Comparison of Analysis Methods for Burnup Credit Applications*

The current approach used for the development and certification of spent fuel storage and transport casks requires an assumption of fresh fuel isotopics in the criticality safety analysis. However, it has been shown (Sanders et al. 1987) that there is a considerable reactivity reduction when the isotopics representative of the depleted (or burned) fuel are used in a criticality analysis. Thus, by taking credit for the burned state of the fuel (i.e., burnup credit), a cask designer could achieve a significant increase in payload. Accurate prediction of keff for spent fuel arrays deJ?ends both on the criticality safety analysis and the prediction of the spent fueltsotopics via a defletion analysis. Spent fuel isotopics can be obtained from detailed multtdimensiona reactor analyses, e.g. the code PDQ (Caldwell, 1967), or from point reactor burnup models. Detailed isotopics resulting from PDQ analyses have been obtained from the Virginia Power Company for Cvcle 5 of the North Anna Unit 1 Reactor. The detailed isotopics are used in KENO V.a (NUREG/CR-Q200 1984) models of these reactors under restart and end-of-cycle critical conditions. These reactor calculations will help verify the adequacy of the isotopics and determine Δkeff biases for various analysis assumptions (with and without fiSsion products, actinide absorbers, burnable poison rods, etc.). New software developed to interface PDQ multidimensional isotopics with KENO V.a reactor and cask models is described. A number of different axial fuel zones are used to determine the effect of the axial variation in bumup on keff· Analyses similar to those performed for the reactor cases are carried out with a representative burnup credit cask model using the North Anna fuel.