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Pastimes : The Case for Nuclear Energy
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To: Hawkmoon who wrote (172)7/7/2008 5:38:43 PM
From: Sam Citron  Read Replies (1) of 312
 
Dunno. I guess these journalists don't read Mechanical Engineering Magazine. <g> Some may also remember that the THTR-300 accident in Germany in 1989 was an early Pebble Bed Reactor. [Germany also built a second pebble bed reactor, the THTR-300, which generated 300 megawatts when it achieved full power operation in 1989. THTR stands for Thorium High Temperature Reactor; it uses thorium to enrich the uranium fuel. Thorium is fertile in that it is not itself very radioactive but can be transformed into uranium fuel. The Th232 absorbs a neutron from the chain reaction of U235 decay, and then the Th233 decays into U233, which is a fissile element that participates in the chain reaction. Thorium is three times as plentiful as uranium in the earth's crust.

In 1986 an operator error caused some of the pebbles to be fractured and the helium gas lock to be jammed. An unknown amount of radioactive materials were released. The THTR-300 was shut down in 1989 following public concerns arising from the Chernobyl accident. Since then Germany has decided to shut down all its nuclear power plants. pebblebedreactor.blogspot.com 4/7/07

The Pebble Bed [excerpted from Pebbles Making Waves:
South Africa is moving forward with a new reactor design that could revolutionize nuclear power memagazine.org]

Nuclear-fueled pebbles are introduced at the top of the reactor vessel and slowly wend their way down through the annular packed bed under the action of gravity to the bottom of the reactor vessel. There, a pebble falls into a defueling chute leading into a tubing system external to the reactor vessel. The pebble is then moved pneumatically by helium gas pressure up to assaying equipment above the reactor vessel, much as pneumatic mail was delivered through pressurized air tubes in department stores and banks during the last century.

Pebbles are individually assayed for burn-up—the nuclear neologism for reactor fuel consumption—and physical integrity. Depending on the results, pebbles are pneumatically routed either back to the top of the reactor core for reuse or to a spent fuel tank. It is expected that a typical pebble will make six loops through the reactor in the three years of its fuel life.

Conventional light-water nuclear power plants must shut down completely every one to two years for refueling, an expensive operation during which the plant is not producing revenue.

A PBMR plant has a continuous fueling process occurring during full load operation. Since fresh pebbles can be added as needed, there's no need for such a shutdown. Plans do call for a 30-day shutdown for maintenance every six years, which amounts to about six or seven shutdowns over the planned 40-year life of a PBMR. That compares favorably to the 20 or 30 shutdowns expected over the life of a conventional nuclear power plant.
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