Safety of plant personnel and the population living near a NPP site is ensured by consecutive implementation of the defence in depth concept in plant design. This concept stipulates the application of several barriers to the release of ionizing and radioactive substances into the environment, as well as application of technical features and administrative measures to protect and maintain the effectiveness of barriers and to protect personnel, the population and the environment.

Effectiveness of the protective barriers under accident conditions is maintained mainly through inherent reactor (self-protection) features based on negative feedback and natural processes, and due to the use of passive safety systems.

Physical barriers for the GT-MHR are:

— Coated fuel particles;

— Fuel compacts;

— Fuel assemblies;

— Leaktight primary pressure boundary (vessel system);

— The containment.

The reliable retention of fission products within fuel assemblies is ensured by:

(1) The design of coated particle fuel and fuel assemblies based on available experience in fuel element design, testing and operation. The GT-MHR utilizes ceramic fuel in the form of 200 pm spherical particles with multilayer pyrocarbon and silicon carbide coatings (coated fuel particles), which are dispersed in the graphite matrix (fuel compact). Silicon carbide is the main barrier preventing a release of gaseous and volatile fission products. Fuel compacts and fuel assemblies are made of graphite, which provides the effective retention of solid fission products;

(2) Design features to prevent fuel overheating under abnormal operation conditions;

(3) Design features to provide a large temperature margin between the operation limit and the safe operation limit; crisis free heat removal from the fuel elements during normal and abnormal operation, including design basis accidents;

(4) Design features ensuring that fuel temperature does not exceed 1600°C in any accident involving failure of heat removal from the reactor, including the failure of all ‘active’ means of reactor shutdown and cooling. In this way, the effectiveness of the main protective barrier (protective coating on fuel kernels limiting fission product release beyond the boundaries of coated fuel particles) is maintained.