Preparation and characterization of UO 2 -based AGR SIMFuel
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Preparation and characterization of UO2-based AGR SIMFuel Zoltan Hiezl1, David Hambley2 and William E. Lee1 1 Imperial College London, Centre for Nuclear Engineering, London, SW7 2AZ, UK 2 Spent Fuel Management and Disposal, UK National Nuclear Laboratory (NNL), Springfields, PR4 0XJ, UK ABSTRACT Preparation and characterization of a Simulated Spent Nuclear Fuel (SIMFuel), which replicates the chemical state and microstructure of Spent Nuclear Fuel (SNF) discharged from UK Advanced Gas-cooled Reactor (AGR) after a cooling time of 100 years is described. Thirteen stable elements were added to depleted UO2 and sintered to simulate the composition of fuel pellets after burn-ups of 25 and 43 GWd/tU and, as a reference, pure UO2 pellets were also investigated. The fission product distribution was calculated using the Fispin code provided by NNL. SIMFuel pellets exhibit a microstructure up to 92% TD. During the sintering process in H2 atmosphere Mo-Ru-Rh-Pd metallic precipitates and grey-phase ((Ba, Sr)(Zr, RE)O3 oxide precipitates) formed within the UO2 matrix. These secondary phases are present in real PWR and AGR SNF, although they are smaller in size than those examined in this study. The grain size of the produced SIMFuel is in good agreement with literature references. INTRODUCTION To engineer a safe geological disposal facility (GDF) for high level nuclear waste (HLW), the behavior of the materials that will be placed in it have to be well-understood. As many different types of nuclear power plant (NPP) exist, a range of wastes arise from their operation and the effects of their different behavior and possible interactions of their degradation products with the environment need characterizing. SNF is one of the types of material that could be consigned to a GDF. As actual SNF is highly radioactive, simulated SNF (SIMFuel) has been developed [1] to provide a convenient method to study high burn-up fuel in the laboratory, without the complication of the intense radiation field. This material is produced by doping a natural or depleted uranium dioxide (UO2) matrix with a series of non-radioactive elements in appropriate proportions to replicate the chemical composition and phases in irradiated fuels. These compositions can be varied to reflect the effects of different burn-ups and cooling times. Nonradioactive elements in SIMFuel can represent most fission products (FP) present in an irradiated SNF. A number of these FPs will segregate from UO2 matrix once their solubility limit is exceeded, forming various metallic and oxide precipitates or being released from the fuel entirely. However, other FPs with high solution limit remain dissolved in the UO2 matrix. While there are a few, slightly different, e.g. [2, 3], classifications for FP typically they are: 1. Metallic precipitates: Ru, Pd, Rh, Tc, Mo, Ag, Cd 2. Oxide precipitates: Ba, Zr, Mo, (Rb, Cs, Te, I) 3. Oxides dissolved in the UO2 matrix: Sr, Zr, Y, La, Ce, Sm, Nd, Pu, Np 4. Inert gases and volatile elements: Xe, Kr, He; I, Br, (Rb, Cs, Te)
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