Goodbye Uranium, Hello Thorium

Kelvin Teo

As the world watches in an admixture of horror and amazement at the events that unfolded at the Fukushima Daiichi nuclear power plant, it seems apparent that such an event unfortunate as it may be has vindicated a certain group of people – naysayers who are completely against nuclear energy. Their argument against nuclear energy is not without basis; there is always the inherent risk of nuclear meltdown, which results in the release of radioactive materials into the environment.

An earthquake at the magnitude of 9.0 off the northeast coast of Japan triggered a tsunami which flooded the plant, and this knocked off the emergency generators that ran the pumps which cool the reactor core, which in the case of the Fukushima comprises of uranium. Uranium is capable of undergoing nuclear fission, a process in which the uranium atom absorbs a neutron and is split into smaller atoms and releasing heat and neutrons in the process. The neutrons produced can be absorbed by other uranium atoms in what is known as a chain reaction. Usually, the reaction may be controlled using control rods (made of cadmium) capable of absorbing neutrons. However, this does not mean that no nuclear fission reaction occurs even when the control rods in place; there is still residual reactions that produces heat, which explains why the reactor core must be continually cooled. When the core overheats, it will melt and the hot radioactive fuel could melt through the vessel and other barriers and be exposed to the environment AKA nuclear meltdown. In the case of the Fukushima Nuclear power plant, this was what triggered the meltdown when the emergency generators were knocked out resulting in the failure of the pump system for the coolant.

One catastrophe can have a domino effect, and the accident has already sent ripples through the mining industry, specifically those concerned with mining of uranium. Australia has 30% of the world’s economically exploitable uranium reserves, and the Federal government of Australia has made the sales of nuclear fuel a national priority. Initially, the climate change issue has prompted national governments to consider nuclear fuel (including the Singapore government) as an alternative. However, with the looming nuclear crisis in Japan, such could plunge confidence in nuclear fuel generated power and stall any developments in this area, which explains jitters within the uranium-mining industry.

The pertinent question is – will this crisis spell the end of the road for nuclear fuels or will it signify the end of one and ascendency of another type of nuclear fuel? It can be argued that it is the latter, as opposed to the former, as the case for the use of thorium as a nuclear fuel gains more ground. The first reason is a matter of statistical odds – it is conceivable from the statistical sense that the current use of fossil fuels to produce electrical power will lead to increasing emissions and climate change with impact such as floods and droughts all of which affect food supply and human health as compared with occurrence of a freak nuclear accident which is of lower probability. The second reason is there are actually safer and reliable alternatives of nuclear fuel such as thorium. Hence, why thorium?

Thorium is relatively abundant, with Australia (again) leading the pack at 425,000 tonnes, followed by the US at 400,000 tonnes and Turkey at 344,000. And it comes relatively cheap, being priced at USD$25.00 per kg according to the US Department of Energy. How thorium works is that it has to absorb a neutron, and undergo a series of reactions to form uranium-233, which is capable of undergoing nuclear fission. Hence, one of its drawbacks is that it requires a neutron source, but this drawback can be turned into an advantage. Plutonium can be used as a neutron source and this is good in a certain sense for the anti-nuclear weapons proliferation process as weapons-grade plutonium can be utilised. And its reactions do not produce any nuclear weapon usable by-products.

In certain aspects, thorium is superior as compared with uranium – it has superior fuel economy, generating 30 times more energy per unit mass as compared with uranium. And it has a superior radiotoxicity profile too; its waste has a radiotoxicity period of 200 years, as compared with that of uranium with a period of greater than 1,000,000 years.

The use of thorium within the concept of a nuclear reactor isn’t actually a new one. The Thorium Molten Salt Reactor (the rest of this article will discuss about the Thorium Molten Salt Reactor) experiment was carried out by the Oak Ridge National Laboratory (USA) through the 1960s which found that such a reactor can be operated safely, reliably and without much difficulty. However, the American authorities didn’t pursue with thorium research because the latter couldn’t produce any nuclear weapon usable material unlike uranium, and this coming during the height of the Cold War, and hence uranium was established as the standard fuel for the industry.

