India's 500 MWe Prototype Fast Breeder Reactor at Kalpakkam Reached First Criticality at 8:25 pm on 6 April 2026, Opening the Second Stage of Homi Bhabha's Three-Stage Nuclear Plan

The Prototype Fast Breeder Reactor (PFBR) at Kalpakkam in Tamil Nadu reached first criticality at 8:25 pm on 6 April 2026, according to a Department of Atomic Energy (DAE) release published on 7 April. Criticality is the point at which a nuclear chain reaction sustains itself, with the neutrons made by fission matching those lost, as the Press Information Bureau (PIB) defines it in its backgrounder. The reactor is rated at 500 MWe, meaning 500 megawatts of electricity. The Indira Gandhi Centre for Atomic Research (IGCAR) developed its technology, and Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI), a public sector company under the DAE, built and commissioned it. The release says it was made mostly with indigenous technologies and components.

The Atomic Energy Regulatory Board (AERB), the nuclear safety regulator, issued its clearance after reviewing the safety of the plant systems, the DAE says. PIB states that India has entered the second stage of the three-stage programme first conceived by Homi Bhabha, and that once the reactor is fully operational India will be the second country after Russia to operate a commercial fast breeder reactor.

What a fast breeder does that other reactors do not

Most power reactors use a moderator, a material such as water that slows neutrons so they split uranium-235 atoms efficiently. A fast reactor has no moderator and relies on fast neutrons alone, according to the World Nuclear Association (WNA). It is cooled by liquid metal, normally sodium, and the WNA says fast reactors run at about 500 to 550 degrees Celsius at close to atmospheric pressure.

The PFBR core uses mixed oxide (MOX) fuel, a blend of uranium and plutonium oxides. PIB says the plutonium is recovered by reprocessing spent fuel from India’s pressurised heavy water reactors. A blanket of uranium-238 surrounds the core. The DAE says fast neutrons convert that material into plutonium-239, so the reactor produces more fuel than it consumes.

The WNA gives the conversion ratio of a normal reactor as around 0.6 and says a fast reactor may exceed 1.0. It adds that fast reactors can use uranium about 60 times more efficiently, and that this pays for the extra cost of building and reprocessing only if uranium prices are reasonably high or the aim is to burn long-lived waste.

Sodium needs careful handling. The WNA says designs must prevent chemical interaction, especially between sodium and water. Japan’s Monju reactor lost about three tonnes of sodium from its secondary cooling system in 1995 and the leak caught fire. Monju was shut for almost 15 years, and Japan decided in December 2016 to decommission it.

Where India’s reactor sits among the world’s fast reactors

The WNA says Russia’s BN-600 has supplied electricity to the grid since 1980 and that the BN-800 began operating with a full MOX core in September 2022. It lists China’s 600 MWe CFR-600 at Xiapu, where construction began in December 2017. It also records that the United States shut its EBR-II fast reactor in 1994, that France closed Superphenix in 1998 on political grounds, and that by the 1980s fast reactors were not commercially competitive with light water reactors.

Why India built one: the thorium arithmetic

The DAE’s three-stage plan was formulated by Bhabha, the Nuclear Power Corporation of India Limited (NPCIL) says, to link two fuel cycles using India’s limited uranium and large thorium reserves. Stage one is pressurised heavy water reactors on natural uranium, whose spent fuel is reprocessed to obtain plutonium. Stage two is fast breeders fuelled by that plutonium, which also breed uranium-233 from thorium. Stage three is reactors using uranium-233 and thorium. Bhabha’s career and the dates of the plan are covered in Homi Bhabha and India’s atomic programme.

The WNA puts India’s reasonably assured thorium resources at 319,000 tonnes, about 13 per cent of the world total. Thorium-232 cannot be used as it comes. In the PFBR design it is meant to sit in the blanket and turn into uranium-233, the fuel for stage three, and the DAE release says only that the reactor is designed to use it “eventually”. A World Nuclear News report of 1 August 2024 says the first core carries a uranium-238 blanket, with a uranium and thorium blanket planned for later.

