Initial validation of a fuel oxidation model with an out-reactor instrumented defected fuel experiment

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An out-reactor instrumented defected fuel experiment using a fuel element simulator (FES) was conducted for the first time. The experiment simulated both reactor coolant pressures and temperatures during and after the sheath (clad) of a single CANDU fuel element (fuel pin) containing UO2 pellets was defected. A mechanistic fuel oxidation model with discrete radial fuel cracks was adapted to represent the defected fuel element for the purpose of model validation. The duration of the experiment lasted ≈5.2 days at a total electrical power of 16.5 kW (or ≈20 kW m−1 of linear power). Post-test coulometric titration (CT) measurements of the fuel oxygen potential and the hyperstoichiometric oxygen, in the out-reactor UO2 fuel were consistent with a 3D fuel oxidation model results in the vicinity of the sheath defect. These results were supported by lattice parameter measurements, using X-ray diffraction (XRD), converted to hyperstoichiometric oxygen x. Away from the sheath defect, in the fuel element axial upstream direction, both the CT measurements and the 3D fuel oxidation model results showed a decline in hyperstoichiometric fuel, though this decline was more pronounced in the model. This discrepancy was believed to be a result of the model not including dynamic radial fuel crack geometry and that additional oxygen was introduced to the samples during sample preparation. Considering that most of the x measurements using the XRD lattice parameter technique closely agreed with the model results, it is believed that the fuel oxidation model was close to simulating the oxidation in the out-reactor instrumented defected fuel experiment after ≈5.2 days of heating.