SPENT FUEL TRANSPORTATION RISK ASSESSMENT: CASK FIRE ANALYSES

The NRC has recently completed an updated Spent Fuel Transportation Risk Assessment, NUREG-21251 . This assessment considered the response of three certified casks to a range of fire accidents in order to determine whether or not they would lose their ability to contain the spent fuel or maintain effective shielding. The casks consisted of a lead shielded rail cask that can be transported either with or without an inner welded canister, an all-steel rail cask that is transported with an inner welded canister, and a DU shielded truck cask that is transported with directly loaded fuel. For the two rail casks, large pool fires that were concentric (fully engulfing), offset from the casks by 3 meters, and offset from the cask by 18 meters were analyzed using the Computational Fluid Dynamics CAFE-3D fire modeling code coupled with the Finite Element Analysis PATRAN-Thermal heat transfer code. All of the fires were assumed to last for three hours. In addition to these extra-regulatory fires, the regulatory 30-minute fire was analyzed using both the regulatory uniform 800 degrees C boundary condition and the more realistic CAFE-3D fire modeling code. For the truck cask, only the engulfing fire case was analyzed, using a one-hour fire duration. In all of the fire analyses the seal region of the cask stayed below the failure temperature: therefor there would be no release of radioactive material. Also, the temperature of the fuel rods stayed below their burst-rupture temperature, providing another barrier to release. For the lead shielded cask, very severe fires cause some of the lead to melt. There is no leak path for this molten lead to exit the shield region, but its expansion during the melting and subsequent contraction due to solidification during cool-down results in a reduction in gamma shielding effectiveness.