Safety aspects of the modular high-temperature gas-cooled reactor (MHTGR)

The Modular High-Temperature Gas-Cooled Reactor (MHTGR) is an advanced reactor concept under development through a cooperative program involving the U.S. Government, the nuclear industry and the utilities. Near-term development is focused on electricity generation and cogeneration uses such as water desalination. The design utilizes the basic high-temperature gas-cooled reactor (HTGR) features of ceramic fuel, helium coolant, and a graphite moderator. However, the specific size and configuration are selected to utilize the natural characteristics of these materials to develop a significantly higher margin of safety than current generation reactors. The qualitative top-level safety requirement is that the plant's operation not disturb the normal day-to-day activities of the public. Quantitatively this requires that the design meet the U.S. Environmental Protection Agency's Protective Action Guides at the site boundary hence precluding the need for sheltering or evacuation of the public. The MHTGR safety response to events challenging the functions relied on to retain radionuclides within the coated fuel particles has been evaluated. A broad range of challenges to core heat removal have been examined which include a loss of helium pressure and a simultaneous loss of forced cooling of the core. The challenges to control of heat generation have considered not only the failure to insert the reactivity control systems, but the withdrawal of control rods. Finally, challenges to control chemical attack of the ceramic coated fuel have been considered, including catastrophic failure of the steam generator allowing water ingress or of the pressure vessels allowing air ingress. The plant's response to these extreme challenges is not dependent on operator action and the events considered encompass conceivable operator errors. In the same vein, reliance on radionuclide retention within the fuel particle and on passive features to perform a few key functions to maintain the fuel within acceptable conditions also reduces susceptibility to external events, site-specific events, and to acts of sabotage and terrorism. (author). 4 refs, 14 figs, 1 tab