The long road from 2003 to 2026

IGCAR’s first sodium-cooled fast reactor, the Fast Breeder Test Reactor at Kalpakkam, reached first criticality in 1985. An IGCAR event on 18 October 2025 marked 40 years since, and Dinamalar reports that the test reactor is rated at 40 MWt (megawatts of heat), or 13.6 MWe, with a mixed plutonium-uranium carbide core.

The government approved BHAVINI in 2003. R. Ramachandran, writing in The Wire in 2020, records that the PFBR project was sanctioned in September 2003 at Rs 3,492 crore, for completion by September 2010. The WNA says the regulator approved construction in 2002 and that building began in 2004.

The dates then moved. Ramachandran reports a revised sanction in April 2012 at Rs 5,677 crore, with completion due by September 2014. BHAVINI gave the tsunami damage, rising material and labour costs, design changes and taxes as reasons for delay, he writes. In a written reply to the Lok Sabha reported by PTI on 15 December 2021, the government said a revised completion cost of Rs 6,840 crore had been approved in 2021, with a target of October 2022. The International Panel on Fissile Materials (IPFM), a research group, reports that a parliamentary standing committee in 2026 put the cost at Rs 8,181 crore.

Core loading began on 4 March 2024 with the Prime Minister present, PTI reported. The official statement that day said more than 200 Indian industries, including small and medium enterprises, contributed to the reactor. The AERB then permitted the first approach to criticality, which covers loading fuel and starting low-power physics experiments, with its statement dated 27 July 2024.

In a written reply on 5 August 2025, the Atomic Energy Minister told the Lok Sabha the delay was “mainly due to first-of-a-kind technological issues” found during integrated commissioning. IPFM adds that in April 2025 DAE officials told a parliamentary committee that first criticality was expected in March 2026 and the plant would be completed by September 2026. Criticality came in the first week of April.

What first criticality does and does not mean

PIB describes criticality as the transition from construction to operation and the first step towards generating heat and, ultimately, electricity. The DAE release gives no timeline for grid connection, commercial operation or thorium use in the blanket. The PIB backgrounder does not either.

The reactor’s spent fuel is meant to be reprocessed and recycled, according to PIB. That needs a separate plant, the Fast Reactor Fuel Cycle Facility, which a December 2021 Lok Sabha reply said stood at 32 per cent financial progress as of 30 November 2021, with completion expected in December 2027. IPFM reported on 7 April 2026 that the parliamentary committee says the facility was first due in 2014 and is expected in December 2029.

Assessments differ on what the delays mean. IPFM calls the reactor more than 15 years behind schedule and treats the delays and cost growth as typical of fast breeders elsewhere. Prateek Tripathi of the Observer Research Foundation wrote in October 2024 that capital and per-unit electricity costs are comparable to other nuclear and conventional plants, and that commercial fast breeders are likely decades away, citing a 2019 DAE statement.

What comes next

The December 2021 Lok Sabha reply said Rs 250 crore had been allocated for pre-project work on two more fast breeder reactors, FBR-1 and FBR-2, at Kalpakkam, with detailed design under way at IGCAR using feedback from PFBR commissioning, and no sanction yet as of that date. The WNA lists two more 500 MWe units for Kalpakkam as announced.

PIB gives India’s nuclear capacity as 8.78 GW as of 7 April 2026, and says nuclear plants generated 56,681 million units in 2024 to 2025, 3.1 per cent of total generation. It projects 22.38 GW by 2031 to 2032 and cites the Nuclear Energy Mission, announced in the Union Budget for 2025 to 2026, with a goal of 100 GW by 2047. Nuclear power’s place beside solar and hydro is covered in India’s non-fossil power capacity.

Stage three reactors had not been built as of the Lok Sabha reply of 5 August 2025, in which the minister reported that the Advanced Heavy Water Reactor was in design validation and not formally launched, and that the Molten Salt Reactor was in proof-of-concept work.