The Modular High-Temperature Gas-Cooled Reactor (MHTGR) is an advanced power plant concept which has been under design definition since 1984. The design utilizes basic high-temperature gas-cooled reactor features of ceramic fuel, helium coolant, and a graphite moderator which have been under development for 30 years. The geometric arrangement of the reactor vessels, the core, and the hear removal components has been selected to exploit the inherent characteristics associated with high temperature materials. The design utilizes passively safe features which provide a higher margin of safety and investment protection than current generation reactors. The design has been evaluated to be economically attractive relative to modern coal-fired plants. The design and development program is a cooperative effort by the U. S. government, the utilities, and the nuclear industry.

The typical MHTGR plant includes an arrangement of four identical modular reactor units located in a single reactor building. The plant is divided into two major areas: a Nuclear Island (NI) containing the four reactor modules and an energy conversion area (ECA) containing two turbine generators. Each of the four modules produces a thermal output of 350 MW(t). Al modules are headered to feed two turbine generators of 300 MW(e) each, operating in parallel.

Each reactor module is housed in adjacent, but separate, reinforced concrete structures located below grade and under a common roof structure. The below-grade location provides significant design benefits by reducing the seismic amplifications typical of above-grade structures and by providing confinement.

Almost all components and systems of each module, which are required to meet regulatory requirements, are independent of other modules and are localized within the individual concrete structures. These include plant protection and decay heat removal systems.

The reactor components are contained within three steel vessels: a reactor vessel, a steam generator vessel, and a connecting cross vessel. The reactor vessel is approximately the same size as that of a large boiling water reactor and contains the core, reflector, and associated supports. A shutdown heat exchanger and a shutdown cooling circulator are mounted on the bottom of the reactor vessel. Top mounted penetrations house the control rod drive mechanisms and the hoppers containing boron carbide pellets for reserve shutdown. The penetrations are also used as access for refueling and inspection.

The heat transfer during power operation or normal core decay heat removal operation is accomplished by helium which is heated as it flows down through the core. It is collected in a plenum below the core and flows through a coaxial hot duct inside the cross vessel to a once-through helical bundle steam generator.

After flowing downward over the steam generator tubes, the cool helium flows upward in an annulus between the steam generator vessel and a shroud leading to the main circulator inlet. The main circulator is a submerged electric motor-driven single stage axial compressor with active magnetic bearings. The helium is discharged from the circulator and flows through the annulus of the cross vessel and hot duct and then upward to the top plenum over the core. In order to meet availability and maintenance requirements, a seperate shutdown cooling system is provided as a backup to the primary heat transport system. The heat removal systems allow hands-on plant maintenance to begin within 24 hours after plant shutdown.

A reactor cavity cooling system (RCCS) is located in the below grade concrete structure external to the reactor vessel to remove plant residual heat. This system is totally passive and provides the alternative safety related heat sink if the forced cooling systems are inoperative. the heat is transferred by means of conduction, convection, and radiation from the core to the RCCS. This system has no controls, valves, circulating fans, or other active components. the RCCS is the only safety related heat removal system utilized by the MHTGR.

The reactor core and the surrounding graphite neutron reflectors are supported on a steel core support plate at the lower end of the reactor vessel. The reactor core primarily contains graphite fuel blocks that are hexagonal in cross-section. The fuel is in the form of coated particles of low enriched fissile uranium oxycarbide and fertile thorium oxide. The fuel particles are bonded together in fuel rods which are contained in sealed vertical holes in the fuel blocks. These fuel blocks are stacked in columns to make up an annular-shaped core. Unfueled graphite blocks form the center of annulus, and surround the active core to from the reflector. The annular shape of the core has been selected to enhance the heat removal capabilities in the event of a loss of all forced cooling.

The MHTGR utilizes a once-through fuel cycle; that is, it does not rely on recycling of spent fuel. Each module is refueled once every 20 months. The refueling is accomplished with the reactor shutdown and depressurized, utilizing a refueling machine accessing the fuel elements through the appropriate control rod penetrations in the top of the reactor vessel. The spent fuel is transported to the spent fuel storage pool for temporary storage before shipping to final storage offsite.

Thermal energy from the four reactor modules is delivered to two steam turbine generators to produce 538 MW(e) net, of electric power. The turbine plant is similar to a modern fossil-fired plant except that the MHTGR plant utilized a nonreheat steam cycle. A mechanical draft cooling tower rejects the condenser heat load to the atmosphere.