Physico-chemical characterization of a spent UO 2 fuel with respect to its stability under final disposal conditions
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Physico-chemical characterization of a spent UO2 fuel with respect to its stability under final disposal conditions Ernesto González-Robles1, Detlef H. Wegen2, Elke Bohnert1, Dimitrios Papaioannou2, Nikolaus Müller1, Ramil Nasyrow2, Bernhard Kienzler1, and Volker Metz1 1 Karlsruhe Institute of Technology (KIT), Institute for Nuclear Waste Disposal (INE), Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany 2 European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany
ABSTRACT Two adjacent fuel rod segments were irradiated in a pressurized water reactor achieving an average burn-up of 50.4 GWd/tHM. A physico-chemical characterisation of the high burn-up fuel rod segments was performed, to determine properties relevant to the stability of the spent nuclear fuel under final disposal conditions. No damage of the cladding was observed by means of visual examination and γ-scanning. The maximal oxide layer thickness was 45 μm. The relative fission gas release was determined to be (8.35 ± 0.66) %. Finally, a rim thickness of 83.7 μm and a rim porosity of about 20% were derived from characterisation of the cladded pellets. INTRODUCTION During irradiation in the reactor and also during the cooling time after discharge, the nuclear fuel, as well as the cladding materials, undergoes transformation due to the temperature, external irradiation and fission reactions. As a consequence of the high temperature the cladding can creep and the generated fission products cause a swelling of the fuel [1]. On the other hand, at burn-up (BU) higher than 40 GWd/tHM the formation of porous and fine grained microstructure is observed , commonly referred as rim structure or high burn-up structure (HBS) [2]. The first step in analysis of irradiated fuel rods is the non-destructive testing (NDT). NDT is an essential set of analyses that allows to consistently acquire reliable data needed for validation of the safety and efficient performance of the fuel rod in pile and to provide a valuable basis of information to plan and implement successful destructive post irradiation examination [3]. Moreover, results of the NDT are of importance for forthcoming studies on the stability of the spent nuclear fuel (SNF) under conditions of deep geological disposal. This paper focuses on NDT as visual examination, γ-scanning, and oxide layer thickness as well as on destructive analysis which allows the quantification of the fission gases release into the plenum and the characterisation of the HBS. EXPERIMENTAL Characteristic data of the spent nuclear fuel Segments N0203 and N0204 of the fuel rod SBS 1108 were irradiated in the pressurised water reactor of Gösgen, Switzerland, during four cycles of 1226 days in total with an average linear power of 260 W/cm achieving an average BU of 50.4 GWd/tHM. The fuel rod segments
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were discharged on the 27th May 1989. Segment N0203 was characterized already after three years cooling time in the “Heiße Zellen” division of KIT (formerly
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