Thorium is an incredibly energy-dense material. A single tonne of (refined) thorium has the potential to produce as much energy as over 200 tonnes of uranium or millions of tonnes of coal. This energy density is due to thorium’s ability to act as fertile material in a nuclear reactor. When bombarded with neutrons, thorium-232 transmutes into uranium-233, an excellent fissile isotope capable of sustaining a nuclear chain reaction.

The breeding of uranium-233 from thorium allows for a much more efficient utilisation of the fuel compared to conventional uranium reactors, which leave the reactor with a significant fraction of their uranium unburned. With thorium, reactors can achieve higher burn-up rates, meaning more of the fuel is converted into usable energy before it is discarded as waste. This efficiency not only reduces the fuel requirements but also minimises the volume of waste generated.

Thorium is the only element capable of enabling a breeder reactor in the thermal spectrum. The thermal spectrum refers to reactors using slowed-down neutrons, while a breeder reactor is one that generates more fissile fuel than it consumes. Most nuclear reactors built to date consume more fissile fuel than they produce. Consequently, they must be continually supplied with new fissile fuel, which is highly expensive.

Versatility of reactor designs
Thorium can be utilised in various innovative reactor designs that offer distinct advantages over traditional light-water reactors. One of the most promising designs is the molten salt reactor (MSR), where thorium is dissolved in molten salt and circulated through the reactor core. MSRs operate at higher temperatures and lower pressures than conventional reactors, improving thermal efficiency and reducing the risks associated with past failures.

Another key advantage of thorium-based MSRs is their potential to operate in a closed fuel cycle, where the fuel salts are continuously recycled into new reactors as new fuel. This contrasts with the open fuel cycle of most uranium reactors, which requires fresh uranium fuel to be mined, enriched and fabricated for each new cycle.

The primary reason why thorium is so important is also why thorium molten salt breeder reactors stand in a class of their own, far surpassing the benefits of all other fission or fusion reactors.

Thorium can be utilised in various innovative reactor designs that offer distinct advantages over traditional light-water reactors – one of the most promising designs is the molten salt reactor, where thorium is dissolved in molten salt and circulated through the reactor core.

Fusion reactors face a similar challenge as fission reactors – they require tritium. Tritium currently costs $30mn/kg. Perhaps, in the future, someone will invent a fusion reactor capable of producing more tritium than it consumes, but I find it highly unlikely to occur within the next 50 years.

There is also a theoretical possibility that a fast breeder reactor could be developed to produce more fissile fuel from uranium than it consumes. However, the world has already invested more than $1tn (in today’s money) in developing fast breeder reactors. Over 20 fast reactors have been built in the past, yet none has been able to operate solely on depleted uranium.

Copenhagen Atomics’ thorium reactor is poised to be the first reactor in history to achieve this groundbreaking milestone. When it does, no other fusion or fission reactor will be able to compete with the energy price of this thorium reactor. Copenhagen Atomics is already more than halfway towards reaching this significant goal. In 2023, it built a full-scale prototype that is planned to be tested in 2026–2027 at Switzerland’s Paul Scherrer Institute.

Abundance and accessibility
Thorium is three to four times more abundant in the Earth’s crust than uranium. However, unlike uranium, where only 0.7% of the naturally occurring reserves is the usable U-235 isotope, almost 100% of the naturally occurring thorium is the isotope Th-232, which is needed. Therefore, thorium does not require the energy-intensive enrichment processes needed for uranium, making its extraction and preparation for use in reactors less resource-intensive and less costly. 

Thorium is widely distributed globally, with significant reserves in countries such as India, the US, Australia and Norway. Because it is often found as a byproduct of rare earth mining, much of the world’s existing thorium stockpile is already being unearthed but remains unused. Leveraging this resource provides an opportunity to make better use of materials that are currently considered waste, adding another layer of economic and environmental efficiency.

Reduction in long-lived radioactive waste
A significant public perception challenge facing traditional nuclear power is the long-lived radioactive waste it produces, which can remain hazardous for tens of thousands of years. Thorium-based reactors have the potential to address this issue by producing waste that has a much shorter half-life. Most of the waste generated in a thorium reactor is composed of fission products with half-lives of up to 300 years, a manageable timeframe compared to the millennia-long lifespans of some uranium-derived isotopes.

Additionally, thorium reactors can be designed to recycle spent nuclear fuel from conventional reactors, further reducing the need for long-term storage. Some designs, such as the molten salt reactor from Copenhagen Atomics, allow for continuous fuel reprocessing within the reactor itself, extracting valuable isotopes and minimising the accumulation of fission products, improving the reactor efficiency significantly.

Economic and geopolitical benefits
Thorium’s abundance and accessibility offer substantial economic and geopolitical advantages. Because thorium is more evenly distributed around the world than uranium, countries can potentially reduce their reliance on foreign suppliers, fostering greater energy independence. This could be particularly beneficial for nations with limited fossil fuel resources or constrained access to uranium supplies.

In conclusion, thorium energy represents a paradigm shift in how we think about nuclear power. Its efficiency, reduced waste profile, and scalability align with the broader goals of reducing environmental impact while meeting the growing energy needs of an expanding global population. By integrating thorium into the energy mix, nations could establish a more sustainable and resilient energy infrastructure that supports both present and future generations. In combination with new advanced designs such as the Copenhagen Atomics Waste Burner, the price of energy could eventually reach $20/MWh, lower than any other energy source, which would significantly increase global prosperity.

The views and opinions expressed in this article are strictly those of the author only and are not necessarily given or endorsed by or on behalf of the Energy Institute.

• Further reading: ‘Why UK government hydrogen fuel research is the 21st century’s hottest energy ticket’. Outside of the EU, the UK is forging its own path towards commercial production of electricity from nuclear fusion. The 2022 UK fusion strategy has sketched out a vision of building a prototype fusion reactor using a domestic design, along with fuel cycle facilities, proposals which attracted ‘significant support’ in the October 2024 budget. 
• The combination of molten salt technology and flexible, load-following operation could prove an essential component of efforts to reach net zero in the UK – writes David Landon, CEO of MoltexFLEX.