New Delhi: India has activated the atomic reactor on board INS Arihant, a development that Prime Minister Manmohan Singh described as a "giant stride" in the progress of the country`s indigenous capabilities.
Sources said the nuclear reactor on Arihant was made critical on Friday night and the submarine was on its own power now.
They said activation of nuclear reactor was "a major landmark" in making the submarine operational.
Sources added the submarine would undergo extensive sea trials before being made operational.
INS Arihant has been undergoing trials at Navy`s submarine base in Visakhapatnam, Andhra Pradesh.
Manmohan Singh said the nuclear reactor`s activation on India`s first nuclear submarine INS Arihant "represents a giant stride in the progress of our indigenous technological capabilities".
In a congratulatory message, he lauded the scientists and defence personnel for activating the reactor on INS Arihant.
"I am delighted to learn that the nuclear propulsion reactor on board INS Arihant, India`s first indigenous nuclear powered submarine, has now achieved criticality," he said.
The DRDO has also readied a medium-range nuclear missile BO-5 for being deployed on the Arihant and its last developmental trial was held on January 27 off the coast of Vishakhapatnam.
The nuclear submarine will help India achieve the capability of going into high seas without the need to surface the vessel for long durations.
Conventional diesel-electric submarines have to come up on surface at regular intervals for charging the cells of the vessel.
India is the only nation in the Indian Ocean region to have a nuclear submarine and the sixth in the world to have the capability to design and construct a nuclear submarine
The orange beacon atop the conning tower came to life on July 26, 2009, as Prime Minister Manmohan Singh`s wife Gursharan Kaur had pressed the button to symbolise the launching of the vessel into water.
In the middle of the body - comprising an outer hull through which the water goes in and an inner pressurised hull - on the starboard side are two rectangular vents, meant to take in water when the vessel dives into the sea. It is like a "cocoon within a cocoon", an official had explained.
INS Arihant is longer than any of the submarine in the Indian Navy`s fleet so far. A nuclear submarine is powered by a nuclear reactor, which generates tremendous heat driving a steam turbine. It has unlimited underwater endurance and speed twice that of its conventional counterparts.
The submarine can carry 12 nuclear missiles K-15. Keeping in line with its "no first use policy", the submarine will help India in developing a "credible second strike capability" in case of nuclear attack, said officials.
The Indian Navy has been operating conventional diesel-electric submarines, which have to surface to charge their batteries.