India has brought a long-delayed but strategically important nuclear project to a key operational milestone. [some emphasis, links added]
The 500 MWe Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, Tamil Nadu, has now reached criticality, signaling that the reactor core has begun sustaining a controlled fission process.
The development places India among a small group of nations operating fast breeder reactor technology at this scale.
Indian Prime Minister Narendra Modi announced the milestone on X, framing it as a major step forward in the country’s nuclear roadmap.
Today, India takes a defining step in its civil nuclear journey, advancing the second stage of its nuclear programme.
The indigenously designed and built Prototype Fast Breeder Reactor at Kalpakkam has attained criticality.
This advanced reactor, capable of producing more fuel…
— Narendra Modi (@narendramodi) April 6, 2026
Built for Fuel Multiplication
Unlike conventional reactors, the PFBR is designed to create more fuel than it burns.
It runs on mixed oxide fuel, combining uranium-238 and plutonium-239, and uses liquid sodium as a coolant.
This configuration allows the reactor to convert fertile material into fissile fuel during operation.
Engineers consider this capability critical for sustaining long-term nuclear energy programs.
The reactor was designed by the Indira Gandhi Centre for Atomic Research and built by Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI).
Both operate under India’s Department of Atomic Energy.
The project has faced delays and cost escalations since construction began in 2004.
Engineers also adapted the fuel-handling system after technical issues with the original transfer mechanism, switching to an alternate approach.
Strategic Shift Toward Thorium
India’s nuclear program follows a phased structure to manage limited uranium reserves.
Fast breeder reactors anchor the second stage of this plan.
The PFBR is expected to generate fissile material that supports a future transition to thorium-based fuel cycles.
India holds some of the world’s largest thorium reserves, making this shift strategically significant.
h/t Steve C
Read rest at Interesting Engineering
The Thorium Fuel Cycle (Th-232 absorbing a neutron and in short period of time decays to U-233 which is fissile) had been proven in a High Temperature Gas Cooled Commercial Reactor outside Platteville, CO, back in the ’80s but has not been used in any other commercial reactors since. The great thing about Thorium is that the only isotope is Th-232 so no enrichment required as compared to Uranium in which the only fissile isotope is U-235 which makes up just 0.72% of Uranium which means it requires expensive enrichment to even get it to approximately 5% that is typical in a commercial reactor.