Thorium mixed with plutonium and other actinide “waste” could continuously power modified conventional reactors almost forever in a reusable fuel cycle, according to a discovery at the University of Cambridge in England.
The discovery, by PhD candidate Ben Lindley working under senior lecturer Geoff Parks, suggests that mixed thorium fuel would outperform mixed uranium fuel, which lasts only for one or two fuel cycles rather than for the “indefinite” duration of the thorium mix.
Ideally, the reactors would be “reduced-moderation water” reactors that work on the same solid-fuel, water-cooled principles of conventional reactors but that do not slow down neutrons as much and thus also offer some of the advantages of fast reactors.
Lindley’s finding, made while he was a master’s candidate in 2011, bodes well for the use of thorium not only as a safe, efficient and clean power source, but also as one that addresses the vexing problem of what to do with nuclear waste from the 430-some conventional light water reactors that make up almost all of the commercial power reactors operating in the world today and that run on uranium.
By mixing thorium with “waste” in a solid fuel, the nuclear industry could eliminate the need to bury long-lived plutonium and other actinides.
Lindley’s work surfaced recently in an article about it in the hard copy edition of Cambridge’s quarterly Engineering Department magazine. An earlier version also appears online.
ACCENTUATE THE NEGATIVE
I interviewed Lindley and Parks recently after the magazine story appeared. They explained the crux of Lindley’s discovery: Uranium/plutonium lasts for only a limited period because after one or two cycles, when the actinide portion increases, the mix displays a “positive feedback coefficient.” In the sometimes counter intuitive world of nuclear engineering, a positive feedback is an undesirable occurrence. To use an unscientific term, the reaction goes haywire.
Parks notes that with uranium, “As the amount of actinides in the mixture increases, you get this tipping point where with the uranium mixed with actinide based fuel – a key feedback coefficient goes from being negative to being positive, at which point the fuel is not safe to use in the reactor.”
Lindley completes the thought. “The idea is that mixing things with thorium rather than with uranium keeps the feedback coefficient negative,” he says.
In a mixed fuel system, reactor operators would allow a batch of fuel rods to stay in a reactor for about five years, roughly the same as with today’s solid uranium fuel. The fuel would then cool for a few years while the shorter-lived fission products decay, and would then be reprocessed over another year, mixing actinide wastes with more thorium before being put back in a reactor.
And just how long could this cycle continue? “You could just keep doing that forever – until the world runs out of thorium,” notes Parks.
Lindley’s proposal is the latest possibility to emerge for using thorium reactors to dispose of waste as well as generate power.
As we wrote here recently, Japan’s Thorium Technology Solution (TTS) is proposing to mix thorium and plutonium in a liquid molten salt reactor. Likewise, Transatomic Power in the U.S. has similar plans, although it is starting first with a liquid mixed uranium fuel rather than with thorium.
Lindley and Parks’ idea differs from TTS and Transatomic in one obvious way: It would allow the nuclear industry to carry on building conventional solid fuel, water-cooled designs. That would be strictly true only in the initial implementation of the technology, which Lindley and Parks say would entail thorium mixed only with plutonium rather than also with other actinides like neptunium, americium and curium. That’s because plutonium is now available from sources such as the Sellafield nuclear waste site in Britain. The other actinides are not as readily available, but would become so as it became clear they could be used as part of a mixed thorium fuel, Lindley and Parks believe.
GO EASY ON THE WATER
Once the other actinides enter the mix, the optimal reactor would be a light water reactor modified to have less water and thus less moderation of neutrons in the reaction process. That, in turn, would allow more burn up of actinides.
Lindley envisions a reactor with about a quarter to half the amount of water as in a conventional LWR – enough to serve as a necessary coolant, but little enough so that the water could not slow down neutrons to the extent they do in a conventional reactor.
“It’s not really a fast reactor, and it’s not really a thermal (conventional) reactor,” notes Lindley. “It’s between the two.”
Hitachi, Toshiba, Mitsubishi Heavy Industries and the Japan Atomic Energy Agency all have reduced-moderation water reactor designs (RMWR), according to the International Atomic Energy Agency.
Lindley described them as similar to LWRs but with different fuel assemblies.
The development – and regulatory approval – of RMWRs is one of several challenges facing the deployment of mixed thorium fuel in a water-cooled reactor.
Another is the development and cost of reprocessing techniques for thorium and for actinides other than plutonium (for which reprocessing already exists).
“Splitting thorium from waste or splitting some of the minor actinides from waste has not been done on an industrial scale,” notes Lindley. “There are processes that are envisaged that can do that, that have been tested on a laboratory scale, but never on an industrial scale.”
Another hurdle: Fabricating fuel that as Lindley notes would be “highly radioactive” given the amount of waste that would go into it. “That would have to be done behind a shield,” Lindley says.
All of that will require significant research and development funding –more than what Lindley currently has at his disposal, which consists of university research funds and academic scholarships. One possible source for additional funding could be Cambridge Enterprise, a commercial arm of the university.
U.S. nuclear company Westinghouse has also been collaborating with Lindley on his thorium research. Lindley hopes to test his fuel at the Halden test reactor in Norway, where Westinghouse is a partner in Thor Energy’s project to irradiate thorium/plutonium fuel.
It will be interesting to see if any of the £15 million that the UK government recently earmarked for nuclear R&D finds it way to Lindley’s project. It’s possible that Sellafield could at least provide plutonium.
FUNDS FROM DECOMMISSIONING?
Given the potential usefulness of thorium as a way of ridding the UK of actinides, it’s not out of the question that funding could also come from the UK’s Nuclear Decommissioning Authority, which has a 2013-14 budget of £3.2 billion and which is responsible for managing nuclear waste, including actinides and shorter lived fission products.
As Parks notes, an ultimate goal for applying Lindley’s discovery “is to come up with a nuclear fuel cycle where the only waste you have to dispose of is the fission product waste.”
Parks encourages the government to “grasp the nettle” and financially back the thorium research. He and Lindley note that a multiple-cycle thorium reactor would save money in the long run for among other reasons: uranium prices, although low now, will rise; and a mixed thorium/actinide fuel would eliminate costs associated with nuclear waste storage.
“There are economic benefits in the future to investing in the reprocessing and fuel fabrication aspects now,” says Parks. “And you would completely change what nuclear waste means as far as the public is concerned, in terms of the volume of it and how long it’s radioactive for.”
Lindley and Parks say that their technology could take hold in a commercial RMWR within 10-to-20 years.
For that to happen, they’ll have to find the right mix of collaborators and financial backers.
Photos from Geoff Parks and Ben Lindley. RMWR diagram from Japan Atomic Energy Agency