According to the proponents of using thorium as nuclear fuel, it would have benefits such as producing much less radioactive waste than uranium fuel. It would make nuclear accidents like nuclear meltdowns all but impossible. And its lobby says thorium would be less likely to lead to nuclear proliferation. Are these claims true? If so, what kind of technical developments must occur to makes these predictions come true?

Thorium is a chemical element with the symbol Th and the atomic number of 90. All of the thorium isotopes are radioactive, and the only natural active isotope is Th-232. Thorium (Th-232) itself is not fissile[1], meaning it can’t produce energy directly in a conventional reactor. But when Th-232 is irradiated with neutrons it transforms to a fissile isotope of uranium, U-233. To use thorium to produce energy, you first have to irradiate thorium in a reactor. The U-233 that’s produced must then either be reprocessed in a chemical process to produce the new fuel, or it can be used in-situ within the same reactor, as in the concept molten salt reactors.

Compared to uranium, thorium is three to four times more abundant in the earth’s crust, giving it the potential to replace uranium as nuclear fuel in the future. But the International Atomic Energy Agency (IAEA) has said there are sufficient uranium resources for the next 150 years based on current reactor requirements.

As there are sufficient uranium resources for the foreseeable future there has been little interest from the traditionally nuclear industry to develop thorium fuel. But some countries like Norway and India have looked into this option, and their interest is based largely based on their domestic thorium resources.

Estimated world thorium resources

Norway

Norway has relatively large thorium resources in the Fen-area in the country’s southeast. Even though the country has a strict non-nuclear policy, there have been discussions on whether Norway should do research on thorium’s potential as a nuclear fuel. At the moment, a privately funded research program on the behavior of thorium fuel in a traditional reactors is taking place at the Norwegian Halden research reactor.

India

Because of India’s nuclear weapons program, the country has not signed the Treaty on the Non-Proliferation of Nuclear Weapons, or NPT. It has therefor been difficult for the country to import uranium to fuel their nuclear reactors. India has a long-term goal to develop a heavy-water reactor fuel cycle for their domestic thorium resources.

The Indian heavy-water reactor fuel cycle consists of three stages. The first stage consists of conventional reactors using uranium fuel to produce plutonium. The plutonium will be used in the second stage, which uses fast neutrons reactors that produce more plutonium as well as fissile uranium (U-233) from thorium (Th-232). The plutonium and thorium will be used to produce the plutonium-thorium fuel for either the Advanced Heavy Water Reactors (AHWR) or molten salt breeder reactors (MSBR) in stage three.

But according to Bhabha Atomic Research Centre (BARC) it is important to build up a significant amount of fissile materiel before stage 3 is implemented. BARC announced in 2013 that the introduction of thorium-based reactor deployment in India is expected happen after 2070.

Thorium in nuclear reactors

With some modifications, thorium can be used in some conventional reactors in operation today, like heavy water reactors. It would be necessary to mix thorium (Th-232) with either U-235 or Pu-239 to produce fissile U-233. The U-233 would then be reprocessed so that the fuel in the reactor would gradually contain more and more fuel comprised of U-233.

The use of thorium in the present reactors would involve reprocessing of the spent thorium fuel in order to use the U-233 products in the fuel. Because spent thorium fuel contains a higher amount of short-lived radioactivity, this would make the reprocessing more challenging than current methods of reprocessing of traditional uranium fuel.

Producing so-called Mixed Oxide (MOX) fuel with a mixture of thorium and plutonium has been proposed as solution to burn some of the huge amounts of plutonium that’s been stockpiled around the world for military and civilian purposes. A thorium-based MOX process would burn plutonium more effectively, as no new plutonium would be produced, unlike the burning uranium and plutonium.

The use of thorium in reactors would produce radioactive waste and spent nuclear fuel that would have to be stored and or treated in the same order of magnitude as traditional uranium fuel. The spent thorium fuel would be more radioactive and more challenging to handle than spent uranium fuel because spent thorium fuel contains the alpha-emitter Th-228, which has a half-life of 2 years.

In a long perspective it’s possible to develop a thorium fuel cycle based on so-called fourth generation nuclear reactors. With fourth generation nuclear reactors it has been envisaged that radioactive waste production will be much lower than with the present technology. These benefits are expected for both for uranium- and thorium fuel cycle if the technology is developed. It’s expected that the fourth generation nuclear reactors would be commercially available around 2030-2040.

Of the fourth generation reactors, the most suitable for thorium would be the Molten Salt Reactor (MSR). In this reactor, thorium and uranium is dissolved in molten fluoride salt at a temperature of 400-700⁰C. This mixture is circulated through the core region and then through a chemical processing circuit that removes unwanted radioactivity produced in the core region. The MSRs are the fourth generation reactor still requiring considerable research and development, and the period of study is planned for completion by 2025.

Conclusion

Even though thorium is three to four times more abundant that uranium, the economic initiative is lacking to develop a thorium fuel cycle – which is a result of predictions by the IAEA and the Organization for Economic Cooperation and Development that uranium resources will last for another 150 years.

There are very few safety benefits to be gained from using thorium in current reactors. Burning thorium in traditional reactors produces spent nuclear fuel that’s more radioactive and more challenging to handle than spent uranium fuel.

The use of thorium in a full scale fuel cycle in the next generation of nuclear reactors is far in the future. India’s active development of a thorium fuel cycle won’t, as noted above, come to fruition for another 55 years.

Thorium won’t be part of the solution in dealing with climate change before 2050. The only tools that can rise to that challenge are renewable energy deployment, energy conservation and CO₂ Capture and Storage.

[1] Fissile isotopes are isotopes that can sustain a nuclear chain reaction with slow neutrons.

Nils Bøhmer is Bellona’s executive director and nuclear physicist.