In 1829, as Swedish chemist Jöns 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 UN 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 kilogramme of it today, then although some will “decay” to other elements plus energy, after 14 billion years there would still be half a kilogramme 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 U232 uranium isotope, several projects are underway 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°C, and running at close to atmospheric pressure. Should any molten salt leak, it would flow into a holding tank, to safeguard against a criticality event such as an explosion.
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 fossil fuel burning 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 Nixon administration 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 desk- and computer-based study of the feasibility of a pilot-scale molten salt reactor had won funding from the UK government’s strategic innovation agency, the Technology Strategy Board. TerraPower, a nuclear company chaired by Bill Gates, is exploring thorium fuel and molten salt reactors among potential alternatives to its currently preferred fast reactor.
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. The initiative was reportedly sparked by Jiang Mianheng – son of former president Jiang Zemin and then a vice president of the Chinese Academy of Sciences [[“vice” appropriate?! – Wikipedia says was “reportedly stripped of his post and accused of bribery and embezzlement in a stock bidding scandal and the misallocation of public funds”]], reading an article espousing such reactors in the July/August 2010 issue of American Scientist. He led a delegation to Oak Ridge National Laboratory to learn of their experience, and the next year the academy CAS announced a US$350 million five-year thorium MSR project engaging 400 people.
This March, the South China Morning Post reported that – propelled by the “war on pollution”, the Shanghai-based project team had their timeframe 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 being abandoned by a government blinded by desire to build bombs.
I’ve seen the China project described as akin to a nuclear “moonshot”. 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 round.