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Conference Article: Simulation of steam ingress accidents with irradiated fuel elements

Abstract

Accident sequences are considered for the gas-cooled High Temperature Reactor (HTR), in which water may enter into the primary circuit and reactor core as a consequence of pipe rupture in the steam generator. Irradiation experiments with intermittent water injections have demonstrated that moisture in the sweep gas lead to an increase of the release of fission gases and iodine from defective/failed particles. A special apparatus KORA was constructed in the Hot Cells of the Research Centre Juelich to study the effects of moisture-related fission product release as a function of temperature and water vapour partial pressure with different fuel samples. Initial experiments with irradiated UO₂ and UCO fuels at 800 deg. C showed an increased of ⁸⁵Kr release with water vapour additions. In contrast, intact particles are not affected even by extremely long water vapour injections. UO₂ kernels obtained by cracking particles from spherical fuel elements correspond to irradiation-induced failures. They show the following release fractions at 800 deg. C after repeated injections of water vapour: with a medium burn-up of 5% FIMA. With a high burn-up of 9% FIMA. Release of 0.4 to 2.6% of the ⁸⁵Kr inventory. Release of 17% of the ⁸⁵Kr inventory. In the case of defective UO₂ TRISO particles, which would dominate the release in an HTR-MODUL, some of the free fuel may have been carburized in the fabrication process during the final heat treatment at 1950 deg. C, which could lead to changed release behaviour. Further studies will have to show whether the release as a consequence of the influence of water vapour is similar to that from UO₂ kernels or possible higher. There was a complete moisture-induced release from high-burnup UCO kernels or designed-to-fail particles with a burnup of 20% FIMA. Together with the knowledge that unirradiated UO₂ kernels show practically no changes due to moisture, the moisture-induced fission gas release - and similar the iodine release - from fuel kernels is a burnup dependent effect. Both the changed structure of the kernel as well as the chemical composition may be of significance.

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Reference:

Technical committee meeting on response of fuel, fuel elements and gas cooled reactor cores under accidental air or water ingress conditions. Beijing (China). 25-27 Oct 1993

International Atomic Energy Agency, Vienna (Austria)

IAEA-TECDOC--784, pp:37-44