How does a molten salt reactor work? Basically, the fuel, thorium and subsequently, uranium-233 is dissolved in a high temperature molten fluoride salt. The unique feature of this type of reactor is that the reactor fuel is also a coolant. Within the core, the flowing fuel salt is heated to a temperature of 700 degrees celcius, which subsequently results in it flowing to a primary heat exchanger where the heat is transferred to a separate coolant, which then circulates to a secondary heat exchanger. The fuel-molten salt mixture meanwhile circulates back to the core of the reactor. The secondary heat exchanger transfers the heat to a facility which transforms the heat into electrical energy.

Why is Thorium Molten Salt reactor considered safe? The first safety aspect has to do with the fact that fuel is dissolved in a molten salt. Unlike the stationary fuel-containing core in conventional nuclear reactors, temperature increase can lead to fluid expansion and expulsion of the fuel from the core, which reduces its reactivity. Secondly, molten salt reactors operate at high temperature and low pressures. Its ability to operate at low pressure (1 atmospheric pressure) eliminates the risk of a pressure-mediated explosion.

In light of the Japanese disaster, a crucial question is how safe would such a reactor be in the event of a worst-case scenario, say a tsunami which knocks out the power generators in the plant? One safety feature is the use of freeze valves as elucidated by Uri Gat and colleague H.L. Dodds from the University of Tennessee. A cool stream of air can be provided by a fan, which cools the liquid below its melting point, which creates a frozen plug. When the plant generators are knocked out and the fan stops working and is unable to keep the plug in a frozen state, it will start melting and the fluid can be directed to passively cooled storage tanks. The valve may in some cases be melted by the fluid, and it can also directed to the storage tanks.

And for those concerned with a strong earthquake (the probabilities are remotely low in places like Singapore), what happens if a high magnitude earthquake damages a vessel or pipe that results in a leak? Basically, the molten salt containing products of the nuclear reaction will solidify and freeze at room temperature, and can be subsequently recovered. And because the fuel is present in the molten salt, meltdown is a complete non-issue unlike the case of the Fukushima Daiichi nuclear power plant.

From an engineering perspective, the issue that has to be addressed is whether the Thorium Molten Salt Reactor is viable, and can be implemented in the design of currently operational nuclear power plants. The answer is yes. Because the reactor has a good safety profile, it doesn’t require the extensive inbuilt safety defense system that the current conventional plants have, and as a result, this keeps the costs low. And due to its inherent simplicity and safety, construction costs will not be prohibitively high. In fact, Moir, in a paper, estimated the potential costs of electricity generated in the 1960 experiment of the molten salt reactor to be 3.8 cents per kWh, as compared with coal at 4.2 cents per kWh. And with latest advancements in turbine systems (Brayton cycle), molten salt reactors can achieve thermal efficiencies at 30% greater than conventional nuclear plants. What this means is that the process in the molten salt reactors is more energetically efficient with a greater percentage of heat produced converted into electricity.

Currently, the Japanese are working on one such reactor dubbed the Fuji Molten Salt Reactor which utilises thorium. India, which is fourth largest in the world in terms of thorium resources has begun development into a prototype of a thorium-based reactor. The US, Russia and China have also commenced research into the use of thorium for nuclear energy.

Thus, for naysayers who believe that that the Japanese disaster spells the end of nuclear energy, it is still considered premature at this point of time. If anything, the disaster provides the impetus into research and development of thorium-based reactors, especially the molten salt types which are inherently safe, reliable and cost-effective. One would suspect that governments (including our Singapore government) may shelve plans for the conventional uranium fuel-based nuclear reactors and play a waiting game by observing the nuclear power industry for developments in the area of thorium-based reactors. And it is time to bid goodbye to uranium and usher in the thorium era.

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Photo courtesy of Reuters.