ichi Nuclear Power Plant. The dose rate of the locations where the walk survey was performed decreased to about 38% of its initial value in the 42 months from June 2013 to the December 2016, which was beyond that attributable to the physical decay of radiocaesium. The ecological half-life of the slow decreasing component was evaluated to be 4.1 ± 0.2 y. The air dose rates decreased depending on the level of the evacuation areas, and the decrease in the dose rates was slightly larger in populated areas where humans are active. The dose rates as measured by walk surveys exhibited a good correlation with those by car-borne surveys, suggesting that car-borne survey data are reflecting the air dose rates in living environments surrounding roads. The comparison of walk survey data with car-borne survey data indicated that the air dose rate varies largely even within a 100 m square area, and the variation is enhanced by human activities. The dose rates measured by the walk surveys were estimated to be medial of those along roads and those of undisturbed flat ground, and they were found to be decreasing quickly compared with the air dose rate from the flat ground fixed-point measurements.

Modifications in the gamma dose rate in air due to downward and lateral mobility of 137Cs in the soil

¹³⁷Cs can be an important environmental contaminant due to fallout from nuclear reactor accidents and atomic weapons testing. Its contribution to the air gamma dose rate at 1 m height above contaminated ground depends on the soil inventory, the migration processes along the soil profile and possible modifications in the spatial distribution. In this paper the diffusion-convection equation is used to approach the ¹³⁷Cs soil vertical migration transport and standard dose rate factors are jointly applied for estimating the air dose rate. In order to calculate the temporal reduction of the external radiation, an empirical attenuation factor is used and its coefficients are analyzed in terms of the effective diffusion coefficient and downward migration rates. Additionally, it is analyze the corrections that should be introduced in dose rate values attributable to soil redistribution processes. If these processes, natural or as a consequence of human activities, are not taken into account, both the air dose rate values and the attenuation period can be noticeably under or overestimated.

ESTIMATION OF EFFECTIVE DOSE FROM EXTERNAL EXPOSURE DUE TO SHORT-LIVED NUCLIDES IN THE PREFECTURES SURROUNDING FUKUSHIMA

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in a release of radionuclides into the environment. Since the accident, measurements of radiation in the environment such as air dose rate and deposition density of radionuclides have been performed by various organizations and universities. In particular, Japan Atomic Energy Agency (JAEA) has been performing observations of air dose rate using a car-borne survey system continuously over widespread areas. Based on the data measured by JAEA, we estimated effective dose from external exposure in the prefectures surrounding Fukushima. Since car-borne survey started a few months after the accident, the main contribution to measured data comes from 137Cs and 134Cs whose half-lives are relatively long. Using air dose rate of 137Cs and 134Cs and the ratio of deposition density of short-lived nuclides to that of 137Cs and 134Cs, we also estimated contributions to the effective dose from other short-lived nuclides.

Decreasing trend of ambient dose equivalent rates over a wide area in eastern Japan until 2016 evaluated by car-borne surveys using KURAMA systems

