Volume 7, Issue 9 (October 2010)
Advanced Zirconium Alloy for PWR Application
Westinghouse is evaluating several advanced zirconium-based alloys, designated collectively as AXIOM™, to achieve improved performance for more demanding fuel management schemes. There are five candidate AXIOM alloys currently being evaluated by Westinghouse. The in–pressurized water reactor (PWR) performance of one candidate alloy, X5A, is reviewed in this paper. The irradiation performance of X5A (previously identified as Alloy A) cladding that was fabricated using high-temperature processing (HTP) was published in 2002. Since then, the fabrication process of X5A was optimized by the use of a low-temperature process (LTP). Cladding tubes with the improved processing have been irradiated in two commercial PWRs (PWR A and PWR B) and in two test reactors (Test Reactor C and Test Reactor D). The irradiation performance of both versions (HTP X5A and LTP X5A) is reviewed in this paper with a primary emphasis on the current LTP cladding. After achieving an intermediate burnup in the range of 48–54 GWd/Metric Ton of Uranium (MTU) in PWR A, the average maximum oxide layer thickness for LTP X5A was about 23 μm or about 27 % lower than the oxide thickness on ZIRLO® clad fuel rods. In addition, the fuel rod axial growth strain for LTP X5A was about 50 % of ZIRLO rod growth. Lead fuel rod irradiation of LTP X5A in PWR B with burnup in the range of 47–53 GWd/MTU showed about 30 % lower corrosion relative to ZIRLO rods and a 7 % lower axial rod growth strain than ZIRLO. An irradiation experiment in Test Reactor C was designed to study breakaway irradiation growth (in the absence of waterside oxidation) of several alloys. LTP X5A cladding showed a growth strain of about 20 % that of ZIRLO cladding at a fluence of 16×1025n/m2. In Test Reactor D, at a burnup of about 44 GWd/MTU, HTP X5A had the same oxide thickness as ZIRLO. However, the post-irradiation hydrogen pick-up was 35 % lower for HTP X5A compared to ZIRLO. In addition to the irradiation experience, the supplemental out-reactor autoclave evaluation of X5A welds indicates adequate weld corrosion resistance. While additional in-PWR exposures are required along with post-irradiation examination, the results to date demonstrate that X5A is a promising alloy for future PWR application.