Post-LOCA ductility assessment of Zr-Nb Alloy from 1100°C to 1300°C to explore variable peak cladding temperature and equivalent cladding reacted safety criteria

Abstract

This study experimentally examines the variable PCT-ECR embrittlement criteria by conducting post-LOCA ductility assessments in the temperature range of 1100∼1300°C. Post-LOCA ductility assessments demonstrate the viability of variable PCT-ECR criteria, allowing either higher PCT for less oxidized specimens, or higher ECR limit for lower PCT specimens. The variable PCT-ECR criteria can be useful for high burnup fuel regulation. For potential burnup uprates that may lead to the violation of the current CP-ECR limit (18%) due to excessive steady-state oxidation, it may be possible to circumvent the regulation limit by allowing PCT less than the current limit (1204°C). This represents a new, yet more rational, safety envelop that may liberate nuclear reactor from sweeping conservatism associated with LOCA safety assessments. For the specimens oxidized at temperatures greater than 1200°C, ductile-brittle transitions take place even with substantially thicker prior-β layer (375 µm) and lower oxygen concentration compared to 1200°C specimens. As such, premature ductile to brittle transition occurs from the view point of oxygen uptake for steam annealing temperature above 1200°C. Premature embrittlement of 1250°C and 1300°C that cannot be explained in terms of the thickness of prior-β phase is relevant to promoted grain growth at elevated temperatures. Unlike lower temperature (≤1200°C), steam annealing temperature of 1250°C and 1300°C made grains continuously grow beyond the average grain size of 50 µm. The enhanced grain growth is considered to degrade cladding ductility as commonly observed in other HCP materials, in addition to solid-solution hardening of oxygen.