ＭＡＮＹＣＡＳＫコードを用いた使用済核燃料運搬船内のγ線線量率分布計算における容器表面線量率の役割

Abstract

A new method is contrived to economize the CPU time in the calcalation of spatial distributions of γ-ray dose rates in ships loaded with spent fuel shipping casks. One of main featnres of this method is that the method requires radiation dose rates on each cask surface as the input data instead of volume source data of radiations, and another is that the method succeeds in economizing the calculation time remarkably in comparison with that of QAD. In this method, the decrease of dose rates behind shieldings due to shielding effect is calculated by the approximation that the way of decreasing is equal to that of γ-rays with energy of 2.18 MeV. It is dednced from a theoretical study that this approximation yields an overestimation of at most 20% to the calculated dose rates behind shieldings in ships. The energy of 2.18 MeV corresponds to that of γ-rays from 144Pr; a fission product isotope. Consequently, the above mentioned approximation is applicable as far as one deals with spent fuel shipping casks.The new calculation method is coded as MANYCASK code. As an example for the demonstration of applicability of this code, this code is applied to one of the most gopular personal computers in Japan; PC-9801 (VX 21), to the calculation of γ-ray dose rate distributions above a ship loaded with 12 casks. Spatial mesh points of 1, 500 are chosen with spatial interval of 50cm. The CPU time required is about 5min, i.e. very tolerable calculation time for a practical usage of a personal computer. As an output of this calculation, one can obtain a very fine contour map for the spatial distribution of γ-ray dose rates in the ship. From a comparison of dose rate distributions between measurements and calculations, it is clarified that one can calculate reliable dose rate distributions very close to actual ones, if one feeds measured dose rates on the surface of each cask as input data to this code.