India becomes second nation after Russia to achieve nuclear criticality: Here's what it means

This marks the beginning sustained nuclear fission chain reaction and brings the reactor one decisive step away from full operation.

On Monday, India took a massive leap in the nuclear race, becoming only the second nation after Russia to reach nuclear criticality. This was achieved at the country's most advanced atomic reactor -- the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam in Tamil Nadu.

This marks the beginning of sustained nuclear fission chain reaction and brings the reactor one step away from full operation.

"This milestone marks a significant step toward strengthening India’s long-term energy security and advancing its indigenous nuclear technology capabilities," the Department of Atomic Energy said in a statement.

With the milestone, India completes second stage of Dr Homi Jehangir Bhabha's three-stage nuclear plan outlined in 1954. The vision aims to achieve self-reliance in energy production by utilising the nation's vast thorium reserves, overcoming limited domestic uranium resources.

What are the three stages?

The first stage uses Natural Uranium (NU) to fuel Pressurized Heavy Water Reactors (PHWRs), producing electricity and Plutonium-239 as a byproduct.

The second stage, that India recently achieved, utilises FBRs fuelled by a mixture of Plutonium-239 and Uranium-238, which produces more fuel than it consumes. The same produced fuel can be reused to produce more fuel and the cycle continues.

The final stage, which is the future of nuclear energy, uses advanced heavy water reactors to harness Uranium-233, produced by converting Thorium-232, to provide sustainable, large-scale electricity. Reports suggest achieving this could power India's electricity needs for centuries.

What is nuclear criticality?

Criticality, in nuclear terms means the point at which the reactor achieves a self-sustaining nuclear fission reaction, where the number of neutrons produced is sufficient to keep the reaction going without external intervention.

However, this does not mean that the reactor is generating electricity at full capacity, but is a crucial milestone in commissioning any nuclear plant.

The PFBR is a 500 megawatt electric sodium-cooled fast breeder reactor and was in the pipeline for more than two decades.

Unlike conventional thermal reactors, the PFBR uses Uranium-Plutonium Mixed Oxide (MOX) fuel. It is surrounded by a blanket of Uranium-238, which, through neutron absorption, is converted into fissile Plutonium-239, the Department of Atomic Energy said in a statement.

"This enables the reactor to generate more fuel than it consumes. The reactor is also designed for future utilization of Thorium-232, which can be transmuted into Uranium-233 to support the third stage of India’s nuclear power programme," the statement added.

In simpler terms, the spent fuel generated in the PFBR will be processed and recycled back into the reactor.

This will pave the way for utilisation of the abundant thorium reserves (India houses about 25 per cent of the world's thorium reserves) in the third stage of the nuclear power programme, comprising of thorium-based reactors.

Achieving criticality involves some of the most complex engineering and materials challenges faced by the Department of Atomic Energy.

Atomic Energy Commission Chairman and Department of Atomic Energy Secretary Dr Ajit Kumar Mohanty said "This 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."

What is Prototype Fast Breeder Reactor?

PFBRs are different from conventional nuclear reactors. Traditional reactors consume uranium fuel, while PFBRs use plutonium as fuel and are designed to produce more fissile material than they consume by converting fertile uranium-238 into plutonium-239.

Plutonium produced in pressurised heavy water reactors is used in fast breeder reactors, paving the way for the third stage where Thorium will be used as fuel.  

This capability has earned PFBRs the description of an "Akshay Patra" of energy, symbolising a near-limitless source of fuel.

India's PFBR is desgined and built indigenously, while also incorporating advanced safety systems, high-temperature liquid sodium coolant technology, and a closed fuel cycle approach, enabling efficient recycling of nuclear materials, improved sustainability, and reduction in radioactive waste.

Once fully operational, the PFBR could be considered for commercial viability of fast breeder technology in India.

Only Russia currently operates a functional commercial fast breeder reactor. Several other countries had pursued breeder reactor programmes but suspended it due to cost, safety concerns, and technological complexity.