Arihant achieves criticality when the boat was "already in the sea"

Capping 25 years of indigenous efforts in a technologically challenging area that only a handful of nations have mastered so far, the reactor on board India’s nuclear-powered submarine, Arihant, went into operation at 1.20 a.m. on Saturday.

Arihant’s reactor achieved “criticality” — the term used to describe the self-sustaining nuclear reaction which is the first step towards the stable production of power — when the boat was “already in the sea.”

The submarine — which is about 111 metres long, 11 metres broad and about 15 metres tall — is designed to be propelled by a pressurised water reactor (PWR) that uses enriched uranium as fuel, and light water as both coolant and moderator. The PWR will generate about 80 MWt.

The main challenge, say the scientists who worked on the project, lay in making the reactor compact enough to fit into a submarine. Besides, the reactor needs to be stable when the submarine is accelerating in the depths of the sea.

The submarine will eventually be fitted with K-15 underwater fired missiles, which can hit targets 700 km away. The K-15 missiles, which will carry nuclear warheads, are already under production. India is building three more nuclear-powered submarines at Visakhapatnam as part of its programme to shore up its second strike capability.

Five countries already possess nuclear-powered submarines: the U.S., Russia, the U.K., France and China. Apart from India, Brazil is working on naval nuclear propulsion.