NEW DELHI: India on Thursday inked yet another mega defence deal worth over $3 billion for the lease of a nuclear-powered attack submarine from Russia, despite the threat of US financial sanctions still looming over the earlier $5.4 billion contract inked for Russian S-400 Triumf missile systems in October last year.Defence sources said the over $3 billion (around Rs 21,000 crore) contract for the Akula-1 class submarine, which will be ready by around 2025, includes a comprehensive package for refurbishment of the nuclear boat lying mothballed at Severodvinsk, its sustenance and spares support for 10 years, as well as training and technical infrastructure for its operations.This submarine will replace INS Chakra, the Akula class submarine taken on a 10-year lease from Russia in April 2012, under a secret over $900 million deal inked way back in January 2004.“INS Chakra’s existing lease will be extended till at least 2025 through another contract till the new submarine, which will be bigger and more advanced than it, becomes operational,” said a source.It was in November last year that India declared its first indigenous nuclear-powered submarine with nuclear-tipped ballistic missiles (called SSBN in naval parlance), INS Arihant, had successfully completed its “first deterrence patrol” .It meant that the country’s long-awaited nuclear triad -- the capability to fire nuclear weapons from land (Agni missiles), air (fighter-bombers) and sea (SSBNs like INS Arihant) – was finally operational to some extent.A nuclear-powered attack submarine (SSN) like INS Chakra, which is not equipped with long-range nuclear missiles due to international treaties, is not meant for “deterrent patrols”.But it can be a deadly “hunter-killer” of enemy submarines and warships. Apart from being equipped with land-attack conventional cruise missiles, it can also be deployed for ISR (intelligence, surveillance, and reconnaissance) missions due to its stealthy nature and capability to stay underwater for extended periods.INS Chakra has also added some much-needed teeth to India’s depleting fleet of just 14 conventional diesel-electric submarines (13 old, and a new Scorpene INS Kalvari out of the six under construction at Mazagon Docks), a SSN (INS Chakra) and a mini-SSBN or “baby boomer” (INS Arihant). Overall, India needs at least 18 conventional submarines, six SSNs and four SSBNs for effective deterrence.The over 8,000-tonne INS Chakra has also served as a training platform for the 6,000-tonne INS Arihant, which is to be followed by three more SSBNs being built at the ship-building center at Vizag under the secretive Rs 90,000 crore Advanced Technology Vessel (ATV) programme.Nuclear-powered submarines can stay submerged for months at end, their endurance only limited by the physical and mental endurance of their crews, but conventional submarines must surface or “snorkel” every few days to get oxygen to re-charge their diesel-electric batteries.India’s deal for the new SSN, however, will further raise the hackles of US, which under its new law CAATSA (Countering America’s Adversaries through Sanctions Act) seeks to prevent countries from buying Russian weapons or Iranian oil.Though India is hopeful of eventually getting US presidential waiver for the S-400 deal, even as New Delhi and Moscow are finalising an alternate payment mechanism, the new submarine deal could complicate matters. “Repeated waivers for India, which the US wants to progressively reduce its dependence on Russian weapon systems, will be a problem,” said an expert.