Yesterday evening, at 8:25 PM Indian Standard Time, the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam in Tamil Nadu achieved first criticality — the initiation of a self-sustaining, controlled nuclear fission chain reaction. It is a milestone that has been a long time coming: more than two decades in the making, marked by repeated delays, cost overruns, and hard-won engineering lessons. But it has arrived. India has now entered Stage II of the three-stage nuclear programme that physicist Homi Bhabha conceived in the early 1960s, and the path to thorium energy just got measurably shorter.

What the PFBR Is and Why It Matters

The PFBR is a 500 MWe sodium-cooled, pool-type fast breeder reactor. Unlike conventional light-water reactors, which use slow (thermal) neutrons and consume uranium-235, the PFBR uses fast, unmoderated neutrons to sustain fission. Its core is loaded with uranium-plutonium mixed oxide (MOX) fuel, and it is surrounded by a fertile blanket of uranium-238. As the reactor operates, fast neutrons convert that uranium-238 into fissile plutonium-239 — producing more fuel than it consumes. That is the “breeder” in its name.

For readers of this blog, the deeper significance is in what comes next. The PFBR is also designed to accept a blanket of thorium-232 in place of uranium-238. When thorium-232 absorbs a fast neutron, it transmutes to uranium-233 — the fissile fuel that will drive Stage III of India’s nuclear programme, a thorium-based closed fuel cycle that could power the country for centuries. The fast breeder reactor is not the end of the road. It is the bridge.

India holds roughly 25 percent of the world’s thorium reserves, mostly in coastal sands, and comparatively modest uranium resources. The Bhabha three-stage strategy was designed precisely for this resource reality: use uranium in Stage I pressurized heavy water reactors to generate electricity and produce plutonium; use that plutonium to fuel Stage II fast breeder reactors that breed yet more fissile material; and finally use the accumulated uranium-233 from thorium blankets to power Stage III advanced heavy water reactors running on a thorium fuel cycle. Kalpakkam’s criticality is the moment Stage II stopped being a plan and became a physical reality.

A Long Road: The PFBR Timeline

The journey to this moment was neither smooth nor swift. Construction began in 2004 with a projected completion date of September 2010. What followed was a sequence of deferrals that stretched across sixteen years, driven by the formidable engineering challenges of working with first-of-a-kind sodium coolant systems, complex fuel handling machinery, and a regulatory environment that rightly demanded thoroughness.

From groundbreaking to criticality: 22 years. From the original target completion date to actual criticality: 16 years. These numbers deserve honest acknowledgment, and they have prompted fair criticism — including from MIT nuclear engineering professor Koroush Shirvan, who noted that China recently built a slightly larger plutonium fast breeder reactor in approximately six years. India’s indigenous approach, working without foreign technology transfer and developing its own sodium systems, fuel handling, and safety protocols, extracted a steep price in time and money. But it also produced something of enduring strategic value: a domestic industrial and scientific capability in fast breeder technology that cannot be embargoed or sanctioned away.

What Comes Next

First criticality is not the same as commercial operation. The PFBR will now enter a phased power ascension process, with low-power physics experiments followed by stepped increases in reactor power, safety validation at each level, and ultimately connection to the electrical grid. Commercial electricity generation is expected by September 2026, pending regulatory approval at each stage.

Beyond the PFBR itself, India’s plans are substantial. Two additional 600 MWe fast breeder reactors are already planned for construction at Kalpakkam, and four more at sites yet to be determined, all contingent on satisfactory PFBR performance. A dedicated Fast Reactor Fuel Cycle Facility (FRFCF) is under construction at the Kalpakkam site to handle reprocessing and fuel fabrication for this expanding fleet. When fully operational, the PFBR will make India only the second country after Russia to operate a commercial-scale fast breeder reactor.

For the thorium community, the most important watch item is the blanket configuration. In its initial operating mode, the PFBR blanket will contain uranium-238, breeding plutonium-239. The transition to a thorium-232 blanket — which will breed the uranium-233 needed for Stage III — will come in a subsequent phase of the programme. That is where the long arc of Bhabha’s vision finally closes: thorium in, uranium-233 out, and a fuel cycle that could sustain India’s energy needs for generations.

A Patient Victory

Twenty-two years is a long time to build a reactor. It is worth pausing to appreciate what India has accomplished nevertheless. The PFBR was designed and built entirely with indigenous technology, by IGCAR (Indira Gandhi Centre for Atomic Research) and BHAVINI (Bharatiya Nabhikiya Vidyut Nigam Limited), without the kind of foreign technology partnerships that other nations have relied upon. The sodium coolant systems, the safety architecture, the fuel fabrication infrastructure — all of it developed in-house, in a country that had no prior commercial fast reactor experience beyond the small FBTR research reactor.

The delays were real. The cost overruns were real. But so is the reactor. On the evening of April 6, 2026, neutrons multiplied in a controlled chain in the core of the PFBR, and India crossed a threshold that almost no other nation has ever crossed. The bridge to thorium is open.

The PFBR achieved first criticality on 6 April 2026. It was designed by the Indira Gandhi Centre for Atomic Research (IGCAR) and built by Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI), both under India’s Department of Atomic Energy. Commercial electricity generation is projected for September 2026.