Commercial Power Generation And The Thorium Conundrum

Plenty of plutonium is also available in the spent fuel of reactors. Still, it is not being used because there is a global consensus that reprocessing spent fuel increases proliferation risks, and hence left untouched

Three isotopes can be used as fuel in nuclear reactors: Uranium-235 (U-235), Plutonium-239 (Pu-239) and Uranium-233 (U-233). These are called fissile isotopes.

U-235 is the only fissile isotope that occurs in nature as 0.7 per cent of natural uranium. Pu-239 and U-233 are not found in nature. Pu-239 can be manufactured by irradiating uranium-238 (U-238) with neutrons, a process known as breeding. Similarly, U-233 can be bred by irradiating thorium-232 (Th-232) with neutrons. Thorium, about four times more abundant than uranium on Earth, presents a promising solution to the world's energy needs. Since the inception of the nuclear age, there has been a growing interest in utilising these thorium reserves to produce energy, particularly in India, which boasts vast thorium reserves. However, realising this potential has proved to be a complex journey.

A majority of commercial nuclear power reactors use enriched uranium as fuel. Natural uranium contains 0.7 per cent of U-235, and the rest is U-238. Enriching uranium is the term used to describe the process of increasing the U-235 content in it. Light water reactors (LWRs) use uranium fuel that has U-235 in the range of 3-5 per cent. These power plants will also produce roughly the same energy if they use U-233 instead of U-235 as their fuel. However, the process used to make enriched uranium fuel doesn't work for U-233 because natural thorium does not contain any U-233, so a different approach has to be followed.

The first step in U-233 production is irradiating thorium with neutrons. Large-scale production of U-233 has only been done in the graphite-moderated, water-cooled reactors at Hanford and Savannah River in the USA during the Cold War years.

Such reactors are not built these days by mutual agreement to prevent the proliferation of nuclear weapons. Any reactor built today for U-233 production has to be proliferation-resistant and suitable for peaceful applications only.

The irradiated thorium fuel rods have to be chemically treated to separate out the U-233 in a process called reprocessing before they can be fabricated into fuel assemblies.

Reprocessed U-233 will usually contain trace amounts of U-232. One of the nuclides in the decay chain of U-232 emits high-energy gamma rays, which is hazardous for workers. So the fabrication of the U-233 fuel assemblies has to be done robotically in shielded hot cells. The reactor, the reprocessing facility and the fuel fabrication facility are the three major investments required for making U-233 fuel.

Once an initial inventory of U-233 is available, it is possible to breed more of it in a thermal water-cooled reactor. This was demonstrated in the Shippingport Atomic Power Station, which operated from 1977 to 1982 using U-233 and thorium fuel. After the plant was shut down, fuel analysis showed more U-233 at the end of core life compared to the beginning, proving that breeding had occurred. The Shippingport reactor core had many special features to achieve breeding. This made its core design more complicated and expensive than the standard design used in light power reactors (LWRs). This plant design was never commercialised, even though it demonstrated that thermal breeders could be made using U-233 and thorium fuel.

Reprocessing of fuel also generates a lot of liquid waste that is both radioactive and highly acidic. It is quite difficult to store this waste safely. Reprocessed fuel can also be used for military applications, though there are proliferation concerns. So, various options to utilise thorium without reprocessing have also been studied. These are called the once-through options.

One such option is to mix plutonium with thorium to make a fuel called Th-MOX. This fuel can be used in existing LWRs without any problem. Few advanced nations have plutonium left over from their weapons programs. Using it to generate electricity in reactors is an attractive option.

Plenty of plutonium is also available in the spent fuel of reactors. Still, it is not being used because there is a global consensus that reprocessing spent fuel increases proliferation risks, and it’s better to leave it untouched.

Another alternative is to mix thorium with low-enriched uranium (LEU) in a 3:1 proportion and use it in existing LWRs and PHWRs. This is a costly option because uranium must first be enriched to increase the concentration of U-235 and then blended with thorium to reduce the concentration. For example, to make one kg of five per cent enriched fuel, we would mix 250 grams of uranium enriched to 20 per cent with 750 grams of thorium. ANEEL fuel, developed by a Chicago-based company called Clean Core Thorium Energy, plans to use this approach.

High-temperature gas-cooled reactors (HTGR) are the most likely candidates for successfully utilising thorium in commercial power generation. Experimental HTGR reactors have been built and operated in the US, Germany, UK, Japan and China for many years. China recently built two small HTGRs of 100 MWe capacity and put them in commercial operation in 2023. The Americans and the German HTGRs used thorium mixed with fissile uranium as fuel. The TRISO fuel used in HTGRs is very robust. Since the fission particles are trapped in three layers of ceramic and carbon materials, there is very little chance of their release into the environment in any accident scenario.

While the reactor design is exceptionally safe, we have to wait for the performance of the Chinese HTGRs to confirm that they have overcome the engineering challenges faced by the American and German HTGRs built in the 1960s and 70s.

Oak Ridge Nation Lab built and operated a molten salt reactor (MSR) from 1965-1969 using U-233 fuel. Although the MSR reactor operated successfully and demonstrated breeding, its research funding was stopped in the early 1970s in favour of fast breeder reactors. Since 2000, there has been renewed interest in developing thorium-fuelled MSRs in China, Japan, Russia, France and the USA. China has built a small pilot plant based on the Oak Ridge design, which achieved criticality in 2023 and was operated at full power in 2024.

To summarise, it looks like the HTGR and MSR designs are the most promising technologies to utilise thorium.

We will not know whether these reactors will be a commercial success until at least a few are built and operated worldwide.

(The writer is a nuclear engineer who has worked in India’s strategic submarine program. He is now Director of the School of Engineering at DY Patil International University, Akurdi, Pune)