NUREG/CR-6672: GENERIC CASKS AND THEIR RESPONSE TO FIRES

In order to complete an overall assessment of the safety of transportation of spent nuclear fuel, four typical cask types were selected. The typical cask designs were then analyzed for both thermal response in longduration fires and structural damage during extra-regulatory collisions. This paper describes the four cask designs chosen, tabulates the cask dimensions, and summarizes the analysis method used to obtain the thermal response of the casks to long-duration, fully engulfing fires. The four cask types included two steel truck casks, one with a lead radiation shield and another with a depleted uranium radiation shield. Two rail casks were also included, one with a lead radiation shield, and a second rail cask with a monolithic steel design. The four designs were selected to represent a wide range of typical spent fuel casks that are in current use or are planned for use in future shipping campaigns. To obtain the thermal response of the casks in long-duration, extra-regulatory fires, the PATRAN-P/Thermal computer code was used to construct onedimensional axisymmetric models of the cask wall structures. Boundary conditions for the computer models included a 1000°C fully engulfing fire that burned for 11 hours. A cargo consisting of three-year-old, high burn-up fuel is assumed for the calculations. The cask computer model is described, and temperatures from cask locations are reported in a time vs. temperature format. Cask locations that approximate seal temperatures and fuel rod bundle temperatures were then used to estimate the fire exposure duration that would result in a degradation of the cask seals or fuel rod failure by burst rupture. These results provided data that were used to estimate the fraction of all fires that would last long enough to cause seal failure or burst rupture of rods. A discussion of underlying assumptions and the sensitivity of the thermal model to the various input parameters is presented.