Safety margins for high-burnup zircaloy-4 cladding behaviour during LOCA in PWRs

Because of major advantages in fuel-cycle costs and waste management, the current trend in the nuclear industry is to increase fuel discharge burnup. At high burnup, pressurized-water reactor (PWR) fuel rod claddings fabricated from low-tin Zircaloy-4 exhibit significant oxidation and related hydriding; hydriding may enhance the embrittlement, directly below the Leidenfrost temperature, and indirectly by enhancing oxygen solubility in the beta phase. Performance of high-burnup low-tin Zircaloy-4 under hypothetical loss-of-coolant (LOCA) situations is not well understood at this time. Therefore, it is important to reevaluate the safety margins for High-burnup fuel cladding. In recognition of this, LOCA-related behavior of high-burnup low-tin Zircaloy-4 is being actively investigated in several countries. The rationale and data bases used to established the current LOCA embrittlement criteria (1973 Emergency Core Cooling System (ECCS) Rule-Making Hearing) are reminded and clarified. Results obtained for hydrided Zircaloy-4 cladding in the 1980s from JAERI slow-ring-compression tests, and more recently from similar AEKI tests are evaluated. They show the importance of clad hydriding during reactor operation and make it feasible to provisionally estimate the limit of equivalent oxidation rate (ECR) which would provide the same safety margin for high-burnup low-tin Zircaloy-4, than that provided by the 17% value, tied with the use of the Baker-Just correlation, for fresh Zircaloy-4. This value is compared with the current methodology of the French and U.S. Safety Authorities, that consists to cumulate the initial oxidation (due to reactor operation) and the transient oxidation (due to LOCA); this comparison is not in accordance with an opinion published last year. Continued testing is necessary to establish embrittlement thresholds that provide sufficient safety margin to high-burnup operation. (author)