NRC SFST ISG-7: Potential Generic Issue Concerning Cask Heat Transfer in a Transportation Accident

Staff raised two major issues concerning the adverse effects of fission gases to the gas-mixture
thermal conductivity in a spent fuel canister in a post accident environment. The two major
concerns were: (1) the reduction of the thermal conductivity of the canister gas by the mixing of
fission gases expelled from failed fuel pins and (2) the resultant temperature and pressure rise
within the canister. Since the fission gas is typically of a lower conductivity than the cover gas,
its mixing with the cover gas tends to lessen the thermal performance of the mixture.
Furthermore, since additional gas is introduced into the canister, the internal pressure will
increase as will the bulk temperature of the gas. The combination of these phenomena, if they
are great enough, would pose a containment issue if the design basis pressure is exceeded.