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Conference Article: Depressurization accident analysis for the HTTR by the TAC-NC

Kunitomi, K.; Nishiguchi, I.; Wada, H.; Takeda, T.; Hishida, M.; Sudo, Y.; Tanaka, T.; Saito, S. (Japan Atomic Energy Research Inst., Oarai, Ibaraki (Japan). Oarai Research Establishment)

Abstract

The two-dimensional thermal analysis code TAC-NC is modified from the analytical code TAC-2D in order to calculate temperature transients in the case of loss of forced cooling accidents of the HTTR (High Temperature engineering Test Reactor) such as a depressurization accident. In these conditions, temperature transients in the core are affected by natural convection between hotter and colder regions in the pressure vessel. The TAC-NC code includes a special function to calculate heat transfer by natural convection in addition to conduction, radiation and forced convection. Verification of the TAC-NC code was carried out by the comparison of the analytical results with the experimental ones of an air ingress test. Analytical results of simulated core temperature were in good agreement with experimental results. Temperature transients during a depressurization accident were evaluated by the TAC-NC code for the HTTR. The maximum fuel temperature decreases rapidly after the reactor scram and increases slightly after that due to decay heat. The maximum fuel temperature, however, remains below the initial maximum fuel temperature since most of the core decay heat is absorbed in the large thermal capacity of graphite in the core and reflector. The peak vessel temperature occurs at about 30 hours after the beginning of the accident and also remains lower than the allowable temperature, even if one of the reactor pressure vessel cooling systems is failed.

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key words: depressurization; graphite; heat transfer; httr reactor; natural convection; pressure vessels; reactor accidents; reactor cores; reactor safety; specific heat; t codes; temperature gradients; transients
Reference:
Technical committee meeting on gas-cooled reactor technology safety and siting Dimitrovgrad (Russian Federation) 21-23 Jun 1989
International Atomic Energy Agency, Vienna (Austria)
IAEA-TC--389.26, pp:41-55