Helium release from neutron-irradiated Li2O sintered pellets

Lithium oxide (Li2O) pellets with various bulk densities (71, 80, 85 and 88% T.D.) were used as specimens. After the irradiation up to 2×1017–2×1019 n cm−2 with thermal neutrons in JRR-4 and JRR-2, helium release from the Li2O specimens during a constant heating rate was continuously measured by a quadrupole mass spectrometer. In the helium release curves during a constant heating of 2 K min−1, four kinds of peaks were observed. They are divided into two groups. One group was relatively wide peaks, of which the peak temperature range is 850–1100 K, appeared for all specimens. Another was a narrow and a very wide peaks, of which the peak temperature is around 1330 K, was remarkably observed only for the specimens with bulk density higher than 85% T.D. From the activation energy of the peaks and the neutron fluence dependencies of the temperatures of the peaks, the former group is considered to be attributed to the helium released through the processes of bulk diffusion, grain boundary diffusion without trapping at closed pores in the grains. On the other hand, from the bulk density dependence of the appearance of the peaks, the latter is considered to be attributed to the helium release with trapping at closed pores in the grains.