Although we have dramatically increased the amount of power generated by wind and solar over the last few decades, there is one inescapable reality which still exists, the sun goes down every night and remains off limits for approximately 12 hours every day and cloud cover can reduce solar output to a fraction. Even in the windiest environments, there can be lulls which last from hours to days or more at a time, which will also coincide with the Solar variability every day.
So, how can we rely upon renewables like wind and solar for a 100% green energy supply, well we can’t, we can have batteries to help store the excess energy, but that’s still a long way from being ideal. What we really need is energy that is created all day, everyday, regardless of whether the sun is shining or the wind is blowing.
With the move away from fossil fuels, there are just a few options left for energy generation that can be relied upon to run on a continuous basis. These are hydroelectric and geothermal power, and hopefully in the future tidal power, but these are only available in certain parts of the world.
The other alternative is nuclear power and hopefully one day Nuclear fusion, if and when that becomes a reality, so up until then we will have to make do with nuclear fission, the power that created the atomic bomb, and has powered every nuclear power station since the 1950s.
Back then, it was thought that by the end of the 20th century, we would be able to run the whole world on nuclear; the fear was that there might not be enough uranium to go around.
But as time went by, we began to realise that the vision of a nuclear-powered world would be far more difficult than anyone imagined and with accidents like, 3-mile island, Chernobyl and Fukushima, nuclear power has been in decline since the 1980s. It’s only in the last decade that it has made a resurgence with the fuel shocks of the Ukraine war escalating it.
And then there is the issue that nuclear power is inextricably linked to the production of nuclear weapons, in fact the only way to create the plutonium for the bomb that was dropped on Nagasaki in 1945 was in a nuclear reactor and the technology used to make a nuclear power station can, with a little modification produce plutonium for weapons.
So, there was a great deal of political support for the uranium cycle reactors, but it wasn’t the only way to create electrical power from nuclear fission.
It was thought in the 1950s, that we might actually run out of uranium for all the nuclear power plants that will be required in the future and was the basis of another experiment using an even more abundant supply of nuclear material, thorium and a different type of reactor, one that didn’t rely upon water to keep it cool enough so that it didn’t overheat and cause a meltdown or “China syndrome”.
As with many things, the military saw a use for this new wonder fuel before the commercial power generation.
In 1946, The US Army Air Forces formally studied the possibility of using a nuclear reactor to power supersonic bombers with effectively an unlimited range.
To do this they would need a small compact nuclear reactor that relied upon liquid sodium metal as the heat transfer agent and coolant instead of water.
In 1954, the ARE or the Aircraft Reactor Experiment was built and tested for four days at the Oak Ridge National Laboratory. This was a prototype for a 350 MW beryllium oxide moderated circulating fuel aircraft reactor.
The ARE had an operational targeted life of 1000 hours and a full power output of 3MW. The fuel was a mixture of sodium fluoride, zirconium fluoride and the nuclear fuel uranium fluoride. This flowed through a series of tubes in the reactor core and was surrounded by an inconel shell.
The design temperature of the liquid fuel mixture was 820°C, and 170 litres of this molten salt and fuel flowed through the reactor at the rate of 570 litres per minute at a pressure of 50 PSI.
The advantage of a molten salt reactor, is that the fuel is already molten in the salt and if it gets too hot this fuel salt mixture expands out of the and the reactivity reduces, this negative temperature coefficient means that it is impossible for a meltdown to occur.
Unlike in a Conventional water cooled reactor, if there is a loss of coolant or if the water flashes to steam and it splits into hydrogen and oxygen, an explosion can occur which is what happened at Chernobyl and Fukushima.
Although the ARE only ran for four days at a maximum of 2.5 megawatts of thermal power, it was sufficient enough for a follow on reactor to be built in the 1960s with the intention of paving the way to a Thorium Molten Salt Reactor that would produce much less radioactive waste and and less in the way of weapons grade material.
The MSRE Molten-Salt Reactor Experiment was also built at Oak Ridge National Laboratory going critical in 1965 and running up until 1969 and was to simulate a liquid Thorium Reactor.
As the salt fuel mix flowed thru the reactor it would be heated up to 650°C by the reaction, this then flowed thru aircooled radiators, there was no electrical generation capabilities and did this at full power for the equivalent of 1.5 years. Although it was operational for 4 years there were lots of time when is was not functional due to faults and external issues.
This was after all the first experimental molten salt reactor, and the material sciences we’re still in their infancy.
The piping, core vat and structural components were made from Hastelloy-N a low chromium, nickel–molybdenum alloy.
The pyrolytic graphite core also served as the moderator, which slowed the neutrons created by the reaction to make them more likely to react with the fuel and increase the reactivity.
If the salt got too hot it would expand and reduce the amount of fuel in the core and reduce the reactivity, so it was not possible to induce a meltdown, the primary safety feature of a molten salt reactor and one which differentiates it from the traditional water cooled reactors.
However, after the reactor was shut down, there found to be a problem with the corrosiveness of the salt and the Hastelloy which was used to create the structure.
As anyone who lives near the sea knows salt and metal don’t always go together well, although some types of metals are much less susceptible to salt corrosion such as nickel.
The problem inside a reactor is not only is there salt it is at a high temperature which increases its corrosiveness, and there is also neutron radiation which also causes embrittlement of metal. Combine all three and even the best alloys at the time would only have had a short life span.
While this was enough for the experimental reactor at four years, in a commercial reactor, it might be expected to run for 40 to 60 years.
This became the Achilles heel of the molten salt reactors, If the metals that made-up the reactor start it fail after maybe ten years, it would not make for a viable commercial reactor.
And although it would have been possible to create better alloys, the whole molten salt reactor project was shut down in favour of the more well-known and more scalable uranium cycle reactors and their ability to create weapons-grade plutonium.
For decades, people have proposed restarting thorium molten salt reactor programmes around the world but it wasn’t until the 2000s that interest increased enough for governments to start looking at creating new thorium reactors projects.
Material sciences have also increased dramatically since the 1960s. So the alloys available today are much more capable, and at MIT, a recent research programme has discovered how the salt and high radiation levels were affecting the original materials.
Metals are made-up of grains, and within the grains are the crystalline structures of atoms but at the boundary of the grains it’s found that salt finds atomic-level openings in the solid, seeps in, and dissolves salt-soluble atoms, pulling them out and leaving a gap in the material, a spot where the material is now weak and further cracks can form allowing in more salt, till the material fails.
High energy neutrons found in reactors also affects the crystalline structure of some atoms, causing voids and swelling to occur, making it become weaker and brittle.
The combination of salt and radiation is therefore thought to be the worst of all worlds. However, the team at MIT have discovered that the radiation also accelerates the salt atoms and instead of concentrating in the voids between the grains it is dispersed more throughout the entire material and in effect slows down the salt reaction.
Although the research has to be verified by other research teams, they discovered that the proportion of chromium which is salt soluble to nickel and the amount of radiation could make significant differences to the corrosion that occurred and that knowing this, new materials could be formulated that would be much more corrosion resistant and eliminating the biggest obstacle to molten salt reactors.
China has already taken the lead and created the first thorium molten salt reactor and showed that this 2MW reactor could power up without a problem. They have already achieved another first by fully reloading it with fuel whilst it was still running.
The US, Russia, Europe and several other countries are also building their own thorium salt reactors, with several of them expected to come online by 2030.
Their contained design also makes them ideal for small modular reactors that are currently being proposed to power towns or cities in remote areas and to backup for when solar and wind are not available.
In all, it might not be the panacea that fusion promises, but it would go a long way to make nuclear a much more acceptable form of power, where renewables are not a viable solution.
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