Restoring areas after a radioactive fallout: A multidisciplinary study on decontamination.

Land remediation is an important part of restoration measures after a radioactive fallout containing long-lived fission products such as 137Cs. In this multidisciplinary study, we focused on three main issues related to remediation of contaminated urban areas. First, we assessed how much decontamination contributes to reducing resident radiation exposure and how much this reduction depends on the timing of implemented measures. Second, we calculated direct and indirect costs of decontamination in an industrialized country such as Sweden. Finally, in a survey study, we considered reactions of Swedish citizens to being given the hypothetical option of moving to a site decontaminated after radioactive fallout and how this predicted response might influence the design of contingency plans. The main findings are that clean-up operations must be done within the first few years after a fallout to contribute significantly to reducing residual dose. If conducted within 1-2 years, large-scale decontamination can, on average, avert 20-200 manSv per km2 residential area and unit ground deposition of 137Cs (1 MBq). The estimated direct costs (in 2020 purchasing power) would amount to 100 million Euro per km2 decontaminated residential area (comparable to Japanese estimates after the Fukushima accident), generating 39,000 m3 of radioactive waste on average, mainly in the form of 137Cs-contaminated topsoil. In our survey study of 2291 Swedish respondents about their willingness to return to decontaminated homes, women, families with resident children, and high-income earners exhibited more skepticism about returning, even if authorities were to deem it safe. The demographic pattern in attitudes was similar to that found among evacuees in the Fukushima prefecture after 2011. We conclude that predefined ranges of measured 137Cs ground deposition can be used as guidance for rescue leaders in the early post-accident phase in long-term planning for affected areas. This planning should include timing and intensity of decontamination measures, duration of evacuation, and risk communication to citizens. Because some citizens expressed both high risk perception and risk aversion, however, timely and dialogic communication is unlikely to limit a shift after the incident to an older and more male-dominated population composition. There is a risk that those who can afford to do so will move away, whereas people whose wealth is locked in property (houses or businesses) will feel stuck. Perceptions of unfairness may fray the social fabric and complicate resettlement, which in some cases may mean inefficient outlay of decontamination costs. We believe that the issue of monetary compensation to affected residents requires priority in future work.

Time-dependence of decontamination efficiency after a fallout of gamma-emitting radionuclides in suburban areas: a theoretical outlook on topsoil removal.

Decontamination of urban areas may be necessary in the case of extensive fallout of radioactive material after a nuclear accident, as removal of contaminated soil and vegetation will significantly reduce doses for the residents in an area affected by fallout. Experience from Japan shows that cleanup operations of urban areas may take years despite investment in ample resources. The time delay between the initial fallout and completion of the decontamination measures allows natural and physical processes to affect the results. The efficiency of the decontamination will therefore depend significantly on time. Radioecological modeling and computer simulation of urban topography with one-story houses were applied in this study to estimate action-influenced time-integrated dose reductions (TDR) of contaminated topsoil removal as a function of time after the fallout. Results indicate that the TDR decreases gradually after the fallout depending on the vertical migration rate of radiocesium and, to some extent, the initial 134Cs/137Cs ratio. Delaying the topsoil removal from 1 to 10 years will result in a TDR decrease by more than a factor of two. Removing the topsoil within one year after fallout results typically in an averted effective dose between 34 and 80 mSv per MBq m-2 deposition of 137Cs for residents in wooden houses. The corresponding values for residents in brick houses are about 50% lower due to higher shielding. Additional modeling is needed to estimate how age and sex influence the averted detriment to affected cohorts. In addition, more in-depth knowledge of how the efficiency of topsoil removal in practice compares with hypothetical models and the effect of incomplete removal of radiocesium is needed to improve calculations of TDR values.

Accuracy of a Bayesian technique to estimate position and activity of orphan gamma-ray sources by mobile gamma spectrometry: Influence of imprecisions in positioning systems and computational approximations.

The purpose of this study was to investigate the effects of experimental data on performance of a developed Bayesian algorithm tailored for orphan source search, estimating which parameters affect the accuracy of the algorithm. The algorithm can estimate the position and activity of a gamma-ray point source from experimental mobile gamma spectrometry data. Bayesian estimates were made for source position and activity using mobile gamma spectrometry data obtained from one 123% HPGe detector and two 4-l NaI(Tl) detectors, considering angular variations in counting efficiency for each detector. The data were obtained while driving at 50 km/h speed past the sources using 1 s acquisition interval in the detectors. It was found that deviations in the recorded coordinates of the measurements can potentially increase the uncertainty in the position of the source 2 to 3 times and slightly decrease the activity estimations by about 7%. Due to the various sources of uncertainty affecting the experimental data, the maximum predicted relative deviations of the activity and position of the source remained about 30% regardless of the signal-to-noise ratio of the data. It was also found for the used vehicle speed of 50 km/h and 1 s acquisition time, that if the distance to the source is greater than the distance travelled by the detector during the acquisition time, it is possible to use point approximations of the count-rate function in the Bayesian likelihood with minimal deviations from the integrated estimates of the count-rate function. This approximation reduces the computational demands of the algorithm increasing the potential for applying this method in real-time orphan source search missions.

