The Pebble Bed Modular Reactor (PBMR) technology may trump the Generation III designs, and possibly compete with Generation IV head-to-head. PBMR is a High Temperature Reactor (HTR) technology conceived in the 1950s, and developed in Germany in the 1970s. It was abandoned in Germany because of politics. The basic reactor design is unusual, but simple. It is a smaller reactor, making it quite flexible. Consequently, the PBMR may become ideally suited for "mass marketing" across the globe.
The reactor provides heat to the coolant gas, which then turns a generator. The uranium or other nuclear fuels are formed inside ceramic balls, comprised of pyrolytic graphite. The "pebbles" look like billiard balls, about the size of a tennis ball. Each pebble is a 60 millimeter hollow sphere, weighing 210 grams and with 9 grams of uranium inside. They are stacked in a steel-cased, bin-shaped reactor and continuously fed, gumball-style, through the helium-cooled reactor, which is lined with graphite, from the bottom to the top. This process repeats over a few years until the fuel inside the ceramic balls is expended.
There are about 360,000 of these graphitic-encapsulated pebbles in the core, depending upon the power of the reactor. For example, a 120 MWe reactor would hold about 380,000 pebbles. About 3,000 pebbles are handled every day, about 350 are discarded daily. The average pebble cycles through the core about 10 to 15 times. Inside the hollow sphere are about 15,000 small seeds surrounding a kernel of fissionables. The pyrolytic graphite, which is the main material of the pebbles, melts at 3000 degrees Celsius – more than double the design temperature of most reactors. It also has a long history of being used in nuclear reactors For example, pyroltic graphite is what space engineers use in constructing the missile re-entry nose cones.
According to the PBMR website, "The current schedule is to start construction in 2007 and for the demonstration plant to be completed by 2011. The fist commercial PBMR modules are planned for 2013." The South Africans are leading the way in bringing the PBMR technology to market. However, the Chinese also licensed the AVR technology (Arbeitsgemeinschaft Versuchsreaktor) and are developing it further at Tsinghua University in Beijing.
