RATIONAL SHIELDING ABILITY EVALUATION FOR A MODULAR SHIELDING HOUSE

In Japan, it is in the planning stage of building interim storage facilities for transportable storage casks. Nuclear spent fuels stored for a prolonged period of time in casks in such facilities would be transported to fuel reprocessing facilities without taking the lid off casks. The criterion of effective dose rate around the boundary of the interim storage facility would be 50μSv/year in Japan, which is one-fifth of the criterion applied in the United States. Though the capacity of the facility heavily depends on the dose rate, any rational methodologies to evaluate dose rate around the interim storage facility have not been developed, because the scale of the system is so large and numerous distributed radiation source terms make calculational procedure complicated. In this paper, we discuss rational methodology to evaluate dose rate at the facility boundary by using two approaches as follows; 1) a method taking into account \"shade effect\" which represents the self-shielding effect of interim storage casks, 2) utilization of newly-developed simplified code, MCNP-ANISN_W. Considering “shade effect\", some radioactive sources in the storage casks are surrounded by other casks in the interim storage facility, thus, the increasing number of the storage casks shield against the radioactive sources much more. Hence, it becomes possible to increase the number of allowable storage casks in the facility. Using the method of shade effect, the dose rate was calculated for a facility stored in 8 or 16 casks and the results of calculation were consistent with that of full Monte-Carlo calculation. In this study, a simple neutron transport code, MCNP-ANISN_W, which incorporates onedimensional transport code, ANISN, into Monte-Carlo code, MCNP, was developed. The contribution of the direct radiation was calculated by MCNP-ANISN_W. The effect of skyshine was separately calculated by using SHINE-III. The total dose of these results was consistent with that of the full Monte-Carlo calculation. Results show that the simple evaluation technique and the code developed in this study would be useful for evaluation of dose rate around the boundary of the interim storage facility. Furthermore, the technique and code is applied to evaluate ships loading many casks.