Thorium – The Fuel of Tomorrow?

Ever since the creation of the first steam-based motors, significant amounts of time, research effort and resources have been spent on the improvement of energy generation by either improving the technology used or looking at alternative sources, with the latter being prioritized in the past decades due to the environmental impact of current fuels like coal, oil, and natural gas.

This is the main motivation behind many projects which are pushing to bring a new type of fuel and technology to the table, which could kickstart a wave of incredible advancements in energy generation and distribution: the use of Thorium and Molten Salt Reactors (MSR) as a substitute for fossil fuels.

The Forgotten Element

Thorium is a radioactive element of same group of more renown elements like uranium and plutonium but is less radioactive and slightly more abundant on Earth. It was first discovered in 1828, nearly 70 years before the discovery of radioactivity, and its first uses involved the incandescence generated when heated by burning gases to power lamps.

However, as the knowledge about radioactivity increased and uses beyond energy generation were becoming more popular, especially in the military sector, radioactive elements reached extremely high demands. Sadly, thorium fell to a minor role behind uranium and plutonium for nuclear weapons.

Still, thorium would continue to be developed, with a functional reactor build in the Oak Ridge National Laboratory and used from 1965 and 1969, until in 1973 the Atomic Energy Join Committee of the U.S. Congress would double down on the financing of uranium technologies by publishing the “Uranium Enrichment Services Agreement”.

As years went by, the scientific community began pushing for the use of alternative fuels due to the growing necessities regarding the environment, and thorium entered the picture once again in the 1990s. The latest piece of support for Thorium as a fuel for the future comes from the World Nuclear Association, in a continuously updated article about the element, which shines a positive light towards it by highlighting its abundance (compared to other radioactive elements), viability and some of the current ways to take advantage of it.

Present and Future

With the resurgence in popularity, multiple technologies have been either revived or developed from scratch, with the most relevant ones being as follows:

Pressurized Heavy-Water Reactors

PHWRs are nuclear reactors which use deuterium oxide (“heavy water”) as a coolant agent, which is kept under pressure to avoid boiling and helps the reactor to reach higher temperatures without forming bubbles. The use of thorium in this type of reactor does benefits from the neutron economy of it, which allows thorium to produce uranium isotope U-233 and is able to generate up to 80% of the total energy from thorium after reaching equilibrium.

This technology has been in development in Canada for over 50 years, with joint R&D efforts being made along with China and achieving good results in terms of material properties. The second phase of this agreement between Canada and China was signed in 2009 and its feasibility and enhanced safety could lead to its implementation in China in the near future.

High-Temperature Gas-Cooled Reactors

HTRs are another kind of reactor suited to reaching high temperatures by embedding capsules of the fuel in a graphite matrix, which keeps stable under these conditions. Thorium specifically is used in coated particles and mixed with plutonium or an enriched variant of uranium, in layers of carbon to contain fission gasses. Since the stability of the fuel is associated to the matrix, the fuel can be irradiated for long periods of time with little to no risks and deeply burning their fissile charge.

These were common in the past, with the most successful example being the one from the U.K. which was operational for 741 full power days from 1964 to 1973, in a cooperative effort involving Austria, Denmark, Sweden, Norway and Switzerland. Simultaneously, Germany also operated an HTR for over 750 weeks between 1967 and 1988, using over 1,300 kg of thorium. Currently, a reactor based on the German one is under development in China.

Molten Salt Reactors

Arguably the most popular technology in the recent years, since they are still on the development stage and companies are taking advantage of the internet to spread their vision and raise funds, MSRs use a mixture of thorium and uranium as fluid fuel as part of a salt which melts at temperatures between 400 and 700 °C, serving the purpose of both a heat transfer fluid and the matrix for the fuel.

A variant of this model which mostly used uranium as a fuel is the one mentioned early in the article, in the Oak Ridge National Laboratory, and studies showed how it could be feasible if certain problems regarding corrosion could be solved.

The current interest from multiple countries like the U.S.A, Russia, China, Japan and France focuses on a new type of MSR known as the Liquid Fluoride Thorium Reactor (LFTR), first introduced in an 2011 R&D publication by the China Academy of Sciences, but efforts are pointed towards a more realistic first step: using solid fuel, salt-cooled reactors as these do not have the high activity levels of molten salt, and can help in developing and testing the necessary technologies to turn this idea into a reality.

Despite many years in the background, thorium appears to be making a comeback in the eyes of the scientific community, which later translates in interest from government, private companies and start-ups looking for ways to contribute to the betterment of society and the environment by taking a chance and investing time and money in this new source of energy.