Thorium: the nuclear fuel of the future?

Thorium in nuclear power is described by its proponents as a “superfuel”. Marion Cromb explores what makes it better than what we have now.

Currently, nuclear reactors use enriched uranium as fuel. It is 96% uranium isotope U-238, and just 4% fissile U-235. Fissile isotopes split when hit by a neutron, and are the only isotopes capable of sustaining a nuclear chain reaction. While uranium power is a thousand times more efficient than fossil fuels, reactors utilise less than 1% of their fuel and generate plutonium waste that is dangerously radioactive for hundreds of thousands of years.

So what is thorium? It is an element found in sand deposits - more common than tin and three times more abundant than uranium. Thorium only has one natural isotope (Th-232) so does not require enrichment before being used as fuel. Technically thorium is not a nuclear “fuel”, because it is not fissile. Instead Th-232 is fertile, which means upon absorbing a neutron it decays to become a fissile isotope, in this case uranium-233. Whether the fissile isotope is U-233 or U-235 makes a difference!

Next generation molten salt reactors (MSRs) using thorium are 200 times more efficient than current uranium reactors. A lifetime supply of thorium is just 650g and can be safely held in the palm of one hand. By recycling fuel, thorium reactors produce a smaller amount of radioactive waste and can even burn up the waste from uranium plants. These reactors are also inherently safer than current water reactors. There is no risk of pressure explosions or dangerous meltdowns because MSRs operate at atmospheric pressure and with liquid fuel. If the reactor overheats, the freeze plug at the bottom of the core melts and the fuel safely drains away.

If thorium power is so good, where is it? In the past, thorium has largely been ignored in favour of uranium with its sideline in nuclear weapons. The uranium industry is well established and uranium reactors are proven technology. Even if uranium starts to run out and the price rises, raw fuel is just 5% of the cost of nuclear plants, so thorium research is yet to be justified by market conditions.

Nevertheless, some countries are investing in thorium. India has one third of the world’s thorium reserves and has had a long-term thorium power plan since the 1950s. China entered the field with a £223 million program in 2011, and aims to have a functioning MSR by 2024. China believes its native thorium could sustain its energy needs for 20,000 years. Yet there is little motivation to switch for countries that have to import nuclear fuel and worldwide use of thorium power may not be for decades, until the reactors are proven commercially.

However, with fossil fuels running out and climate change a current worry, it seems likely that research into thorium will continue as people look for alternative energy sources. Who knows, in 100 years’ time we may look back on uranium-235 power as a costly, wasteful diversion.

From Issue 10