The main technical parameters of HTR-200 can be found in Table 18.1 and Figures 18.1 and 18.2.

18.2.2 Engineered safety feature plan

The safety design philosophy of HTR-200 is to realize the required high level of safety and, at the same time, to simplify the design of the systems required only for safety purposes, to the greatest extent possible. Emergency measures outside the plant boundary should be made technically not necessary or reduced to a minimum level. The HTR-200 safety design is to a large degree based on the inherent and/or passive safety features, while still adhering to the defense-in-depth principles. The following three features characterize the basic safety concept of HTR-200:

• Radioactive materials are confined through the implementation of multiple barriers with a strong emphasis on fuel elements, especially in accidents. Fuel elements with coated particles serve as the first barrier. Every fuel kernel of about 0.5 mm diameter is coated with three layers of pyro-carbon and one silicon carbon (SiC) layer. A large number of coated particles are dispersed in the graphite matrix of 5 cm diameter to form the fuel-containing part of a fuel element, which in turn is protected by a 0.5 cm thick fuel-free graphite layer. The fuel elements used for HTR-200 were demonstrated to be capable of confining fission products within the coated particles under temperatures of ~1600 °C that are not expected for any plausible accident scenario. The second barrier is the primary pressure boundary, which consists of a pressure vessel that hosts units of the primary components. The third barrier is a reactor building and some additional auxiliary buildings, which house the primary helium-containing components.

• The decay heat is automatically removed under accident conditions. In the case of an accident, the primary helium circulator is stopped. Because of the low power density and the large heat capacity of the graphite structures, the decay heat in fuel elements will dissipate to the outside of a reactor pressure vessel by means of heat conduction and radiation within the core internal structures, without leading to unacceptable fuel temperatures.

• The overall negative temperature feedback is guaranteed under all conditions. The reactor nuclear design assures that the temperature reactivity coefficients of fuel and moderator are always negative under all operating and accident conditions. Together with the protection action of stopping the primary helium blower, will lead to an automatic reactor shutdown in an accident.