ZIRLO TM Cladding Improvement

Improvements to fuel design have been made in recent years to meet the challenges of increases in fuel duty in terms of linear power and operating temperature. Improved cladding material is one of these design improvements. Specifically, Westinghouse has developed an improved version of ZIRLOTM called Optimized ZIRLO and denoted as OPT ZIRLO. The Sn level in this improved material is reduced from the nominal standard previous level of about 1 % to a range of 0.6 % to 0.8 %. The reduced Sn level has been optimized to produce a higher corrosion resistance and provide adequate in-reactor creep resistance. Out-reactor diameter creep tests have shown that decreasing Sn increases out-reactor creep, suggesting that decreasing Sn may also increase in-reactor creep. An in-reactor testing program in the Vogtle Unit 2 pressurized water reactor (PWR) was performed to confirm the predicted in-reactor behavior. The test samples were suspended as segmented rods inside fuel assembly thimble tubes. In-reactor diameter creep data confirmed that decreasing Sn increases in-reactor creep. As a result of the correlation between in-reactor and out-reactor creep, an extensive out-reactor diameter creep program was performed in order to develop methods to fabricate OPT ZIRLO with the same in-reactor creep properties as the currently used stress-relief annealed standard ZIRLO (denoted as SRA STD ZIRLO). The level of in-reactor diameter creep of SRA STD ZIRLO was achieved for OPT ZIRLO by two methods. One method involved changing the final microstructure from SRA to partially recrystallized (PRXA). The other method kept the final microstructure as SRA and changed the tube reduction sequence to decrease the final tube area reduction. In order to develop these methods, a series of material variation tests was performed. Some of the material variations included different final heat treatments, different tube reduction sequences, and different pre-charged hydrogen levels. These tests were performed for both out-reactor and in-reactor. In addition, the out-reactor and in-reactor creep were observed to directly correlate for OPT ZIRLO material fabricated with different final anneal temperatures. Thus, out-reactor creep may be used to predict in-reactor creep properties for different final anneal temperatures. These results show that fabrication changes may be used to control in-reactor creep. In this study, fabrication changes were used to compensate for the reduction in in-reactor creep strength associated with lower tin content in OPT ZIRLO.