To the great surprise of the established nuclear industry, thorium and molten salt reactors could be on the verge of making a comeback. Tim Probert cuts through the growing hype to explore whether good ideas can ever become good business. This article was first published in the May 2013 edition of Energy World.
Thorium has enjoyed a remarkably high profile of late given its chequered history. Despite the fact most development programmes by commercial entities (though not research institutes) were canned in the 1970s and 1980s, an increasing amount of hyperbolic articles are being published in serious and not-so-serious outlets extolling the virtues of thorium.
In reading these articles, the reader may be led to believe thorium is something of an energy panacea. Thorium is a ‘green’ source of nuclear energy, they say, abundant, cheap, safe, without the drawbacks of core meltdowns and copious amounts of radioactive waste.
The truth, of course, is far more complex. While thorium does offer highly attractive potential advantages as a source of atomic power, there are also several considerable barriers to the development of thorium-based nuclear reactors.
Uranium’s head start
Some view thorium as the Wankel rotary versus four-stroke piston engine or Betamax versus VHS battles of the energy industry: despite its significant technical advantages over its rivals, the ‘lesser’ product won. There is a great deal of truth in this, and a short history lesson is required to explain why.
It is essential to point out Thorium-232 (T232), like Uranium-238 (U238), is a fertile isotope, not a fissile isotope. Both these isotopes have to be first irradiated in a reactor to produce their derivative respective fissile isotopes U233 and Plutonium-239 (Pu239) resulting from neutron capture decay chains, neither of which exist in nature.
To begin its nuclear weapons programmes in the 1940s, the United States decided to produce both enriched uranium in U235, the only naturally occurring fissile isotope in nature, and Pu239, produced from neutron irradiation of fertile U238 in a reactor.
The United States recognised but declined the third option to produce U233 from irradiation of Th232 in U235-enriched reactors for two main reasons. Firstly, it was known that U233 was a hard gamma emitter which would be both difficult to reprocess and to machine and handle from a personnel protection standpoint.
Secondly, and more importantly, it was demonstrated that the 0.7% level of fissile U235 in natural uranium was sufficient to be the basis for a nuclear reactor system, with the 99.3% U238 content in natural uranium hence being partially converted to produce around 0.8% of the new fissile element plutonium – unknown in nature – which could then be chemically separated through reprocessing.
Had the United States gone down the thorium route, there would have been a significant weapons programme delay in producing sufficient quantities of the required enriched uranium driver. Unlike uranium, thorium cannot be driven by a natural uranium reactor (since neutrons are absorbed by the U 238), only by an enriched uranium reactor.
Uranium, therefore, got a head start for civil nuclear power from which thorium has never recovered. Thorium research became a side show as a means to an end to produce U-233, the only other fissionable feed isotope.
The light water nuclear reactors (LWR) deployed in nuclear power stations developed simply as scaled-up versions of nuclear submarine reactors, again driven by military impetus. As a result, virtually the entire nuclear energy industry has been optimised around the uranium and, to a lesser extent, plutonium fuel cycles.
While thorium is three to four times more abundant than natural uranium, there is no shortage of the latter. At current usage of 68,000 tonnes/year, global proven resources of 5.3 million tonnes mean there is enough natural uranium for at least 80 years, notwithstanding spent fuel.
While there are many good arguments in favour of thorium, energy security is one of the weaker ones and governments may only turn to thorium if uranium economics or its geopolitical abundance becomes problematic, says P.K. Doshi, Director of International Business Development at nuclear consultancy Excel in Maryland, USA.
Doshi, who has 45 years of nuclear industry experience and also thorium fuel manager when Westinghouse Electric operated the world’s first pressurized water reactor with thorium cores (at Shippingport, near Pittsburgh) in the 1970s, is highly sceptical about thorium’s chances.
“The infrastructure is in place to support LWRs on the uranium cycle,” says the Indian. “It’s expensive to compete with an established infrastructure, particularly when the timeframe to develop an alternate is so long. In this sense, thorium is kept down by ‘the system’. But I don’t see it as a conspiracy, rather as an economic fact of life.”
Westinghouse’s 60 MW experimental light water breeder reactor at Shippingport, Pennsylvania, which successfully transmuted Thorium 232 to Uranium 233
Molten salt reactors: The next big thing?
It is the job of the Weinberg Foundation to challenge ‘the system’. Founded in 2011 to promote thorium energy, the Foundation is named after thorium advocate Alvin Weinberg, research director of Oak Ridge National Laboratory (ORNL), which produced plutonium for the Manhattan Project.
Weinberg Foundation’s headquarters are at Somerset House on London’s Strand. In keeping with thorium’s image as a ‘green’ nuclear energy source, its patron is Baroness Bryony Worthington, founder of emissions trading lobby group Sandbag and a key architect of the 2008 Climate Change Act.