Maximum detection distances for gamma emitting point sources in mobile gamma spectrometry.

In this study an algorithm was developed for calculating maximum detectable distance (MDD) for mobile gamma-ray detection of lost sources in-situ for some common mobile spectrometer systems and gamma sources (137Cs and 60Co). The MDD is a function of detector efficiency, vehicle speed, acquisition time interval, radiation background level and accepted frequency of false alarms. To test its accuracy in predicting the MDD experiments were conducted in-situ. Results indicate agreement with experimental mobile detection distance measurements for 60Co and 137Cs point sources. Both the algorithm and the field test show that at a vehicle speed of 50 km/h, the optimal acquisition time intervals range between 3 and 10 s when searching for unshielded sources with activities of 137Cs and 60Co in the order of 100 MBq. Longer acquisition time intervals (20-30 s) are better to detect higher activity sources at the same speed. However, at higher speeds, shorter time intervals should be selected to increase the MDD. The developed algorithm can help select the optimal combination of detectors, vehicle speed, and instrument settings when using mobile searching in the event of loss of gamma-ray sources.

Maximizing avertable doses with a minimum amount of waste for remediation of land areas around typical single family houses after radioactive fallout based on Monte Carlo simulations.

The uncontrolled release of long-lived radioactive substances from nuclear accidents can contaminate inhabited land areas. The removal of topsoil is an important method for reducing future radiation exposure but can also generate a large amount of waste that needs safe disposal. To the best of our knowledge, previous studies have determined the optimal depth of topsoil removal but not the size of the area designated for this measure. For this purpose, this study performed Monte Carlo simulations of hypothetical 137Cs surface contamination on various ground areas in a typical northern European suburban area. The goal was to study the size of the areas needed and amount of waste generated to achieve a certain relative and absolute dose reduction. The results showed that removing the topsoil from areas larger than 3000 m2 around the houses in the study neighbourhood results in only marginal reduction in radiation exposure. If, on average, 5 cm of topsoil is removed over 3000 m2, then 150 m3 of waste would be generated. However, in this scenario adjacent properties benefit from each other's decontamination, leading to a smaller amount of waste for a given reduction in future radiation exposure per inhabitant of these dwellings. Additionally, it was shown that topsoil removal over limited areas has a higher impact on the absolute dose reduction at an observation point inside or outside the houses with higher initial dose.

Bayesian algorithm to estimate position and activity of an orphan gamma source utilizing multiple detectors in a mobile gamma spectrometry system.

To avoid harm to the public and the environment, lost ionizing radiation sources must be found and brought back under the regulatory control as soon as possible. Usually, mobile gamma spectrometry systems are used in such search missions. It is possible to estimate the position and activity of point gamma sources by performing Bayesian inference on the measurement data. The aim of this study was to theoretically investigate the improvements in the Bayesian estimations of the position and activity of a point gamma source due to introduction of data from multiple detectors with angular variations of efficiency. Three detector combinations were tested-a single 123% HPGe detector, single 4l NaI (Tl) detector and a 123% HPGe with 2x4l NaI (Tl) detector combination-with and without angular efficiency variations for each combination resulting in six different variants of the Bayesian algorithm. It was found that introduction of angular efficiency variations of the detectors did improve the accuracy of activity estimation slightly, while introduction of data from additional detectors lowered the signal-to-noise ratio threshold of the system significantly, increasing the stability and accuracy of the estimated source position and activity, for a given signal-to-noise ratio.

Monte-Carlo simulations of external dose contributions from the surrounding ground areas of residential homes in a typical Northern European suburban area after a radioactive fallout scenario.

