Scientists including in China study thorium-fuelled nuclear power
In 1829, as Swedish chemist Jons Jakob Berzelius named a metallic element he had discovered after the Norse god of thunder and lightning, he surely had no inkling of its potential for unleashing enormous power. Yet today there's growing interest in thorium, which just might prove key to a new generation of nuclear reactors.
If so, it could slash our dependency on fossil fuels, greatly helping to limit global warming, as well as reducing air pollution. Both benefits could be significant for Hong Kong, which ranks among Asia's most vulnerable cities to climate change, and is frequently smothered in smog.
"I am told that thorium will be safer in reactors - and it is almost impossible to make a bomb out of thorium," Dr Hans Blix, formerly a United Nations weapons inspector and director general of the International Atomic Energy Agency, told the BBC. "These are very major factors as the world looks for future energy supplies."
Though thorium is radioactive, its naturally occurring isotope is stable, with a half-life of 14 billion years. This means that if you have a kilogram of it today, then although some will "decay" to other elements plus energy, after 14 billion years there would still be half a kilogram remaining. So thorium is in itself no use as nuclear fuel.
But, add a neutron to a thorium atom, and it will transmute, mainly becoming an unstable thorium isotope that swiftly decays to protactinium, which in turn decays to an isotope of uranium, U-233. This isotope is "fissile" - capable of sustaining a nuclear chain reaction, in which the nuclei absorb neutrons, decay, and emit sufficient neutrons to sustain the process plus carry additional energy.
Reading this, you might think, "Great - let's get started! We can solve the world's energy problems, stabilise the climate, and move on to eliminating poverty and finding a cure for cancer." But there are challenges to overcome, and no one yet knows if these will prove insurmountable.
Issues include the process involving isotopes that could be used in nuclear bombs, such as the plutonium or enriched uranium required to convert the thorium and get the reactor started. Also, there will be dangerously radioactive products, requiring safe storage for perhaps tens of thousands of years.
There's as yet no agreement regarding the best technology for managing the process, without radioactive and chemically reactive substances plus heat causing damaging to containment vessels. Costs could be prohibitive.
Yet with advantages including thorium being about as abundant as lead, plus severe difficulties for making a nuclear bomb from a thorium reactor - partly as it will include the dangerous and easily detectable U-232 uranium isotope, several projects are under way around the world, involving both theoretical and practical work.
India is aiming to build thorium-based reactors, favouring designs akin to typical nuclear plants, with solid fuel plus heavy water - which has deuterium rather than hydrogen atoms. A Norwegian project is pioneering use of thorium in a light-water reactor.
But the main excitement around thorium centres on the possibility of using salt mixtures with thorium fluoride plus other chemicals, which can become molten during operation. Advantages over reactors utilising water would include higher efficiency as temperatures could be around 800 degrees Celsius, and running at close to atmospheric pressure.
Pioneering work on molten-salt reactors was conducted at the US Oak Ridge National Laboratory. Rather than include thorium as envisaged for working reactors, experiments were conducted with uranium isotopes. Though some issues arose, the five-year trial was a success, achieving all objectives.
Laboratory director Alvin Weinberg - who had studied the absorption spectrum of carbon dioxide for his master's thesis - warned about the burning of fossil fuels leading to climate change, and believed there could be a solution in nuclear power, particularly using thorium. He was also concerned about safety, which evidently helped lead to him being fired - six years after which came the partial meltdown at Three Mile Island, Pennsylvania.
With the US government wanting nuclear reactors that could create plutonium for making bombs, attention shifted away from thorium and molten-salt reactors.
These reactors were little known, and thorium became akin to a forgotten fuel, until the recent resurgence of interest. This has been spurred partly by the Weinberg Foundation, which was established in 2011 and is "dedicated to driving awareness, research and the commercialisation of cleaner and safer nuclear technologies, fuelled by thorium".
Last month, the foundation reported that a study of the feasibility of a pilot-scale molten-salt reactor had won funding from the British government's strategic innovation agency, the Technology Strategy Board.
But never mind shilly-shallying with computer studies and the like, China is leaping into action with an intensive programme to create thorium-powered molten-salt reactors.
In March, the South China Morning Post reported that, propelled by the "war on pollution", the Shanghai-based project team had their time frame for achieving this goal shortened from 25 to 10 years.
The US Department of Energy - especially its Oak Ridge laboratory - is said to be "quietly collaborating" on the project, and we can only guess at the frustration some of its scientists may feel given previous work was abandoned by a government blinded by desire to build bombs.
I've seen the China project described as akin to a nuclear "moon shot". It's indeed ambitious, and may fail. Yet we need something monumental to stave off calamitous climate change, and thorium may yet help us realise Alvin Weinberg's vision of the "Second Nuclear Era". If so - if! - we in Hong Kong may yet experience a stable climate and, whisper it, smog-free skies year-around.
Martin Williams is a Hong Kong-based writer specialising in conservation and the environment, with a PhD in physical chemistry from Cambridge University.