MHTGR licensing approach and plant response to off normal events

The MHTGR design meets stringent top-level regulatory and user safety requirements that require that the normal and off-normal operation of the plant not disturb the public's day-to-day activities. Quantitative, generic top-level regulatory criteria have been specified from US NRC and EPA sources to guide the design. The user has further specified that these be met at the plant boundary. A probabilistic risk assessment has been utilized to select licensing basis events that cover a wide spectrum of events. The events have been grouped into three frequency regimes governed by the 10CFR50 App. I, 10CFR100, and PAG risk and dose criteria. The MHTGR fuel has been designed to limit the primary circuit activities to levels that, even if completely released, are within those allowed by 10CFR100. The focus of the safety approach has then been centered on retaining the radionuclide inventory within the fuel by removing core heat, controlling chemical attack, and by controlling heat generation. The core geometry, core power, core power density, heat removal geometry, and heat sinks are designed to provide passive core temperatures during accidents. To limit the potential for chemical attack of the MHTGR core, the primary coolant boundary is designed to make large ingress of air and water very unlikely. Furthermore, the fuel particle coatings are highly impervious to oxidizing agents that do enter the primary coolant. The core heat generation is controlled by the large negative temperature coefficient and by insertion of control material to maintain a subcritical core configuration during the span of the accidents. Thus, the three key safety functions have been accomplished by relying on the inherent characteristics of the MHTGR so that the response is largely passive and relatively simple. (author). 4 refs, 5 figs, 1 tab