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.