APPLICABILITY OF THE MONTE CARLO METHOD TO THE EVALUATION OF DOSE EQUIVALENT RATES FROM METAL CASKS IN AN INTERIM STORAGE FACILITY

Dose equivalent rates at the premises of an interim storage facility have been evaluated by using the continuous energy Monte Carlo code MCNP-4B[1]. It is confirmed that the direct and skyshine dose equivalent rates are evaluated with sufficient accuracy by using the appropriate detailed source biasing and weight window parameters. The wall thickness required to meet a dose equivalent limit in Japan has been roughly estimated. Furthermore, the shadow effects caused by other surrounding casks and streaming effects through an opening in the ceiling have been evaluated for realistic shielding design of such facility. The direct dose equivalent rates may reduce to about 1/3 by the shadow effects and the skyshine dose equivalent rate may increase by over 10 times by the streaming effects. These results show that neutron skyshine dose equivalent rates may be critical for the shielding design of it. The coupling calculation method based on MCNP-4B code was applied to perform calculations efficiently. The calculation results by the coupling method well agreed with the ordinary method. It is confirmed that the coupling method based on MCNP code is useful to save the calculation time.