HTGR Knowledge Base

Conference Article: Modeling and analysis of heat transfer from the MHTGR core through a steel reactor vessel to the reactor cavity cooling system

Dilling, D.A.; Berkoe, J.M.; Ghose, S.K. (Bechtel National, Inc., San Franciso, CA (United States)); Dunn, T.D. (General Atomics, San Diego, CA (United States)); Caspersson, S.A. (ABB/Combustion Engineering, Inc., Windsor, CT (United States))

Abstract

The commercial Modular High Temperature Gas-Cooled Reactor (MHTGR) achieves improved reactor safety performance and reliability by utilizing an integrated sequence of completely passive thermal storage and heat transfer mechanisms to reject decay heat in the event that all its active cooling systems fail to operate. During such events, the initial heatup transient in the core is followed by a quasi-steady state cooldown process which, if uninterrupted, can continue for several days. A buoyancy-driven natural convection cooling system called the RCCS facilitates the continuous heat removal by circulating ambient air through the reactor cavity, where it is heated and then exhausted to the outside environment. The peak thermal load on the RCCS occurs approximately at the time that the vessel reaches its highest temperature. To confirm the adequacy of the RCCS design, detailed analytical models were developed to simulate the decay heat removal process and predict the maximum vessel temperature at this condition. Due to the integrated nature of the MHTGR thermal characteristics and the complexity of the RCCS configuration in the reactor cavity, a three-dimensional computer model of the 350 MWt plant design which extends from the reactor core to the outside environment was created to simulate the entire decay heat removal process. The results of the analyses confirm that the MHTGR core temperatures remain below their prescribed design limits, and that the RCCS adequately maintains peak vessel temperatures within acceptable limits. In providing increased understanding of the passive heat transfer mechanisms which prevail during MHTGR accident conditions, the analysis results show that the combined effects of thermal radiation, free convection, and non uniform RCCS geometry can play significant roles in the decay heat removal process and strongly influence the reactor vessel temperature profile.

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key words: Gas Cooled Reactor, Nuclear Technology

Reference:: Specialists meeting on decay heat removal and heat transfer under normal and accident conditions in gas cooled reactors. Juelich (Germany). 6-8 Jul 1992; International Atomic Energy Agency, Vienna (Austria); IAEA-TECDOC--757, pp:111-116