Microstructure and density changes of Li2O during irradiation in BEATRIX-II, Phase 1

Abstract

BEATRIX-II, Phase 1 was an in situ tritium recovery experiment carried out on Li{sub 2}O to lithium burnups in excess of 4%. Solid Li{sub 2}O specimens were irradiated under a temperature gradient in the range from 700 K at the outer diameter to 1,270 K at the center. A ring specimen of Li{sub 2}O operated at a nearly uniform temperature with the capability of changing temperature in the range from 780 to 920 K. Li{sub 2}O single crystals having two different {sup 6}Li enrichments (0.07 and 1.8 at%) were irradiated at 650 K in nonvented canisters. Microstructures of these specimens, before and after irradiation, were investigated using optical and scanning electron microscopy. Substantial microstructure change was found to have occurred in the solid specimen during irradiation and this change was characterized by the development of large columnar grains, lenticular porosity and a center void. The development of columnar grains and the formation of the center void was attributed to the migration of lenticular pores up the thermal gradient by a vaporization/condensation process. Morphological changes on fracture surfaces were observed both for the polycrystalline (solid and ring) specimens and the single crystal specimens. Measurements of density were used to further characterizemore » the effects of irradiation on the open and closed porosity distributions in all the specimens. Although extensive microstructural and density changes occurred during irradiation, these changes had no significant impact on the integrity of Li{sub 2}O during irradiation, and therefore these results support the use of Li{sub 2}O as fusion solid breeders.« less