India has reached a major milestone in its civil nuclear energy programme with its most advanced atomic reactor, the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam in Tamil Nadu, attaining criticality. This marks the beginning of a sustained nuclear fission chain reaction inside the reactor and brings it a step closer to full-scale power generation. The development is seen as a crucial breakthrough after years of effort in building and testing the complex system.

Prime Minister Narendra Modi called the achievement a defining moment in India’s nuclear journey. "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," the Prime Minister said in a statement. He added that the achievement reflects "the depth of our scientific capability and the strength of our engineering enterprise," and is a decisive step towards harnessing India's vast thorium reserves in the future.

In nuclear science, criticality refers to the stage where a reactor can maintain a self-sustaining chain reaction. At this point, the number of neutrons produced is enough to continue the reaction without external support. This does not mean the reactor has started generating electricity at full capacity. However, it is one of the most important milestones in the commissioning process of any nuclear plant.

The PFBR is a 500 megawatt electric sodium-cooled fast breeder reactor. It has taken over two decades to develop due to the technological and engineering challenges involved. The reactor has been built and is operated by Bharatiya Nabhikiya Vidyut Nigam Limited under the Department of Atomic Energy. It is located at the Indira Gandhi Centre for Atomic Research campus.

Describing the development as historic, Atomic Energy Commission Chairman and Department of Atomic Energy Secretary Ajit Kumar Mohanty said, "Today, India achieved yet another historic milestone in the country's peaceful usage of atomic energy by entering the second stage of the three-stage nuclear power programme envisioned by Dr. Homi Jehangir Bhabha, the architect of our country's nuclear power programme. The Prototype Fast Breeder Reactor, a 500 megawatt electrical nuclear reactor, constructed by Bharatiya Navbhikiya Vidyut Nigam Ltd., Kalpakkam in Tamil Nadu, has achieved its first criticality i.e. initiation of sustained nuclear chain reaction. The achievement reinforces Department of Atomic Energy's contribution to India's commitment to achieve the net zero emission target by 2070, as announced by Prime Minister Narendra Modi. The Prototype Fast Breeder Reactor utilises fissile material recovered from the reprocessing of spent fuel from first-stage Pressurised Heavy Water Reactors (PHWR). The Prototype Fast Breeder Reactor will produce more fuel than what it consumes," Mohanty said.

He added, "The spent fuel generated in the Prototype Fast Breeder Reactor (PFBR) will, in turn, be processed and recycled back into the reactor, thereby enabling closure of the second stage fuel cycle. This will pave the way for utilisation of the abundant thorium reserves in the third stage of the nuclear power programme, comprising of thorium-based reactors. The Prototype Fast Breeder Reactor was designed and built indigenously, reflecting decades of dedicated efforts by our scientists, engineers and industry partners."

Fast breeder reactors work differently from conventional nuclear reactors. Traditional reactors mainly use uranium as fuel and gradually consume it. In contrast, breeder reactors use plutonium and are designed to generate more fissile material than they consume. They do this by converting uranium-238 into plutonium-239. This unique feature allows them to produce additional fuel during operation. Because of this ability, the PFBR is often described as an "Akshay Patra" of energy, suggesting a nearly endless fuel source.

Globally, only Russia currently operates a commercial fast breeder reactor. This highlights the complexity and exclusivity of the technology. Several other nations had earlier explored similar programmes but discontinued them due to high costs, safety concerns, and technical challenges.

The achievement of criticality officially marks India’s entry into the second stage of its three-stage nuclear power programme. This long-term strategy was originally planned by Homi Jehangir Bhabha. In the first stage, pressurised heavy water reactors produce plutonium. In the second stage, this plutonium is used in fast breeder reactors like the PFBR. The third stage will focus on thorium-based reactors. India has one of the largest thorium reserves in the world, making this stage crucial for long-term energy security.

The milestone also supports India’s climate goals. The government aims to expand nuclear power capacity to 100 gigawatts by 2047. This is part of a broader effort to shift towards cleaner and low-carbon energy sources. Fast breeder reactors are expected to play an important role in this transition by ensuring energy security while reducing carbon emissions.

Once fully operational, the PFBR will demonstrate the commercial viability of fast breeder technology in India. It is also expected to serve as a model for future reactors across the country. The success of this project represents not just scientific progress but also a major step towards energy self-reliance.

Calling it a moment of national pride, the Prime Minister congratulated scientists and engineers across the country. "A proud moment for India," he said. "Congratulations to our scientists and engineers."