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Friday, March 25, 2011

Nuclear Power's Next Fuel Is a Blast from the Past

By Jeremy Hsu, InnovationNewsDaily Senior Writer

American efforts to develop nuclear-powered aircraft never took off during the Cold War, but they led to a nuclear reactor design that looks virtually meltdown-proof. Now China wants to use a similar design to build a nuclear future upon thorium fuel rather than uranium fuel.

Thorium-fueled nuclear power offers the chance to minimize nuclear weapon proliferation, produce less nuclear waste and create ultra-safe reactors, several experts say. Safer reactor designs look especially enticing in view of Japan's nuclear plant disaster.

There is no shortage of thorium – it's more than three times as plentiful as uranium, with large reserves around the world. The United States sits on the largest reserve. But despite having abundant thorium and having pioneered the reactor technology to use it, the U.S. has yet to explore the possibilities of a thorium-based nuclear future.

Cold War relic

The U.S. chose uranium for nuclear fuel in part because the material that can be used to produce nuclear weapons is difficult to extract from thorium. That looked like a downside for the U.S. military when it wanted to stockpile nuclear weapons during the Cold War, but it sounds appealing these days.

U.S. scientists and engineers did create a reactor that could use thorium — called a molten salt reactor — at Oak Ridge National Laboratory in 1954. That Aircraft Reactor Experiment, part of a peculiar effort to make a reactor for nuclear-powered bombers, later led to the Molten Salt Reactor Experiment that ran from 1965 to 1969.

Such molten salt reactors can produce more heat from less fuel by mass than traditional uranium-fueled nuclear reactors can. About 100 grams, or 8 tablespoons, of thorium could provide the energy used by an American during his or her lifetime, Kirk Sorensen, chief nuclear technologist at Teledyne Brown Engineering, said in a 2009 presentation at Google.

Pros and cons

Thorium by itself cannot start or sustain a nuclear chain reaction that generates power. Uranium-235 or plutonium-239 is needed to kick-start the reaction until enough of the thorium has converted into uranium-233. A critical mass of uranium-233 can then sustain the chain reaction.

That starter requirement offers a way to neutralize some stockpiles of plutonium, according to some advocates of thorium-fueled nuclear power.

"If you put weapons-grade plutonium in [the reactor], you can transmute it into a form where it's not usable," said Jim Hedrick, a thorium expert formerly with the U.S. Geological Survey.

But processing the thorium fuel isn't easy. Thorium does not change instantaneously into the necessary uranium-233. It first converts into protactinium-233. That element must be chemically separated so that it can slowly transform into uranium-233 over several months.

Engineers must figure out a cost-effective chemical separation for protactinium-233, says Larry Miller, a nuclear engineer at the University of Tennessee in Knoxville. Highly radioactive byproducts, such as uranium-232 and thorium-228, also can complicate the fuel processing and recycling.

Still, Miller sees thorium-fueled molten salt reactors as the most promising nuclear reactors for the future.

"If you poured substantial resources into it, I think you could have a fully operating molten salt reactor producing power in 10 to 15 years," Miller told InnovationNewsDaily.

The safer nuclear power

Molten salt reactors have several important safety features. They use thorium dissolved within sodium fluoride salt, which is very chemically stable and can go to high temperatures while at low pressure. That would mean no high-pressure- or steam-related explosions, such as those that plagued the water-cooled Fukushima nuclear plant in Japan.

Molten salt reactors are also highly self-controlled. If the salt fluid expands due to heating up, the nuclear chain reaction slows down because less fuel is available in the core. If the salt fluid cools and became denser, that allows the chain reaction to increase once more.

U.S. researchers even devised a safety system to keep the fuel cool in case of loss of power at a nuclear plant, the scenario that led to Japan's nuclear plant disaster. A small frozen plug of salt would normally be kept chilled by an electric fan. If the power ever died, the frozen plug would melt and allow the molten salt to drain into a tank that was passively cooled.

All of that means a thorium-fueled molten salt reactor is virtually guaranteed never to suffer a catastrophic meltdown.

Waste not, want not

A final benefit is that the thorium fuel cycle produces less radioactive waste, which lasts just a few hundred years. By comparison, waste from uranium-fueled nuclear power remains dangerous for thousands of years and requires extreme long-term thinking about safe storage.

India, which has the world's third greatest thorium reserve (after the U.S. and Australia), has begun a decades-long effort to build its nuclear industry upon thorium-fueled "heavy-water" reactors. The country's urgency stems from its lack of major uranium reserves.

More recently, during a Chinese Academy of Sciences conference in January 2011, China announced its intention to develop thorium-fueled molten salt reactor technology.

Whether or not the U.S. government and nuclear power industry choose to take up where they left off during the Cold War remains up in the air.

This article was provided by InnovationNewsDaily, a sister site of TechNewsDaily.

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