Effect of cooling rate on the residual ductility of Post-LOCA Zircaloy-4 cladding

Abstract

Effects of cooling rates on mechanical behavior of steam-oxidized Zircaloy-4 are systematically investigated with the cladding material oxidized to the current ductility-based regulatory limits (17% ECR at 1204 °C). Three different cooling rates associated with distinctively different quenching environments are tested; ambient air cooled (<7 °C/s), boiling water (∼400–500 °C/s), and room temperature water (∼1600–2000 °C/s). Cooling rates of post-LOCA Zircaloy-4 have (1) limited effects on average cladding residual ductility for non-hydrided cladding, (2) pronounced effects on statistical variation of ring compression test results, and (3) effects on fracture strength reduction on pre-hydrided cladding (765 wppm). The limited effect of cooling rate on average residual ductility of non-hydrided cladding is due to the cooling-rate insensitive resulting phase thicknesses and their oxygen contents, both of which dictate the level of cladding residual ductility. Homogenizing prior-β phase with finer lamellar structures and reduced α incursion, fast cooling (i.e., 25 °C water quenching) effectively reduces statistical variation of RCT results. The presented results support regulatory validity of past experiments conducted irrespective of cooling rates for Zircaloy-4 with no pre-transient hydrogen content. The observed insensitivity of cooling rates on the ECCS criteria for non-hydrided Zircaloy-4 may represent a gap between ductility-based regulation and integral fuel rod behavior. Cooling rate effect is shown to become increasingly important for increasing pre-transient hydrogen embrittlement, as shown by the fracture strength changes.