In his message, Dr. Singh said: "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. I extend my congratulations to all those associated with this important milestone, particularly the Department of Atomic Energy, the Indian Navy and the Defence Research and Development Organization.
Dr. Singh further said that Saturday's development represents "a giant stride in the progress of our indigenous technological capabilities."
"It is testimony to the ability of our scientists, technologists and defence personnel to work together for mastering complex technologies in the service of our nation's security," he added.
The Prime Minister said that he looked forward to the early commissioning of the INS Arihant.
The INS Arihant achieved nuclear criticality early on Saturday morning.
Sources were quoted by the media, as saying that the 83-megawatt 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.
The INS Arihant was till now 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. (ANI)