The reactor vessel is about 50 feet high and 15 feet in diameter, containing approximately 360,000 fuel pebbles. Pebbles are introduced at the top and flow down and out at a rate of 3,000 per day. As each fuel pebble is removed it is examined to determine how much of the fuel has been used up. After a few years to reach steady state operation about 350 pebbles will be replaced daily with fresh ones.
A nuclear chain reaction takes place within the reactor, creating heat. The heat is taken up by circulating helium gas. Helium is chemically inert and it does not form radioactive isotopes, so it can safely be pumped outside the reactor vessel and its containment building to directly power a gas turbine generator to produce electric power. The helium gas enters at 500 degrees Celsius and exits at about 900 degrees Celsius.
Helium cools the reactor and powers the generator
The helium circuit contains pressure modifying turbine-compressors and heat exchangers between the reactor and the generator. This results in a high, 44% transfer of thermal energy to electrical energy. In comparison, a typical coal or nuclear power plant has a 33% efficiency.
The mechanisms for removing and introducing the fuel pebbles are important to the overall safety of the reactor. In 1988 the German AVR pebble bed reactor experienced a pebble jam and released some radiation. Coming shortly after the Chernobyl disaster, public fears led to the AVR shutdown. Germany decided to turn off all its 19 nuclear power plants by 2025.
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