“Building a reactor for propelling a nuclear submarine is a different ball game altogether”

The compact pressurised water reactor (PWR) aboard INS Arihant, the first Indian nuclear-powered submarine, achieving criticality today (Saturday), marks a significant milestone in India’s capability in nuclear technology. While it does provide a platform for launching a parallel indigenous PWR-based nuclear power programme in the country, building a reactor for propelling a nuclear submarine is a different ball game altogether involving several scientific and technological challenges.

Unlike the natural uranium and heavy water-based Pressurised Heavy Water Reactors (PHWRs), which constitute the mainstay of our nuclear power programme today, PWR technology uses enriched uranium and ordinary water as both the moderator and coolant. (Natural uranium contains 0.7 per cent of the fissile isotope U-235 and the rest is the non-fissile but fertile isotope U-238.) In the normal PWR power reactors, enrichment is 3-5 per cent. For submarine propulsion reactors, however, much higher enrichment is needed.

A critical requirement for a submarine propulsion reactor is its compactness. “The engineering for a compact reactor system is very complex,” pointed out Anil Kakodkar, former Chairman of the Atomic Energy Commission (AEC). One of the important challenges is to design the reactor for a moving system unlike the stationary reactors built on land for power generation. During its motion, the submarine will be subjected to different kinds of rolling and pitching motions. High accelerations can also result from explosions of depth charges or striking anti-submarine torpedoes. The system should remain stable and continue to perform even with the kind of acceleration loads that the reactor components will experience, Dr. Kakodkar said.

“Fabrication of reactor components for the compact reactor, too, involved serious manufacturing challenges. In fact, new machines were required,” Dr. Kakodkar said. “These were developed jointly by the Bhabha Atomic Research Centre (BARC) and the Indian industry, and credit should be given to them,” he added.

Since the reactor has to occupy a much smaller volume for a given power output, the corresponding energy density becomes high. To achieve this, as Dr. Kakodkar had pointed out, in an earlier interview, a large amount of power has to be exchanged in a small volume calling for a host of challenges in material technology. For compactness and high energy density requirements, highly enriched uranium is used.

The Arihant reactor is stated to have uranium enriched to above 25 per cent, which was produced at the Rare Materials Plant (RMP) of the Department of Atomic Energy (DAE) at Ratnahalli, Karnataka. This high fissile density also means that the fuel can last for extended periods without requiring refuelling.

While on cruise, because of rapid speed changes, the power requirements of a submarine can vary suddenly and the power level would need to be correspondingly ramped up rapidly. This means that the reactor should be a fast responding system resulting in rapid power changes. In a normal power reactor, the response needed is much slower, which is achieved slowly by controlling the rate at which control rods are introduced into the reactor. Here it calls for major material challenges in the core, Dr. Kakodkar said.

One of the considerations, Dr. Kakodkar pointed out, is the requirement of good thermal conductivity of the fuel material, to respond quickly to rapid changes in power requirement. Uranium oxide fuel, the form usually used in power reactors, being a ceramic compound, has low thermal conductivity. So a different material needs to be used, he said, though he declined from identifying the uranium compound that is being used in the Arihant reactor.