For [approximately equal to]15 yr Dominion Generation's Surry Nuclear Station 15 × 15 Westinghouse pressurized water reactor (PWR) fuel was stored in a dry inert-atmosphere Castor V/21 cask at the Idaho National Environmental and Engineering Laboratory at peak cladding temperatures that decreased from ~350 to 150°C. Before storage, the loaded cask was subjected to thermal-benchmark tests, during which time the peak temperatures were greater than 400°C. The cask was opened to examine the fuel rods for degradation and to determine if they were suitable for extended storage. No fuel rod breaches and no visible degradation or crud/oxide spallation from the fuel rod surface were observed. The results from profilometry, gas release measurements, metallographic examinations, microhardness determination, and cladding hydrogen behavior are reported in this paper.
It appears that little or no fission gas was released from the fuel pellets during either the thermal-benchmark tests or the long-term storage. In the central region of the fuel column, where the axial temperature gradient in storage is small, the measured hydrogen content in the cladding is consistent with the thickness of the oxide layer. At ~1 m above the fuel midplane, where a steep temperature gradient existed in the cask, less hydrogen is present than would be expected from the oxide thickness that developed in-reactor. Migration of hydrogen during dry storage probably occurred and may signal a higher-than-expected concentration at the cooler ends of the rod. The volume of hydrides varies azimuthally around the cladding, and at some elevations, the hydrides appear to have segregated somewhat to the inner and outer cladding surfaces. It is, however, impossible to determine if this segregation occurred in-reactor or during transportation, thermal-benchmark tests, or the dry storage period. The hydrides retained the circumferential orientation typical of prestorage PWR fuel rods. Little or no cladding creep occurred during thermal-benchmark testing and dry storage. It is anticipated that the creep would not increase significantly during additional storage because of the lower temperature after 15 yr, continual decrease in temperature from the reduction in decay heat, and concurrent reductions in internal rod pressure and stress. This paper describes the results of the characterization of the fuel and intact cladding, as well as the implications of these results for long-term (i.e., beyond 20 yr) dry-cask storage.