Thorium, an element named after the Norse god of thunder, may soon contribute to the world’s electricity supply

Thorium reactors

Apr 12th 2014 | From the print edition

WELL begun; half done. That proverb—or, rather, its obverse—encapsulates the problems which have dogged civil nuclear power since its inception. Atomic energy is seen by many, and with reason, as the misbegotten stepchild of the world’s atom-bomb programmes: ill begun and badly done. But a clean slate is a wonderful thing. And that might soon be provided by two of the world’s rising industrial powers, India and China, whose demand for energy is leading them to look at the idea of building reactors that run on thorium.

Existing reactors use uranium or plutonium—the stuff of bombs. Uranium reactors need the same fuel-enrichment technology that bomb-makers employ, and can thus give cover for clandestine weapons programmes. Plutonium is made from unenriched uranium in reactors whose purpose can easily be switched to bomb-making. Thorium, though, is hard to turn into a bomb; not impossible, but sufficiently uninviting a prospect that America axed thorium research in the 1970s. It is also three or four times as abundant as uranium. In a world where nuclear energy was a primary goal of research, rather than a military spin-off, it would certainly look worthy of investigation. And it is, indeed, being investigated.

India has abundant thorium reserves, and the country’s nuclear-power programme, which is intended, eventually, to supply a quarter of the country’s electricity (up from 3% at the moment), plans to use these for fuel. This will take time. The Indira Gandhi Centre for Atomic Research already runs a small research reactor in Kalpakkam, Tamil Nadu, and the Bhabha Atomic Research Centre in Mumbai plans to follow this up with a thorium-powered heavy-water reactor that will, it hopes, be ready early next decade.

China’s thorium programme looks bigger. The Chinese Academy of Sciences claims the country now has “the world’s largest national effort on thorium”, employing a team of 430 scientists and engineers, a number planned to rise to 750 by 2015. This team, moreover, is headed by Jiang Mianheng, an engineering graduate of Drexel University in the United States who is the son of China’s former leader, Jiang Zemin (himself an engineer). Some may question whether Mr Jiang got his job strictly on merit. His appointment, though, does suggest the project has political clout. The team plan to fire up a prototype thorium reactor in 2015. Like India’s, this will use solid fuel. But by 2017 the Shanghai Institute of Applied Physics expects to have one that uses a trickier but better fuel, molten thorium fluoride.

Thorium itself is not fissile. If bombarded by neutrons, though, it turns into an isotope of uranium, ²³³U, which is. Thorium can thus be burned in a conventional reactor along with enriched uranium or plutonium to provide the necessary neutrons. But a better way is to turn the element into its fluoride, mix that with fluorides of beryllium and lithium to bring its melting-point down from 1,110ºC to a more tractable 360ºC, and melt the mixture. The resulting liquid can be pumped into a specially designed reactor core, where fission raises its temperature to 700ºC or so. It then moves on to a heat exchanger, to transfer its newly acquired heat to a gas (usually carbon dioxide or helium) which is employed to drive turbines that generate electricity. That done, the now-cooled fluoride mixture returns to the core to be recharged with heat.

This is roughly how America’s experimental thorium reactor, at Oak Ridge National Laboratory, worked in the 1960s. Its modern incarnation is known as an LFTR (liquid-fluoride thorium reactor).

Date: 2015-12-24; view: 922