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India’s first indigenous Fast Breeder Reactor (FBR) at Kalpakkam has attained criticality, marking a crucial milestone in the country’s long-term nuclear energy roadmap.
The development indicates that the reactor has successfully achieved a self-sustaining nuclear chain reaction, which is a key step before it can begin generating electricity at scale.
This milestone is being seen as a defining moment because it advances the second stage of India’s three-stage nuclear programme, which is designed to ultimately unlock the country’s vast thorium reserves for energy generation.
What Does ‘Criticality’ Mean?
Criticality refers to the stage where a nuclear reactor achieves a stable, self-sustaining fission process. In simple terms, each atomic split produces
enough neutrons to trigger further reactions at a steady rate.
This does not mean the reactor is producing electricity yet, but it confirms that the core is functioning as intended. The reactor will now undergo further testing and calibration before being connected to the power grid.
A great accomplishment! This is a high-tech and powerful reactor, bigger than the Clinch River Breeder the US tried but failed to deliver in the 70s-80s. Breeder technology has long been considered the ultimate goal for nuclear fission. Often demonstrated, not yet done… https://t.co/BrndnTx1ZM
— Nick Touran (@whatisnuclear) April 7, 2026
What Makes The Kalpakkam Reactor Unique?
The 500 MWe Prototype Fast Breeder Reactor stands apart from conventional reactors in how it uses and produces fuel.
Unlike standard reactors that rely on slowing down neutrons, this reactor uses fast-moving neutrons and liquid sodium as a coolant. It runs on a mix of uranium and plutonium fuel, while also converting surrounding material into new fuel through a process known as nuclear transmutation.
In effect, it produces more fuel than it consumes, a defining feature of breeder reactors and a capability that allows for more efficient use of nuclear material.
Why This Is Key To India’s Three-Stage Plan
India’s nuclear programme has been designed in three stages to maximise limited uranium reserves while tapping into abundant thorium deposits.
Stage 1: Pressurised heavy water reactors using natural uranium, which also generate plutonium as a by-product
Stage 2: Fast breeder reactors like the one at Kalpakkam, which use plutonium and generate more fissile material
Stage 3: Thorium-based reactors, which will use the fuel generated in earlier stages
The West poured $50 billion into fast breeder nuclear reactors and abandoned every single one. India poured $900 million and just achieved criticality on the first commercially viable one outside Russia.
The US spent $15 billion. Gave up. Japan spent $12 billion. Their Monju… https://t.co/29D1ufZ9kR
— Aakash Gupta (@aakashgupta) April 7, 2026
The Kalpakkam reactor is central to bridging the first and third stages, as it helps to build the necessary fuel base for future thorium reactors.
Why Thorium Matters
India has limited uranium reserves but one of the world’s largest thorium deposits, especially in coastal regions.
The three-stage programme is designed to convert this thorium into usable nuclear fuel over time, reducing dependence on imported uranium and strengthening long-term energy security.
PM’s Praise For Project
The Bigger Picture
The successful criticality of the Kalpakkam FBR reflects decades of scientific effort and indigenous capability in nuclear technology. It also places India among a select group of countries with advanced breeder reactor programmes.
More importantly, it moves India closer to a long-term goal of achieving energy independence through efficient use of its own nuclear resources.
