I think nearly all of us are aware of the urgent need to reduce carbon emissions and ideally create a carbon-neutral society. There’s a major focus on renewable energy such as wind, tidal, hydroelectric, solar and geothermal. I’m making my own small contribution through solar panels on my house. However, when it comes to talking about reducing carbon emissions, there’s an elephant in the room. A source of electricity that is not renewable but can generate large amounts of electricity with negligible carbon emissions: nuclear power.
There are obvious and understandable reasons why discussions on reducing emissions tend to ignore the nuclear option. Windscale, Three Mile Island, Chernobyl and Fukushima are four of them. The management of nuclear waste which may remain radioactive for hundreds of thousands of years is another; as is the safe decommissioning of reactors at the end of their working life. But it didn’t have to be this way.
In the late 1940s, when research was going on into the potential use of nuclear energy for generating electricity, two possible fuels were investigated: thorium and uranium. When bombarded with neutrons, thorium is converted into uranium-232, which will then undergo nuclear fission, releasing heat. This can then be used to create high pressure steam to drive turbines. The other fuel, uranium-235, also undergoes nuclear fission to release heat.
It was clear from the beginning that thorium had some very significant advantages over uranium as a fuel. Firstly, thorium is much more abundant than uranium, and it is a lot easier and safer to mine and process. In fact, massive amounts of thorium are created as a waste product from mining other metals. In addition, nearly all thorium is ‘fertile’ – meaning it can be used as fuel. Uranium, by contrast, is mostly the non-fertile isotope uranium-238; only 0.7% is uranium-235, which can undergo nuclear fission. This means that thorium does not require expensive fuel enrichment processes.
Another benefit is that a thorium reactor produces much less radioactive waste than a uranium reactor; up to 1000 times less, in fact. In addition, those small amounts of waste products remain radioactive for between 1 and 100 years; by contrast, the waste products from uranium remain radioactive for tens or even hundreds of thousand years. The decommissioning of thorium plants is also safer and easier.
Thorium reactors are much, much safer than uranium reactors. In fact, they are literally melt-down proof. The reason for this is that they use a liquid fuel – molten thorium fluoride. In the bottom of a thorium reactor is a plug which will melt if the temperature of the fuel exceeds a set limit. The fuel will then be drained into a tank for safe storage. Since thorium must be bombarded with neutrons to undergo nuclear fission, once removed from the reactor, nuclear fission ceases and the fuel is safe.
By contrast, uranium reactors use a solid fuel and rely on control rods which can be lowered into the reactor to absorb neutrons and slow down or stop nuclear fission. In addition, any failure in the cooling system can lead to a meltdown. If the cooling system of a thorium reactor fails, the plug described in the previous paragraph will melt and the fuel will drain from the reactor.
Thorium reactors are also highly efficient. It is estimated that one ton of thorium can generate the same amount of electricity as 200 tons of uranium or 3,500,500 tons of coal.
There are some disadvantages. One is the cost of processing thorium ore into thorium fluoride fuel; however, uranium also requires extensive processing and enrichment before it can be used in a reactor. another disadvantage is that the molten thorium fluoride fuel is highly corrosive. However, it would be perfectly feasible to develop corrosion-resistant materials that could be used to line the reactor. There are a few other potential problems which I won’t go into as the science is quite complex; all of these can be solved if there is sufficient investment into thorium technology.
Between 1965 and 1969, an experimental thorium reactor was successfully developed and tested at the Oak Ridge National Laboratory in the USA. Nevertheless, in 1973 the US government made the decision to abandon thorium technology in favour of uranium. Other governments such as the UK and France had already made the decision in favour of uranium.
So, why did governments abandon a cheap, clean and safe fuel in favour of one that is more dangerous and generates more waste? The official reason was that uranium reactors are supposedly more efficient. The true answer lies in what is either thorium’s biggest advantage or its biggest disadvantage, depending on your point of view. Quite simply, a thorium reactor does not produce plutonium, which is needed to create nuclear weapons. In the climate of the Cold War, governments made the choice to go with the fuel that produced the plutonium they needed for their nuclear weapons programmes. In the late 1960s and early 70s, Alvin Weinberg, the director of Oak Ridge, argued passionately for the adoption of thorium as the fuel for nuclear power. His refusal to abandon what he considered to be a safer, cleaner option in favour of one with weapons applications eventually cost him his job.
So, what is the future? Many countries, including the USA and UK, are reluctant to provide the funding that is needed to develop thorium technology into a viable method of generating electricity. A notable exception is India, which has abundant supplies of thorium and is aiming to generate 30% of its electricity from thorium by 2050. In addition, despite the ending of the Cold War, many countries including the UK continue to maintain a nuclear deterrent. This means that the demand for plutonium is unlikely to end any time soon.
A final note. Two of the most influential scientists in the development of nuclear power were Irene and Frederic Joliot-Curie, subjects of one of my previous posts. They oversaw the development of France’s first nuclear power station. Both died prematurely from radiation-related illnesses; Irene in 1956 and Frederic in 1958, when the nuclear program was in its infancy. As ardent pacifists, it is possible, even likely, that they would have championed the development of thorium as a nuclear fuel. Had they lived longer, would we now have a safe source of low-carbon nuclear energy?