Assessment of fuel rod failure propagation in light water reactors: model development, calculational results, and conclusions

This report summarizes analyses of fuel rod failure propagation (FFP) during normal and off-normal conditions in commercial light water reactors (LWRs). FFP is defined to occur when the failure of one fuel rod causes the failure of an adjacent rod, which causes the failure of another rod adjacent to it, etc. Potential FFP initiators are examined, including gas ejection from a failed fuel rod, molten and solid fuel release from a failed fuel rod, during normal and off-normal conditions, and local coolant flow constrictions of channel blockage, rod ballooning, and rod bowing. FFP induced by departure-from-nucleate-boiling (DNB) propagation is also discussed. Results for gas ejection and limited fuel release from a failed rod indicate no FFP potential. If a significant fraction of the fuel inventory from a failed rod should desinter into fine particles that simultaneously interact with the coolant, then FFP could be possible. Because cladding ruptures large enough to permit a large-scale interaction have not been observed in commercial reactor experience, FFP is still very unlikely. The data base relevant to DNB propagation is sparse; however, the fact that DNB propagation has never been observed strengthens the theoretical arguments against such a propagation mode.