New Delhi: India’s first indigenous nuclear submarine INS Arihant was commissioned into service in August, a report said today.

By inducting the 6,000-tonne submarine into service, India has completed its nuclear triad, reported The Hindu.

The daily quoted defence sources as saying that INS Arihant was formally commissioned by Navy Chief Admiral Sunil Lanba in August. Sources added that in a bid to maintain secrecy, it is not being referred to as INS Arihant.

After being inducted, the submarine completes India's nuclear triad, giving it the capability to respond to nuclear strikes from sea, land and air-based systems.

INS Arihant gives second-strike capability to India, which has a clearly declared policy of "no first-use" of nuclear weapons.

INS Arihant is India's first indigenous nuclear submarine, and the lead ship of Arihant class of nuclear-powered ballistic missile submarines launched in 2009.

The vessel's miniaturised nuclear reactor, built with Russian help, went critical in 2013.

While its 100-member crew has been trained by Russian specialists, Indian scientists at the Bhabha Atomic Research Centre have received significant expertise in reducing the reactor size to help it fit into the 10-metre diameter hull of the submarine.

Nuclear submarines have the capability to stay out at sea longer, and don't need to surface for a long duration. Conventional diesel-electric submarines have to come up to the surface at regular intervals for charging their batteries.