As part of the investigation of the distribution of ambient dose equivalent rates around the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), car-borne surveys using Kyoto University RAdiation MApping (KURAMA) systems have been conducted over a wide area in eastern Japan since 2011. The enormous volume of measurement data collected until 2016, including those until 2012 which were reported in the previous paper, was analyzed, and dependencies of the decreasing trend of the dose rates in regions within 80 km of the FDNPP on land-use categories, evacuation order areas and magnitude of the dose rates were examined. The air dose rates within 80 km of the FDNPP tended to decrease considerably with respect to the physical decay of radiocaesium. The decrease of the dose rate in the "forest" was slower than its decrease in other regions, while that in "urban area" was the fastest. The decrease in the air dose rate from 2011 was the fastest outside the evacuation order area until 2015, and it was the slowest in the "difficult-to-return zone". However, the decreasing trend starting from 2013 showed that the decrease in the "zone in preparation for the lifting of the evacuation order" and in the "residence restriction area" was the fastest. It was found that the air dose rates decreased depending on the magnitude of the dose rates and elapsed time from the FDNPP accident, i.e. the decrease in air dose rates in areas with relatively low dose ranges (such as 0.2-0.5 μSv/h) was the largest during a period relatively early after the accident, and the decreasing rate in the dose rate ranges of 1.9-3.8 and 3.8-9.5 μSv/h were the fastest after 2013. The averaged ratios were analyzed to obtain the ecological half-lives of the fast and slow decay components, and those in whole area within 80 km of FDNPP were estimated to be 0.44 ± 0.05 y and 6.7 ± 1 y, respectively. The ecological half-lives with respect to the land use categories, evacuation order areas and magnitude of the dose rates were also evaluated. The decrease in the dose rates obtained by the car-borne survey was larger than that obtained on flat ground with few disturbances using the NaI(Tl) survey meter during approximately 1.5 y after the FDNPP accident. Thereafter, the difference of decreasing tendencies in the air dose rates between both the measurements was negligibly small, with the ratio of dose rates by the car-borne survey to those by the fixed-point measurement of 0.72-0.77.

Measurement of ambient dose equivalent rates by walk survey around Fukushima Dai-ichi Nuclear Power Plant using KURAMA-II until 2016

Ambient dose equivalent rates in various environments related to human lives were measured by walk surveys using the KURAMA-II systems from 2013 to 2016 within an 80-km radius of the Fukushima Dai-ichi Nuclear Power Plant. The dose rate of the locations where the walk survey was performed decreased to about 38% of its initial value in the 42 months from June 2013 to the December 2016, which was beyond that attributable to the physical decay of radiocaesium. The ecological half-life of the slow decreasing component was evaluated to be 4.1 ± 0.2 y. The air dose rates decreased depending on the level of the evacuation areas, and the decrease in the dose rates was slightly larger in populated areas where humans are active. The dose rates as measured by walk surveys exhibited a good correlation with those by car-borne surveys, suggesting that car-borne survey data are reflecting the air dose rates in living environments surrounding roads. The comparison of walk survey data with car-borne survey data indicated that the air dose rate varies largely even within a 100 m square area, and the variation is enhanced by human activities. The dose rates measured by the walk surveys were estimated to be medial of those along roads and those of undisturbed flat ground, and they were found to be decreasing quickly compared with the air dose rate from the flat ground fixed-point measurements.

Modelling the external radiation exposure from the Chernobyl fallout using data from the Swedish municipality measurement system

In connection with the Chernobyl fallout and the subsequent deposition of radionuclides in Sweden, Swedish municipalities launched a measurement program to monitor the external radiation exposure. This program encompasses measurements of the ambient dose equivalent rate 1 m above ground at selected locations, and repeats those measurements at the same locations at 7-month intervals. Measurement data compiled from the seven locations with the highest deposition were combined with data from aerial surveys since May 1986 of ground deposition of ¹³⁷Cs, high-resolution gamma spectrometry performed at four locations in May 1986, and measurements from fixed continuous air gamma rate monitoring stations from 28 April to 15 May 1986. Based on these datasets, a model of the time pattern of the external dose rate in terms of ambient dose equivalent rate from the Chernobyl fallout was developed. The decrease in the ambient dose equivalent rate could, on average, be described by a four-component exponential decay function with effective half-times of 6.8 ± 0.3 d, 104 ± 26 d, 1.0 ± 0.02 y and 5.5 ± 0.09 y, respectively. The predominant contributions to the external dose rate in the first month were from short-lived fission products superseded by ¹³⁴Cs and then ¹³⁷Cs. Integrated over 70 y and using extrapolation of the curve fits, our model predicts that ¹³⁷Cs contributes about 60% and ¹³⁴Cs contributes about 30% of the external effective dose at these seven locations. The projected time-integrated 70 y external effective dose to an unshielded person from all nuclides per unit total activity deposition of ¹³⁷Cs is estimated to be 0.29 ± 0.0.08 mSv/(kBq m^-2). These results are in agreement with those found in Chernobyl contaminated Russian forest areas, and emphasize the usefulness of maintaining a long-term and regular measurement program in contaminated areas.

