Safety objectives for the GT-MHR are first achieved by relying on the inherent safety features incorporated into plant design, which are described below.

Thermal stability of the reactor core

Thermal stability of the reactor core is ensured by the use of:

— Fuel in the form of small particles with several coating layers, which can effectively retain fission products at high temperatures (up to 1600°C) and high fuel burnups (up to 70% of fissile materials for Pu fuel);

— Graphite as the structural material for the core. Graphite has a sublimation temperature of about 3000°C and, therefore, can withstand high temperatures. Graphite structures maintain their strength even at temperatures higher than those possible in accidents. This feature ensures stability of the reactor core configuration and prevents fuel redistribution over the core volume in accidents;

— Annular reactor core with a relatively low power density (6.5 MW/m3).

TABLE VII-1. MAIN DESIGN CHARACTERISTICS

FIG. VII-3. Reactor building.

Neutronic stability of the reactor core

Neutronic stability of the reactor core is ensured by:

— High degree of reactor power self-control and self-limitation owing to negative feedback on reactor core temperature and reactor power;

— Self-shutdown capability of the reactor core at temperatures below the minimum level allowable from the viewpoint of reliable operation of the fuel particles;

— The fact that the coolant has no impact on the neutron balance because of ‘zero’ neutron absorption and scattering cross-sections. The latter prevents an uncontrolled increase of reactor power during variations in coolant density as well as under coolant loss in accidents.

Chemical stability

Chemical stability of the plant is ensured by the helium coolant being:

— Chemically inert;

— Not prone to phase changes, which rules out sharp variations of heat removal conditions in the core. Structural stability

Structural stability of the plant is attributed to:

— No large diameter pipelines used in the primary circuit;

— No steam generator (with associated complexities related to operation using a two phase coolant); no large diameter steam lines, and no steam condensing circuit existing in the plant;

— By-design prevention of large scale depressurization of vessel system components.

Dynamic stability

Dynamic stability of the reactor core is secured by:

— Core cooling by natural processes; prevention of a core meltdown in all credible accidents including primary circuit depressurization without compensation for coolant loss;

— Plant capability to switch to a safe state without control actions if all power supply sources are lost;

— Plant capability to maintain such a safe state over a long time period (dozens of hours) in hypothetical critical situations without emergency protection (EP) actuation and with no organized heat removal from the reactor.

Activity localization

Passive localization or radioactivity is provided mainly by containment designed for the retention of helium-air fluid during accidents with primary circuit depressurization. The containment is also designed for external loads, which may apply to seismic impacts, aircraft crash, air shock waves, etc. Radioactivity release from the containment into the environment is determined by the containment leakage level, which is about 1% of the volume per day at an emergency pressure of 0.5 MPa. Results of safety analyses carried out at the preliminary design stage are being used to elaborate technical measures in an effort to reduce the requirements of containment characteristics.