India’s PFBR attains criticality at last

Prime Minister Narendra Modi proclaimed it "a proud moment for India" when on April 6 the 500-MWe, sodium-cooled Prototype Fast Breeder Reactor (PFBR) achieved initial criticality. This milestone, which comes some 22 years after the continually delayed PFBR project began, marks India's entrance into the second stage of its three-stage nuclear program, which has the ultimate goal of supporting the country's nuclear power program with its significant thorium reserves.

The context: The story of the PFBR starts all the way back in the 1950s-but first, a look at India's modern-day nuclear sector can illustrate why the country has invested so much time into this project.

Today, there are 25 commercial power reactors operating in India, the world's seventh largest nuclear fleet (by number of reactors). However, with 1.4 billion people, India is the most populous country in the world, meaning that, despite the size of its fleet, it actually has the lowest nuclear capacity per capita of all nuclear-powered countries. Its operating capacity per 1 million persons is about 6 MWe. (For context, the U.S. capacity per million persons is 285 MWe, while France's is 921 MWe.)

What's more, India's small reserve of uranium forces the country to rely heavily on imports to fuel these reactors. Taken in sum, India's nuclear sector currently has neither the scale nor independence that the country is aiming to build.

These present challenges, however, are anything but unforeseen. India is currently leaving the first of a three-stage nuclear program that was initially developed in the 1950s, primarily by Homi Bhahba, who served as the first chair of the Indian Atomic Energy Commission, and who the Indian Department of Atomic Energy (DAE) calls the "architect" of the country's nuclear sector.

In contrast to its relatively small uranium deposits, India has a plentiful amount of thorium. The end goal of the country's nuclear plan is to leverage that thorium to reduce its reliance on imported uranium and create a (theoretically) closed fuel cycle. The first stage of the plan focuses on building pressurized heavy water reactors (like much of India's current fleet) that use natural and low-enriched uranium fuel. In the second stage, spent fuel from these PHWRs, along with thorium, would be used in a fast breeder reactor (like the PFBR) to breed fissile uranium-233. In the third stage, India aims to achieve large-scale power production from thorium by way of the U-233 generated in the breeder reactors, which would be used to fuel a new generation of advanced heavy water reactors.

The PFBR: Criticality at the PFBR represents a major step in India's transition from the first to the second stage of its plan. It's important to note, however, that this is not India's first breeder reactor. The PFBR builds directly on the lessons learned from the construction and operation of the Fast Breeder Test Reactor (FBTR), a significantly scaled down, 40-MWt design that entered operation in 1985.

Both the FBTR and the PFBR are located at the DAE-operated Indira Gandhi Center for Atomic Research (IGCAR), which was responsible for designing the reactor. The PFBR uses uranium-plutonium mixed oxide (MOX) fuel, and its core is surrounded by a blanket of uranium-238. Fast neutrons will convert this fertile U-238 into fissile plutonium-239, enabling breeding. The reactor design also allows for that blanket to use fertile thorium-232, which will be similarly converted into U-233.

In its announcement of the PFBR's initial criticality, the DAE called fast breeder technology "the vital bridge between the current fleet of pressurized heavy water reactors and the future deployment of thorium-based reactors, leveraging the country's abundant thorium resources for long-term clean energy generation."

The delay: No mention was made in that announcement of the milestone coming more than 15 years behind schedule. Construction of the PFBR began in 2004. In 2008, Nuclear News reported that the PFBR's reactor vessel had been put in place and was slated for operation in 2011. The article further stated that the DAE planned to have five other fast breeder reactors up and running by 2020.

Repeated and difficult-to-track delays beset the project, with supply-chain issues, regulatory requirements, and the delayed development of the closely related Fast Reactor Fuel Cycle Facility all cited as causes over the years.

In 2025, Jitendra Singh, India's minister of state for science and technology, said the project was experiencing "first-of-a-kind technological issues . . . in the integrated commissioning phase of the project."

Despite the PFBR's troubled path to criticality, it is being celebrated, with Modi saying it reflects "the depth of our scientific capability and the strength of our engineering enterprise." The DAE has yet to announce when the reactor will enter operation.