Mechanical Property Testing of Irradiated Zircaloy Cladding Under Reactor Transient Conditions

Abstract

Specimen geometries have been developed to determine the mechanical properties of irradiated Zircaloy cladding subjected to the mechanical conditions and temperatures associated with reactivity-initiated accidents (RIA) and loss-of-coolant accidents (LOCA). Miniature ring-stretch specimens were designed to induce both uniaxial and plane-strain states of stress in the transverse (hoop) direction of the cladding. Also, longitudinal tube specimens were also designed to determine the constitutive properties in the axial direction.Finite-element analysis (FEA) and experimental parameters and results were closely coupled to optimize an accurate determination of the stress-strain response and to induce fracture behavior representative of accident conditions. To determine the constitutive properties, a procedure was utilized to transform measured values of load and displacement to a stress-strain response under complex loading states. Additionally, methods have been developed to measure true plastic strains in the gauge section and the initiation of failure using real-time data analysis software. Strain rates and heating conditions have been selected based on their relevance to the mechanical response and temperatures of the cladding during the accidents.

On the Embrittlement of Zircaloy-4 Under RIA-Relevant Conditions

“…As in previous studies [13,14] The grain structure consists of elongated grains with ≈10:1 aspect ratio, oriented parallel to the tube axis. The elongated grains are ≈10-15 µm long and ≈1-2 µm thick.…”

“…Therefore, we have utilized a specially designed ring-stretch specimen geometry to impose a near plane-strain state of stress such that we can produce cladding deformation with hoop extension followed by a fracture path of through-thickness slip, similar to the hoop deformation that would be expected during an RIA. As described elsewhere [13,14], a "transverse plane-strain" specimen with double edge notches, shown in Fig. 1a, was used.…”

“…The uniaxial ring-stretch specimen geometry, shown in Fig. 1b, has a gauge length-to-width ratio of four in conjunction with lubrication between the specimen gauge section and loading grips to promote uniform deformation within the gauge section [14]. As in previous studies [13,14,17], microhardness indentations were used to determine strains on a local basis, as shown in Fig.…”

“…1a [13,14]. For the uniaxial specimen testing, which has been described elsewhere [14,17], similar "D-shaped" loading fixtures were used along with a well-lubricated (fine film of tungsten disulfide with Teflon tape and vacuum grease) central insert to mitigate the effects of bending and friction between loading grips and the specimen gauge section.…”

“…More specifically, the thermal expansion mismatch considerations of the PCMI imply a stress state of equal biaxial tension (σ θ /σ z = 1); however, the axial path of cladding cracks in RIA tests [15,16] suggests σ θ > σ z or σ θ /σ z > 1. We believe a stress state relevant to the analysis of cladding behavior during an RIA is that imposed by the transverse, plane-strain, ring-stretch tension test, for which a procedure has been recently developed [13,14]. In this case, owing to the plastic anisotropy of the Zircaloy cladding (R = 2.3 for unirradiated Zircaloy-4 tubing, where R = widthstrain/thickness-strain in uniaxial hoop tension), a plane-strain deformation path imposes multiaxial tension such that σ θ /σ z ≅ 1.4.…”