Kalpakkam PFBR Hits Nuclear Criticality, But Thorium Payoff Still Years Away

India marked a significant milestone in its civil nuclear journey as the indigenously built Prototype Fast Breeder Reactor (PFBR ) at Kalpakkam achieved criticality, with Prime Minister Narendra Modi describing it as a defining step in advancing the country’s long-term energy strategy.

Criticality signifies that the reactor has reached a self-sustaining nuclear chain reaction, the first essential step before power generation. While this milestone has been achieved globally, the Kalpakkam reactor stands out as a Fast breeder reactor, enabling India to move beyond conventional uranium-based systems.

At the core of this technology is the conversion of Uranium-238 into Plutonium-239. When plutonium undergoes fission, it releases neutrons that not only generate energy but also enable the next stage of India’s nuclear programme converting Thorium-232 into Uranium-233 . This newly formed fuel is fissile and can sustain further reactions, producing energy while breeding more fuel, forming the basis of a closed, regenerative nuclear cycle.

A key advantage of this approach is that it does not rely on enriched Uranium-235, which is required in most conventional reactors. Instead, it uses unenriched Uranium-238 and abundant Thorium-232, allowing India to potentially avoid the costly process of uranium enrichment or dependence on imported enriched fuel.

However, while achieving criticality is a major technological breakthrough, experts caution that full fuel self-sufficiency will take time. The reactor is expected to begin electricity generation of about 500 MW within the next couple of years, following phased commissioning. Achieving a stable breeding cycle, where the reactor produces more fuel than it consumes, could take close to a decade.

In the broader energy context, the PFBR’s capacity remains modest compared to India’s total installed power capacity of roughly 500 GW, which continues to grow across fossil and renewable sources. Even so, nuclear energy is seen as an important component of the long-term energy mix, particularly for its reliability and low carbon footprint.

In his post, the Prime Minister highlighted the achievement as a “defining step” that advances the second stage of India’s nuclear programme and paves the way for harnessing thorium reserves. Senior Congress leader Jairam Ramesh noted that India joins Russia among nations with commercial fast breeder capability, and underscored that the milestone reflects over three decades of sustained effort by the country’s nuclear establishment, calling for recognition of the continuity behind the achievement.