Characterization of local hydride re-orientation in high burn-up PWR fuel rods induced by high pressure at high temperat
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Characterization of local hydride re-orientation in high burn-up PWR fuel rods induced by high pressure at high temperatures Y. Yan, T. Smith, Z. Burns, B Bevard Oak Ridge National laboratory, Oak Ridge, TN-37831 ABSTRACT Hydride re-orientation in high burn-up PWR fuel rods was induced by high pressure at high temperatures. The high-burnup specimens were sectioned from PWR rods taken from a 15×15 assembly of the H.B. Robinson (HBR) Unit 2 reactor. Out-of cell benchmark tests performed on unirradiated hydrided Zircaloy-4 specimens were conducted to determine the appropriate temperature, pressure, cooling rate, and number of cooling cycles for the reorientation of the irradiated in-cell specimens. The in-cell hydride reorientation tests were performed using highburnup fuel specimens under a hoop stress 145 MPa at 400C. The specimens were heated to the target temperature of 400°C, held for 3 hours, cooled at 1°C/min to 170°C,and then heated at 1°C/min to the target temperature again for five cycles. Post test metallographic examinations showed that a significant amount of radial hydrides were induced in the HBR fuel rods. The length of radial hydride was up to 60 m. For unirradiated materials, the ductility of the radial hydride treated specimens is significantly reduced as compared to the as-hydrided specimens having the same hydrogen concentration (300 wppm in this work). The mechanical testing on irradiated fueled samples with hydride reorientation experiments have been performed, and will be reported separately in the near future. I. INTRODUCTION Hydrogen embrittlement of zirconium alloys is a phenomenon of interest in the United States due to spent nuclear fuel (SNF) remaining in storage for longer time frames than initially envisioned. Normal operation of nuclear fuel in a reactor results in the formation of a waterside corrosion layer and the introduction of hydrogen into the zirconium cladding, which can cause cladding ductility and failure energy to decrease [1, 2]. The processes used during the drying of SNF as it is transferred from wet storage to dry storage can expose the SNF cladding to temperatures and pressure-induced tensile hoop stresses high enough that could allow radial hydrides to precipitate during subsequent cooling. These radial hydrides could provide an additional embrittlement mechanism as the cladding temperature decreases below the ductile-tobrittle transition temperature. To simulate this behavior, unirradiated Zircaloy-4 samples (hydrided by a gas charging method) and defueled irradiated cladding has been tested under high pressures at high temperatures to generate radial hydrides at several laboratories [3-6]. In this experiment, we expanded on the earlier work [7] by performing hydride reorientation tests using fueled high-burnup irradiated SNF rod segments. The post-test examinations were conducted to characterize the microstructure of hydride reoriented samples. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of E
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