Neutronic analysis of the burning of transuranics in fully ceramic micro-encapsulated tri-isotropic particle-fuel in a PWR

Abstract

Calculations are performed to assess the neutronic behavior of fully ceramic micro-encapsulated (FCM) fuel in otherwise-conventional pressurized water reactor (PWR) fuel pins. The FCM fuel contains transuranic (TRU) oxide fuel, solely, encapsulated in tri-isotropic (TRISO) particles. The TRU loading originates from the used fuel of a conventional LWR following 5 years of cooling. Use of the TRISO particle fuel provides an additional barrier to fission product release in the event of cladding failure. Depletion calculations are performed to evaluate reactivity-limited burnup of the TRU-only FCM fuel. These calculations show that because of the relatively small space available for the fuel meat, the achievable cycle lengths (i.e., elapsed time between required refueling shutdowns) of the fuel with these pins alone is prohibitively short. Various reactivity parameters are also evaluated at each burnup step. These include the moderator temperature coefficient (MTC), Doppler coefficient, and the soluble boron worth. These are compared to reference UO2 and MOX unit cells. The TRU-only FCM fuel exhibits degraded MTC and Doppler coefficients relative to those of UO2 and MOX. Also, the reactivity effects of coolant voiding suggest that the behavior of this fuel would be similar to that of MOX fuels of very high plutonium fraction, which are known to possess positive void reactivity coefficients. In general, loading of TRU-only FCM fuel into an assembly without the concomitant presence of significant quantities of uranium induces significant challenges into the reactor design. However, if such FCM fuel pins are included in a heterogeneous assembly alongside LEU fuel pins the overall reactivity behavior is dominated by the uranium pins while net TRU destruction performance levels are achieved in the TRU-only FCM fuel pins. Though preliminary, this work demonstrates that the use of heterogeneous assemblies such as these appears feasible from a reactor physics standpoint.