January 31, 2002. Copyright 2001. Graphic News. All rights reserved.

Billiard-ball-sized ÒpebblesÓ power new generation reactors

LONDON, January 31, Graphic News: The rising demand for power, last summerÕs rolling blackouts in California, and increasing interest in reducing greenhouse gas emissions are leading to innovative designs for nuclear power generation.

There are 438 nuclear power plants, including 103 in the United States, which generate 16-20 percent of the worldÕs electricity. Now, 24 years after the last new nuclear plant was ordered in America, a nine-nation project is focussing on the design of a new generation of nuclear reactors: the so-called Generation IV program.

Among Generation IV nuclear technologies -- new gas-cooled, water-cooled and fast-spectrum reactors -- a fourth design, called the pebble-bed modular reactor (PBMR), is especially exciting.

The PBMR uses hundreds of thousands of billiard-ball-sized carbon spheres (pebbles), each containing 15,000 grains of uranium dioxide, the size of poppy seeds, to generate heat.

Teams in the United States, South Africa, the Netherlands, Russia, Japan and China are developing PMBR technology and the first large prototype pebble-bed reactor is expected to begin operation in South Africa in 2006.

The 110 megawatt demonstration plant to be built at Koeberg, near Cape Town, has been funded and designed by a consortium including British Nuclear Fuels, ChicagoÕs Exelon Corp., and the national South African utility ESKOM. The plant will have a power output equal to about one-tenth that of todayÕs large, light water reactors.

In the U.S., a joint research team composed of the Massachusetts Institute of Technology (MIT) and the Department of EnergyÕs Idaho National Engineering and Environmental Laboratory (INEEL), are also developing a 250-megawatt pebble. At MIT, a team including Professor Andrew Kadak has received research grants of more than $1 million to build a demonstration reactor. The group also plans to cooperate with the Chinese researchers at Tsinghua University who fired up a small 10-megawatt prototype in December 2000.

The 16-metre-high PBMR, to be built next year by MIT at the Idaho laboratory, will use about 330,000 fuel pebbles contained in a metal reactor vessel, surrounded by a shield of graphite blocks. Each graphite-coated fuel pebble will contain some 15,000 uranium fuel kernels which will generate heat by nuclear fission. Each kernel will produce enough energy to power a 100-watt light bulb for a dozen years. Once filled with pebbles a reactor could produce the same energy as a typical gas-fired power plant, potentially for similar costs. 

In order to have a self-sustaining or ÒchainÓ reaction, each 0.5mm-wide fuel kernel, known as a Trisco microsphere, contains uranium dioxide enriched by about 8 percent with U-235 -- the isotope of uranium that undergoes the fission reaction in the core.

Each kernel is coated with a layer of porous carbon and two high-density layers of pyrolytic carbon (a very dense form of heat-treated carbon) separated by a layer of silicon carbide. 

The porous carbon absorbs any mechanical deformation of the kernel as well as containing fission products released from the kernel. The pyrolytic carbon and silicon carbide layers provide an impenetrable barrier containing the fuel and the radioactive products that result from the nuclear reactions -- in effect a miniature containment vessel. 

The pebble reactor operates at around 900 degrees Centigrade and is cooled by inert helium gas. The helium is heated to 850 degrees and is used to directly drive a power turbine which in turn drives a conventional generator to produce electricity.

Kadak believes the concept will prove simpler, safer and cheaper to operate than todayÕs ageing fleet of commercial reactors. For one thing, he says, the fuel spheres are meltdown-proof.

The Òburn-upÓ or extent to which the enriched uranium can be used before depletion is so much greater in the PBMR than in conventional power reactors that it is unusable as weapons-grade material. This, coupled with the level of technology and cost required to break down the pebbles makes used PBMR fuel Òproliferation proofÓ.

ÒWe wanted to get rid of the big bogeyman of the technology,Ó Kadak said of the design, which he refers to as Òthe politically correct reactor.Ó

ExelonÕs Vice President Ward Sproat is bullish on the future of pebble-fuelled reactors: ÒIf it can be made to work reliably, it could blow the pants off any competing electric source.Ó
/ENDS
Source: Eskom, Massachusetts Institute of Technology, American Nuclear Society, Exelon