Individual external dose monitoring of all citizens of Date City by passive dosimeter 5 to 51 months after the Fukushima NPP accident (series): II. Prediction of lifetime additional effective dose and evaluating the effect of decontamination on individual dose

In the first paper of this series, we showed that the ratio c of individual dose to ambient dose did not change with time in Date City, Fukushima Prefecture, after the Fukushima Daiichi Nuclear Power Plant accident. The purpose of the present paper, the second in a series, is to estimate the lifetime doses of the Date City residents, based on continuous glass badge monitoring data, extrapolated by means of the ambient-dose-rate reduction function obtained from the airborne monitoring data. As a result, we found that the external exposure contribution to the mean additional lifetime dose of residents living in Date City is not expected to exceed 18 mSv. In addition, effects of decontamination on the reduction of individual doses were not evident. This method of combining individual doses and the ambient doses, as developed in this study, has made it possible to predict with reasonable certainty the lifetime doses of residents who continue to live in this radiologically contaminated area.

Coupling the advection-dispersion equation with fully kinetic reversible/irreversible sorption terms to model radiocesium soil profiles in Fukushima Prefecture

Radiocesium is an important environmental contaminant in fallout from nuclear reactor accidents and atomic weapons testing. A modified Diffusion-Sorption-Fixation (mDSF) model, based on the advection-dispersion equation, is proposed to describe the vertical migration of radiocesium in soils following fallout. The model introduces kinetics for the reversible binding of radiocesium. We test the model by comparing its results to depth profiles measured in Fukushima Prefecture, Japan, since 2011. The results from the mDSF model are a better fit to the measurement data (as quantified by R²) than results from a simple diffusion model and the original DSF model. The introduction of reversible sorption kinetics means that the exponential-shape depth distribution can be reproduced immediately following fallout. The initial relaxation mass depth of the distribution is determined by the diffusion length, which depends on the distribution coefficient, sorption rate and dispersion coefficient. The mDSF model captures the long tails of the radiocesium distribution at large depths, which are caused by different rates for kinetic sorption and desorption. The mDSF model indicates that depth distributions displaying a peak in activity below the surface are possible for soils with high organic matter content at the surface. The mDSF equations thus offers a physical basis for various types of radiocesium depth profiles observed in contaminated environments.

Assessment of residual doses to population after decontamination in Fukushima Prefecture

Large quantities of radioactive materials were released into the environment as a result of the Fukushima Daiichi Nuclear Power Station accident. Many inhabitants residing in the affected areas are now exposed to radiation in their daily lives. In an attempt to manage this radiation dose, an additional radiation dose of 1 mSv/y was adopted as a long-term dosimetric target. An activity level reading of 0.23 μSv/h was then determined as a guidance value to achieve the target by implementing decontamination measures. The objectives of this study are to assess the effects of decontamination based on this guidance value and to predict any possible future problems with the decontamination strategy. Using a probabilistic approach, we assessed the annual effective dose of indoor workers, outdoor workers, and pensioners in the Fukushima Prefecture. Our probabilistic model considers the variabilities in behavioral patterns and Cs-137 surface-activity levels. Five years after the initial contamination, the 95th percentiles of indoor workers and pensioners in 53 of the 59 municipalities were found to receive annual effective doses of below 1 mSv/y (0.026-0.73 mSv/y). However, for outdoor workers in 25 municipalities, the annual doses were over 1 mSv/y (1.0-35 mSv/y). Therefore, the guidance value is effective for indoor workers and pensioners; to determine whether additional countermeasures for outdoor workers should be implemented, a detailed assessment that uses more realistic assumptions is required.