The Foundation’s CEO Laurence O’Hagan is under no illusions about the scale of the challenge of taking on a uranium fuel cycle industry worth an estimated £200bn a year. “The argument is it will take too long,” he says. “That the established nuclear industry doesn’t want to change. But if we don’t start now, we’ll never get there. The potential advantages are so significant that we believe they are worth pursuing.”
O’Hagan sees considerable promise in molten salt reactors (MSR). Indeed, one of Alvin Weinberg’s major projects at Oak Ridge was the Molten Salt Reactor Experiment of 1965-1969, a 7.4 MW thorium-based demonstration reactor using U-233 as the main fissile driver.
Unlike conventional nuclear reactors which use solid fuel, usually rods or pellets, MSRs use a mixture of fluoride salts in a molten state. The salt mixture includes fissile material, i.e. fissile isotopes of uranium and/or plutonium, together with fertile material, such as T232 or U238.
The front-running thorium-based MSR is the liquid fluoride thorium reactor (LFSR). In this design, the molten salt also serves as the primary coolant, carrying heat away from the reactor, and delivering it to a secondary cooling circuit and ultimately to the steam turbines that generate electricity.
One of the main advantages is the reactor and its cooling circuits operate at near atmospheric pressure, reducing the chance of any explosion. As fuel in a LFTR is already in liquid form, it cannot melt down, as can solid fuel rods in a LWR.
Furthermore, the LFTR’s large negative temperature coefficient means that regulation of the reactor’s temperature is passive, so there is no need for control rods. The molten salt expands as a result of the heat generated by fission, which slows the rate of fission.
The reduction in fission heat cools the salt, which in turn leads to an increase in the rate of fission. In other words, as the reactor temperature rises, the reactivity decreases. The reactor thus automatically reduces its activity if it overheats.
In the event of a reactor overheating, the fuel and salt drains into a holding tank, where the fuel spreads out enough for the reactions to stop. The salt then cools and solidifies, encapsulating the radioactive materials.
Another major advantage over uranium is fuel efficiency. Due to the degrading effect of neutron bombardment on solid fuel rod metal cladding in LWRs, between only 2-4% of the energy contained within can be used before they have to be removed. An LFTR reactor, however, would continuously recirculate nuclear fuel, thus greatly improving efficiency and radically diminishing the produced volume of nuclear waste and proliferation risk.
Corrosion: The killer blow?
On paper at least, LFTRs are highly attractive and it is easy to see why they have a growing number of proponents. In practice, however, there are major drawbacks yet to be overcome.
While Weinberg’s work at Oak Ridge’s relatively short-lived Molten Salt Reactor Experiment successfully demonstrated the potential for thorium molten salt reactors, a number of serious problems were highlighted, particularly pertinent for power generation reactors with an operational lifetime of several decades.
Researchers at Oak Ridge found that the Hastelloy-N metal used for the containing molten salts cracked and corroded under intense radiation. The development of corrosion-resistant materials capable of surviving exposure to neutron bombardment and fluoride salts at high temperatures for decades is essential if LFTRs are ever to be a commercial proposition.
“Corrosion was one of the problems at Oak Ridge but since then material science has considerably progressed,” says O’Hagan. “We are now two generations beyond Hastelloy.
“MIT is researching using salt as a coolant and they are about to patent a corrosion-resistant material. We are not claiming all the engineering problems have been solved and there has to be work done to resolve them, but all the signs are pointing in the right direction.”
The main stumbling block, though, is economics. “LFTRs are theoretically beautiful but making it work in practice is a different matter,” says Johann Lindner, who heads up Excel’s European division. “In the end, they will not get off the ground because building a full-size unit will prove to be economically unviable,” he says.
“Building an LFTR requires more than just a reactor design. It requires a new fuel cycle with fuel fabrication facilities, remote handling equipment, and new back-end spent fuel management methods and technologies.
“It’s a non-starter; Darwinian selection has chosen LWRs. There may be some prototypes built but you’ll never convince utilities to buy them for commercial power generation. The first 50-100 will be too expensive compared to LWRs.”
Thorium fuel has so far been used in about 30 operational reactors. Most of these were located in the USA, Germany, Netherlands and India. A single example operated in the UK, from 1965 to 1976: the Dragon Reactor at Winfrith, a helium-cooled test reactor.
Aside from Oak Ridge, perhaps the most significant use of thorium was the German THTR-300 (Thorium High Temperature Reactor), a 300 MW pebble bed modular reactor, which used a mixture of thorium and highly enriched uranium, between 1983 and 1989. The plant, constructed in the North Rhine Westphalia town of Hamm-Uentrop by Hochtemperatur-Kernkraftwerk, was not a success.
On 4 May 1986, within just six months of generating grid-connected power and days after the Chernobyl accident, the THTR-300, which had no containment building, was found to have released radioactive dust into the environment due to an erroneously open valve. It was later found that pebble friction had generated so much dust that several pebbles became jammed, and the attempts to unclog the blockage merely aggravated the release.
