It was noted that, in a nuclear reactor, 35Cl present as an impurity in nuclear fuel was activated by thermal neutron capture. In order to elucidate possible diffusion mechanisms, both irradiation and thermal effects had to be assessed. The thermal diffusion of Cl in depleted UO2 was considered here. For this purpose, sintered UO2 pellets were implanted with 37Cl to an ion fluence of 1013/cm2 and successively annealed at 1175 to 1475K. The implanted Cl was used to simulate the behavior of that displaced due to recoil and to interactions with fission fragments during reactor operation. The behaviors of pristine and implanted Cl were investigated during thermal annealing. Secondary ion mass spectrometry and μ-XAS (at the Cl–K edge) analyses showed that thermal migration of implanted Cl became significant at 1275K. This temperature level, and the calculated activation energy (4.3eV) emphasized the marked ability of Cl to migrate in UO2 at relatively low temperatures. The behavior of implanted Cl which aggregated into so-called hot spots, before effusion, was clearly different to that of the pristine Cl; which remained homogenously distributed after annealing. The hot-spot and pristine Cl seemed to be present in different structural environments. Both types of Cl were assumed to have a valence state of −1. A comparison of an U2O2Cl5 reference compound with the pristine Cl environment revealed a contribution of U2O2Cl5 to the pristine Cl.

Thermal Diffusion of Chlorine in Uranium Dioxide Studied by Secondary Ion Mass Spectrometry and X-ray Absorption Spectroscopy. Y.Pipon, N.Toulhoat, N.Moncoffre, L.Raimbault, A.M.Scheidegger, F.Farges, G.Carlot: Journal of Nuclear Materials, 2007, 362[2-3], 416-25