Fission Gas Behaviour in a Multiscale modelling Context: 10 years of research at SCK-CEN

Fission gas behaviour has a major effect on fuel rod performance and the phenomenon is even more marked at high burnup considering their accumulated yield. The fraction of the produced gas that precipitates inside the grains or at grain boundaries presents a larger effective volume than the atomically-dissolved fraction, enhancing as such fuel swelling. Gas that could escape from the fuel has a beneficial effect on fuel swelling, but still imposes constraints on the cladding through an increase of rod internal pressure. An accurate inventory of the gas in its different forms would therefore enable to better estimate the margin to rod failure both under normal and accident conditions. Over the past decade, SCK•CEN has investigated fission gas behaviour at the meso- and nano-scale through experimental (Scanning Electron Microscopy, SEM; Electron Probe Micro-Analysis, EPMA; rod puncture; gamma-spectrometry) and theoretical work. These studies have recently been supported by up-to-date atomic scale computer experiments which shed some light on the underlying primary mechanisms at play. The findings also provided an atomistic base for the existence of a well-defined threshold for fission gas release, commonly known as the "Halden threshold", well known to nuclear fuel researchers and engineers.