Helium behavior in UO2 polycrystalline disks

AbstractThe behavior of helium implanted in sintered uranium dioxide disks has been investigated as a function of annealingtemperature. UO 2 disks have been implanted with 1 MeV 3 He at a nominal fluence of 5·10 16 3 Hecm 2 using a Van deGraaff accelerator. The 3 He(d,a) 1 H nuclear reaction analysis method was used to determine the helium depth profile inthe UO 2 disks. Partial flaking was observed after annealing at 500 C for local He concentration of 1 at.%. Afterannealing at 600 C flaking has affected the whole surface. The formation of helium bubbles is discussed. 2003 Elsevier B.V. All rights reserved. PACS: 66.30.h; 61.82.m; 81.05.Je 1. IntroductionThe release of fission gases, mostly Xe, from UO 2 and spent fuels has been extensively studied since themiddle of the 1960s [1–5]. The solubility of these gases isextremely low in UO 2 and, as a consequence, the gasatoms tend to precipitate into bubbles. In irradiatedfuels, a high density ( 10 17 cm 3 ) of small intragranularbubbles of about 2 nm uniformly distributed in thematrix are observed [6,7]. The size of the bubbles in-creases and the concentration decreases slightly withincreasing temperature and burnup. At higher burnupand/or temperatures a second bubble population is cre-ated with a larger mean diameter (10–20 nm) [7]. Theprecipitation of bubbles has also been demonstrated foruranium dioxide samples implanted with Kr and Xe evenat temperatures as low as 300–350 C [8,9].Fewer experiments have been performed on the be-havior of helium gas produced by alpha decay of acti-nides [10,11]. Yet the amount of helium produced afterirradiation are large in particular in the case of MOXfuels: the amount of helium produced in MOX (burnup47.5 GWd/tU) after 10000 years is evaluated at 6700cm