Study of the kinetics of fission gases inside fuel rods of nuclear power plants using the ALCYONE fuel performance code.

The aim of this work was to simulate the experimental irradiations of PWR-type fuel rods in experimental reactors using the ALCYONE fuel performance code developed by CEA. It took place within the framework of the P2M project, which is an international irradiation program, led by the core group CEA, EDF and SCK-CEN (Belgium). This irradiation program consists in studying the behaviour of an instrumented fuel rod submitted to a power ramp, until reaching a molten volume fraction in the fuel centreline. The final objective was to calculate the fission gas kinetics and more precisely the fission gases detected inside the plenum for the dimensioning of the P2M fuel rod, with particular focus on the evaluation of the pressure inside its plenum due to burst releases of fission gases. At the moment, its cause is still unknown: it could be due to fuel fragmentation or by the fuel-cladding gap reopening, even though this is unlikely as the gap is supposed to be closed at high burn-up. This study will help evaluate how those gases will impact the P2M experiment, to know if the expected pressure is within the sensor’s range of measurement, to address possible safety issues and for the dimensioning of the P2M-Q2 fuel rod’s free volumes. Three other past experiments have been simulated in order to calibrate the software and identify its capabilities and limitations by comparing the results with available experimental data: REMORA 1, AN3 and AN10. These experiments have a wide range of objectives, but in general they are done to better understand fuel behaviour during power transients to improve the safety of reactors while also increasing the lifetime of the fuel inside the reactor, allowing to use more of their available energy before needing to be changed. The challenge of simulating the burst release in order to have a proper evaluation of the plenum pressure has proven to be difficult. While ALCYONE is a precious and powerful tool, it still has its limitations due to the complexity of the phenomena that it has to describe; unfortunately, this is common for every fuel performance codes currently being developed around the world. Nonetheless it has shown its great reliability in its depiction of thermal and mechanical aspects by correctly predicting cladding deformations and the temperatures measured by the thermocouples during past experiments. These calculations have been used in October 2023 for the P2M phase 2 simulation exercise for comparison with the other partners’ fuel performance codes.