Advanced characterization of MIMAS MOX fuel microstructure to quantify the HBS formation

• An advanced characterization of MIMAS MOX fuel based only on fresh fuel pellet characterization.
• A probabilistic approach to model the High Burnup Structure formation in oxide fuels.
• Validation of the method by comparing to experimental data obtained on fuel irradiated in the Halden reactor.

Fission gas behaviour in accidental situations is closely related to the location of fission gas before the accident. More precisely, most of the fission gas in intergranular position is released during the accident and HBS zones contribute a lot to this intergranular quantity. So a methodology to characterize the HBS zones a priori from examination of unirradiated pellet has been developed at CEA.Characterization of plutonium distribution in MIMAS MOX fresh fuel pellets can be performed by image analysis on 1 mm2 X-ray mappings of plutonium acquired using Electron Probe Micro Analysis (EPMA). The specific software developed to describe the fuel using Pu X-ray mapping (ANACONDA) has been improved in order to simulate the fission products (FP) production and recoil during a given irradiation of the fuel, taking into account the evolution of the plutonium due to neutron irradiation. This simulation results from calculations with our fuel performance code ALCYONE combined with image processing. The final result is a mapping of local burn-up, but also the distribution of the relative FP concentration as a function of the local burn-up. A validation of this simulation process has been done by comparing the simulated mapping of neodymium to one measured on the same fuel batch after irradiation.Using previous studies of mechanisms for HBS formation, a probabilistic criterion for HBS formation has been proposed, based on the EPMA measurements of the decrease of the xenon signal as a function of the local burn-up.Combining the simulated FP cartography with this probabilistic HBS formation criterion, it is possible to calculate the surface fraction of the fuel which will be restructured at a given average burn-up, and the fraction of the total FP contained in these areas. These quantities are given as random variables.This approach has been extended to the whole fuel pellet to take into account the radial distribution of burn-up and the radial distribution of temperature (at high temperature, HBS formation is impossible) using the CEA fuel performance code PLEIADES/ALCYONE. And so, knowing the irradiation history and the initial plutonium repartition of a MOX fuel before irradiation, this methodology allows us to calculate the total fraction of HBS zones in the pellet, and the associated FP quantity.The application on an experimental fuel pellet (Pu content about 12%) irradiated in the Halden reactor demonstrates that the calculated quantities can be considered as upper bounds for the fission gas release of MOX fuel in accidental situation.