A slow positron beam coupled with a Doppler broadening spectrometer was used to measure the low- and high-momentum annihilation fractions, S and W, respectively, as a function of positron energy in UO2 disks implanted with various 1MeV 3He fluences and annealed in ArH2 or in vacuum. The S(E) and W(E) behaviors indicated that, for fluences of 2 x 1014 to 2 x 1016 3He/cm2, the vacancy defect distribution evolved upon annealing at 264 to 700C under ArH2. This evolution was found to be dependent upon the 3He fluence implanted into the sintered UO2 disks. For the lowest fluence of 2 x 1014 3He/cm2, the S(W) plot, with positron energy as the running parameter, suggested that only the concentration of vacancy defects decreased when the annealing temperature was increased. For the highest implantation fluences (from 5 x 1015 to 2 x 1016 3He/cm2) the S(W) plot suggested that the nature of the vacancy defects changed in the annealing range from 260 to 400C. Measurements performed on implanted UO2 disks annealed in vacuum revealed a partial recovery of the vacancy defects; due possibly to their recombination with mobile O interstitials.

Thermal Evolution of Vacancy Defects Induced in Sintered UO2 Disks by Helium Implantation. H.Labrim, M.F.Barthe, P.Desgardin, T.Sauvage, G.Blondiaux, C.Corbel, J.P.Piron: Applied Surface Science, 2006, 252[9], 3262-8