Can fast reactors speedily solve plutonium problems?

The U.K. is grappling with how to get rid of weapon-grade plutonium and may employ a novel reactor design to consume it.

The U.K. has nearly 100 metric tons of plutonium--dubbed "the element from hell" by some--that it doesn't know what to do with. The island nation does not need the potent powder to construct more nuclear weapons, and spends billions of British pounds to ensure that others don't steal it for that purpose. The unstable element, which will remain radioactive for millennia, is the residue of ill-fated efforts to recycle used nuclear fuel.

One solution under consideration is to recycle the plutonium yet further--by using it as fuel in a pair of new, so-called "fast" reactors. Such nuclear reactors can actually "consume" plutonium via fission (transforming it into other forms of nuclear waste that are not as useful for weapons). The U.K. is considering a plan to build two of General Electric's PRISM fast reactors, the latest in a series of fast-reactor designs that for several decades have attempted with mixed success to handle plutonium and other radioactive waste from nuclear power. The idea remains that fast reactors, which get their name because the neutrons that initiate fission in the reactor are zipping about faster than those in a conventional reactor, could offer a speedy solution to cleaning some nasty nuclear waste, which fissions better with fast neutrons, while also providing electricity as a by-product.

The U.K. is not alone in considering fast reactors as a solution for eliminating plutonium. Pictured here: plutonium 238. Courtesy of U.S. Department of Energy

"If they really want to get rid of plutonium, a fast spectrum reactor is safer and gets rid of more of it," than other options, argues nuclear engineer Eric Loewen, chief consulting engineer at GE Hitachi Nuclear Energy. "It just seems like humans are grappling with the question: 'How do we do it better?'"

The U.K. is hardly alone in struggling to cope with nuclear waste, whether plutonium or otherwise. The U.S. remains a nation in search of a solution for what to do with its nearly 70,000 metric tons of spent nuclear fuel, which has a small fraction of plutonium mixed in it. A recent blue ribbon commission impaneled by President Obama suggested looking for communities that would volunteer to take the waste, for a fee.

Nor is the U.K. alone in considering fast reactors as a solution for eliminating plutonium. Japan has built a fast reactor known as Monju to recycle its used nuclear fuel. France had one for awhile, too, but it has since been shut down due to difficulty operating the plant as designed. In fact, most such fast reactors have proved difficult to run reliably. "At one time or another, [fast reactors] were a priority program in the U.S., Japan, France, Germany, Italy and Russia," notes physicist Thomas Cochran of the Natural Resources Defense Council, an environmental group. "They were largely failures in all those places and in two nuclear navies, so one should think twice before trying it again."

Novel design
The trouble with fast reactors has largely been related to what's used to cool them--liquid sodium in the case of GE's PRISM and many others. The better half of table salt, this element cools a fast reactor nicely and also ensures there is no perpetual chain reaction. And, thanks to a more than 800-degree Celsius boiling point, it can operate at low pressures, unlike conventional reactors. But sodium also reacts explosively with either air or water, necessitating elaborate safety controls in places where it must get close to water in order to create steam to turn a turbine to make electricity, such as steam generators. As a result of numerous fires from leaking systems, operating sodium-cooled fast reactors to date have been shut down more than they have run. "You can't take the top off and look down in the reactor and correct any problems," Cochran notes. "You have heroic maintenance issues any time you need to go into the reactor."

And that's why GE has decided that the solution is to keep such reactors small, to minimize safety concerns as well as the size of systems, among other design changes. For example, the piping in the PRISM that carries the liquid sodium coolant around the reactor has two layers. As soon as the inner pipe that actually carries the sodium springs a leak, sensors in the outer layer shut down its flow. "We have learned from the past," Loewen says.

In fact, the PRISM is based on a government research project design that successfully operated in Idaho for decades, under the name the Experimental Breeder Reactor II, or EBR-II. And, although sodium does not play well with air or water, it gets along with metals. "At EBR-II, you could still see [engineering] chalk marks inside the [reactor] vessel when it was drained," Loewen says. That suggests an impressive degree of stability, given that such chalk marks are rapidly eroded in the hundreds of light-water reactors in operation today.

