NEW DELHI: The miniature reactor on board India's first indigenous nuclear submarine INS Arihant has gone "critical", which marks a big stride towards making the country's long-awaited "nuclear weapons triad," an operational reality.

Sources, in the early hours of Saturday, said the 83 MW pressurized light-water reactor attained "criticality" after several months of "checking and re-checking" of all the systems and sub-systems of the 6000-tonne submarine at the secretive ship-building centre at Visakhapatnam .

INS Arihant, till now, was being tested in the harbor on shore-based, high-pressure steam. With the reactor going critical now, the submarine will eventually head for open waters for extensive "sea- acceptance trials", which will include firing of its 750-km range K-15 ballistic missiles. The sea trials will take at least another 18 months before INS Arihant can become fully operational.

When that happens, India will finally get the long-elusive third leg of its nuclear triad — the capability to fire nuclear weapons from the land, air and sea. The first two legs — the rail and road-mobile Agni series of ballistic missiles and fighters like Sukhoi 30MKIs and Mirage-2000s capable of delivering nuclear warheads — are already in place with the armed forces.

The capability to deploy submarine-launched ballistic missiles (SLBMs) is crucial since India has a declared "no first-use policy" for nuclear weapons, and hence needs a robust and viable second-strike capability.