Energy-hungry India, the world's most populous country and third-largest emitter of greenhouse gases, has ambitious plans to expand nuclear power capacity from its current eight to 100 gigawatts by 2047.

Prime Minister Narendra Modi said the Prototype Fast Breeder Reactor (PFBR) at Kalpakkam attained "criticality", the stage at which a self-sustaining nuclear chain reaction starts.

"Today, India takes a defining step in its civil nuclear journey, advancing the second stage of its nuclear programme," Modi said in a statement late Monday.

"This advanced reactor, capable of producing more fuel than it consumes, reflects the depth of our scientific capability and the strength of our engineering enterprise."

Modi called it a "decisive step towards harnessing our vast thorium reserves", a potential fuel for nuclear reactors.

The reactor does not yet generate electricity for the grid. That comes in the next stages, once the reactor moves to full power operation.

The development also comes at a time of global energy uncertainty, including war in the Middle East, which has heightened concerns about fuel supply disruptions.

India remains heavily dependent on coal, but has pledged to reach net-zero emissions by 2070.