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 nucleus of an atom into two lighter nuclei in a process called fission, releasing large volume of heat. This heat is used to make steam, which runs turbines to produce electricity. All heavy nuclei have the ability to fission, but only a few do so readily. These are called fissile. The most commonly used fissile nuclides in the nuclear industry are uranium and plutonium. While India has limited reserves of uranium, it has one of the world’s largest reserves of thorium—a weaker radioactive element which cannot be directly used in fission. But thorium can be turned into a variant of uranium that can be used to produce electricity.
India has a three-stage nuclear energy programme that aims to use thorium as a fuel. In Stage 1, uranium is used as a fuel to generate power in pressurised heavy water reactors. The spent fuel from the first stage produces plutonium, which can be used as fuel in FBR in Stage 2. The Kalpakkam FBR marks India’s entry into this stage. These reactors will be used to breed uranium from thorium, laying groundwork for Stage 3 (thorium-based reactors), which will run on the thorium-derived uranium from Stage 2.
India started construction of FBR in 2004, with the target to attain criticality by 2010. The fact that it took 22 years, when it was supposed to take six years, is “an indication that these reactors are quite complicated”, M V Ramana, professor and Simons Chair in Disarmament, Global and Human Security at the School of Public Policy and Global Affairs, University of British Columbia, Canada, tells Down To Earth (DTE). “This is a very important step. It was visualised by Homi Bhabha as the second step towards not only energy security, but towards energy independence for our country,” Shah Nawaz Ahmad, senior adviser, India, Middle East and South-East Asia at London-based World Nuclear Association, tells DTE.
According to the World Nuclear Association, about 9 per cent of the world’s electricity is produced from nuclear energy, while the figure for India is 3 per cent, as of March 31, 2026, according to Central Electricity Authority data. India’s current nuclear capacity stands at ...
This article was originally published in the May 16-31, 2026 print edition of Down To Earth