Spent nuclear fuel characterization in view of long-term storage

The capability of the spent fuel rod to contain radionuclides during the cooling, storage and handling / transportation stages determines to a large extent the safety of nuclear fuel after discharge from the reactor. In addition to the property evolution of intact fuel rods, the behaviour of defective rods and the consequences of potential accidents causing rod failure must be considered. This paper presents the most recent results of ongoing spent fuel characterization studies focused on long-term storage and transportation performed at JRC-ITU. Irradiated fuel alterations as a function of time under the cumulative effect of alpha-decay damage and radiogenic He accumulation are investigated at the microstructural level and at the level of macroscopic properties (e.g. hardness). Properties measured on LWR fuel (UO2, MOX) are complemented by tests on alpha-doped UO2 which simulates ageing processes under accelerated conditions. Hardness results obtained on a 67 GWd/t fuel are compared to previous data on similar fuel and to UO2 containing different fractions of short –lived alpha-emitters. The generic trend observed for all the measured property changes indicates that a saturation level is reached after an initial alteration stage. At very high dose, transmission electron microscopy analysis provides evidence for grain polygonization and microstructure reorganization. In addition to intact fuel rods, data on the behaviour of defective rods and the consequences of potential accidents are presented. These tests are part of a program of studies aimed at characterizing safety relevant aspects potentially affecting LWR fuel rods with different burnup, and simulating different scenarios (ages and thermal histories).JRC.E.2-Safety of Irradiated Nuclear Material