Cladding Behaviour During Postulated Design-Basis LOCAs

A previous report by Argonne provided results on cladding embrittlement, breakaway oxidation, and ballooning with rupture under conditions of postulated loss-of-coolant accidents (LOCAS). This paper updates those results, provides additional results in those areas, and adds results of mechanical testing of cladding after ballooning, rupture, oxidation at elevated temperature, and quench. Significant conclusions are as follows. Embrittlement of high-exposure cladding is accelerated by hydrogen that is absorbed during normal reactor operation (i.e., pre-transient) and the acceleration has been quantified as an oxidation limit vs. pre-transient hydrogen content in the cladding metal. Breakaway oxidation, which leads to excessive hydrogen pickup and embrittlement during a LOCA transient, can occur during times that are relevant for these accidents. Conditions that lead to breakaway are only partially understood. Factors that contribute to increased susceptibility to breakaway oxidation - especially surface finish and scratches - have been investigated. Ballooning strains increase as rupture temperature decreases within the temperature range explored in this work. Rupture temperature depends primarily on internal gas pressure and to some extent on pre-transient hydrogen content, alloy composition, and heating rate. Four-point bend tests are effective for measuring post-quench failure limits for ballooned, ruptured, and oxidized cladding. The results indicate that strength and failure energy degrade significantly with increasing pre-transient hydrogen and transient oxygen levels, based on cross-section-average values for these parameters. Further, using the oxidation limit vs. hydrogen content derived for non-ballooned regions would ensure the preservation of strength for ballooned regions. (author)