Chlorine Diffusion in Uranium Dioxide : Thermal Effects versus Radiation Enhanced Effects
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0985-NN05-03
Chlorine Diffusion in Uranium Dioxide : Thermal Effects versus Radiation Enhanced Effects Yves Pipon1, Nelly Toulhoat1,2, Nathalie Moncoffre1, Nicolas Bererd1, Henri Jaffrezic1, Marie France Barthe3, Pierre Desgardin3, Louis Raimbault4, Andre M. Scheidegger5, and Gaelle Carlot6 1 Université de Lyon / Université Claude Bernard Lyon 1 / IUT A Chimie, CNRS/IN2P3/IPNL, 4 rue Enrico Fermi, Villeurbanne, 69622, France 2 Commissariat a l'Energie Atomique, DEN, CEN Saclay, Gif sur Yvette cedex, 91191, France 3 CNRS, Centre d'Etudes et de Recherches par Irradiation, 3A rue de la Ferollerie, Orleans cedex2, 45071, France 4 Ecole des Mines, Centre d'Informatique Geologique (CIG), 35 rue Saint Honore, Fontainebleau cedex, 77305, France 5 Paul Scherrer Institut, Nuclear Energy and Safety Department (NES), Laboratory for Waste Management, Villigen, 5235, Switzerland 6 Commissariat a l'Energie Atomique, DEN/DEC/SESC/LLCC, Centre de Cadarache, Saint Paul lez Durance, 13108, France
ABSTRACT During reactor operation, chlorine (35Cl), an impurity of the nuclear fuel, is activated into 36 Cl, a long lived mobile isotope. Because of its long half life and its mobility, this isotope may contribute significantly to the instant release fraction under disposal conditions. Thermal annealing of Cl implanted UO2 sintered pellets show that it is mobile from temperatures as low as 1273 K (Ea = 4.3 eV). Chlorine diffusion induced by irradiation with fission products preserves a thermally activated contribution. The radiation induced defects significantly enhance chlorine migration. INTRODUCTION Chlorine is present as an impurity in the nuclear fuel (< 5 ppm). During reactor operation, Cl is activated into 36Cl, a long-lived isotope (T = 3.01.105 years). In the case of interim storage or disposal of the spent fuel, 36Cl may significantly contribute to the instant release of activity, thereby contaminating the bio/geosphere [1]. The evolution of spent nuclear fuel with time before water access to the waste form in a repository must be assessed, because it may influence the subsequent radionuclide release. This study provides information on the migration behavior of chlorine in a sintered depleted uranium dioxide. During in-reactor life, part of the 36Cl may be displaced from its original position, due to recoil or to collisions with fission products. In order to study the behavior of the displaced chlorine, 37Cl has been implanted into UO2 pellets. The thermal effects on the migration of the implanted and initially present (or pristine) Cl (35Cl = 75.78 % and 37Cl = 24.22 %) have been investigated on one hand [2-4]. Moreover, preliminary results on the effects of the irradiation defects induced by fission fragments have been studied [5]. In this work, we discuss the respective effects of temperature and defects (remaining from polishing or induced by 37Cl implantation or heavy ion irradiation) on the migration of Cl. 35
EXPERIMENTAL Sintered depleted UO2 (0.3 % of 235U) pellets have been provided by the CEA Cadarache (8.2 mm in dia
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