Thorium reactors – Asgard’s fire | Science & technology
April 12, 2014
WELL started; half done. This proverb – or rather its front – sums up the problems that civil nuclear power has pursued since its inception. Nuclear energy is viewed by many, and with good reason, as the misshapen stepchild of the world’s atomic bomb programs: badly started and badly done. But a clean board is a wonderful thing. And that could soon be delivered by two of the world’s emerging industrial powers, India and China, whose energy needs prompt them to consider the idea of building reactors that run on thorium.
Existing reactors use uranium or plutonium – the material used in bombs. Uranium reactors require the same fuel enrichment technology as bomb manufacturers and can therefore cover secret weapons programs. Plutonium is made from non-enriched uranium in reactors, the purpose of which can easily be switched to the manufacture of bombs. However, thorium is difficult to turn into a bomb; not impossible, but uninviting that America stopped thorium research in the 1970s. It’s also three or four times as common as uranium. In a world where nuclear power has been a primary research target rather than military secession, it would certainly be worth investigating. And it is actually being investigated.
India has abundant reserves of thorium and the country’s nuclear power program, which will ultimately deliver a quarter of the country’s electricity (currently 3%), plans to use it as fuel. This will take some time. The Indira Gandhi Center for Atomic Research already operates a small research reactor in Kalpakkam, Tamil Nadu, and the Bhabha Atomic Research Center in Mumbai plans to develop a thorium-powered heavy-water reactor that will hopefully be ready early next decade.
China’s thorium program looks bigger. The Chinese Academy of Sciences claims the country now has “the world’s largest national thorium effort” and employs a team of 430 scientists and engineers. The number is expected to rise to 750 by 2015. This team is also led by Jiang Mianheng. An engineer from Drexel University in the USA, the son of former Chinese leader Jiang Zemin (himself an engineer). Some may wonder if Mr. Jiang got his job entirely for merit. However, his appointment suggests that the project has political influence. The team plans to commission a prototype thorium reactor in 2015. As in India, solid fuel is also used here. By 2017, however, the Shanghai Institute of Applied Physics expects one that uses a more difficult but better fuel, molten thorium fluoride.
Thorium itself is not fissile. However, when it is bombarded by neutrons, it turns into an isotope of uranium, 233U. Thorium can thus be burned in a conventional reactor together with enriched uranium or plutonium to provide the neutrons required. A better way, however, is to convert the element to its fluoride, mix it with fluorides of beryllium and lithium to lower its melting point from 1,110 ° C to 360 ° C, and melt the mixture. The resulting liquid can be pumped into a specially designed reactor core, where the fission raises its temperature to around 700 ° C. It then goes to a heat exchanger to transfer its newly recovered heat to a gas (usually carbon dioxide or helium) that is used to power turbines that produce electricity. Then the now cooled fluoride mixture returns to the core to be recharged with heat.
This is how America’s experimental thorium reactor worked at Oak Ridge National Laboratory in the 1960s. Its modern incarnation is known as the LFTR (Liquid Fluoride Thorium Reactor).
The benefits of fluoridation
One of the smartest things about LFTRs is that they operate at atmospheric pressure. This changes the profitability of nuclear energy. In a light water reactor, currently the most common type, the cooling water is under extremely high pressure. As a result, light water reactors must be jacketed in steel pressure vessels and housed in fortress-like security buildings in the event their cooling systems fail and radioactive vapor is released. An LFTR doesn’t need any of these.
Thorium is also easier to prepare than its rivals. The fissile isotope 235U is only 0.7% of natural uranium. The remainder is 238U, which is heavier because it has three more neutrons, and doesn’t fission because of the stability these neutrons bring. Therefore, uranium has to be enriched through the complicated centrifugation process. Plutonium is made by bombarding 238U with neutrons in a manner similar to converting thorium to 233U. In his case, however, this requires a separate reactor from the one in which the plutonium is ultimately burned. In contrast, once thorium has been extracted from its ore, it is reactor ready.
It takes a seed of uranium or plutonium to provide neutrons to get the ball rolling. However, once enough of it has been converted to 233U, the process becomes self-sustaining, with neutrons from fission of 233U converting enough thorium to replace the 233U when it is consumed. The seed material is then superfluous and, since the fuel is liquid, it can be flushed out of the reactor together with the fission products that arise when 233U atoms are split up. Similarly, more thorium fluoride can be injected as needed. The result is that, unlike light water reactors, thorium reactors can run for years without interruption. These must be shut down every 18 months to replace lots of fuel rods.
Thorium has other benefits as well. Even the waste products from LFTR are less dangerous than those from a light water reactor. There is less than a hundredth of the amount and its radioactivity falls to safe levels within centuries rather than the ten millennia for light water waste.
Paradoxically, however, given the history of thorium, the difficulty in arming thorium is what many view (so to speak) as a killer app in civilian power plants. One or two 233U bombs were tested in the Nevada desert in the 1950s, and perhaps another was detonated by India in the late 1990s. But if the American experience has anything to offer, such bombs are spirited and prone to premature detonation, as the intense gamma radiation creates 233U french fries and makes the weapons dangerous to handle. American efforts were abandoned after the Nevada Tests.
The gamma ray problem arises from a peculiarity of the process that turns thorium into 233U. A small amount goes a different route and ends up being radioactive thallium – which is very radioactive indeed. Its gamma rays are so strong that they can penetrate concrete one meter thick. Extracting, melting, and manipulating material that contains even traces of it is impossible for all but a handful of national weapons laboratories. Rogue states interested in an atomic bomb are likely to leave thorium reactors alone with so much poorly monitored plutonium scattered around the world. Technology that was abandoned because it could not be converted into weapons may now reappear, in part for that reason.
This article appeared in the Science & Technology section of the print version under the heading “Asgard’s Fire”.