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Conference Article: Safety analysis of small and medium HTRs under core-cooling accident conditions

Rehm, W.; Altes, J.; Breitbach, G.; Nabbi, R.; Verfondern, K. (Kernforschungsanlage Juelich G.m.b.H. (Germany, F.R.). Inst. fuer Nukleare Sicherheitsforschung)

Abstract

For the safety analysis of HTR's the core-cooling accidents are of great importance because of their influence on the retention behaviour of the different fission product barriers. The consequences on the surroundings depend on reactor design and accident sequences. In the modular HTR with 200 MW(th), a core heatup leads to maximum temperatures of 1100 deg. C in the reactor under pressure and of 1500 deg. C in the depressurized reactor. Inadmissable temperatures of the components only result from the loss of the surface cooling system and from a failure of blower shutdown for ATWS. In the HTR-500 with 1250 MW(th), the maximum core temperatures will rise in a core heatup to 1650 deg. C in the reactor under pressure and up to 2550 deg. C in the depressurized case. With an intact liner cooling system the reactor can be cooled over a long term. The top reflector and thermal shield will fail because of the high temperatures. In spite of the increase of reactivity by subsidence of the top reflector and reactor scram by reflector control rods only, the reactor remains subcritical up to 210 h. Afterwards the thermal power is stabilized at a low level. Without liner cooling the liner insulation fails after 210 h and a limited thermal decomposition of the concrete of the PCRV occurs. The release of fission products from the primary circuit into the containment appears as result from opening the safety valve. Because of the relatively low accident temperatures in small HTR's no significant additional fission product release from the core occurs, so that the activity which is deposited in the primary circuit under normal operating conditions will become more important. But this inventory is too small to cause severe consequences. In medium HTR's temperatures are reached with an increasing failure of particles and a higher diffusive release of metallic fission products. The analysis, however, shows that the transport of activity out of the containment is strongly controlled by the high retention properties and the advanced safety features of HTR's.

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key words: after-heat removal; atws; coolants; fission product release; htgr type reactors; loss of flow; safety analysis; very high temperature; accidents; design basis accidents; gas cooled reactors; graphite moderated reactors; reactor accidents; reactors
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:109-119