SHIELDING ABILITY OF A MODULAR SHIELDING HOUSE KEEPING SPENT FUEL TRANSPORTABLE STORAGE CASKS IN AN INTERIM STORAGE FACILITY

The shielding ability of a modular shielding house in which 2 unit of 4 spent fuel transportable storage casks are set aside is analyzed with the Monte Carlo coupling technique. The coupling technique is available with the SSW card and the SSR/CRT card in the continuous energy Monte Carlo code MCNP 4C as the “SSW-SSR/CRT calculation system”. The effective dose rate distributions in an interim storage facility are obtained as far as 300 m from the center of the shielding house. In the present Monte Carlo calculations, the total effective dose rate at 300 m from the center of the shielding house is estimated as 0.083 (mSv/y/4 casks). Accordingly, when the distance from the center of the shielding house to the site boundary of the storage facility takes 300 m at least, it might be possible to keep 600 casks approximately in the modular shielding houses, under the Japanese severe criterion of 50 mSv/y at the site boundary of an interim storage facility. Moreover, it is cleared that the secondary gamma rays account for more than 60 % of the effective total dose rate at all the calculated points around the shielding house, and the most of it is produced from the water in the steel-water-steel shielding system of the house. The rest of the dose rate is mostly by neurons, and fission product and 60Co activation gamma rays are a few percents of it. That is the reduction of the secondary gamma rays is a critical matter to improve not only the shielding ability of a shielding house but also the shielding safety of an interim shielding facility