Why does India want fast breeder nuclear reactors? | Explained

Explore India's pursuit of fast-breeder nuclear reactors, focusing on criticality, efficiency, and long-term energy security.

In an important milestone, the prototype fast breeder reactor (PFBR) at Kalpakkam achieved criticality on April 6. The term ‘criticality’ is familiar to India: over the decades, it has been associated with the slow and tedious successes of India’s nuclear power programme. At the same time, in keeping with many terms in the nuclear vocabulary, ‘criticality’ is also often mistaken as an end goal. In reality, it is actually the first step.

A nuclear reactor becomes critical when its chain reaction is able to sustain itself. That is, when an atom’s nucleus undergoes nuclear fission, it releases neutrons that trigger at least one more fission reaction in the surrounding nuclei. Reactor engineers ensure this happens by controlling the composition of the fuel (the material whose nuclei undergo fission), how well the neutrons are able to ‘access’ more nuclei, and the temperature of the reactor.

Once a reactor is critical, it also means it is in a kind of stable state. However, it does not mean that it is operating in a commercially viable way. That comes much later. After criticality, the operators keep the reactor running as it produces a low amount of power, for months if necessary, while they check if its operating parameters are within design limits. If an operator is sure that the parameters are, they can go to the next stage.

Most of India’s currently operating nuclear reactors are pressurised heavy water reactors (PHWRs). They are designed to support the fission of natural uranium. Natural uranium consists of 99.3% of uranium-238 and 0.7% of uranium-235. ‘235’ and ‘238’ denote the total number of protons and neutrons in the nucleus. In a PHWR, neutrons are introduced into the reactor, where a device called a moderator slows them down. This is necessary for the neutrons to cause uranium-235 to undergo fission. When it does, it releases heat, which the PHWR converts to electricity; a small amount of plutonium; and a few neutrons.

PHWRs are inefficient because only a small fraction of the fuel, around 1%, undergoes fission before it becomes unusable.

A fast-breeder reactor (FBR) is more efficient, achieving a fuel use rate of around 10% or more. Mainly, the fuel consists of plutonium, not uranium. The reactor core is surrounded by a ‘blanket’ of depleted uranium, like the unusable fuel produced by PHWRs. When a fast neutron bombards the blanket, the uranium nuclei are transmutated to plutonium nuclei, which are reprocessed as nuclear fuel. The plutonium-based fuel also uses the fast neutrons to undergo fission, releasing more fast neutrons.