Burnup in Criticality Safety for Spent Fuel Casks

Criticality safety margins must be based upon the combination of the best available prediction of the margin and all uncertainties in the prediction. Inclusion of the effects of burnup in the evaluation of spent fuel shipping or storage casks must be based upon a thorough understanding of the prediction of the effects of burnup and the uncertainties in the measurements (or predictions) of burnup and predictions of the effects. A preliminary estimate of the effects of burnup and its uncertainties is presented. This will serve as a first step in the effort to develop acceptance criteria that assure public safety. An assembly average burnup of 20,000 MWD/MTU represents an increase in the criticality safety margin of about 20% (δk/k), and the current estimate of the uncertainty in this value is close to 4% (δk/k). The uncertainties in the components of the effects of burnup were based upon relevant literature citations and — where no other information was available — upon estimates. Consequently, the margins and uncertainties in the margin presented here should be considered as initial estimates upon which more refined analyses should build to develop a defensible basis for predicting and reviewing the criticality safety margins which include the effects of burnup.