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Post irradiation examination results from commercial PWR fuel rods with average burn-ups up to 98 MWd/kgU are presented. The fractional fission gas release increased exponentially with rod burn- up from less than 10% at 50 MWd/kgU to around 25% at 98 MWd/kgU. It is argued that most of the gas released to rod free volume originated in the central region of the fuel. The penetration of the high burn-up structure in the outer region of the fuel increased markedly with burn-up, and at a pellet burn- up of 102 MWd/kgU it extended over a distance of 1.15 mm to about r/ro= 0.75. The porosity of the high burn-up structure increased linearly with the local burn-up, and in fuel pellets with burn-ups greater than 80 MWd/kgU the pores in the vicinity of the pellet rim showed a distinct increase in size. Above 70 MWd/kgU, the widespread formation of the high burn-up structure caused the fuel-swelling rate to increase from 1 to 1.5% per 10 MWd/kgU. The concentration of Xe retained in the grains of the high burn-up structure did not increase with burn-up, but remained at a low level of around 0.25 wt% over the whole bum-up range studied. It is concluded that radiation-enhanced diffusion is responsible for the release of fission gas from the recrystallised grains and that there is no build-up of gas in the grains with burn-up because equilibrium exists between the rate at which gas atoms are produced and the rate at which they diffuse to grain boundaries. The diffusion coefficient required to obtain an equilibrium Xe concentration of 0.2 to 0.3 wt% in grains 0.1 to 0.3 J um in diameter is of the order of 10-24 to 10-22 m2s-1.

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Authors: BREMIER S, JRC Institute for Transuranium Elements, Karlsruhe (DE);WALKER C.T, JRC Institute for Transuranium Elements, Karlsruhe (DE);BREMIER S, Siemens Nuclear Power GmbH, Postfach (DE)
Bibliographic Reference: Oral Report delivered at: International Seminar on Fission Gas Behaviour in Water Reactor Fuels: hosted at: Les Journees de Cadarache (FR), 26-29 September 2000
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