Processing and microstructural characterisation of UO2-based simulated spent nuclear fuel ceramics for the UK`s advanced gas-cooled reactors

Processing and characterisation of depleted UO2-based Simulated Spent Nuclear Fuel (SIMFuel), which aims to replicate both chemistry and microstructure of Spent Nuclear Fuel (SNF) discharged from a UK Advanced Gas-cooled Reactor (AGR) after a prolonged cooling time is described in this thesis. Thirteen fission product surrogates were blended with depleted UO2 and sintered to simulate the composition of fuel pellets after burn-ups of 25 and 43 GWd/t U. Pure depleted UO2 pellets were also investigated as a reference. The fission product (FP) inventory was calculated using the FISPIN code provided by the UK National Nuclear Laboratory. Experiments were conducted in two phases, during which SIMFuel pellets were sintered for 5 and 12 h at 1730 °C in reducing atmosphere. Some pellets were also heat-treated to simulate microstructural changes in SNF while in the reactor. SIMFuel pellets were up to 92% dense, with grain sizes between 1.5 μm and 5 μm and porosity 4% and 10%. Undoped reference pellet density was ~96.5%, with grain size of 10.3±3.0 μm and ~4.5 area% porosity. Heat treatment of the UO2 samples increased grain size by ~50%, while little change occurred in the doped samples. The chemistry of the various FPs was reproduced with limitations. Notably, during the sintering process oxide precipitates ((Ba,Sr)ZrO3 perovskite phase) and Pd-Ru-Rh-Mo metallic precipitates formed within the UO2 matrix, as originally sought. Spherical oxide precipitates measured up to 30 μm in diameter, while the metallic precipitates were 0.8±0.7 μm. FPs with high solubility in UO2, such as La, Nd and Y, dissolved into the UO2 matrix. ICP-MS analysis showed that some dopants, e.g. Cs and Te, evaporated from the pellets, while the concentration of other elements had also changed during sample fabrication. Very scarce information on real PWR and AGR SNF are reviewed and compared to AGR SIMFuel fabricated in this project. Thorough analysis reveal the severe limitations of the SIMFuel technique in general and call for more experimental work and accessible publications on SNF.Open Acces