Factory-build PBRs, ship, and assemble on site

A pebble bed reactor unit can be assembled within a containment building approximately 25 x 40 meters in size. Such a unit would generate approximately 100 megawatts, at a capital cost of roughly $200 million. Most large coal or nuclear electric power plants in the US are much larger, about 1,000 megawatts. An advantage of the relatively smaller PBR unit is that the modules can be grouped and added to as demand increases. From a business investment point of view, a utility company would be making a series of $200 million investments, adding to them as profitability is proven, rather than making a $2 billion investment all at once.

Distributed 100 MW power plants would have advantages over centralized 1,000 MW plants. About 8% of all generated US electrical power is lost in transmission. Having generation closer to consumption reduces these costs. Such localization of power during disruptions can lessen the chance of massive power blackouts caused by cascading failures of the transmission system. Today's electric power grid has become more strained and lossy with deregulation, which can encourage customers to buy power generated a long distance away. Utility companies have difficulty getting site approvals for new nuclear power plants. However, gaining ten times more approvals for smaller plants would not be possible today. Therefore ganged, centralized PBRs would be the most attractive to uilities. Public attitudes about the safety of PBR power plants will have to change before the nation can take advantage of the distributed power generation benefits.

Prefabricated units can be shipped by truck and rail



Marc Berte and Andrew Kadak of MIT have published a presentation Modularity Approach to the Modular Pebble Bed Reactor. The conceptual design of the heat exchangers, turbines, intercoolers, recuperators, and manifolds have been constrained to sizes that can be put into boxes and shipped standard truck and rail facilities. The large reactor vessel, approximately 14 feet in diameter by 50 feet long, will require special transportation. All units can be brought together on site and assembled. One of the design goals is that the units bolt together, without the requirements for specialized welding at the site. Central, factory production of standardized PBR components allows improved quality control, lower costs, and rapid on-site assembly.

Pebbles circulate within the reactor

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.