That compatibility with metals is also why GE has chosen to make an alternative nuclear fuel as well, as part of its "Advanced Recycling Center" concept that includes PRISM reactors. Rather than the pellets of uranium oxide used in other fast reactors and conventional reactors as fuel, GE would fabricate metal alloy fuels, with the plutonium or uranium mixed with zirconium metal. That might also allow GE's metal fuels to incorporate the full spectrum of radioactive elements in spent nuclear fuel. "From the experience of making just one oxide, it's tough," Loewen argues. "Add in all the other elements and it's a science project." Such "mixed oxide" fuel has not proved popular in the broader nuclear reactor fleet, although France continues to pursue it, with the U.S. soon to follow.

The challenge is that the metal fuel gets hot--and unlike oxide-based fuels, when it heats, it swells. If the fuel expands too much, it can crack the surrounding cladding, and that presents a big problem. GE's solution is to put in less fuel: "Let's not put in 100 percent of the volume, let's put in 75 percent," Loewen says.

Metal's ability to transfer heat more efficiently means that PRISM's ultimate heat sink--where the 500-degree C heat from the liquid sodium gets dumped--is air rather than water. Natural circulation in the reactor alone is enough to remove all the heat generated by the radioactive decay of the elements in the reactor fuel. "You don't need any human action," Loewen notes. "You don't need valves to open or any automatic safety systems. That's the most significant safety feature."

Fit for purpose
Of course, there is a simpler solution to the U.K.'s plutonium problem: bury it. The PRISM proposal, however, would transmute the plutonium before burying it, as an additional level of security. "We're going to take plutonium oxide that's a powder, turn it into fuel form, put it in the reactor, make it more radioactive, and then put that into the ground," Loewen admits, which would also render it unfit for nuclear weapons. "That's what the customer is asking for."

That additional level of transmutation might prove too costly, both in terms of getting the technology licensed to operate in the U.K. and in constructing the reactor itself. Such fast reactors are more expensive than even traditional reactors, such as Westinghouse's new AP-1000 under construction in China and the U.S., which are estimated to cost roughly $7 billion apiece. Conventional light-water reactors can also "consume" plutonium, if need be. "If I was going to try to get rid of 100 tons of plutonium, I'd burn it in a light-water reactor," Cochran says, by making it into the mixed oxide fuels. And "the cheapest thing to do is vitrify it [convert it to glass] and mix it with other nuclear waste."

Plus, the U.K. has a poor record in the past with its own experimental fast-reactor designs--the Dounreay Fast Reactor and the Prototype Fast Reactor--including multiple sodium leaks. Dounreay also suffered an explosion at its dumping ground for used sodium coolant that may have contributed to radioactive particles from spent fuel turning up on nearby beaches. The Dounreay and Prototype cleanup and decommissioning continue today, despite both having been shut down for decades.

Originally, such fast reactors were developed to solve a problem that never panned out: scarcity in the global supply of uranium. The idea was to create fuel within the reactors themselves once fission began, in effect making more than they consumed. But, factoring in inflation, uranium prices remain the same today as they were at the dawn of the nuclear era. "Like all minerals, improvements in the efficiency of extraction and the ability to dig for deeper ores outpaces the depletion of the resource over 100 years or more," Cochran notes. "Economically, fast reactors are not competitive and they're never going to be competitive."

"We're not going to run out of uranium," Loewen admits. "Here's a solution for this stuff that's piled up."

Ultimately, however, the core problem may be that such new reactors don't eliminate the nuclear waste that has piled up so much as transmute it. Even with a fleet of such fast reactors, nations would nonetheless require an ultimate home for radioactive waste, one reason that a 2010 MIT report on spent nuclear fuel dismissed such fast reactors. Or, as Cochran puts it: "If you want to get rid of milk, don't feed it to cows."

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