The emissions of [Formula: see text] into the environment from the nuclear accidents in Chernobyl in 1986 and Fukushima in 2011 led to the need to decontaminate large areas to avert radiation doses to the population in the affected areas. To be able to perform cost-effective and sustainable remediation, knowledge is needed about how radiation doses can be minimized through optimized interventions such that the greatest possible reduction in radiation dose is obtained with the smallest possible negative impact on the area. Theoretical calculations have been performed to determine how radiation doses in single family houses in a typical Swedish residential suburb arise from a hypothetical [Formula: see text] deposition on the ground. The intention was to highlight how remediation of different parts of the surroundings affects the radiation dose to the residents in a particular property. A Monte Carlo model of the houses and the environment in a suburban area was set up to allow calculations of the dose contributions from different contaminated ground areas such as their own property, neighbouring properties, streets and surrounding recreational areas. Calculations were performed for eleven observation points inside different rooms of the house and one observation point in the garden outside the house, for four houses in the neighbourhood, and for two types of building construction material. The influence of the time spent in different rooms of the house and the contamination of areas surrounding the house was studied. The results show that in general the main dose contribution originates from their own property, but that a significant part (30-80%, depending on the observation point) can come from other areas, showing the importance of considering the surroundings in remediation actions. More detailed analysis of the results showed that the dose contribution from a source region is in general highly dependent on the position of windows in a brick house, whereas for a wooden house the distance to the source region is also of relevance.

Influence of the migration of radioactive contaminants in soil, resident occupancy, and variability in contamination on isodose lines for typical Northern European houses.

In the remedial phase following an accidental radioactive release, it is important that soil decontamination measures are carried out on the areas that contribute most to the radiation dose. In this study, the newly developed concept of isodose lines was applied to the area around typical Swedish dwellings to identify these areas. The influence of the most common building materials in Sweden, wood and brick, and the importance of the positions of doors and windows on the isodose lines were demonstrated for specific positions inside the houses, as well as for the entire house, assuming the residents exhibit typical resident occupancy. Decontamination of the areas within certain isodose lines was shown to result in a greater dose reduction than decontaminating the same area of soil within a certain distance of the house. Furthermore, the impact of vertical migration of the radioactive contaminants in the soil on the isodose lines was studied, showing that the area enclosed by isodose lines decreases over time as the contaminants migrate deeper into the soil. The resulting isodose lines and their change over time are dominated by the downward movement of the contamination in the upper layer of soil. The impact of the variability in contamination on the final isodose lines and their dependence on building materials are demonstrated.

Comparison of experimental and calculated shielding factors for modular buildings in a radioactive fallout scenario.

Experimentally and theoretically determined shielding factors for a common light construction dwelling type were obtained and compared. Sources of the gamma-emitting radionuclides 60Co and 137Cs were positioned around and on top of a modular building to represent homogeneous fallout. The modular building used was a standard prefabricated structure obtained from a commercial manufacturer. Four reference positions for the gamma radiation detectors were used inside the building. Theoretical dose rate calculations were performed using the Monte Carlo code MCNP6, and additional calculations were performed that compared the shielding factor for 137Cs and 134Cs. This work demonstrated the applicability of using MCNP6 for theoretical calculations of radioactive fallout scenarios. Furthermore, the work showed that the shielding effect for modular buildings is almost the same for 134Cs as for 137Cs.

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.

A rotating-slit-collimator-based gamma radiation mapper.

For situations with radioactive material out of control where it may be physically difficult or prohibited to access areas close to the source, measurements from distance may be the only way to assess the radiation environment. Using collimated detectors will provide means to locate the direction of the radiation from the source. To investigate the possibilities of mapping gamma emitting radioactive material in a closed non-enterable area, a tentative system for mapping radioactive materials from a distance was built. The system used a computer controlled cylindrical rotating slit collimator with a high purity germanium detector placed in the cylinder. The system could be placed on a car-towed trailer, with the centre of the detector about 1.4 m above ground. Mapping was accomplished by the use of a specially developed image reconstruction algorithm that requires measurements from two or more locations around the area to be investigated. The imaging capability of the system was tested by mapping an area, 25 by 25 m2, containing three 330 MBq 137Cs point sources. Using four locations outside the area with about 20 min measuring time in each location and applying the image reconstruction algorithm on the deconvoluted data, the system indicated the three source locations with an uncertainty of 1-3 m. The results demonstrated the potential of using collimated mobile gamma radiometry combined with image reconstruction to localize gamma sources inside non-accessible areas.

A deviation display method for visualising data in mobile gamma-ray spectrometry.

A real time visualisation method, to be used in mobile gamma-spectrometric search operations using standard detector systems is presented. The new method, called deviation display, uses a modified waterfall display to present relative changes in spectral data over energy and time. Using unshielded (137)Cs and (241)Am point sources and different natural background environments, the behaviour of the deviation displays is demonstrated and analysed for two standard detector types (NaI(Tl) and HPGe). The deviation display enhances positive significant changes while suppressing the natural background fluctuations. After an initialization time of about 10min this technique leads to a homogeneous display dominated by the background colour, where even small changes in spectral data are easy to discover. As this paper shows, the deviation display method works well for all tested gamma energies and natural background radiation levels and with both tested detector systems.