Shielding analysis considerations for spent-fuel transport and storage casks

Spent-fuel transport and storage casks are expected to be major components of the solution to the problem of spent-fuel disposal in the United States. In keeping with as low as reasonably achievable dose levels, these casks are designed to incorporate shielding for neutrons and gamma rays to minimize the radiation exposure associated with spent-fuel cask storage, transport, and maintenance. Nuclear Assurance Corporation (NAC) has obtained good results from the QAD-CG computer code for essentially monoenergetic {sup 60}Co gamma-ray calculations. The QAD-CG code is also used in the evaluation of the dose rate contribution from the active fuel region. The energy distribution of the neutrons and gamma rays emitted in the fuel region must also be taken into account. The dose rate contribution from the fuel region is both spatially and energy dependent. The NAC method of determining the dose rate contribution assumes that the energy dependence and the spatial dependence can be treated separately. An XSDRNPM analysis is performed to take into account the energy dependence. As the technology associated with cask designs improves, the shielding analyses required to obtain accurate dose rates will become more complex.