Helium and xenon behaviour in irradiated Am-containing MgAl2O4 (reactor experiment EFTTRA-T4)

Magnesium aluminate spinel (MgAl2O4) containing 241Am was irradiated for 358 days in the High Flux Reactor in Petten (NL) with the objective of effectively transmuting this highly radiotoxic actinide. The feasibility of the process was demonstrated; however, a major drawback was observed as material swelling, probably due to the large quantity of helium emitted from formed short-lived actinides. The behaviour of helium and of fission gas in the matrix was investigated by post-irradiation thermal annealing experiments in a Knudsen-cell. The observed gas release was related to the microstructure of the matrix investigated by scanning and transmission electron microscopy. Helium was found to be present in large pores, whereas the less mobile xenon was distributed partly in the same pores and partly in small intragranular bubbles. During post-irradiation thermal annealing, almost complete helium release took place abruptly at a rather high temperature (∼1600 K), whilst only part of the xenon was released during this stage. The release of the remaining xenon was associated with a bulk thermal restructuring of the MgAl2O4 target at higher temperatures.