Helium release from neutron-irradiated Li2O single crystals

Lithium oxide (Li2O) single crystals with various sizes (0.15-5 mm) were us ed as specimens. After the irradiation in JRR-4 and JRR-2 (thermal neutron fluence: 2 x 10(17)-2 x 10(19) n/cm(2)), and fast neutrons in FFTF (fast ne utron fluence: 4 x 10(22) n/cm(2)), helium release from the Li2O specimens during the heating at a constant heating rate was continuously measured wit h a quadrupole mass spectrometer. The helium release curves from JRR-4 and JRR-2 specimens have only one broad peak each. From the dependence of the p eak temperature on the neutron fluence and the crystal diameter, and the co mparison with the results of sintered pellets, it is concluded that the hel ium generated in the crystal is released through the processes of bulk diff usion with trapping by irradiation defects such as some defect clusters. Fo r the helium release from FFTF specimens, two broad peaks were observed in the release curves. It is considered that two different migration paths exi st for helium migration in the specimen, that is, bulk diffusion and diffus ion through the microcracks formed due to the heavy irradiation. (C) 1998 E lsevier Science B.V. All rights reserved.