Hightemperature gascooled reactor

The high-temperature gas-cooled reactor (HTGR) is similar in concept to the AGR. It uses uranium fuel, a graphite moderator and a gas as coolant. In this case, however, the gas is helium.

Several attempts have been made to build reactors of this type but none has so far entered commercial service. Early development work was carried out in the USA. The US design utilised fuel elements in the shape of interlocking hexagonal prisms of graphite containing the fissile material. HTGR fuel is often much more highly enriched than the fuel in a water-cooled reactor, with up to 8% uranium-235. The arrays of hexagonal graphite prisms contain shafts for control rods and passages for the helium to pass through and carry away the heat generated by fission.

Another design, developed in Germany, uses uranium oxide fuel which is sealed inside a graphite shell to form a billiard ball-sized fuel element called a pebble. This gives the reactor its name, the pebble-bed reactor. Development of this in Germany was eventually abandoned but the idea was taken up during the 1990s by the South African utility Eskom which is still developing the design. Japan and China have experimental programmes too.

The advantage of the HTGR is that both the moderator, graphite, and the coolant, helium, can operate at high temperature without reacting or deteriorating. A typical hTGR will operate at a pressure of 100 atm and at a higher temperature than a water-cooled reactor. This enables better thermo-dynamic operation to be achieved. The reactor is designed so that in the event of a coolant failure it will be able to withstand the rise in internal temperature without failing.

The HTGR can use a dual cycle system in which the helium coolant passes through a heat exchanger where the heat is transferred to water and steam is generated to drive a steam turbine. This arrangement is around 38% efficient. However a more advanced system uses the helium directly to drive a gas turbine. This arrangement is sometimes called a gas turbine modular helium reactor (GT-MHR). In theory the GT-MHR can achieve an energy conversion efficiency of 48%.

One of the attractions of the HTGR is that it can be built in relatively small unit sizes. Modules can have generating capacities of between 100 MW and 200 MW, making it attractive for a wider variety of applications. The modular form of most designs also makes it easy to expand a plant by adding new modules. However no reactors of this design have yet entered commercial service.

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