The HTR-200 design has the following remarkable technical features [5]:

• Spherical fuel elements with tristructural isotropic-type (TRISO) coated particles are used, which have a proven capability of fission product retention under 1600 °C in accidents.

• Ceramic materials, i. e. graphite and carbon bricks that are resistant to high temperatures, surround the reactor core.

• The decay heat in fuel elements is assumed to be dissipated by means of heat conduction and radiation to the outside of the reactor pressure vessel, and then taken away to the ultimate heat sink by water cooling panels on the surface of the primary concrete cell. Therefore, no coolant flow through the reactor core would be necessary for the decay heat removal in loss of coolant flow or loss-of-pressure accidents. The maximum temperature of fuel in accidents will be limited to 1600 °C.

• Spherical fuel elements are charged and discharged continuously in a so-called ‘multi-pass’ mode, which means the fuel elements pass through a reactor core several times before reaching the discharge burn-up.

• Two independent reactor shutdown systems are foreseen. Both systems are assumed to be located in the graphite blocks of the side reflector. When called upon, neutron absorber elements will fall into the designated channels located in the side reflectors, driven by gravity.

• The reactor core and steam generator are housed in two steel pressure vessels, which are connected by a connecting vessel. Inside the connecting vessel, a hot gas duct is mounted. All pressure-retaining components, which constitute the primary pressure boundary, are in touch with the cold helium of the reactor inlet temperature.

• Under an accident with complete loss of pressure, the primary helium inventory will be released into the atmosphere due to the radioactive material in the primary helium loop being very low and then will be no fuel failure during a loss-of-coolant accident (LOCA).

• Several HTR-PM (pebble bed module) modules could be built at one site to satisfy the power capacity demand of a utility. Some auxiliary systems and facilities could be shared among the modules.

After the Chernobyl Unit 4 accident in 1986, there was little new reactor installation. In 2000, the Generation IV reactor development was proposed by the US DOE (Department of Energy). This stimulated R&D activity in Japan, including that on SMRs. At this time, the economic aspects were considered to be the most important point in the development, and hence R&D was aimed at overcoming the scale demerit to achieve the same level of the construction cost (per kWe) and the electricity generation cost (per kWh) as for the existing large LWRs. In order to accomplish high economic performance, the design simplification of the system is considered to be the most effective approach. Optimizing the simplification, three SMR design concepts were proposed and developed in Japan based on the LWR concept. They are IMR (integrated modular water reactor), CCR (compact containment water reactor) and DMS (double MS (modular simplified and medium small) reactor) (Okubo, 2011). In addition to these, other SMR concepts based on the HTGR (high-temperature gas-cooled reactor) were also proposed, utilizing their excellent characteristic of the higher thermal efficiency in HTGR than in LWRs. One of them is GTHTR 300 (Gas Turbine High Temperature Reactor 300). Furthermore, an SMR concept based on the SFR (sodium-cooled fast reactor) concept was also proposed, especially for local use in isolated area conditions, such as in Alaska. This is named as the 4S (super-safe, small and simple). In the following sections, these five SMR concepts are presented briefly.

Water precarity or scarcity is exacerbated by the effect of climate change on rainfall patterns. In Ghana, for example, this has affected the reliability of hydropower resources with a resulting shortfall in electricity supply (IBRD, 2010).

The use of coal plant requires large quantities of water, which may affect its availability for other needs, and additionally creates downstream pollution. The water component in energy planning has become increasingly important in this perspective, with more stringent requirements for water usage being put in place when funding and planning for large plants is done. An example is the Medupi coal plant in South Africa, discussed in Section 20.5.4. Some SMR designs are air-cooled, obviating the need for a great deal of water usage for electrical power production. This would greatly expand the range of suitable sites.

In addition, the desalination capability of SMRs could provide water for desert or polar sites. Indeed, as after the Haiti earthquake of 2010, SMR desalination could be applied temporarily in disaster situations where sites are accessible to the sea and nuclear vessels like the USS Carl Vinson could share their excess distilled water (Padgett, 2010).

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.