Development of an External Radiation Dose Estimation Model for Children Returning to Their Homes in Areas Affected by the Fukushima Nuclear Accident.

On 1 April 2017, around 6 y after the Fukushima Daiichi nuclear power station accident, evacuation orders for large affected areas were lifted, and areas to which people could return were expanded. In the current study, a dose estimation model based on a probabilistic approach has been developed to estimate the external radiation doses children would receive after returning to these areas. The target groups are children from infants to high school students, and the target areas are nine municipalities including evacuation areas as of 5 September 2015. The estimation period is for 4 y starting 1 April 2017. Validation of the model in an area for which individual personal dosimetry measurements were available showed that it is valid for infants, kindergarteners, 3rd and 4th grade elementary school students, 5th and 6th grade elementary school students, and junior high school students. Considering the statement of the International Commission on Radiological Protection, the estimated radiation doses for these five age groups were taken to be the 95th percentiles of the predicted distributions as an index of conservative judgement. As a result of our estimations, the 95th percentile doses to all age groups were less than 20 mSv y in all periods and in all areas. The 95th percentile doses in some areas were less than 1 mSv y, which is the long-term dosimetric target set by the Japanese government. It should be noted that our results are preliminary, being based on several assumptions and limitations regarding environmental contamination conditions and the behavioral patterns of children. To estimate the children's doses precisely, further considerations for these assumptions and limitations will be needed.

Development and application of a method for discriminating the influence of radon progenies in air from aerial radiation monitoring data.

The influence of gamma-rays from natural nuclides (particularly the radon progenies, 214Pb and 214Bi) must be excluded from aerial radiation monitoring (ARM) data to accurately estimate the deposition of artificial radionuclides. A method for discriminating the influence of the radon progenies in air from the ARM data was developed. Two types of detectors with different crystal sizes were installed in a helicopter. The gamma-ray responses of these detectors were simulated using EGS5. The influence of the radon progenies in air was excluded using the relation between the count rates of six NaI (Tl) detectors and a LaBr3 detector. The discrimination method was applied to the ARM data obtained from around the Sendai and Fukushima Dai-ichi Nuclear Power Stations. To verify the validity of the discrimination method, the dose rate estimated from the ARM data was compared with the dose rate measured using a NaI survey meter at a height of 1 m above the ground. The application of the discrimination method improved the dose rate estimation, showing the validity of the discrimination method.

Dose-reduction Effects of Vehicles against Gamma Radiation in the Case of a Nuclear Accident.

Self-evacuation by a private vehicle is one of the most commonly used methods of public evacuation in the case of a nuclear accident. The aim of this paper is to evaluate the dose-reduction effects of vehicles. To achieve this aim, a model for calculating the dose reduction factor was developed based on the actual shape and weight of Japanese vehicles. This factor is defined as the ratio of dose rate inside a vehicle to that outside. The model was developed based on weight of vehicle to take into account the dose-reduction effects due to not only the steel plate of the vehicle body but also the other assemblies. In addition to model calculation, the dose reduction factors were evaluated by actual measurements in the areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. A comparison between the simulated and the measured results revealed that the dose reduction factors obtained using the developed models were in good agreement with the results of actual measurements. Using this model, we also evaluated the dose reduction factors for cloudshine and groundshine in the case of a nuclear accident. The evaluations were performed for four vehicle models whose weights were 800-1,930 kg. The dose reduction factor for cloudshine with photon energy of 0.4-1.5 MeV was 0.66-0.88, and that for groundshine from Cs was 0.64-0.73. Although these results were obtained under the assumption that Cs is placed only on the ground surface, according to these considerations, if Cs migrated into the ground corresponding to the relaxation mass depth of 10 g cm, the dose reduction factors would be almost 8% less than those for the ground surface.

Application of topographical source model for air dose rates conversions in aerial radiation monitoring.

After the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in 2011, aerial radiation monitoring (ARM) using a manned helicopter was conducted to rapidly measure air dose rates and the deposition of radioactive nuclides over a large area. Typically, the air dose rate is obtained by conversion from the count rate using conventional flat source model (FSM). The converted dose rate obtained via aerial monitoring poorly matches the results of ground measurement in the mountain and forest areas because FSM does not consider topographical effects. To improve the conversion accuracy, we developed new methods to analyze aerial monitoring data using topographical source model (TSM) based on the analytical calculation of the gamma-ray flux. The ARM results converted using both FSM as well as TSM were compared with ground measurement data obtained after the FDNPS accident. By using TSM, the conversion accuracy was improved. In addition, to determine a parameter sensitive to topographical effects, we examined five parameters and it was clear that the difference between the elevation just below the helicopter and the mean elevation within the measurement area was the most influential.