India's Fast Breeder Reactor Achieves First Criticality at Kalpakkam, Opening Second Stage of Country's Nuclear Program

India's Prototype Fast Breeder Reactor at Kalpakkam in Tamil Nadu achieved first criticality on April 6 — a self-sustaining nuclear fission chain reaction — marking a milestone that scientists and engineers at the country's Department of Atomic Energy had pursued for more than two decades and that moves India meaningfully closer to a nuclear energy strategy built around its own vast thorium reserves.

The reactor reached criticality at 8:25 PM local time on Sunday, April 6. Department of Atomic Energy Secretary Ajit Kumar Mohanty confirmed the achievement in an official statement, calling it "a moment of national pride and scientific achievement." Prime Minister Narendra Modi praised the breakthrough in a post on X, writing that India's scientists had "proved once again that this nation has the talent, determination, and resolve to achieve what others said could not be done."

The Prototype Fast Breeder Reactor — rated at 500 megawatts of electrical output — uses a fuel core of uranium-plutonium mixed oxide surrounded by a blanket of uranium-238. Where conventional nuclear reactors burn uranium fuel and leave behind radioactive waste, a breeder reactor is designed to convert that uranium-238 blanket into fissile plutonium-239 through neutron bombardment — potentially generating more fuel than it consumes. In theory, a fully operational fleet of fast breeder reactors fed eventually by India's thorium deposits could provide carbon-free power for centuries.

India's thorium reserves are the largest in the world, estimated at around 650,000 metric tons. The country has relatively little natural uranium, which has historically made it dependent on imported fuel. The fast breeder program is designed to break that dependency by using thorium and home-produced plutonium to close the fuel cycle — eventually burning thorium directly in what would become the world's most self-sufficient civilian nuclear program.

The path to first criticality was not smooth. Construction on the Kalpakkam reactor began in 2004 with an original target of completion by 2010. A succession of technical challenges, procurement difficulties, and design revisions pushed that deadline back repeatedly — ultimately by more than 15 years. The original cost estimate of ₹3,492 crore grew to ₹8,181 crore, a 134 percent overrun.

India becomes only the second country after Russia to achieve an operational commercial fast breeder reactor. Russia's BN-800 reactor at the Beloyarsk Nuclear Power Plant has been running since 2016 and BN-1200 is under construction. France, Japan, Germany, and the United States all pursued breeder programs in the 20th century but ultimately abandoned them due to cost, safety concerns after accidents at experimental facilities, and the relatively low cost of conventional uranium on world markets.

The Kalpakkam reactor still has a lengthy commissioning process ahead before it generates power for the grid. Following criticality, engineers will conduct a phased series of low-power physics experiments over the coming months to characterize the reactor's behavior before gradually increasing output. Commercial electricity generation is expected by September 2026 at the earliest, pending approval from India's Atomic Energy Regulatory Board.

India's nuclear vision is divided into three stages. Stage one — conventional pressurized heavy water reactors burning natural uranium — is largely complete, with 24 operational reactors across the country. Stage two, now beginning at Kalpakkam, uses fast breeders to produce plutonium and eventually breeds uranium-233 from thorium. Stage three, still decades in the future, would close the fuel cycle entirely by burning thorium directly. Nuclear experts say that if all three stages succeed, India would have one of the world's most energy-secure electricity systems, independent of global uranium markets.