A critical review of the practice of equating the reactivity of spent fuelto fresh fuel in burnup credit criticality safety analyses for PWR spent-fuel pool storage

This research examines the practice of equating the reactivity of spent fue l to that of fresh fuel for the purpose of performing burnup credit critica lity safety analyses for pressurized water reactor (PWR) spent-fuel pool (S FP) storage conditions. The investigation consists of comparing k(eff) esti mates based on reactivity "equivalent" fresh fuel enrichment (REFFE) to k(e ff) estimates using the calculated spent-fuel isotopics. Analyses of select ed storage configurations common in PWR SFPs show that this practice yields nonconservative results (on the order of a few tenths of a percent) in con figurations in which the spent fuel is adjacent to higher-reactivity assemb lies (e.g., fresh or lower-burned assemblies) and yields conservative resul ts in configurations in which spent fuel is adjacent to lower-reactivity as semblies (e.g., higher-burned fuel or empty cells). When the REFFE is deter mined based on unborated water moderation, analyses for storage conditions with soluble boron present reveal significant nonconservative results assoc iated with the use of the REFFE. Finally, it is shown that the practice of equating the reactivity of spent fuel to fresh fuel is acceptable, provided the conditions for which the REFFE was determined remain unchanged. Determ ination of the REFFE for a reference configuration and subsequent use of th e REFFE for different configurations violates the basis used for the determ ination of the REFFE and, thus, may lead to inaccurate, and possibly, nonco nservative estimates of reactivity A significant concentration (similar to 2000 ppm) of soluble boron is typic ally (but not necessarily required to be) present in PWR SFPs, of which onl y a portion ( less than or equal to 500 ppm) may be credited in safety anal yses. Thus, a large subcritical margin currently exists that more than acco unts for errors or uncertainties associated with the use of the REFFE. Cons equently, the findings presented here do not represent a significant safety concern unless/until the subcritical margin associated with the soluble bo ron (that is not currently explicitly credited) is offset by the uncertaint ies associated with burnup credit and/or the expanded allowance of credit f or the soluble boron.