Cladding Embrittlement During Postulated Loss-of-Coolant Accidents (NUREG/CR-6967)

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Publication Information

Manuscript Completed: June 2008
Date Published: July 2008

Prepared by:
M. Billone, Y. Yan, T. Burtseva, R. Daum

Nuclear Engineering Division
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439

H. Scott, NRC Project Manager

NRC Job Code N6281

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

The effect of fuel burnup on the embrittlement of various cladding alloys was examined with laboratory tests conducted under conditions relevant to loss-of-coolant accidents (LOCAs). The cladding materials tested were Zircaloy-4, Zircaloy-2, ZIRLO, M5, and E110. Tests were performed with specimens sectioned from as-fabricated cladding, from prehydrided (surrogate for high-burnup) cladding, and from high-burnup fuel rods which had been irradiated in commercial reactors. The tests were designed to determine for each cladding material the ductile-to-brittle transition as a function of steam oxidation temperature, weight gain due to oxidation, hydrogen content, pre-transient cladding thickness, and pre-transient corrosion-layer thickness. For short, defueled cladding specimens oxidized at 1000-1200ºC, ring compression tests were performed to determine post-quench ductility at ≥135ºC. The effect of breakaway oxidation on embrittlement was also examined for short specimens oxidized at 800-1000ºC. Among other findings, embrittlement was found to be sensitive to fabrication processes – especially surface finish – but insensitive to alloy constituents for these dilute zirconium alloys used as cladding materials. It was also demonstrated that burnup effects on embrittlement are largely due to hydrogen that is absorbed in the cladding during normal operation. Some tests were also performed with longer, fueled-and-pressurized cladding segments subjected to LOCA-relevant heating and cooling rates. Recommendations are given for types of tests that would identify LOCA conditions under which embrittlement would occur.