Changes in the microstructure and density of Li2O during irradiation in BEATRIX-II, phase I

BEATRIX-II, phase I was an in situ tritium recovery experiment carried out on Li2O to lithium burnups in excess of 4%. Solid Li2O specimens were irradiated under a temperature gradient in the range from 700 K at the outer diameter to 1270 K at the center. A ring specimen of Li2O operated at a nearly uniform temperature with the capability of changing the temperature in the range 780–920 K. Li2O single crystals with two different 6Li enrichments (0.07 and 1.8 at.%) were irradiated at 650 K in non-vented canisters. The microstructures of these specimens before and after irradiation were investigated using optical and scanning electron microscopy. Substantial microstructural 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 pores 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 an evaporation-condensation process. Morphological changes on fracture surfaces were observed for both polycrystalline (solid and ring) specimens and single-crystal specimens. Measurements of density were used to characterize further 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 Li2O during irradiation, and therefore these results support the use of Li2O as a fusion solid breeder.