NEW DELHI: India is set to sign a $3-billion deal with Russia this week to lease another nuclear attack submarine that will be customised and fitted with indigenous communications systems and sensors.The deal for the Akula class submarine — dubbed Chakra III after the first two similar vessels India obtained from Russia — will be the biggest signed with Moscow since the $5.5-billion contract for the S-400 air defence system was finalised last year.Sources told ET that the intergovernmental agreement for the submarine lease is likely to be inked on March 7 and that the vessel will be ready by 2025 after an extensive building programme on mothballed hulls at a Russian shipyard.The Chakra III will be in service for at least 10 years and will replace the Chakra II that was obtained under similar conditions in 2012. It is believed that the lease of the Chakra II, which is to expire in 2022, could be extended for a fiveyear period till the new vessel gets built and tested.Operating a nuclear attack submarine or SSN — it’s powered by a nuclear reactor but armed with conventional weapons — gives India significant strike capability in the region. These vessels can remain underwater for months, making them almost impossible to detect and are a big deterrence for enemy vessels in the region.While the mothballed nuclear submarine had been shipped to the Russian port town of Severodvinsk in 2014, sources said that it will be akin to a new vessel once ready, given the extent of work required to make it operational.The work requires a complete build-up and activation of the nuclear reactor that powers it and the fitment of a large number of unspecified Indian original systems for communication and domain area awareness.Indian personnel involved in the project are likely to get significant expertise on working on such submarines that will come in handy for the indigenous nuclear attack submarine programme that has been approved. Design work on a new class of Indian SSNs has commenced but there is no clarity about when it will be completed. The build process started at a domestic shipyard.Talks on the Chakra III lease have been on since 2013 but were given a boost in 2015 when both sides initiated technical and price discussions to specify what will go into the inter-governmental agreement.This will be the third Russian nuclear submarine to be operated by India, starting from 1988 when the original INS Chakra was taken on a three-year lease. The second was inducted in 2012 after a prolonged refit that saw a time delay. The Akula submarines are considered to be next only to newer US nuclear submarines when it comes to stealth and attack capabilities.