Determination of the pressure in micrometric bubbles in irradiated nuclear fuels

In oxide nuclear fuels, at high burn-up or during high temperature periods such as ramp tests, out-of-pile heating tests, or any irradiations at high linear heat rates, fission gases can form micrometric or quasi-micrometric bubbles. During nominal operations, these bubbles participate to the pellet swelling, to the decrease of the fuel thermal conductivity and are involved in the mechanisms leading to fission gas release. During events involving a temperature increase, the resulting increase in the internal pressure of the bubbles might play a role in fuel fragmentation and in the opening of grain boundaries. The gas densities inside these bubbles are therefore one of the useful experimental information for the understanding of the fuel behaviour, and for the fuel behaviour code progress and validation. Two methods were developed to evaluate the gas density in the quasi-micrometric bubbles, using electron probe micro analyser, secondary ion mass spectrometry and focused ion beam scanning electron microscope together. The first method provides a mean gas density for all quasi-micrometric bubbles in a given area. The second method provides a gas density in a single selected bubble. In addition to the gas density, the 3D size and shape of the selected bubble is measured and can be related to the gas density result. In this work, these methods were applied to the bubbles formed in the centre of a PWR Cr doped UO₂ at 38.8 GWd/t_U after a ramp test in the Osiris reactor, with a 12 h plateau at 470 W/cm, and to the bubbles formed in a PWR Cr doped UO₂ at 62.8 GWd/t_U in the centre of the pellet and on the bubbles of the high burn-up structure on the rim. Both show the high pressures reached in these bubbles.