A Probabilistic-Micromechanical Methodology for Assessing Zirconium Alloy Cladding Failure
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0985-NN02-03
A Probabilistic-Micromechanical Methodology for Assessing Zirconium Alloy Cladding Failure Y.-M. Pan1, K. S. Chan2, and D. S. Riha2 1 Center for Nuclear Waste Regulatory Analyses, San Antonio, TX, 78238 2 Southwest Research Institute, San Antonio, TX, 78238 ABSTRACT Cladding failure of fuel rods caused by hydride-induced embrittlement is a reliability concern for spent nuclear fuel after extended burnup. Uncertainties in the cladding temperature, cladding stress, oxide layer thickness, and the critical stress value for hydride reorientation preclude an assessment of the cladding failure risk. A set of micromechanical models for treating oxide cracking, blister cracking, delayed hydride cracking, and cladding fracture was developed and incorporated in a computer model. Results obtained from the preliminary model calculations indicate that at temperatures below a critical temperature of 318.5 °C [605.3 °F], the time to failure by delayed hydride cracking in Zr-2.5%Nb decreased with increasing cladding temperature. The overall goal of this project is to develop a probabilistic-micromechanical methodology for assessing the probability of hydride-induced failure in Zircaloy cladding and thereby establish performance criteria. INTRODUCTION Most of the nuclear fuel cladding for power-generating water reactors is made of two zirconium alloys, Zircaloy-2 (Zr-1.5Sn-0.12Fe-0.1Cr-0.05Ni in wt%) and Zircaloy-4 (Zr-1.5Sn0.2Fe-0.1Cr-0.007Ni in wt%), because of their low neutron cross section and inherent resistance to a variety of environmental conditions. Zircaloy cladding is susceptible to hydride-induced embrittlement as a result of the presence of hydride platelets aligned in the radial direction, which can significantly reduce the tensile ductility of cladding [1]. Cladding failure of the fuel rods caused by hydride-induced embrittlement is one of the major reliability issues of spent nuclear fuel after extended burnup. High burnup increases the thickness of the oxide layer on Zircaloy cladding, the amount of absorbed hydrogen in cladding, and the internal fuel rod pressure. The mechanical integrity of Zircaloy cladding degrades as the burnup increases because of a higher susceptibility to premature fracture resulting from hydride-induced embrittlement and wall thinning by oxidation. For hydride reorientation to occur, the existing circumferential hydrides in the microstructure must first dissolve at temperatures above the hydride solvus temperature and then reprecipitate radially during cooling under a tensile stress [2]. The critical hoop stress required for hydride reorientation depends on many parameters including temperature, material characteristics, and residual stress [2,3]. Values ranging from 35 to 138 MPa [5.1 to 20.0 ksi] for Zircaloys have been reported by Einziger and Kohli [4] and from 35 to 200 MPa [5.1 to 29.0 ksi] by Pescatore, et al. [5]. Uncertainties in the cladding temperature, cladding stress, oxide layer thickness, and the critical stress value for hydride reorientation preclude an assess
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