India’s Prototype Fast Breeder Reactor at Kalpakkam Reaches First Criticality

India’s long-delayed Prototype Fast Breeder Reactor at Kalpakkam in Tamil Nadu has reached first criticality, a significant engineering milestone that moves the country closer to operating a commercial-scale fast breeder reactor.

The achievement does not mean the reactor is already generating electricity for regular commercial use. First criticality means the reactor has begun a controlled, self-sustaining nuclear fission chain reaction at very low power — an early but important commissioning step before higher-power tests, grid connection and, eventually, commercial operation.

According to a Press Information Bureau statement issued by India’s Department of Atomic Energy, the 500 MWe Prototype Fast Breeder Reactor, or PFBR, attained first criticality on April 6 at 8:25 p.m. IST. The department called it “a landmark achievement for India’s nuclear energy programme.” The reactor is an indigenously designed, sodium-cooled, pool-type fast reactor that uses mixed uranium-plutonium oxide, or MOX, fuel. It was designed by the Indira Gandhi Centre for Atomic Research, a leading nuclear research center under the Department of Atomic Energy, and built and commissioned by Bharatiya Nabhikiya Vidyut Nigam Ltd., or BHAVINI, a state-run nuclear power company under the same department.

The milestone matters far beyond a single reactor. Fast breeder reactors are central to India’s three-stage nuclear power program, a long-term strategy meant to stretch limited uranium resources and eventually support wider use of thorium, a fuel source India has long viewed as strategically important. In simple terms, fast breeder reactors are designed to produce more fissile material than they consume, which is why they occupy such an important place in Stage II of the plan and are seen as a bridge to a later thorium-based Stage III.

Government statements framed the milestone as internationally significant as well. In an official statement, Union minister Jitendra Singh said India would become the second country after Russia to operate fast breeder reactors commercially. That remains a government claim rather than an uncontested fact. Russia clearly operates commercial-scale fast reactors, notably the BN-600 and BN-800, while the International Atomic Energy Agency’s PRIS reactor database still listed China’s CFR-600 project at Xiapu as “under construction” as of March 23, 2026.

The PFBR still has several steps to clear before it can begin supplying power. India’s Atomic Energy Regulatory Board, the country’s nuclear regulator, issued permissions on Oct. 16, 2025, for initial fuel loading, first approach to criticality and low-power physics experiments. With first criticality achieved, the reactor must now complete those low-power physics experiments and then move through stepwise increases in power. Further regulatory approvals will be required before any grid connection or commercial operation.

That long checklist helps explain why the moment is both consequential and carefully defined. The PFBR project has been under development for decades: construction began in 2004, and the reactor went through years of delays before reaching this point. Even so, the government and India’s nuclear establishment have presented first criticality as proof that one of the country’s most ambitious atomic-energy projects has finally crossed from prolonged construction into active commissioning. Sreekumar G. Pillai, director of the Indira Gandhi Centre for Atomic Research, said in a statement on April 7: “The attainment of first criticality of the Prototype Fast Breeder Reactor (PFBR) on 6th April, 2026 marks a defining moment and a major milestone in our nuclear power programme.”