India, May 19 -- After over two decades of delay, redesigns and technical changes, the 500 MW Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, Tamil Nadu, attained criticality on April 6, 2026, becoming India's first operational fast breeder reactor (FBR) and a long-awaited milestone in the country's three-stage nuclear energy programme. Criticality is the point at which a sustained, controlled nuclear fission chain reaction begins.

FBR is especially attractive to India because the reactor breeds more fuel than it uses while generating power. It will allow India to use its huge thorium reserves to produce uranium to fuel the final phase of its three-stage nuclear energy programme (see 'Energy plans',).

A nuclear reactor splits the nu...