Evaluation of neutron flux of a PWR dry storage cask

In korea, twenty-four nuclear power plants are currently under operation. As the capacity of spent fuel storage facilities becomes saturated, a dry storage facility will be constructed. To verify the safety of spent fuels in dry storage facility, several studies have been conducted. Most previous studies have proposed a technique based on the detection of gamma-rays. However, since gamma-rays may be emitted from sources other than spent fuel, the detection of gamma-rays have uncertainty. To reduce uncertainty, we developed an evaluation technique based on the neutron detection. The objective of this study was to evaluate neutron flux on the surface of a PWR dry storage cask. TN-32 dry storage cask was selected for this evaluation study. TN-32 dry storage cask is a canister-based cask which was developed by AREVA-TN. Neutron flux on the surface of a dry storage cask was evaluated using computer code. To define source term, ORIGEN-APR code was used to calculate neutron emission rate and energy spectra from a WH 17x17 type spent fuel. Axial distribution of neutron emission rate from a fuel rod was calculated considering axial burnup distribution. MCNP code was used to simulate geometry and material properties of spent fuel assemblies and dry storage cask. Neutron flux for TN-32 was calculated on the surface of top, side and bottom of cask after seven years of cooling time. For TN-32 dry storage cask, average neutron fluxes were evaluated to be 1.08x103 neutrons/cm2·sec, 4.50x102 neutrons/cm2·sec, and 1.04x103 neutrons/cm2·sec on the surface of top, side, and bottom of cask, respectively. Axial neutron flux distributions on the side part varied with cask structure. The reason for high neutron flux on the surface of top and bottom is that neutron shields were designed to have little effect on the top and bottom. This study can be applied to evaluate the characteristic of spent fuel and neutron flux of a dry storage cask. Furthermore, this study can be conducted with measurement data from neutron flux of a dry storage cask.