Kalpakkam fast breeder reactor attains criticality

India’s first prototype fast-breeder reactor (PFBR) in Tamil Nadu‘s Kalpakkam on Monday (April 6, 2026) came another step forward in its civil nuclear journey as it attained criticality.

First criticality in a fast breeder reactor is the moment when the nuclear chain reaction becomes self-sustaining. A breeder reactor is a nuclear reactor that produces more fissile material than it consumes. In a ‘fast’ breeder reactor, the neutrons aren’t slowed, allowing them to trigger specific fission reactions.

The PFBR is a machine that produces more nuclear fuel than it consumes. Its core-loading event is being hailed as a “milestone” because the operationalisation of the PFBR will mark the start of stage II of India’s three-stage nuclear power programme.

The PFBR in Kalpakkam is the first-of-its-kind nuclear reactor to use plutonium-based mixed oxide as fuel and liquid sodium as coolant. It will also utilise the spent fuel of Pressurised Heavy Water Reactors, which form the mainstay of nuclear power in India at present.

In a post on X, Prime Minister Narendra Modi on Monday said this is a proud moment for the country. Today, India takes a defining step in its civil nuclear journey, advancing the second stage of its nuclear programme,” he said.

“The indigenously designed and built Prototype Fast Breeder Reactor at Kalpakkam has attained criticality,” he added.

Mr. Modi said this advanced reactor, capable of producing more fuel than it consumes, reflects the depth of Indian scientific capability and the strength of the country’s engineering enterprise. “It is a decisive step towards harnessing our vast thorium reserves in the third stage of the programme. A proud moment for India. Congratulations to our scientists and engineers,” he said.

The loading of fuel at the Kalpakkam PFBR had started in October last year.

(With inputs from PTI)