Nuclear returns from the dead


1. Time for nukes?

2. A safe reactor

3. On a bed of pebbles

4. Nuclear willies



These spheres contain uranium inside heat-resistant coatings.
Courtesy Eskom.


A road paved with pebbles
atom imageThe pebble bed modular reactor (PBMR) is designed to skirt some of the biggest headaches of nuclear power: Pausing to refuel (which takes on average, about 40 days). Complex piping. And melting down. But the design does not address every objection to nuclear, and it raises some problems while solving others.

A stack of black spheres with a tennis ball for comparison.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:

They are supposed to survive temperatures of 1650 ° Celsius, far hotter than the worst foreseeable accident.

The pebbles would be stacked inside the reactor, so older ones would be removed from the bottom as new ones were added on top. Benefit: No need to stop the reactor for gas -- or new fuel.

The downward-moving pebbles would allow more complete fission. Ralph Bennett, who works on pebble bed design as director of advanced nuclear energy at the Idaho National Engineering and Environmental Laboratory, says that outside rods in a conventional reactor receive fewer neutrons and "burn" less completely than inside rods. (We say "burn," but the fuel does not oxidize. Instead, a uranium atom absorbs a neutron from the splitting of a nearby atom and fissions, releasing more neutrons and heat.) Uniform irradiation improves efficiency.

It's a mod, mod, mod reactor
atom imageThe South Africans aim to build 100 million-watt pebble bed reactors that fit on a railroad car. Unlike the one-of-a-kind reactors built decades ago, these could be cranked out on an assembly line, making them more like factory-built Fords than existing reactors, which resemble hand-built Ferraris -- with price tags to match.

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.

Cool under the collar
atom imageThe biggest advantage of pebble-bed, according to its boosters, is that you'd never have to worry about overheating. The fuel is sheathed in materials tested for high-temperature work, giving a "very large margin" between normal operating temperature and fuel-damaging temperature, says Bennett. "Even in a postulated accident, you'd have a large margin. It takes a very long time, weeks, before extra cooling would be needed."

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.

Caution: Flammable
atom imageIs the reactor safe? Perhaps, but "safety" is never absolute. Unexpected things seem to happen if you run enough reactors long enough, and "improvements" may trade one hazard for another. Take the graphite that pebble-bed reactors use instead of water to slow neutrons. Graphite adds mass, slowing heat-up in an emergency and giving operators more time to respond.

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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