Decoding India's Kalpakkam milestone

By achieving a self-sustaining reaction, the Prototype Fast Breeder Reactor paves the way for large-scale thorium use, writes Dennis C Joy

On April 6, 2026, India achieved a historic breakthrough when the indigenously built Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, Tamil Nadu, reached its first criticality. The milestone signifies India’s official transition to the second stage of its nuclear energy programme, making it only the second country in the world, after Russia, to operate a commercial-scale fast-breeder reactor.

India’s pursuit of nuclear energy was first formalised in 1954, when physicist Homi J Bhabha, often called the father of the Indian nuclear programme, presented a three-stage nuclear energy plan to secure the nation’s long-term energy independence.

“The aim of the long-range atomic power programme in India must be to base the nuclear power generation as soon as possible on thorium rather than uranium”, exclaimed Bhabha. The strategy was born of the reality that, while India possesses only about 1–2% of the world’s uranium, it holds approximately 25% of global thorium reserves.

The first stage of this plan relies on Pressurised Heavy Water Reactors (PHWRs), which use natural uranium as fuel and deuterium oxide, commonly known as heavy water, as the coolant.

Natural uranium is a mixture of different isotopes of uranium, such as U-238, U-235, and U-234, which are the same chemical element with an equal number of protons but different numbers of neutrons in their atomic nucleus. This fuel is used to generate electricity while producing plutonium-239 as a vital byproduct.

India successfully developed a series of sequentially larger, indigenously designed reactors under the Indian PHWR (IPHWR) series, ranging from 220 MegaWatts to the more modern 700 MW designs. Currently, India’s installed nuclear capacity stands at 8.78 Gigawatts, contributing roughly 3% of the national electricity mix.

Stage 2 of India’s nuclear programme, which commenced in 2006, centres on the deployment of Fast Breeder Reactors (FBRs). Unlike conventional PHWR reactors, FBRs use a Mixed Oxide (MOX) fuel composed of plutonium-239, recovered from the reprocessing of spent fuel from Stage 1, and natural uranium. FBRs produce more Plutonium-239, and crucially, another isotope of uranium, U-233, as byproducts. U-233 is a fissile material that can readily sustain a nuclear fission chain reaction and forms the base fuel for stage-3 of our nuclear roadmap.

Because FBRs consume the plutonium waste from Stage 1 and generate their own fuel, they require significantly less natural uranium to produce electricity than PHWRs. Furthermore, by using spent fuel as an input, FBRs also reduce the volume of nuclear waste that requires long-term geological disposal.

After 20 years of research and engineering, the Kalpakkam reactor finally reached criticality, meaning it produced more fissile material than it consumed. This sets the stage for India to enter the final stage of our nuclear programme.

The challenges

The third stage aims to deploy thorium-based reactors. These will consume the uranium-233 bred in the previous stage, along with our vast reserves of thorium as its fuel. The thorium reactors would also produce more U-233, which would be fed back into the reactor, further reducing
uranium use.

The path to reaching stage-2, however, has been marked by technical challenges and financial hurdles, with the PFBR costing more than twice its original estimate and delayed by over a decade.

“India faced technological isolation after the Pokhran tests. Therefore, technologies such as fuel reprocessing, Mixed Oxide Fuel (MOX) fabrication, and sodium-cooling systems had to be developed indigenously. Each of these technologies is quite complex and has long gestation periods,” remarks Suneet Singh, a professor at the Department of Energy Science and Engineering at the Indian Institute of Technology Bombay.

FBRs are also notoriously difficult to operate, with a troubled history worldwide due to inherent technical characteristics that often lead to low operating efficiency. One of the major hurdles has been the use of liquid sodium as the coolant for PFBRs.

“Liquid Sodium poses challenges due to its chemical reactivity with air and water, and activation into radioactive sodium-24,” remarks Singh. “India’s FBR addresses these risks through a pool-type design of thermal stability and intermediate sodium loop to isolate radioactive coolant, and double-wall steam generators,” he adds.

Despite the costs, the long-term economic rationale remains strong, as the Department of Atomic Energy estimates that India’s extractable thorium could power the country with 500 GWe for at least four centuries. To proceed to the third stage, India must now scale its stage-2 capacity to approximately 50 Gigawatts from the 500 Megawatts prototype. This threshold is necessary to build the required inventory of fissile U-233 for thorium-based reactors.

The priorities

According to Singh, “India’s immediate priority is to start building a fleet of commercial FBRs and expanding reprocessing infrastructure. Progress towards stage 3 requires demonstration of Thorium utilisation in Advanced Heavy Water Reactors (AHWRs) and scaling the U-233 fuel cycle.”

The government has accelerated this mission through the Nuclear Energy Mission, which allocates Rs 20,000 crore for developing new technologies. It also introduced the Sustainable Harnessing and Advancement of Nuclear Energy for Transforming India (SHANTI) Act of 2025, which allows for limited private participation.

This strategic progression is essential for India to meet its net-zero emissions commitment, while insulating its fast-growing economy from the volatility of global fossil fuel markets.

While other nations, including the United States, France, the United Kingdom, Japan, China, and Russia, have pursued FBR technology, Russia was the only country to operate a commercial-scale facility until India’s recent milestone. The widespread implementation of FBRs has historically been hindered by technical challenges and the unfavourable economics of pursuing the technology.

However, if India successfully transitions its prototype at Kalpakkam into a viable commercial energy model, it could serve
as a global catalyst, encouraging other nations to reconsider and adopt similar technologies.

“If India succeeds, then it could change the perception about fast breeder technology. This technology can then play an important role in decarbonization and fuel sustainability strategies,” concludes Singh.

(The author is a science communicator associated with Research Matters)