The statistical ductility of embrittled Zircaloy-4 and its regulatory implications

The statistical nature of ductility of embrittled Zircaloy-4 cladding and its implications on the ECCS criteria (10 CFR 50.46) were investigated. Ductility of embrittled cladding specimens (ECR: 9-30%, and H: 171-1759 wppm) was measured by offset strains obtained from Ring Compression Tests (RCT) conducted in compliance with U.S NRC's guidelines. This study demonstrates the Weibull distribution of offset strains for the tested Zircaloy specimens. The current ECCS criteria (ECR 17% with 2% offset strain) ensure ∼97% ductile population, which may serve as a reference statistical confidence to determine the ductility criteria of high burnup fuel cladding. Characterized with low Weibull modulus (m), intermediate levels of embrittlement notably increase statistical uncertainties of the offset strain due to the possibility of having two distinctively different failure modes (i.e., ductile and brittle). Such a mixed failure mode can be used as a flag that instrumentally helps avoid excessive conservatism of the ECR limit being in a highly ductile regime. Griffith theory was employed to understand the mixed failure modes by comparing the critical flaw size of the ductile phase (prior-β) and pre-existing flaw size estimated by the thickness of the brittle phases (ZrO2+α-Zr(O)). Three guidelines were proposed to determine physically sound, yet statistically sufficing, offset strain criterion tied to ECR limit for cladding ductility. It was demonstrated that the current ECR 17% limit satisfies the proposed guidelines. The developed guidelines are readily applicable to the high burnup fuel cladding to revisit the statistical and physical sanity of the current draft rule.