Public attitudes towards nuclear power have been filled with scepticism, especially in the wake of the nuclear disasters in Chernobyl and Fukushima. Yet, in the face of the global energy crisis as well as climate change, countries are finding every potential alternative to replace their dependence on fossil fuels. Although the universal adoption of renewable energy is something that we must strive to achieve, its current underdeveloped status has led to renewed interest and consideration of nuclear energy, of which the fuels required are abundant and arguably ‘clean’.” We look at one of the many Generation IV nuclear reactors including molten salt reactions, and examine their potential pros and cons.

Traditionally, a nuclear power plant utilises nuclear fission to generate thermal energy and convert it into electricity by producing steam through the medium of boiling water or pressurised water. Yet, the energy that nuclear fission generates will not automatically stop accumulating without control. Therefore, a nuclear disaster is possible when heat builds up uncontrollably and melts down the reactor.

Scientists have started researching unconventional ways to generate nuclear energy, and recent experiments have shown the possibility of eliminating its safety concern. These unconventional reactors are also named Generation IV nuclear reactors, as they are mostly introduced and researched by the Generation IV International Forum (GIF), “a co-operative international endeavour seeking to develop the research necessary to test the feasibility and performance of fourth generation nuclear systems, and to make them available for industrial deployment by 2030

Molten Salt Reactor

The first experiment of the molten salt reactor (MSR) took place in the United States in the 60s. However, experiments in this field were halted following difficulties in handling the extremely high temperature of the fuel. However, global interest in it has revived after China invested about RMB 3 billion on the MSR in 2011, hoping to be the first country to commercialise this technology. The construction of the first prototype was already finished in the late 2021, and it will start its experiments soon.

MSR uses liquid fuel instead of the solid fuel that we see in conventional nuclear reactors. The liquid fuel used in MSR is called molten salt, a salt that is solid under standard temperature but will turn into liquid due to elevated temperature. Molten salt will also replace water as the coolant of the reactor, which is designed to carry the heat away. Molten salt can be composed in many different ways; one possible component of molten salt is thorium, a nuclear chemical element whose energy can only be extracted in new-generation reactors, and which carries many benefits: its supply on the planet is at least three times that of the conventional nuclear fuel, uranium; its waste can take 100 years to decay compared to uranium whose waste decay in 10,000 years.

MSR can also be built as a small modular reactor. A small modular reactor is a reactor physically much smaller than a conventional nuclear reactor which produces 300 megawatts per unit at the maximum and of which the components can be manufactured in factories and easily transported for installation. The location of a large nuclear power plant is strongly restrained by geographical limitation whereas a small modular reactor has  a wide range of choices on its location. The construction time for a small modular reactor however, is much shorter as its components can be manufactured in an assembly line. The reason that a MSR can be built on a smaller scale is that it does not require an enormous and expensive machine to maintain water at high pressures.

Advantages of Molten Salt Reactors

In energy production, there is a principle called “energy return on investment” (EROI), in which a higher number indicates better efficiency, when the energy generated is divided by the energy required to generate that energy. Solar energy has a score of 10, while coal scores between 18 and 43. Shockingly, the estimated EROI of MSRs is at about 1,200, meaning its energy efficiency scores higher than every other method of energy production.

MSR is much safer than conventional nuclear reactors due to its inherent system of self-regulation. Conventional nuclear reactors require constant intervention to ensure that the reactors are operating at a safe level. Yet, MSR does not require the same since temperature and reactivity correlate negatively inside MSR. When the temperature goes up in the reactor, the number of nuclear fissions goes down.

Moreover, MSR is not operated under high pressure. Most conventional nuclear reactors require high pressure to increase the boiling point of water, which requires a coolant so that the water will not damage the reactors. On the contrary, MSR can simply operate at atmospheric pressure since the boiling point of molten salt is much higher than the temperature of the reactor during operation. As high pressure is not required, the possibility of an explosion is eliminated from the safety concern. Therefore, any radioactive leak will not be in the form of an explosion and will be limited to the area around the reactor; nuclear disasters in Chernobyl and Fukushima that required the evacuation of the whole region are theoretically impossible.

In addition, the emergency braking system of MSR is much more efficient. The emergency braking system of conventional nuclear reactors is based on the control rods which are built with chemical elements that can absorb neutrons. However, it is always possible that the control rods fail to capture the neutrons if there are too many of them. MSR functions in a different way. Instead of having control rods that can stop the operation by force, MSR has something called graphite rods that moderate the neutrons. In other words, if the graphite rods are removed, there will be no longer nuclear reactions.

Criticisms of Molten Salt Reactors

There are two major reasons for which MSRs is under criticism. The first one being the fact that MSRs are still in their development stage. Albeit workable theoretically, there is not enough empirical evidence to prove that they work in real life, as the most recent MSR that was built dates back to the experiment in the United States in the 60s.  The challenges in handling the high temperature of the fuel remains the same today.

The most challenging problem however, is still the risk of corrosion. MSR operates at about 700C, yet there is yet to be a material that can withstand this high temperature for a period of time. Therefore, some components of the reactors may need to be frequently replaced every few years. Yet, compared to the 60s, today’s technology has significantly improved. Although we are still using the same alloy – Hastelloy-N that was also used in the experiment in the 60s – the resistance of corrosion of Hastelloy-N has improved throughout the years.  An experiment led by the University of South China has proven that the rate of corrosion is only about 35% of that of the original Hastelloy-N after putting a metal coating on it. In other words, today’s resistance of corrosion can be about at least three times stronger than the experiment in the 60s.

The Future of Nuclear Fission

Molten salt reactor is only one of the many new unconventional nuclear reactors, there are also High-Temperature Gas-Cooled Reactor, Very-High-Temperature Reactor, and Sodium-Cooled Fast Breeder Reactor, which all aim to create a system of self-regulation so that all reactors are “walk-away safe” and to maximise the output of electricity. Yet, they are still criticised for the faults of their ancestors. Although they still carry the name of nuclear power, it is crucial to note the difference between these new Generation IV nuclear reactors from their conventional versions, as they are subject to different laws of physics and have largely improved in both safety and efficiency. Should MSR or other new generation nuclear reactors be a part of the renewable energy transition? At the moment, this is unclear, but we should definitely be open-minded to the possibilities and potentials of these new nuclear reactors. The ongoing experiment of MSR in China will eventually illustrate it.