Thorium reactors: Asgard’s fire


Flickr / EMSL

Thorium, an element named after the Norse god of thunder, could soon help power the world.

Well started; half done. This proverb – or rather its reverse – sums up the problems that civil nuclear power has pursued since its inception. Atomic energy is viewed by many, and with good reason, as the miscarriage of the world’s atomic bomb programs: badly started and badly done.

But a clean slate is a wonderful thing. And that could soon be supplied by two of the emerging industrial powers India and China, whose energy requirements prompt them to think about building reactors with thorium operation.

Existing reactors use uranium or plutonium – the stuff of bombs. Uranium reactors require the same fuel enrichment technology as the bomb makers and can therefore cover secret weapons programs. Plutonium is made from unenriched 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 sufficiently uninviting, a prospect that America abandoned thorium research in the 1970s. It is also three to four times as abundant as uranium. In a world where nuclear energy was a primary research target rather than a military spin-off, it would certainly seem worthy of investigation. And it is actually being investigated.

India has ample thorium reserves, and the country’s nuclear power program, which will eventually provide a quarter of the country’s electricity (3% so far), plans to use it as fuel. This will take 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 is planning a thorium-powered heavy water reactor that should be ready soon for the next decade.

China’s thorium program looks bigger. The Chinese Academy of Sciences claims the country is now engaged in “the world’s greatest national thorium effort,” employing a team of 430 scientists and engineers, a number expected to rise to 750 by 2015.

This team is also led by Jiang Mianheng, an engineering graduate from Drexel University in the United States who is the son of former Chinese leader Jiang Zemin (himself an engineer). Some may wonder if Mr. Jiang got his job solely because of merit.

However, his appointment suggests that the project has political clout. The team plans to put a thorium reactor prototype into operation in 2015. As in India, this is also powered by solid fuel. But by 2017 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, {+2} {+ 3} {+ 3} U, which is. Thorium can be burned in a conventional reactor together with enriched uranium or plutonium to provide the necessary neutrons. 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 a manageable 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 generate electricity. The now cooled fluoride mixture then returns to the core to be recharged with heat.

This is roughly 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 cleverest things about LFTRs is that they operate at atmospheric pressure. This changes the economy of nuclear power. In a light water reactor, currently the most common type, the cooling water is under extremely high pressure. Therefore, light water reactors must be encased in steel pressure vessels and housed in fortress-like security buildings if their cooling systems fail and radioactive vapor is released. An LFTR doesn’t need any of this.

Thorium is also easier to prepare than its competitors. Only 0.7% of natural uranium is the fissile isotope {+2} {+ 3} {+ 5} U. The rest is {+2} {+ 3} {+ 8} U, which is heavier because it has three more neutrons and won’t split because of the stability these neutrons bring with them.

Therefore, uranium has to be enriched through the complex centrifugation process. Plutonium is made by bombarding {+2} {+ 3} {+ 8} U with neutrons, similar to the conversion of thorium into {+2} {+ 3} {+ 3} U. However, in this case this requires a separate reactor from the one in which the plutonium is ultimately burned. In contrast, once thorium is extracted from its ore, it is reactor ready.

It is true that it takes a uranium or plutonium seed to provide neutrons to get the ball rolling. However, as soon as enough of it has been converted into {+2} {+ 3} {+ 3} U, the process becomes self-sustaining, with neutrons from the fission of {+2} {+ 3} {+ 3} U converting enough thorium, to replace the {+2} {+ 3} {+ 3} U when it is consumed.

The seed material is then superfluous and, since the fuel is liquid, can be flushed out of the reactor together with the fission products that arise when {+2} {+ 3} {+ 3} U atoms are split up. If necessary, more thorium fluoride can also be bled. The result is that, in contrast to light water reactors, thorium reactors can run for years without interruption. These must be shut down every 18 months in order to exchange fuel rod batches.

Bombs gone?

Thorium has other advantages as well. Even the waste products from LFTRs are less dangerous than those from a light water reactor. There is less than a hundredth of the amount, and its radioactivity declines to safe levels over centuries, rather than millennia, as is the case with light water waste.

Paradoxically, however, given the history of thorium, the difficulty in using thorium as a weapon is what many see (so to speak) as their killer app in civilian power plants. One or two {+2} {+ 3} {+ 3} submarine bombs were tested in the Nevada desert in the 1950s and, perhaps ominously, another was detonated in India in the late 1990s.

But if you look at the American experience, bombs like this are temperamental and prone to premature detonation, as the intense gamma radiation {+2} {+ 3} {+ 3} U deep-fries the trigger circuits and makes them dangerous to use. American efforts were abandoned after the Nevada Tests.

The gamma ray problem arises from a peculiarity of the process that turns thorium into {+2} {+ 3} {+ 3} U. A small amount takes a different route and ends up as radioactive thallium – which is very radioactive, in fact. Its gamma rays are so strong that they can penetrate meter-thick concrete. Extracting, melting, and manipulating material that contains even traces of it is beyond the scope of any national weapons laboratory except a handful.

Rogue states interested in a nuclear bomb are therefore 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 turned into weapons may now re-emerge, in part for that reason.

Click here to subscribe to The Economist.

Comments are closed.