Helium Migration Mechanisms in Polycrystalline Uranium Dioxide
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0985-NN05-02
Helium Migration Mechanisms in Polycrystalline Uranium Dioxide Guillaume Martin1, Pierre Desgardin1, Philippe Garcia2, Thierry Sauvage1, GaÎlle Carlot2, and Marie-France Barthe1 1 CERI, CNRS, 3 A rue de la Ferollerie, ORLEANS, 45071, France 2 DEN/DEC/SESC/LLCC, CEA Cadarache, Saint Paul Lez Durance, 13108, France ABSTRACT This study aims at identifying the release mechanisms of helium in uranium dioxide. Two sets of polycrystalline UO2 sintered samples presenting different microstructures were implanted with 3He ions at concentrations in the region of 0.1 at.%. Changes in helium concentrations were monitored using two Nuclear Reaction Analysis (NRA) techniques based on the 3He(d,α)1H reaction. 3He release is measured in-situ during sample annealing at temperatures ranging between 700∞C and 1000∞C. Accurate helium depth profiles are generated after each annealing stage. Results that provide data for further understanding helium release mechanisms are discussed. It is found that helium diffusion appears to be enhanced above 900∞C in the vicinity of grain boundaries possibly as a result of the presence of defects. INTRODUCTION The α-decay of actinides in oxide nuclear fuels during long term and interim storage or indeed when the fuel is operating, is liable to produce substantial quantities of helium. Therefore predicting fuel element behaviour requires understanding the behaviour of the radiogenic helium it contains. The aim of this paper is to identify some of the more relevant mechanisms related to helium release in UO2 between 700∞C to 1000∞C. Two different procedures were used to prepare the sintered polycrystalline UO2 disks. Both resulting sets reveal different microstructures. All UO2 disks are implanted with 3He at concentrations in the region of 0.1 at.%. Samples from each set are then annealed at temperatures ranging between 700∞C and 1000∞C. Changes in helium concentrations were monitored in the DIADDHEM experimental set-up [1] using two different NRA techniques based on the 3 He(d,α)1H reaction. Firstly, in-situ 3He desorption measurements were performed during sample annealing. Then, helium depth profiles were characterised after each annealing stage from α particle energy spectra using a method described in [2]. A one dimensional diffusion model described in [3] enabled the interpretation of the depth profiles. Results are discussed in the light of a previous study [4] relating to helium intergranular and volume diffusion in UO2. EXPERIMENTAL Sample preparation and characteristics Eight polycrystalline UO2 samples were cut to a thickness of approximately 300 µm from a thick UO2 pellet and polished on one side using a one micron grain. The disks were then all annealed at 1700∞C for 24 hours in a H2 (1.7 vol.% H2O) atmosphere to remove polishing defects and to restore the samples to a stoichiometric composition.
Of the eight samples, four (samples A1 through A4) were then further polished using a colloidal silicon suspension (O.P.U) enabling grinding with a grain of approximately 50 nm. The
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