Benchmark experiment for radio-activation of ex-core structural materials using NCA and analysis with three-dimensional transport codes

In this study, a radio-activation experiment was conducted using stainless steel outside the active fuel region (active core) in the Toshiba Nuclear Critical Assembly (NCA) in order to verify the homogenization method by simulating the NCA experimental reactor system and understand the effects of this method on the analysis accuracy. In order to validate homogenization method, we simulated the system using the continuous energy Monte Carlo code MCNP, which allows heterogeneously modeling, and examined application of the homogenization method used for modeling commercial boiling water reactors (BWRs) with the TORT code. The calculation results of activation rate obtained by using the MCNP code with either heterogeneous or homogeneous models do not affect the calculation result of activation rate outside the active core. As the homogenization method was validated, the calculation of activation rate using the TORT code was performed with the same homogeneous model as in the MCNP calculation. The results of the activation rate calculation using the TORT code gave values 20 to 30% larger than the calculation results obtained via MCNP for ⁵⁵Mn. This is considered to be caused by thermal energy group structure which is treated as one group. (author)