Russian submarine K-152 Nerpa

The INS Chakra is a 8140 t Project 971 (or Project 518 ) Akula class submarine (NATO: Akula II  ) type nuclear-powered attack submarine. Construction was started in 1993, but suspended due to lack of funding. The Indian Navy sponsored the building and sea trials of the submarine provided it was given to the Indian Navy on lease for 10 years. It was launched as the K-152 Nerpa in October 2008 and entered service with the Russian Navy in late 2009. The submarine was leased to the Indian Navy in 2011, after extensive trials and was formally commissioned into service as the INS Chakra II at a ceremony in Visakhapatnam on 4 April, 2012. The INS Chakra joins the Eastern Naval Command at Visakhapatnam.

While K-152 Nerpa was undergoing sea trials in the Sea of Japan on 8 November 2008, an accident caused the deaths of some twenty sailors and injury to twenty-one others. A fire suppression system discharged gas in the bow of the sub, suffocating civilian specialists and navy crew members.

Construction
The Nerpa was laid down at the Komsomolsk-on-Amur shipyard in 1993, but its completion was delayed for nearly a decade due to a lack of funds caused by the economic crisis of the early 1990s. The partly constructed vessel was mothballed until 2004, when Rosprom (the Federal Agency for Industry) signed an agreement with the Indian government to complete the submarine and lease it to the Indian Navy. The vessel was intended to be completed by 2007, but underwent further delays. In 2007, it was transferred to the Vostok shipyard in the closed city of Bolshoy Kamen, Primorsky Krai, for fitting-out. It was launched in October 2008 for sea trials, following which it was due to be handed over to the Russian Defence Ministry. Reports in the Indian media suggest that the resumption of construction was underwritten with Indian funding.

The standards of the vessel's construction were criticised by several commentators. Alexander Golts, defence editor of the Yezhednevny Zhurnal newspaper, said that in the 1980s, the Amur shipyard turned out submarines "one after another, like pancakes," but from 1993 to 2008 had produced just one. "The old specialists had left, and the new ones lacked professionalism." An unnamed worker at the Amur shipyard told Komsomolskaya Pravda that there were "questions about the quality of the metal that was used in building the nuclear submarine", some of which had been bought from China, and alleged that "when the first trials of the submarine were carried out water was leaking in between the seams! So it is not surprising that the work dragged on."

During May 2009, the repairs were reported to be almost complete and new sea trials were planned for June 15–20. However, by October 2009, the work had still not been completed due to the shipyard's electrical supply having been disconnected. Nikolai Povzyk, the head of the shipyard, complained they had not been paid the 1.9 billion roubles (63.8 million dollars) owed for the work carried out on the Nerpa.

Lease to India
, Russia had an agreement pending with India worth US$2 billion for the lease of Nerpa and another Project 971 Shchuka-B class submarine. Of this, K-152 Nerpa will be leased for 10 years to India at an estimated cost of US$670 million. The submarine was finally handed over to India on 30 Dec 2011. After being handed over to the Indian Navy, it would be commissioned as INS Chakra. Nerpa is the Russian word for the Baikal seal, and Chakra is a wheel or a disc.

Indian naval crews earlier trained to operate the submarine near St. Petersburg and another group of sailors was expected to arrive in Vladivostok in late 2008 for joining sea trials. The training of the crew was viewed as crucial to India's own nuclear submarine program, known as the Arihant class submarine.

After the 2008 accident, there were conflicting reports over the status of the lease. A Russian defence industry official denied that talks had been held with India on the delivery of the nuclear submarine. "Russia did not launch talks on a contract to supply India with the Nerpa nuclear-powered submarine." General of the Army Nikolai Marakov stated that Russia would commission the Nerpa and that it would join seven other Akula class submarines in Russia's Pacific Fleet. "The sum of $650-780 million, which Rosoboron export and the Amur Shipbuilding Plant had negotiated over a long period of time with the Indian Ministry of Defence, will now be found in Russia," he said.

However, in May 2009, both Russian and Indian defence officials confirmed that the Nerpa would be joining the Indian Navy by the end of 2009, after Russian President Vladimir Putin visited the yard and announced an immediate release of 1.2 billion roubles, for the submarine construction.

On December 28, 2009, Nerpa was commissioned and joined the Russian Navy. The submarine underwent further adjustments in February 2010. As of August 2010, Russia was training a crew from the Indian Navy to sail the ship to India in fulfilment of the lease agreement. INS Chakra was expected to be commissioned into the Indian Navy before October 2011.

On July 1, 2011, Russian Navy chief Admiral Vladimir Vysotsky has been quoted as holding that "the Indian crew is now absolutely prepared for operating the submarine'', which will be on a 10-year lease.

On January 23, 2012, home voyage under Indian control from the Russian port of Vladivostok to its Indian base Visakhapatnam, commenced. Official Russian announcement of the transfer was still pending at that time. The ship was identified as "INS Chakra II" of the "Akula II" class.

The similarly named Chakra was a Charlie I-class submarine leased by India from Russia 1988-1992.

INS Chakra was officially inducted into the Indian Navy on April 4, 2012.

2008 accident
An accident aboard K-152 Nerpa occurred at 8:30 PM local time on 8 November 2008, during an underwater test run in the Pacific Ocean. A total of 208 people - 81 military personnel and 127 civilians - were on board at the time of the accident. At least 20 people were killed by asphyxiation and at least 21 more were injured, marking the worst Russian submarine disaster since the Kursk sank in 2000. Three of the dead were military personnel and the rest were civilians from the Vostok, Zvezda, Era and Amur shipbuilding yards who were members of the acceptance team.

The incident involved the accidental triggering of a fire extinguishing system which sealed two forward compartments and released freon R-114B2 (Dibromotetrafluoroethane) gas into them. According to survivors, those affected by the gas release were caught off guard and may not have been alerted in time due to warning sirens sounding only after the gas had already begun pouring in. Some of the victims were reported to have been unable to turn on breathing kits before they suffocated.

On 10 November, a Russian Navy statement blamed the disaster on an "unsanctioned operation" of the fire suppression system aboard the Nerpa. Preliminary investigations concluded that the system had triggered automatically without human intervention. On 13 November, naval investigators announced that a crewman had turned on the system "without permission or any particular grounds".

Fiction
The K-152 Nerpa finds a mention in the Hollywood film The Darkest Hour as the nuclear submarine hiding in the Moscow River to take survivors of alien attack to safety.