The THTR-300 was a thorium high-temperature nuclear reactor rated at 300 MWe
Thorium in the UK
Keith Perron, a former nuclear submarine reactor specialist at Rolls-Royce, believes the only viable usage of thorium in the UK is its use in existing LWRs rectors or to incinerate plutonium as a final disposal route as an alternative to the type of MOX reprocessing plants as developed at Sellafield.
“Uranium MOX fuel designs, whereby plutonium is mixed with uranium oxide, do not remove plutonium, they just recycle it,” he says. “Such fuel actually produces more plutonium than it started with. Judged as a means to incinerate separated plutonium and to rid it from the world, MOX is a dismal failure.”
The traditional problem with uranium MOX is chemical changes within the fuel rod result in a positive feedback coefficient, i.e. radioactivity increases over time. By mixing thorium with plutonium oxide, so-called ‘THROX’ would not react that way, it stays negative, which means in theory you could continuously recycle and reprocess the fuel rods until the actinides have been burnt up, says Perron.
Again, however, the medium-term prospect for thorium usage in conventional reactors is extremely poor, although a company called Thor Energy, a division of Scatec, has made an experimental thorium fuel rod to be loaded into a test reactor in Norway this year.
Current development of molten salt reactors
The only operational reactors using thorium currently are in India, which possesses abundant thorium reserves but little uranium. These are conventional, solid fuel LWR reactors.
It is expected China will be the first nation to develop a new MSR. The China Academy of Sciences in January 2011 launched a $350 million R&D programme on LFTR, known locally as the thorium-breeding molten-salt reactor (TMSR).
The 2 MW test unit, developed by the Shanghai Institute of Nuclear Applied Physics, is currently expected to be operational by 2020. However, the programme has been delayed by two years as it is taking longer than expected to train the 700 scientists required, says O’Hagan.
India is also slowly developing a thorium programme at its Bhabha Atomic Research Centre in Mumbai, although primarily for usage in an advanced heavy water reactor rather than MSRs. Meanwhile, Russia is believed to be developing thorium as part of a generic nuclear research programme.
Reborn in the USA?
The USA does not have a concerted thorium programme at present, although there are interesting developments in Cambridge, Massachusetts, home to Transatomic Power. An offshoot of MIT, Transatomic Power is developing the WAMSR (Waste-Annihilating Molten Salt Reactor) designed to run not on thorium, but on the United States’ considerable stockpile of radioactive waste.
The company’s ultimate aim is to produce a 500 MW unit. It estimates that it can build such a plant for $1.7 billion, roughly half the cost per megawatt of current LWRs.
Transatomic Power, backed by the founder of E Ink, Russ Wilcox, appears to be serious. It has recruited highly experienced nuclear engineers from Westinghouse and MIT scientists and it recently scooped the top award at the Department of Energy’s 2013 Energy Innovation Summit.
So far, it has raised a fraction of the $200 million needed to build a small prototype. Aside from materials science, the lack of funding – from both the private and public sectors – is likely to continue to thwart ambition for thorium.
Steve Kidd, Deputy Director General of the World Nuclear Association, cannot foresee a bright future for thorium. “It’s only when the likes of Westinghouse, GE Hitachi and Areva come into the frame will thorium get going,” he says.
“Most of the current development is by physicists and other scientists at government research centres and they are not too fussed about commercial development. It may be so that the thorium cycle is superior to uranium and using thorium in reactors is perfectly technically feasible but nobody is going to commercialise it because in the 1950s we decided to go down the uranium route.”
The Weinberg Foundation refuses to be downhearted in the face of cynics in the conventional nuclear industry. “In my opinion, a commercial MSR is quite likely within 20 years,” says researcher David Martin. “There are many reasons for this, firstly the economics of the current nuclear industry. They don’t have a product they can build on time and to budget.
“An MSR is basically a chemical set with some surrounding concrete, and it operates at atmospheric pressure so there’s no need for multiple redundant safety systems and high-pressure vessels. In theory, the key components of MSRs could be constructed in a modular fashion, rather than built in situ on a bespoke basis.
“Furthermore, fuel fabrication is more chemistry than physics. It’s far easier to manufacture the few cubic metres of fluoride or chloride salts than fabricate fuel rods. Given the safety of operation I just can’t see how it could be more expensive than light water reactors.”
Martin thinks it is high time the Government put its hand in its pocket to fund more research. “There should be a vigorous programme in order to recapture some of the optimism that characterised the first wave of nuclear reactors. In the UK, nuclear R&D is a mess. The fragmentation of the industry was an act of vandalism which has left us without a nuclear research base.”
Had there been 60 years of development of molten salt reactors, the nuclear industry may have been in a very different position. Given the prevalence of the uranium fuel cycle, however, only an optimist could believe that the world will give thorium a second chance.
Schematic of Transatomic Power’s Waste Annihilating Molten Salt Reactor (WAMSR)