Helium behaviour and defect evolution in amorphous spinel during thermal annealing

MgAl2O4-spinel has been widely investigated as inert matrix for actinide-transmutation. Under impact of fission fragments, it becomes amorphous. During reactor irradiation, crystalline and amorphous spinel are expected in the fuel, containing, among others xenon and helium. Gas-release measurements and positron beam analysis (PBA) were performed on amorphized specimens and compared to results on crystalline spinel. Helium was released in two stages between 575 and 800 K in first-order desorption processes with activation enthalpies of 1.9 and 2.7 eV and attempt frequencies of about 1013 s−1. Xenon was released between 1050 and 1450 K. PBA experiments indicated that defect clustering occurred near the surface at low temperatures and in deeper regions at about 950–1200 K. Recrystallization was observed between 1000 and 1350 K and defect annealing was completed at 1600 K. PBA indicates two different damage zones, corresponding to displacement zones with and without xenon.