spheres contain uranium inside heat-resistant coatings.
road paved with pebbles
Small pebble bed reactors ran in Germany in the 1970s, and China has recently started one. A larger version is now being designed by South Africa's state utility, with investments from British Nuclear Fuels, owner of the reactor maker Westinghouse, and Exelon Corp., the largest U.S. operator of power reactors. A decision on construction is expected this fall.
The design uses a bunch of related advances that appear -- on paper -- to produce a small reactor that can be built cheaply and operated safely.
Instead of the typical rod-shaped fuel, the fuel is formed into "pebbles" about the size of a pool ball. Each pebble is made of grains of uranium sheathed in heat-resistant graphite and silicon carbide. The 100 million-watt reactor is supposed to use 310,000 fuel pebbles.
The pebbles confer a number of advantages:
a mod, mod, mod reactor
Moving heat from the core to the generating turbine with helium rather than boiling water is a key simplifying step, says Bennett. "You don't have all those different regimens of heat transfer, all the accident scenarios become much easier to analyze." Although boiling-water carries more heat, gas is simpler, Bennett stresses, since it never becomes liquid in the reactor.
under the collar
Even if the helium disappeared, air would cool the reactor -- so we are assured, preventing a meltdown that could damage the reactor or irradiate the environment. The reactor will not use plumbing for emergency cooling. This plumbing is complicated to design and test and, according to the engineers' "less-is-more" attitude, if you don't need it, you can pitch it out and forget about testing it.
But graphite burns, and water doesn't. As University of Wisconsin-Madison nuclear engineer Michael Corradini observes, "Nothing is totally foolproof. You're trading the problem with air oxidation to the problem of overheating with water." (Remember: air is the emergency coolant if the helium disappears.)
The pebble-bed design contains a lot of graphite, and not just in the fuel, says David Lochbaum, a nuclear safety expert at the Union of Concerned Scientists. The design, he says, raises "at least a question of a graphite fire, as at Windscale (Great Britain) in 1957 and at Chernobyl in 1986." And while the pebble-bed reactor will store spent fuel temporarily, it would only exacerbate the persistent inability to store radioactive waste safely and in a politically acceptable manner.
Radwaste is just one reason for the nuclear willies
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