The VHTR builds on high-temperature reactor (HTR) systems (Fig. 13.5). The system is characterized by its unique fuel form, consisting of tiny coated fuel particles embedded in a graphite matrix and located in a graphite core cooled by helium (Fig. 13.6). The refractory nature of the fuel and core materials permits core outlet temperatures higher than 900 °C (with an ultimate goal of 1000 °C). Advantages of the VHTR include its potential for high burn-up (even higher than 150-200 GWd/tHM), safety, low operation and maintenance costs, and modular construction.

The VHTR is seen as a system for the co-production of electricity and hydrogen and the supply of process heat for industrial applications (see Fig. 13.7). The

Fuel kernel

Porous buffer

Inner pyrocarbon

13.6 VHTR fuel particles and the two types of fuel elements (Brossard et al., 2009).

Power conversion unit 13.7 VHTR with electricity and hydrogen production alternatives.

high core outlet temperature allows both hydrogen and electricity to be generated with high efficiency (Romanello, 2003). The latter may use either a direct (helium gas turbine) or indirect (gas mixture turbine) Brayton cycle. Where process heat is to be supplied, however, an intermediate heat exchanger will need to be connected to the primary loop. This will also serve the hydrogen production process and could use a working fluid such as helium, a gas mixture or a molten salt. The VHTR will be developed using existing materials at first, but new more advanced materials will be needed if it is to reach its full potential.

A number of experimental and prototype HTR and VHTR units have been built, including:

• the US Peach Bottom Reactor (40 MWe, operated 1967-74)

• the Fort Saint Vrain Reactor (330 MWe, operated 1976-89)

• the Dragon Reactor (20 MWft, operated 1965-76)

• the German Arbeitsgemeinschaft Versuchsreacktor (AVR, 15 MWe, operated 1967-88)

• the Thorium Hochtemperature Reaktor (THTR, 300MWe, operated 1983-9).

Several projects to build new prototype high-temperature gas-cooled reactors are described by the Generation IV International Forum (2009). These include experimental reactors in Japan (HTTR, 30 MWft) and China (HTR-10, 10 MWth).