Outline of the national mapping projects implemented after the Fukushima accident

The national mapping projects were implemented with the collaboration of many organizations in order to obtain reliable and detailed information on radiological conditions due to the Fukushima Daiichi Nuclear Power Plant accident. In the projects, repeated large-scale environmental monitoring has clarified the distributions of radionuclide deposition densities and air dose rates over wide areas and their time-dependent tendencies. In parallel, migration of radiocesium was investigated in the test sites having different environmental conditions to draw a comprehensive picture on the movement of radiocesium in the environment. It turned out that the migration velocity of radiocesium drastically varies depending on conditions and the variation was reflected in air dose rates. The radiocesium migration was slow in undisturbed fields and forest; while monitoring data in urban and water areas implied fast migration velocity of radiocesium in these areas. It was confirmed that radiocesium movement through rivers plays an essential role in long-distance migration of radiocesium. In order to securely store the obtained environmental data and open them to the public, a database was constructed and has been maintained. In the latter half of the projects, we started construction of a numerical model to predict contamination conditions according to the statistical analysis of accumulated monitoring data, as well as numerical models to simulate migration of radiocesium.

Development of prediction models for radioactive caesium distribution within the 80-km radius of the Fukushima Daiichi nuclear power plant

Preliminary prediction models have been studied for the radioactive caesium distribution within the 80-km radius of the Fukushima Daiichi nuclear power plant. The models were represented by exponential functions using ecological half-life of radioactive caesium in the environment. The ecological half-lives were derived from the changes in ambient dose equivalent rates through vehicle-borne surveys. It was found that the ecological half-lives of radioactive caesium were not constant within the 80-km radius of the Fukushima Daiichi nuclear power plant. The ecological half-life of radioactive caesium in forest areas was found to be much larger than that in urban and water areas.

A two-step concept to derive a consistent set of intervention levels for radiation emergency planning and to translate international recommendations into practical guidance

A two-step concept is proposed in order to derive a consistent set of intervention levels for early (sheltering, evacuation) and late (relocation/resettlement, returning) protective actions that have to be considered in radiation emergency planning. In the first step, the dose ratios of the projected effective doses have to be calculated for four defined time periods, which correspond with the integration times for sheltering and relocation. In the second step, it is necessary to adopt an intervention level for one protective action or a more general reference level for a certain time period as, for example, the reference level recommended by the International Commission on Radiological Protection (ICRP) Publication 103 in 2007. The adopted intervention level or reference level and the relationships between the projected effective doses are used to derive a consistent set of intervention levels for early and late protective actions. To illustrate the two-step concept, four sets of intervention levels are exemplarily derived for two accidental releases from nuclear power plants.

Movement of radionuclides in terrestrial ecosystems by physical processes

Physical processes that effect the movement of radionuclides in the temperate environments post-deposition are considered in this paper. The physical processes considered include the interception of radionuclides by vegetation, resuspension, and vertical migration in soil. United States and Russian results on the interception of radionuclides are reviewed and defined in terms of models that are currently undergoing evaluation and revision. New results on resuspension are evaluated, and a preliminary new model for the time-dependent resuspension factor is proposed. Chernobyl-related results on the movement of radionuclides into the soil column are presented, as is a revised model for this process based upon recent results from Ukraine.

Attenuation effects on the kerma rates in air after cesium depositions on grasslands

Since the reactor accident of Chernobyl, cesium depth profiles and nuclide-specific kerma rates in air have been determined for various grassland sites in south Bavaria and in Ukraine. The sites are described by soil characteristics, annual precipitation, distance from release point, mode of deposition, and activity per unit area. The effects of surface roughness and migration of cesium into the soil on the kerma rate in air over grasslands was determined by two methods. The kerma rates in air obtained by the evaluations of in situ gamma-ray spectrometry results and of measured activity distributions in the soil showed only negligible differences for the observation period of 6 years after deposition. For the sites in Ukraine the kerma rate in air per activity per unit area was found to be systematically 40% higher than in Bavaria. The results from Bavaria on the attenuation of the kerma rate and a data set, including experiences from the weapons test fallout, are analytically approximated as a function of time up to 25 years after deposition.