Axial strength of Zircaloy-4 samples with reduced thickness after a simulated loss of coolant accident

Jean Desquines,Tatiana Taurines

doi:10.1016/j.net.2021.01.001

Abstract

To investigate wall-thinning impact on axial load resistance of Zircaloy-4 cladding rods after a LOCA transient, axial tensile samples have been machined on as-received tubes with reduced thicknesses between 370 and 580 μm. After high temperature oxidation under steam at 1200 °C with measured ECR ranging from 10 to 18% and water quenching, machined samples were axially loaded until fracture. These tests were modeled using a fracture mechanics approach developed in a previous study. Fracture stresses are rather well predicted. However, the slightly lower fracture stress observed for wall-thinned samples is not anticipated by this modeling approach. The results from this study confirm that characterizing the axial load resistance using semi-integral tests including the creep and burst phases was the best option to obtain accurate axial strengths describing accurately the influence of wall-thinning at burst region.

Highlights

During a LOCA, the fuel cladding deforms by internal pressurization inducing ballooning and can lead to burst during the heat up of the first phase of the LOCA
The influence of wall thinning on a cladding mechanical resistance was both experimentally studied and compared to linear elastic based fracture mechanics modeling
The modeling slightly failed since when decreasing thickness a slight increase of fracture stress was predicted whereas no dependency to cladding thickness was experimentally observed

Summary

Introduction

During a LOCA, the fuel cladding deforms by internal pressurization inducing ballooning and can lead to burst during the heat up of the first phase of the LOCA. This phase is followed by high temperature oxidation under steam environment and the rod is quenched during core reflooding. Loading the rod during quench can lead to cladding fracture [4,5,6,7,8,9] Such fractures are usually observed at the ballooned region where the cladding bursts and sometimes at the secondary hydriding regions. To assess the resistance to axial load of the deformed and high temperature oxidized rod, one can evaluate the Equivalent Cladding Reacted (ECR) describing the fraction of oxidized cladding during steam oxidation

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Conclusion