Mechanistic modeling of chemically reactive fission product release during power ramps

This work presents and validates a novel approach for modelling non-inert fission products behaviour during a power transient. The proposed model, MARGARET Active Fission Products, leverages all the microstructural quantities computed by MARGARET-a code originally developed to evaluate the release of inert fission products under normal and incidental loading sequences-to simulate the production, decay, and transport of non-inert fission products. The MARGARET Active Fission Products model is capable of handling 75 different isotopes, ranging from short-lived to long-lived species. The tendency of non-inert fission products to form different chemical compounds in different phases within the fuel compared to inert ones (always gaseous at equilibrium), requires the incorporation of thermochemistry in the calculations, since it directly impacts their release kinetics from the fuel. The thermochemical behaviour of fission products is included by performing calculations with the Open-Calphad thermochemical solver and the TAF-ID database. In this coupled thermochemistry-fission gas release approach, thermochemistry leads to the assessment of the quantity of non-inert fission products in the gas phase, that will percolate towards the free volume and, consequently, be released in the fuel rod. To validate the model, a power transient simulation was performed, replicating an experiment conducted on a refabricated fuel rod in the OSIRIS experimental reactor. The simulation results are compared with experimental data, including the total xenon release (measured by puncturing), the release of 137Cs, 131I, and 132Te at each interpellet (obtained from gamma spectrometry), and the radial distribution of xenon in the pellet, before and after the power ramp (analysed by Secondary-Ion Mass Spectrometry measurements). Furthermore, a detailed discussion on the thermochemical results is provided.