Determination of alpha dose rate profile at the UO 2 /water interface
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Determination of alpha dose rate profile at the UO2/water interface Poulesquen A.1, Jégou C.2, Peuget S.2 1 CEA Nuclear Energy Division, Department of physics and Chemistry, DPC/SECR, F-91191 Gif Sur Yvette, France 2 CEA Nuclear Energy Division, Waste Confinement and Engineering Department, DTCD/SECM, F-30207 Bagnols Sur Cèze Cedex, France ABSTRACT In this paper, an evaluation of the alpha dose rate profile at the UO2/water interface is presented. The calculation is performed in spherical geometry by considering the position of alpha emitters in the material. Moreover it proceeds to an iterative way. The fuel matrix and water is divided in spherical layers of 0.1 µm of thickness. The alpha emitters have an average energy of 5.3 MeV corresponding to a maximum distance of 13.2 µm in UO2 matrix and 42 µm in water. The direction of emission and the energy deposition, described by the Bragg curve, in the nuclear fuel are also taken into account. It appears that the alpha dose rate strongly decreases towards the calculated maximum penetration depth in water namely 42 µm. The concentrations of oxidising and reducing species are largely higher at the UO2/water interface than in solution. This alpha dose profile is used as input file in a model coupling a radiolysis kinetic model and a diffusion model. This kind of calculation is particular suitable for HTR kernels which have a spherical form of few hundreds microns of diameters (200-500 µm). INTRODUCTION In the option of direct disposal of spent nuclear fuel in a deep geological formation, the redox conditions of the groundwater in contact with the irradiated fuel may be modified by the radiolysis of water. The radiolysis of water produces both molecular and radical oxidants and reductants. UO2 solubility can increase significantly at the interface due to alpha radiation which can lead to the onset of oxidizing conditions and accelerate the dissolution of the spent fuel matrix under disposal condition [1-3]. The concentration of the different species formed during the radiolysis depends on both the nature of the ionizing radiation and the dose rate in water. In the present study, the alpha dose rate profile is calculated by taking into account that the energy deposition in matter is represented by Bragg curve. The results of calculations are confronted to Sunder’s calculation [4]. In addition, some calculations carried out on spherical HTR kernels having a diameter of 400 microns are presented. MATHEMATICAL DESCRIPTION Procedure of calculation Calculations of alpha dose rate profiles are performed in spherical geometry. The direction of emission and the energy loss in water and UO2 matrix is taking into account and the calculation proceeds to an iterative way. The UO2 matrix and water layer are divided into spherical layers of thickness 0.1 µm. Alpha particles have an average energy of 5.3 MeV. This energy leads to a maximum path range of 13.2 µm (L) in UO2 and 42 µm in water. Only the
emitters located at less than 13.2 µm of the solid surface and emitted in the solid ang
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