Property changes and thermal recovery in self-irradiated MOX

In plutonium compounds, self-irradiation induces lattice defects with storage time. Properties such as the lattice parameter and thermal conductivity are changed by accumulation of lattice defects, and the changed properties are recovered by annealing. In this study, the effects of self-irradiation and annealing on properties of mixed oxides (MOX) were investigated by using MOX pellets which had undergone long-term storage in the ambient atmosphere. The maximum storage period was 32 years. Plutonium contents in the MOX samples were 20-48 %. Properties investigated were lattice parameters, densities, microstructures and thermal conductivities. The lattice parameter increased with amount of self-irradiation and became saturated after increasing 0.29%. The expansion of lattice parameter was formulated as a function of the self- irradiation amount: Δa/a0 = 2.9x10-3 x (1 - exp(-12200λ't)). Here, λ' is the average of decay constants of isotopes, and t is the storage time. The 0.29% expansion of lattice parameter corresponded approximately to a 1% density decrease. The expanded lattice parameter was recovered to those of as-fabricated pellet by annealing at 1473K for 2h because of recoveries of lattice defects. The thermal recovery were analyzed from a relationship between recovery rate and annealing time. From the results, the recovery rates of lattice parameter had three stages, which occurred in the temperature range of below 673K, 673 - 1073 K and over 1073 K. The changes of densities due to annealing were measured at 1073, 1473K, 1673K and 1873K for 2h. The change of densities indicated that up to 1473K densities increased more with annealing temperature. The highest densities were found after annealing at 1473K, and the densities varied within the range of 1% corresponding to density decrease calculated from the expanded lattice parameter. The density decreased with annealing temperature above 1473K. After annealing at 1873K, the densities decreased about 1%. It seemed that the interstitial helium atoms migrated to pores due to the annealing, and the pores were expanded by the helium gas. Ronchi and Hiernaut reported that 1073K had been reported as the starting temperature of helium gas release. However, as shown, no significant microstructure changes of annealed samples could be observed. Regarding to the thermal conductivity of the annealed MOX samples, three recovery stages were seen, similar to the lattice parameter recovery. After annealing at 1000K, thermal conductivity was recovered to 80% of the value of the as-fabricated pellet. Thermal conductivity was almost completely recovered by annealing at 1473 K for 30 min