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Conference Article: Air and water ingress accidents in a HTR-module of side-by-side concept

Abstract

Since the possibility of a temperature induced release of fission products from the core has been eliminated by the design of the HTR-Modul, the question arose whether corrosion by air can result in a significant release of fission products. Massive air ingress is only possible by natural convection and requires three large leaks in the pressure boundary at suitable positions (chimney effect). Such an event is hardly imaginable, since the occurrence of one leak already eliminates the mechanism for the formation of additional leaks. Nevertheless this highly hypothetical event was considered in order to get knowledge about the behaviour of the system under these extreme conditions. The results show that the maximal degree of corrosion of fuel elements remains low for a long period of time, since the corrosion zone moves through the pebble bed core. A massive release of fission products is significantly delayed, thus plenty of time is available for counter measures. Water ingress accidents proved to be dominant for the risk of a HTR-Modul. This is due to the fact that fission products deposited on the surface of the primary circuit can be remobilized by reaction with water and steam and can be released from the primary circuit via two release paths. One path leads via the relief valve and the ventilation system to the stack, the other via the steam generator leak and the dump line, if a dump line valve fails to close

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Reference:

Specialists' meeting on safety and accident analysis for gas-cooled reactors. Oak Ridge, TN (USA). 13-15 May 1985

International Atomic Energy Agency, Vienna (Austria)

IAEA-TECDOC--358, pp:237-249