The effect of hydride morphology on the failure strain of stress-relieved Zircaloy-4 cladding with an outer surface pre-crack under biaxial stress states

Abstract

ABSTRACT Hydride precipitates are considered to affect cladding integrity adversely during pellet-cladding mechanical interaction (PCMI) in a reactivity-initiated accident (RIA). This study aims to clarify the role of hydride precipitates in cladding failure under the biaxial stress condition. A displacement-controlled loading method was applied with a biaxial expansion-due-to-compression (biaxial-EDC) test apparatus to maintain a constant ratio of axial to hoop strains. The tests were conducted at room temperature on unirradiated stress-relieved Zircaloy-4 cladding tube samples with various hydrogen contents, pre-crack depths, and strain ratios. The amount and distribution of hydride precipitates (hydride morphology) were evaluated quantitatively and hydrogen content was measured to assess its effect on the decrease in outer surface hoop strain at failure (failure strain) of the samples. The decrease in failure strain of the hydrided sampleswas found to be more significant under lower strain ratios in the samples with shallower pre-crack. The failure strain of sample tended to be more sensitive to hydrogen content under the strain ratio with a higher axial component in the case of samples with hydrogen contents higher than ~150 wppm. This tendency might be explained by a scenario that considered the formation of micro-cracks in the hydride precipitates during the EDC test.