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HTGR Reactor

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Are you interested in HTGR reactor? This on-line interactive 3D model contains educational descriptions and specifications, discover everything from Calandria to Feeder pipes.

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A total of 24 control rods are located at the top of the reactor to control reactivity in the reactor core. These rods can be used to start up, operate and shut down the reactor. The control rods, containing several boron carbide rings in a stainless steel sleeve, are arranged around the core and move in the surrounding graphite reflector. The rods are cooled by a bypass flow of helium.

Control rod drives

Control rod drives are located at the top of the reactor pressure vessel for sealing purposes. The control rods are suspended from the drives by cables which lower them into channels in the radial reflector.

Nuclear fuel

The fuel in HTGR reactors is uranium dioxide particles enriched to 8.5%. The fuel particles are coated with layers of silicon carbide and carbon and placed in a spherical or prismatic graphite matrix. In the case of spherical fuel, the fuel particles are dispersed in the center of the matrix, leaving a 5mm layer containing only graphite at the edges. Each 60 mm diameter fuel sphere contains approximately 15 000 fuel particles, equivalent to about 9 grams of uranium.

Fueling line

Fuel in the form of spheres is continuously fed into the reactor from above by a specialized fuel handling system. The spheres are freely fed through the fuel channel into the reactor core. The spheres then travel for about 150 weeks to the lower part of the zone, where they are gradually removed. Continuous refueling allows operation with a very low reactivity margin.

Defueling tube

Partially spent fuel spheres are removed from the core through a central drainage channel. After removal, they are first tested for mechanical damage and damaged fuel spheres are removed. Depending on the online burnup measurement, the spheres are either returned to the reactor core or removed as spent fuel.

Reactor pressure vessel

The reactor vessel is a steel cylindrical pressure vessel made of materials capable of operating at higher temperatures. In the lower third, it is connected to a pressure vessel that serves to transport the gas to the steam generator and back. Here it is also fitted with three supports which fix it firmly into the structure of the reactor building. In the center of the bottom is a drain pipe about 1 m wide to allow the removal of spent fuel elements from the core.

Pebble bed fuel core

The core of the Pebble Bed High Temperature Gas Cooled Reactor has a cylindrical shape with a diameter of about 3 meters and a height of 11 meters. It contains over 400 000 fuel spheres, which are continuously replaced during operation. Cooling helium at a pressure of 7 MPa flows through the gaps between the spheres. The parameters of the core and the reactor ensure that the fuel cannot melt in any conceivable accident.

Side reflector

A graphite reflector, placed around the active zone and also above and below it, reflects back neutrons that would otherwise escape from the zone. By limiting neutron leakage through the reflector, the uneven power distribution in the peripheral fuel elements is reduced, the reactivity of the entire core is increased and the amount of fuel needed to maintain the critical condition of the reactor over the long term is reduced.

Helium coaxial duct

Cold and hot helium flows between the reactor and the steam generator via a connecting coaxial duct. On the reactor side, the cold helium enters the pressure vessel from the outer ring of the duct and rises up through channels in the side reflector into the chamber above the core. It is heated by passing between the fuel balls and exits at the bottom into the hot gas chamber and from there flows through the central channel of the coaxial duct to the steam generator.

Steam generator

The construction of the steam generator consists of a bundle of thin tubes spirally wrapped around a central tube. Each tube is connected to the feed water at the bottom and superheated steam exits at the top. The hot helium at 700°C moves downwards through the heat exchanger. It gradually gives up its heat and cools down to 250 °C. At the bottom of the vessel, the helium turns and flows upwards along the vessel wall. Above the outlet of the tube bundle, it is directed into a helium gas circulator which pushes it through the outer part of the coaxial duct back into the reactor.

Containment

The containment in the case of the HTR-PM600 is a ventilated, low-pressure, circular concrete structure, similar to the containments in PWR plants. However, due to the parameters of the high-temperature reactors, the gas-tightness requirements for the containment are less stringent than for a PWR plant. In this concept, all six reactor modules are located in the containment vessel in separate hermetic cavities.

Fuel building

The fuel building primarily houses shafts for spent fuel containers. The total storage capacity is 240 containers, with each container holding approximately 40,000 spent fuel spheres. The container shafts use closed ventilation cooling, but if this fails, the system automatically switches to natural air circulation ventilation mode. Decay heat removal from the spent fuel storage area is thus ensured at all times.

Auxiliary building

The auxiliary building next to the containment houses the helium storage and purification system as well as the flow piping and small fuel handling system circulators.

Electrical building

This building houses the plant's main control room, the reactor protection system, and other important electrical safety equipment.

HTGR Fuel

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HTGR Fuel

Fuel kernel

The fuel kernel is highly sintered uranium dioxide with an enrichment of 7-9 % ²³⁵U. The kernel of the particle is made of either highly sintered uranium dioxide or a mixture of UO₂ and UC₂ known as UCO. In principle, the kernels can also be made from thorium and plutonium, but UO₂ and UCO are the most commonly used. The diameter of a pebble fuel kernel is about 500 microns. Fuel enrichment in the kernel is around 8% ²³⁵U.

Pyrolytic carbon

There are two layers of pyrolytic carbon in TRISO fuel. The inner layer reinforces the porous buffer layer and prevents fission products from escaping the fuel kernel. It also protects another layer of silicon carbide from the chemical attack of fission products. The second outer layer of pyrolytic carbon in turn strengthens the carbide layer and protects it from chemical action from the outside.

Silicon carbide

The silicon carbide layer acts as a tiny pressure vessel for the coated particle. It contributes to the overall mechanical strength of the fuel particle and forms an impervious barrier against escaping of solid and gaseous fission products from the particle.

Porous carbon buffer

A buffer layer of porous pyrolytic carbon surrounds the fuel core. Its main role is to provide sufficient void space for the swelling fuel kernel and for the fission product gases released during fuel irradiation. It also protects the other layers of the fuel cover from damage due to these effects.

TRISO particle

Tristructural isotropic particles (TRISO) consist of fissile material in the kernel surrounded by multiple coatings. The TRISO coating forms a tight seal around the uranium kernel, which helps retain the fission products and gases produced during reactor operation. The TRISO particles are extremely robust and incredibly small (about 1 mm in diameter).

Graphite matrix

TRISO fuel particles are fixed in a matrix of graphite powder and a binder in the shape of a sphere with a diameter of about 5 cm, which is surrounded by a 0.5 cm outer layer of pure matrix material without fuel. This outer layer protects the fuel particles from impacts associated with refuelling and from particle contact with each other as the pebbles flow through the core.

Fuel sphere

Spherical fuel elements, called pebbles, are the basic fuel design in high-temperature reactors. Thousands of these fuel pebbles, about 6 cm in diameter, make up the reactor core. The spheres contain thousands of coated fuel particles called tristructural isotropic particles (TRISO) randomly distributed in a matrix of pyrolytic graphite. The graphite also acts as a neutron moderator.

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