Nuclear fusion is many decades away, and current nuclear technology carries severe drawbacks such as waste, meltdowns, proliferation, and cost. Is there anything that can be done to address these issues now? A common solution thrown around is ‘thorium’.
From my research I have come across a long list of alternate nuclear reactor designs, each with a different acronym and a different claim to fame. It can be quite challenging to separate the FBRs from the MSRs, the SFRs from the IFRs. Each one has different combinations of fuel and coolant materials, resulting in different fuel efficiencies and levels of safety. Some are more familiar while some are highly experimental. After sifting through all the jargon, my favourite design must be the LFTRs.
Liquid Fluoride Thorium Reactors are just about the farthest you can go from the standard reactor design. They are fuelled by thorium instead of uranium, the fuel is liquid instead of solid, and they don’t require liquid cooling.
Let me explain the current issues with nuclear reactors and how thorium reactors address them.
As with all elements, uranium can exist in multiple isotopes. This is dependent on how many neutrons each atom has. 99% of the earth’s uranium exists in the U-238 isotope. This variant is not fissile, which means it cannot sustain a reaction to generate electricity. It is effectively dead weight. Only 0.72% of natural uranium is the fissile U-235 isotope. This is the stuff that feeds our existing reactors. Before being fed into a reactor, uranium fuel must be enriched to boost its U-235 content from 0.72% to 5%.
Still, 95% of the fuel is the unreactive U-238. While the reaction takes place, with neutrons flying everywhere, a lot of the neutrons are absorbed by all this U-238. It turns to U-239 which itself undergoes radioactive decay, eventually transforming into the highly radioactive plutonium-239: the stuff nuclear bombs are made of. This understandably makes people uneasy, and there are concerns about some countries gaining access to nuclear weapons via nuclear reactors. Worse still, plutonium-239 has a half-life of 24,000 years, which is why storage of nuclear waste is such a nightmare.
Thorium solves a lot of these issues. Thorium is a similar element to uranium in many ways, but with some critical differences. Thorium is four times more abundant than uranium in the earth’s crust, making it a much more reliable source of energy. More importantly, 100% of this thorium is the Th-232 isotope, which can be used to fuel a nuclear reactor. Thorium is not fissile, which means it cannot be turned into a bomb. However, it is fertile. Fertile means it cannot start a chain reaction by itself, but it can be a part of an existing chain reaction. When hit by neutrons during a chain reaction, Th-232 becomes U-233, which is fissile. This is a process called breeding, and it’s great. So called Fast Breeder Reactors are nothing new – they are useful for converting uranium into plutonium, so have seen development for decades. In fact, this was the primary objective of the experimental reactors constructed at Dounreay Power Station on the north coast of Scotland in the 50s. Dounreay went on to become the first Fast Breeder Reactor in the world to provide electricity to the public. Since 100% of the thorium can be bred into fissile uranium-233, there is very little waste. Many sources claim that the nuclear waste from a thorium reactor is radioactive for only 300 years.
The other big difference with Liquid Flouride Thorium Reactors is the coolant.
A regular nuclear reactor pumps water around the reactor to keep things cool. However, the reactors run at over 300°C, so the water must be highly pressurised to prevent it from boiling into steam. In fact, the pressure is typically about 150 bar. This is a big problem. Maintaining 150 bar requires the reactor to be designed as a pressure vessel with thick steel walls, adding significant cost to the project. If the vessel were to fail at any location, the pressure would immediately release with explosive force, potentially damaging critical components and spreading radioactive material. For this reason, all modern nuclear power plants have a concrete containment structure built around the reactor. The internal volume of this structure must be very large to accommodate the expansion of the pressurised water into steam in the event of a failure. This requirement significantly increases the size of nuclear power plants.
This headache is avoided entirely with LFTRs. In these, the fuel and coolant are one and the same. The thorium and uranium exists within a molten fluoride salt mixture, which is basically a fluid with a very high boiling temperature. This is advantageous because the entire system can operate at atmospheric temperature, which is much safer and cheaper than constructing a high-pressure vessel. With atmospheric pressures, the confinement structure does not need to be so large, so in theory nuclear power plants can be much smaller. Further, the temperature may be increased from about 300°C to 800°C without fear of boiling, which increases the heat-to-electricity conversion efficiency from 30% to 45%.
This all sounds amazing, but of course there are downsides. This technology is extremely under researched and underdeveloped. With only one large-scale reactor having ever been created (Molten-Salt Reactor Experiment, Oak Ridge National Laboratory, 1965), the risk is seemingly too high for investment. The upfront costs are astronomical to develop a new reactor design, and it doesn’t make financial sense in the current nuclear climate.
There are unanswered technical questions too. Can enough neutrons be generated to sustain the breeding of thorium into fissile uranium-233? If thorium is not fissile, how is the chain reaction started? Presumably with uranium-235, which invariably introduces the nasty U-238 along with it. Most fluoride salts freeze between 300°C and 600°C, so how do you ensure the temperature stays well above that level? These are valid concerns which are skimmed over by the thorium advocates.
In my opinion, these technical concerns can be overcome. I see them as challenges that can be engineered away. Even if we must introduce a tiny bit of U-235 during startup, that’s still way better than running exclusively on it like we do currently. There just has to be enough money.
I’m optimistic in this regard too. Countries including France, the UK, Canada, and China have expressed interests in developing this new type of reactor. Most of all, India could be the driving force of this new technology. The country is sitting on a third of the world’s thorium supply and has tasked itself with supplying 30% of its energy from thorium reactors by 2050.
There are also numerous startup companies seeking to revolutionise the nuclear industry. Transatomic is seeking to develop a commercially viable molten-salt reactor. I wish them luck, but I have also read about many failed nuclear startups. I still believe that countries have the best shot at bringing this technology to fruition. They have money, they just need the public support. My next and final blog will look into the public opinion on nuclear and how it could be changed.
Sources
https://www.youtube.com/watch?v=kMCf7XquYKw&ab_channel=VICENews
https://www.youtube.com/watch?v=poPLSgbSO6k&ab_channel=Vox
https://www.youtube.com/watch?v=kybenSq0KPo&ab_channel=TEDxTalks
https://www.americanscientist.org/article/not-so-fast-with-thorium
McEwan Design 
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