For high temperature gas cooled reactors (HTGRs) with pebble bed or pin-in-block tristructural-isotropic (TRISO) fuel and helium coolant, smaller reactor capacity facilitates:

— Long term passive decay heat removal from the core to the outside of the reactor vessel based on natural processes of conduction, radiation and convection, with natural convection based heat removal from the outside of the reactor vessel to an ultimate heat sink;

— Achievement of a large temperature margin between the operation limit and the safe operation limit, owing to inherent fission product confinement properties of TRISO fuel at high temperatures and fuel burnups;

— De-rating of accident scenarios rated as potentially severe in reactors of other types, including loss of coolant (LOCA), loss of flow (LOFA), and reactivity initiated accidents; for example, helium release from the core in the GT-MHR can be a safety action and not the initiating event for a potentially severe accident;

— Achievement of increased reactor self-control in anticipated transients without scram, without exceeding safe operation limits for fuel;

— Relatively high heat capacity of the reactor core and reactor internals and low core power density, resulting in slow progression of the transients.

It should be noted that, in view of currently known reactor vessel materials, an HTGR unit capacity below ~600MW(th) is a necessary condition to ensure long-term passive decay heat removal from the core as described in the first item of this listing. Therefore, all currently known concepts of HTGR with TRISO based fuel and gas coolant belong to the SMR range [2].