Next-generation nuclear technology leverages thorium

Thorium-232 (Th-232) represents a significant advancement in nuclear technology. When converted to Uranium-233 (U-233), it facilitates a chain reaction with an energy density a hundred times higher than conventional U-235.

In India, the Bhabha Atomic Research Centre (BARC), in collaboration with the Nuclear Power Corporation of India Limited (NPCIL), is developing Small Modular Reactors (SMRs) that use relatively small quantities of Th-232 fuel. These reactors, designed to generate up to 300 MWe (megawatts electrical), are compact enough to be deployed in remote regions, industrial clusters, and disaster-resilient infrastructure, with the potential to power an entire district.

Conventional nuclear power uses natural uranium, of which only 0.7% is the fissile isotope U-235. This means one kilogram of uranium contains just seven grams of U-235, with the remaining 993 grams becoming waste. Despite this inefficiency, the energy generated from that small amount of U-235 is immense, equivalent to the thermal energy from coal carried in two large freight trains. While powerful, uranium-based generation produces significant long-lived radioactive waste, necessitating robust safeguards against leaks.

The global SMR market, as projected by the IEA, is expected to grow from $5.8 billion in 2022 to over $18 billion by 2030. However, uranium deposits are not universally abundant, leading many countries, including the United States, to rely on imports for both defence and power generation. This limitation has driven research into alternative fuels, such as Th-232. For two to three decades, nations including China, India, Japan, the UK, and the USA have actively pursued thorium-based reactor technology, and this work is now yielding tangible results.

Th-232 is a slightly radioactive, silvery metal found in igneous rocks and heavy mineral sands. It is three to four times more abundant in the Earth’s upper crust than uranium, with an average concentration of 10.5 parts per million (ppm) compared to uranium’s 3 ppm. Crucially, thorium-fuelled reactors offer the potential to be more environmentally friendly. Although Th-232 is fissionable, it is not fissile on its own; it requires a high-energy neutron bombardment to initiate fission and release energy for electricity generation.

India’s three-stage nuclear programme is a strategic approach to exploiting its limited uranium and vast thorium reserves. Stage I utilises natural uranium in pressurised heavy-water reactors. Stage II employs fast breeder reactors that use plutonium-based fuel to breed more fuel. The final, Stage III, involves advanced reactors that utilise U-233 bred from Th-232.

New achievement

India has now reached this stage, with BARC engineers recently announcing the development of small nuclear reactors capable of supplying electricity to an entire district for years using only a small quantity of Th-232.

A thorium-fuelled reactor requires an initial critical quantity of a fissile driver material—such as U-233, U-235, or plutonium-239 (Pu-239)—to begin the process. This can be likened to lighting wet wood: it won’t ignite on its own, but if placed on a sufficient bed of burning coal (the driver), it dries and eventually sustains its own fire. In India, Pu-239 from its fast breeder programme will be used to drive the thorium chain reaction.

The driver material provides the initial neutrons to convert fertile Th-232 into fissile U-233 through the following steps:

1. Th-232 (atomic number 90) captures a neutron to become Th-233.

2. Th-233 undergoes beta decay to become protactinium-233 (Pa-233) (atomic number 91).

3. Pa-233 undergoes beta decay to become fissile U-233 (atomic number 92).

The U-233 then sustains a nuclear fission chain reaction: U-233 + 1 neutron fission into products like strontium-94 and xenon-137, releasing additional neutrons and approximately 197.9 MeV of energy. These new neutrons can convert more Th-232 into U-233, creating a self-sustaining cycle.

The energy output is enormous. One kilomole of Th-232 (232 kg) contains Avogadro’s number of atoms (6.022 × 10²⁶). Complete fission of this amount would generate approximately 1.9 × 10¹⁶ joules of energy. Factoring in a realistic reactor efficiency of 33%, 232 kg of Th-232 could power a district consuming 40 MW of electricity for several years.

In an era of climate disruption, the global transition away from fossil fuels is urgent. While solar and wind are crucial, nuclear energy provides essential baseload power. India, a rapidly growing economy with daily power consumption currently at 500 GW and projected to rise to 700 GW by 2030, must diversify its energy mix. Currently, nuclear power accounts for only about 4% of its capacity, with solar contributing approximately 25%.

Although a large nuclear plant can take a decade to build—compared to under a year for a solar farm—SMRs promise quicker deployment. The greatest bottleneck for solar energy remains expensive and potentially polluting storage technology for nighttime use. While future advancements may solve this, the development of Th-232 SMRs by BARC presents a significant opportunity to reduce fossil fuel dependence immediately. As with all nuclear technology, this must be pursued with stringent precautions against radioactive leakage to ensure safety and public confidence.