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Nuclear Nirvana: Could thorium someday power a safe, clean, cheap reactor?

November 1, 2013
by Ben Christopher

Imagine nuclear power, only without the prospect of meltdowns, dirty bombs, and millennia after millennia of nuclear waste. That’s the carbon-free energy of harnessed fission, but free from the specters of Fukushima, Chernobyl, and Three Mile Island. Oh, and the nuclear reactors are special too—they run on a material nearly as abundant as lead.

For the many environmental, political and economic complications that result from our dependence on fossil fuels and the high cost of producing reliable energy, this sure sounds like a silver bullet.

Make that a thorium bullet.

Found residing in the more menacing regions of the periodic table beside neighbors like plutonium and uranium, thorium—its fearsome Norse namesake not withstanding—is a surprisingly innocuous-seeming substance. A silvery metal that was once used to make gas mantles in lanterns (its slight radioactivity ultimately proved to be a turnoff for consumers) is now being touted by some as a potential fuel for a new golden age in nuclear power.

Counted among its devotees is Hans Blix. The Swedish diplomat most famous as the weapons inspector the U.S. government decided to ignore in the lead-up to the 2003 invasion of Iraq, Blix touted the potential benefits of thorium in an interview with BBC News on Oct. 31.

“I’m a lawyer not a scientist,” he said, “but in my opinion we should be trying our best to develop the use of thorium. I realize there are many obstacles to be overcome but the benefits would be great.”

Blix may not have the academic pedigree to back up the claim, but he is in well-credentialed company. Take Per Peterson, as an example. A nuclear engineer at UC Berkeley, Peterson is leading Cal’s effort into developing a working prototype of a Pebble Bed Advanced High Temperature Reactor, a next generation nuclear power source that could be designed to run on thorium.

“It does require further development,” cautions Peterson. “But I concur with what Hans Blix said at this meeting. Thorium may be the better option.”

To understand why that might be the case, it helps to understand some of shortcomings of the current way we harness the power of split atoms. For those who might be a little rusty on their nuclear physics, the typical nuclear reactor runs on a source substance called uranium-235. When a U-235 atom is shot with a neutron and breaks apart, it releases energy, along with two or three rogue neutrons. Should any of those neutrons collide with another atom of uranium, it too will burst apart, releasing even more energy and two or three more neurons. Get this process to repeat itself over and over and you have a chain reaction.

Controlled, with coolants and other neutron disrupting rods, and the reaction is called “a nuclear reactor.”

Uncontrolled, it’s called “a meltdown.”

Power plants designed to run on thorium, on the other hand, are theoretically meltdown-proof. These reactors could be constructed with heat-resistant ceramic materials that will not liquefy in extreme temperatures, Peterson explains. They could also be cooled with liquid salt which, unlike the water used to cool most contemporary nuclear reactor cores, can operate at safe—and much more efficient—pressures. Plus, thorium itself must be actively agitated to sustain a nuclear reaction. That means that, in the event of an electrical malfunction such as  the one that led to the Fukushima-Daiichi meltdown in 2011, that agitation would automatically halt, the chain reaction would stop, and the thorium slough would drain into a separate storage tank.

Equally compelling is the amount of nuclear waste generated in such a reactor: very little, says Peterson. In a reactor that is “properly designed,” he says, you could activate a reaction in which much of the most unstable material is recycled within the reactor itself.

The waste produced is of a different sort than what’s currently being squirreled away in places such as Yucca Mountain, “burning out” in hundreds of years rather than the hundred-millennia time scale associated with contemporary nuclear waste. That waste also would prove too complicated and too volatile to safely and easily make use of for weapons. 

So why aren’t we running the world on thorium today?

When it comes to boutique nuclear reactor fuels, thorium, it turns out, isn’t the only game in town, says Peterson. There are many reactor designs and proposed fuel sources being researched. These include reactors with much higher concentrations of radioactive material for a more efficient “burn,” long-lasting reactors that run on depleted uranium, and more traditional reactors with the addition of thorium. All of these avenues are promising and all of them need much more study.

“The big caveat here is that you can’t really say for sure which path would be preferable,” says Peterson. “We currently don’t really have an economically workable reactor that can do many of these things.”

The notion of running a reactor based on thorium-fuel is not a new idea. The concept was first proposed in the 1950s at Oak Ridge National Laboratory in Tennessee, where a test-reactor was built a decade later. In the intervening years, says Peterson, “more effort and money has been invested in the uranium path than in thorium by at least an order of magnitude.”

For the most adamant thorium-boosters—and, as with all advocates of the esoteric and panacean, the Internet has been a boon for them—this is where the story takes a conspiratorial turn. They contend that thorium-reactors, the peaceful, green alternative to uranium, were snuffed out in the early 1970’s by Nixon and his Cold War bureaucrats, who recognized that they couldn’t be used to produce weapons-grade plutonium.

Thorium, in other words, was too beautiful for this world.

Peterson, for his part, thinks the explanation might be a bit more nuanced. “I would reverse (the argument) around,” he says. “Because uranium cycles were being used for military purposes, it had the head start.”

Plus, he says, thorium-fueled chain reactions are more complicated to start. Those complications are still being worked out and require many more years, if not decades, to tease out.

But more importantly, says Peterson, the nuclear power industry needs to enter the 21st century, regardless of the kind of reactor. “Frankly, we have to get to the point where we can actually build reactors much smaller and at cost,” he says, pointing to the four new reactors being constructed in the American southeast.

“These are using construction models that are very novel and very modular. They are being built prefabricated in Louisiana and shipped to the sites,” he says. “Sort of like IKEA for a nuclear plant.”

While that description might not inspire much confidence in home furniture shoppers, Peterson’s argument is that nuclear power must reverse the half-century-long trend of building enormous plants that cost hundreds of millions of dollars and decades to build. Those kinds of limitations will severely restrict the ability of new nuclear technologies to be commercialized when they’re ready.

“I think it would be very imprudent not to develop a very wide range of different energy technologies,” says Peterson. Although he applauds research into renewable energy technologies such as solar and wind, intermittency (the fact that the sun doesn’t always shine, the wind doesn’t always blow, and it’s very difficult to store vast amounts of energy in the meantime) is a perennial problem.

Coal and natural gas are the typical sources used to provide the steady, predictable base-load of energy required to complement renewables, but Peterson sees a role for nuclear there. With or without thorium.

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