Fission Gas Release and Microscopic Swelling in Highly Rated Advanced Fuels

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Abstract
Mixed carbide, carbonitride, and nitride fuels have been irradiated in DFR and Rapsodie to a maximum burnup of 7.8 at.% at a maximum linear power of 135 kW·m−1. At low burnup, xenon release from helium-bonded fuels was found to be dependent on the chemical composition of the fuel Release was greatest from carbide (75%) and least from nitride fuels (35%). At medium burnup, improved gap conductance led to a fall in the fuel centerline temperature and consequently a decrease in gas release. For nitride and carbonitride fuels, over 75% of the retained fission gas was contained in bubbles (<1 µm in diameter) and in the fuel matrix. For all three fuels, xenon release from the outer unrestructured region of the fuel was <15%, whereas release from the central porous region was 50% or more. In the restructured region, gas was released to the plenum by way of interconnected pores. Gas in pores contained proportionally more krypton than the bonded gas, and consequently, it is proposed that atomic diffusion is the principal mechanism of gas transport within the fuel.