Progress of air oxidation tests on spent light-water reactor fuel in an imposed gamma field

The Spent Fuel Oxidation Program at Pacific Northwest Laboratory (PNL) is directed towards providing a data base for the oxidation behavior of spent light-water reactor (LWR) fuel. This information will establish the basis for (1) determining the temperature above which monitoring for loss of an inert storage atmosphere is required, (2) assessing fuel conditions in the case of an abnormal event (e.g., air inleakage into a cask with an inert cover gas), and (3) determining acceptable exposure and temperature conditions for fuel storage in air. An oxidation test matrix was designed to provide an adequate number of specimens and testing conditions to determine the statistical significance of the following variables: temperature, exposure period, and moisture of the test atmosphere. Various spent fuels are being tested, including pressurized-water reactor (PWR) and boiling-water reactor (BWR) fuels from different reactors. To simulate dry storage conditions, a gamma field of about 10/sup 5/ R/h was imposed. Test results to date reveal that higher levels of irradiation, as indexed by burnup level, increase the stability of the basic UO/sub 2/ crystal structure. Consequently, oxidation of specimens with higher burnup levels resulted in larger weight gains and longer periods before low-density powder formed. For all burnupmore » levels tested, the spent UO/sub 2/ fuel crystal structure was more resistant to low-density powder formation than the nonirradiated UO/sub 2/ pellet crystal structure.« less