Metallography Examination and Hardness Measurements of High-Burnup Spent Nuclear Fuel Claddings during Simulated Drying Conditions

In this study, a series of MET examination and microindentation tests were conducted on various as-received and heat-treated HBU spent fuel claddings as a part of the DOE NE High-Burnup Spent Fuel Data Project. For this purpose, selected HBU UNF rods were heated and held at 400oC for ~8 hours, and slow cooled at ~4oC/hr. The key findings are:• The heat treatment of HBU UNF resulted in the reorientation of circumferential hydrides to radial hydrides for some of the HBU cladding alloys.• In the HT-M5 cladding, long radial hydrides werefound, especially near the clad ID. In ZIRLO and Zr4 claddings, very small radial hydrides were found both in the baseline and in the HT claddings as such their lengths were limited by the high density of circumferential hydrides. No strong evidence of hydride reorientation was found after the heat treatment. • M5 cladding has the lowest hardness, followed by ZIRLO and LT-Zr4. The radial indentation profile shows hardening near the clad OD in ZIRLO and LT-Zr4 cladding where the microstructure consists of a high density of hydride rim. The hardness profile was uniform for the M5 cladding.• The HT of HBU UNF reduced the hardness of M5 and ZIRLO claddings, possibly due to the annealing of irradiation-induced damage.