SHIELDING CALCULATIONS FOR A U.S. DEPARTMENT OF ENERGY TRANSPORT CASK

Shielding calculations were performed for a prototype National Spent Nuclear Fuel Program transport cask for the shipment of Department of Energy spent nuclear fuel and high-level waste. This analysis is intended for use in the selection of the cask shield material and for an estimate of shielding thickness. The results are intended to provide a technical basis for determining an optimal design for the prototype cask. The radiation source term was modeled as cobalt-60 with radiation exposure strength of 100,000 R/hr. Cobalt-60 was chosen as a surrogate source because it simultaneously emits two high-energy gammas. This gamma spectrum will upper bound the spectra of all the various spent nuclear fuels types expected to be shipped within the prototype cask. Point-kernel shielding calculations were performed for a range of shielding thicknesses of lead and depleted uranium. The shielding calculations were performed with the use of an automated computational code (SHIELD), written specifically for these analyses. Sensitivity results were obtained for a wide variation of radiation source gamma energies. The code was designed to generate results for inclusion of stainless steel liners as shielding material and for use in estimating the cask weight as a function of shielding thickness. The computational results were compared to three shielding limits: 1) 200 mrem/hr dose rate at the cask surface, 2) 50 mR/hr exposure rate at one meter from the cask surface, and 3) 10 mrem/hr dose rate at two meters from the cask surface. The results indicate that a shielding thickness of 8.5 cm is required for depleted uranium and 14 cm for lead in order to satisfy all three shielding requirements. The system analysis also indicates that required shielding thicknesses are strongly dependent upon the gamma energy spectrum from the radiation source term. This latter finding suggests that shielding material thickness, and hence cask weight, can be significantly reduced if the radiation source term can be shown to have a softer gamma energy spectrum than that due to cobalt-60.