HTGR Knowledge Base

Conference Article: The structure of risk caused by accidents in small and medium HTRs

Kroeger, W.; Mertens, J.; Wolters, J. (Kernforschungsanlage Juelich G.m.b.H. (Germany, F.R.). Inst. fuer Nukleare Sicherheitsforschung)

Abstract

Modern small and medium HTR concepts differ in their design features and safety concepts from each other, but especially from a large LWR, to such an extent that essential differences in accident topology and in the damage structure of risk are to be expected. This is confirmed by small-effort probabilistic analyses for the HTR-500 (thermal power 1250 MW, power density 6 MW/m3 and HTR-MODUL (200 MW, 3 MW/m3).

The dominant design-relevant accident phenomenon for the risk of a medium HTR is core heat-up since loss of core cooling involves temperatures in the fuel elements (maximum 2250 deg.C) which significantly impair their retention properties. The greatest contributions to risk and at the same time also the greatest consequences result from event sequences initiated by loss of preferred power and comprising failure of the afterheat and liner cooling systems (expected frequency of occurrence < 10-7/a).

In the case of small HTR's, failure of core cooling system is not radiologically significant due to lower maximum temperatures of the fuel elements ( < 1600 deg.C). In this case the most important accident phenomenon is remobilization of activity deposited on metallic surfaces. This can be associated with an ingress of water into the primary circuit. The dominant case for risk and consequences is initiated by a leak in the steam generator and involves failure of the depressurization line to close after opening (< 6x10-6/a).

If one compares the reference values for the risk on the basis of equal power then no to decisive differences result, taking into consideration existing uncertainties; it is, however, more essential that the numerical values are composed differently (release quantity vs. release frequency).

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key words: biosphere; fission product release; htgr type reactors; probability; radiation doses; reactor accidents; risk assessment; accidents; gas cooled reactors; graphite moderated reactors; 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:105-108