Fast/Instant Radionuclide Release: Effects inherent to the experiment
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Fast/Instant Radionuclide Release: Effects inherent to the experiment B. Kienzler, A. Loida, E. González-Robles, N. Müller, V. Metz Karlsruhe Institute of Technology (KIT), Institute for Nuclear Waste Disposal (INE), Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ABSTRACT The release of radionuclides measured during washing cycles of spent nuclear fuel samples in a series of experiments using different solutions are analyzed with respect to the fission products Cs, Sr, and Tc and the actinides U, Pu, and Am. Based on the concentrations of the dissolved radionuclides, their release rates are evaluated in terms of fraction of inventory in the aquatic phase per day. The application of this information on the fast/instant radionuclide release fraction (IRF) is discussed and following issues are addressed: Duration of the wash steps, solution chemistry, and radionuclide sorption onto surface of the experimental vessels. Data for the IRF are given and the correlation between the mobilization of the various elements is analyzed. INTRODUCTION A recent publication by Johnson et al. [1] summarized results on the release of 137Cs and I from high burn-up UOx and MOX fuels in the range between 58 and 75 MWd/kgU over leaching periods of ~100 days. According to this publication, the ratio of fission gas release (FGR) to the fractional release of 137Cs and 129I was related to the power history of the spent nuclear fuels (SNF) and resulting heterogeneities in the SNF. Johnson et al. concluded, increased power ratings resulted in higher releases of fission gases, 137Cs and 129I. Still, the contribution of the rim region (fission gas ,137Cs and 129I accumulation) and the contribution of grain boundary release remains unclear. Besides these conclusions related to fuel inherent properties, the authors addressed effects caused by the experimental set-up: x The preparation of the SNF samples significantly influenced the determination of IRF / FGR, e.g. experiments with SNF fragments resulted in relatively high IRF values. x Presence of cladding lead to an underestimation of the IRF. In previous studies a relatively small fraction of 90Sr was found to be fast released in comparison to the IRF of 137Cs and 129I, e.g., [2-5]. For evaluation of SNF leaching experiments, in many cases the 90Sr release is used as an indicator for the dissolution of the UOx matrix of the fuel. Since the IRF of 90Sr depends on the power history of the studied fuel, it has to be discussed, whether 90Sr is an adequate proxy for UOx matrix dissolution of the specific SNF under investigation. The EURATOM FP7 Collaborative Project “Fast / Instant Release of Safety Relevant Radionuclides from Spent Nuclear Fuel (CP FIRST-Nuclides)” addresses the influence of experimental conditions on IRF determination and the relation between uranium release from the SNF matrix and 90Sr release, beyond other issues. During discussions of the project work program with waste management organizations the question of the effects of solution chemistry, especially
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