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Conference Article: HTR fuel: prediction of fission product release in accidents

Abstract

The basic fuel unit of the HTR is the coated particle of about 1 mm diameter. An oxidic fuel kernel is surrounded by a low density buffer layer and a silicon carbide coating sandwiched between high density pyrocarbon coatings. The total release of fission products during accidents is determined not only by the transient-induced and the irradiation-induced failure of the coatings, but also by the levels of manufacturing defects and the level of heavy metal contamination in the fuel matrix material. Modern coated fuel particles are designed so that the fission gas pressure-induced stress in the SiC coating remains small relative to the strength of the SiC even under full design burnup conditions. Therefore the pressure vessel failure of the particles is insignificant both in normal operations and in accidents. Silicon carbide thermal decomposition becomes the dominant failure mode as temperatures exceed 2000 deg. C. Interaction of fission products with silicon carbide leading to SiC corrosion is the dominant failure mechanism below 2000 deg. C. Laboratory simulations of HTR transients have usually measured the release of Cs 137 and Kr 85 as indicators of the coating failure. Once the silicon carbide fails by corrosion or decomposition, Cs 137 is released and is taken as the direct indicator of SiC failure in fuel performance modeling studies. In the case of Kr, an additional delay beyond the Cs release is found due to the time required for Kr to diffuse through the remaining outer pyrocarbon coating. The delay between the SiC failure and gas release is analyzed to yield data on the diffusion coefficient of Kr in pyrocarbon. The present data suggest that, in terms of expected values, the fission product release during a modular reactor system transient to 1600 deg. C is dominated by the manufacturing defects and heavy metal contamination rather than irradiation-induced or transient-induced coating failure.

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Reference:

Specialists' meeting on fission product release and transport in gas-cooled reactors Berkeley (United Kingdom) 22-25 Oct 1985

International Atomic Energy Agency, International Working Group on Gas-Cooled Reactors, Vienna (Austria)

IWGGCR--13, pp:155-169