India's first indigenous n-submarine INS Arihant to operate from 2015

Technically, a nuclear boat can stay under water for very, very long periods but the limit to human endurance is generally put at about three months.

It may be noted that conventional diesel-electric boats can stay underwater for three days to a couple of weeks only, as they have to surface periodically to draw air to recharge their batteries.

The Indian Navy has some 45 vessels on order but at present, its submarine arm is very weak as the boats are old - acquired from mid-1980s - except for the nuclear-powered INS Chakra leased from Russia. INS Arihant has an 80 MW pressurised water reactor, based on Russian subs. Some of the crew trained by Russia for INS Chakra have reportedly been helping in the test procedures.

Notably, the navy is looking for three aircraft carriers in the coming years, and it is imperative to have nuclear-powered boats as part of the overall strategy. The carriers, which are like floating islands, themselves need 360 degree protection up, down, around and underwater and SSN boats are a basic requirement if a country goes in for CBGs.