At 8.25 pm on April 6, inside a heavily shielded complex along Tamil Nadu’s coast, a quiet but defining moment unfolded in India’s nuclear journey. The 500 MWe Prototype Fast Breeder Reactor (PFBR) at Kalpakkam attained “first criticality”, the point at which a nuclear reactor sustains a controlled chain reaction on its own. There were no dramatic outward signs, no immediate surge of electricity into the grid, but in nuclear engineering terms, this was the instant the reactor came alive, a moment some scientists liken to an “Akshaya Patra” of energy, a system that, once stabilised, can keep producing more fuel than it consumes. In common parlance, the term “critical” often suggests danger, but in nuclear science it signifies stability.
Criticality is achieved when each fission event releases enough neutrons to sustain exactly one more fission event, creating a steady, self-sustaining chain reaction.
At this point, the reactor no longer depends on external inputs to keep the process going. It marks the transition from a constructed system to an operational one.
As veteran nuclear scientist V S Ramamurthy explains, “Once the system becomes critical, it is self-sustaining - you don’t have to introduce a new neutron.” However, he cautions that this is only the beginning. “At this stage, the power is very low because the number of neutrons participating is small. What is required is to increase that number.” This distinction is crucial.
Achieving criticality does not mean the reactor is producing electricity at scale. Instead, it confirms that the physics of the system is functioning as designed. The reactor will now undergo a series of physics experiments to understand its behaviour in controlled conditions before operators gradually increase output. The transition from a stable chain reaction to high-power electricity generation is technically complex and fraught with challenges. “The jump from criticality to full power is very large,” Ramamurthy told TNIE. “As the power increases, temperatures go up. You must be able to take out that heat and convert it into electricity. That is where unknowns can arise.”
He added that this phase represents largely uncharted territory, where realworld conditions can differ from theoretical expectations. What makes the PFBR particularly significant is not just that it works, but how it works. Unlike conventional nuclear reactors that consume uranium fuel, the PFBR is designed to produce more fuel than it burns.
It uses a mix of uranium and plutonium, known as mixed oxide or MOX fuel, at its core, surrounded by a blanket of uranium-238. When fast neutrons from the fission process interact with this blanket, they convert uranium-238 into plutonium-239, effectively generating new fuel in the process. This is why such systems are known as “breeder” reactors.
Scientists estimate that the reactor can achieve a breeding ratio greater than one, meaning it produces more fissile material than it consumes. In a country like India, where uranium reserves are limited, this capability is strategically transformative because it allows far greater energy extraction from available resources.
To fully appreciate the importance of this milestone, one must return to the long-term vision articulated by Homi Jehangir Bhabha in the early years after Independence. India’s nuclear programme was designed as a threestage pathway tailored to its resource profile. The first stage relies on pressurised heavy water reactors that use natural uranium and produce plutonium as a by-product.
The second stage, which India has now entered with the PFBR, uses that plutonium in fast breeder reactors to generate more fissile material. The third and final stage aims to use thorium, abundant in India, to produce uranium-233, which can serve as a long-term nuclear fuel.
The central logic of this strategy lies in India’s resource constraints and advantages. While uranium deposits are modest, the country possesses some of the world’s largest thorium reserves, particularly in coastal regions such as Tamil Nadu, Kerala, Odisha and Andhra Pradesh. Thorium, however, cannot be used directly as a nuclear fuel. It must first be converted into uranium-233 through nuclear reactions.
W Selvamurthy, president of the Amity Science, Technology and Innovation Foundation, said thorium cannot be used by itself. It has to be converted into uranium-233 before it can produce power. That is where this system comes into the picture. “Thorium offers India the possibility of long-term energy security spanning centuries once the full fuel cycle is realised. The PFBR thus plays a pivotal role as the bridge that enables this conversion pathway, linking India’s current uranium-based reactors to its future thorium-based ambitions.”
The journey to this point has been long and complex. The PFBR project, designed by the Indira Gandhi Centre for Atomic Research and built by Bharatiya Nabhikiya Vidyut Nigam Ltd, was expected to be completed over a decade ago. Instead, it faced repeated delays due to technical challenges, regulatory scrutiny and the inherent difficulty of developing a first-of-its-kind reactor. Engineers had to overcome challenges related to handling liquid sodium at high temperatures, developing specialised materials, sensors and control systems, and generating entirely new datasets where no prior operational experience existed.
“This is not something you can purchase,” Ramamurthy noted. “You have to develop it yourself. Only a few countries have mastered this technology.”
The Atomic Energy Regulatory Board granted clearance for the reactor’s “First Approach to Criticality” only after extensive multi-layered safety evaluations, reflecting the cautious and iterative approach adopted throughout the project. One of the distinctive features of the PFBR is its use of liquid sodium as a coolant instead of water.
Sodium enables operation at higher temperatures and improves thermal efficiency, but it also reacts vigorously with air and water, leading to concerns about safety. Scientists involved in the programme say these risks are mitigated through a “defence-indepth” approach, with multiple layers of containment and specialised systems designed to ensure sodium remains isolated within the reactor circuit.
Decades of experience in sodium handling at Kalpakkam have contributed to the development of sensors, materials and mitigation techniques tailored to such environments. Ramamurthy dismissed alarmist interpretations of risk.
“Nothing is absolutely safe,” he said. “Safety is about ensuring that the system does not go out of control. Criticality and safety are not the same.”
In that sense, the achievement of criticality reflects not just reactor physics, but also the robustness of the safety architecture built around it. Over the coming months, the PFBR will undergo a carefully staged process of power escalation. Engineers will monitor system behaviour, test safety mechanisms and validate performance under increasing loads.
Even under favourable conditions, this process could take considerable time, as each increment in power introduces new operational variables that must be carefully managed. If successful, however, the implications are far-reaching.
India would join a small group of countries, led by Russia, that have demonstrated fast breeder reactor technology at a commercial scale. More importantly, it would strengthen India’s control over the nuclear fuel cycle, reduce dependence on imported uranium and bring the country closer to realising its long-term thorium-based energy strategy.
The closed fuel cycle, in which spent fuel is reprocessed and reused, is central to this capability, allowing the country to maximise the utility of its nuclear materials. At a time when India is seeking to expand its clean energy portfolio, nuclear power offers a reliable source of low-carbon electricity.
Fast breeder reactors, by improving fuel efficiency and enabling resource sustainability, could play a critical role in this transition while supporting the country’s broader climate commitments, including its net-zero target. Yet, experts remain cautious about timelines and scalability. Commercialising the technology, building multiple reactors and developing the infrastructure for thoriumbased fuel cycles will require sustained effort over decades. For now, the achievement of criticality stands as both a culmination and a beginning. It represents the realisation of a vision conceived in the early years of Indian independence and pursued through decades of scientific and engineering effort.
At the same time, it marks the start of a new and more demanding phase in which theory must be translated into reliable, large-scale power generation. The chain reaction inside the Kalpakkam reactor is now self-sustaining. But the larger reaction, the transformation of India’s energy future, is only just gathering momentum.