INS Arihant is India’s first indigenously designed and built nuclear-powered ballistic missile submarine (SSBN), commissioned into secret service in August 2016 and publicly acknowledged as completing its first deterrence patrol in 2018. Arihant is the lead boat of what has grown into a four-submarine class built under the classified Advanced Technology Vessel (ATV) programme, and represents the sea-based leg of India’s nuclear triad alongside land-based missiles and aircraft-delivered weapons.[1]
The ATV programme began in the 1980s as a highly classified effort to develop indigenous nuclear submarine technology, drawing on decades of experience gained from operating the leased Soviet/Russian Charlie-class submarine INS Chakra in the 1980s and a later leased Akula-class boat. The programme, reportedly valued at around ₹90,000 crore across its full scope, involved the Defence Research and Development Organisation, the Bhabha Atomic Research Centre, and the Indian Navy working together to design a compact naval nuclear reactor and a submarine hull capable of housing it.[2]
Arihant is powered by a pressurised light-water reactor generating around 83 megawatts, based on Russian-assisted reactor design work, driving the boat submerged for essentially unlimited endurance limited only by crew provisions and maintenance needs — a fundamental advantage of nuclear over diesel-electric propulsion for a strategic deterrent patrol role. The boat displaces approximately 6,000 tonnes submerged and can dive to depths supporting sustained strategic patrols.[3]
Arihant is fitted with four vertical launch tubes, each capable of carrying either three K-15 (Sagarika) submarine-launched ballistic missiles with a range of roughly 700–750 km, or a single, much longer-ranged K-4 missile with a range of approximately 3,500 km — giving commanders flexibility to tailor the missile loadout to the strategic requirement of a given patrol.[4] The shorter-ranged K-15 requires the submarine to patrol relatively close to adversary coastlines to hold key targets at risk, increasing exposure to hostile anti-submarine warfare assets, whereas the K-4’s far greater range dramatically expands the safe patrol areas available to the boat while still covering the same target set.[4]
Arihant has since been joined by three sister boats under the same broad programme: INS Arighat, commissioned in 2024 and reported to be the first of the class fitted to carry K-4 missiles operationally, followed by INS Aridhaman — India’s third SSBN, quietly commissioned in April 2026 at the Ship Building Centre in Visakhapatnam — and a fourth boat, INS Arisudan, which was undergoing sea trials as of mid-2026 with commissioning targeted for 2027.[5] The newer boats are reported to displace around 7,000 tonnes, somewhat larger than Arihant and Arighat, reflecting incremental design improvements across the class.[6]
On 25 December 2025, India test-fired a K-4 submarine-launched ballistic missile from INS Arighat in the Bay of Bengal off Visakhapatnam, marking the first reported launch of the longer-range missile from an operational SSBN rather than a test platform, and a significant milestone in operationalising the extended-range leg of the sea-based deterrent.[7] Analysts described the test as making India’s nuclear triad meaningfully more credible, since it demonstrated an operational capability to strike far beyond the K-15’s limited range, effectively rendering the shorter missile increasingly a secondary or training weapon relative to K-4.[8]
Indian naval planners have generally assessed that a fleet of at least four SSBNs is required to sustain a Continuous At-Sea Deterrent (CASD) posture — ensuring that at any given time, at least one boat is on patrol, one is transiting, and others are in maintenance or workup — a threshold the class is expected to approach once Arisudan is commissioned in 2027.[6]
India’s SSBN fleet operates under the country’s declared “No First Use” nuclear doctrine, under which India commits not to be the first to use nuclear weapons in a conflict but reserves the right to a massive retaliatory strike if attacked with nuclear weapons. A survivable, sea-based second-strike capability is considered essential to making this doctrine credible, since land-based missiles and aircraft are comparatively more vulnerable to being neutralised in a first strike, whereas a submerged, difficult-to-detect SSBN is expected to survive to deliver a retaliatory strike.[9]
Consistent with the sensitive nature of the programme, the Indian government has historically disclosed very little detail about Arihant-class specifications, patrol patterns, or precise missile performance, with most technical details emerging through indirect official statements, budget disclosures, and independent analyst assessments rather than direct government confirmation.[2]
As the platform that completes India’s nuclear triad, Arihant and its sister boats provide the least vulnerable leg of India’s deterrent, complicating any adversary’s calculation of a disarming first strike and reinforcing the credibility of India’s No First Use doctrine. The programme also represents one of the most technologically demanding achievements of India’s defence-industrial base, requiring mastery of naval nuclear propulsion — a capability held by only a handful of nations.[1]
| Type | Nuclear-Powered Ballistic Missile Submarine (SSBN) |
| Displacement | ~6,000 tonnes (submerged) |
| Propulsion | Pressurised light-water reactor (~83 MW) |
| Missile Tubes | 4 vertical launch tubes |
| Armament | K-15 (~750 km) or K-4 (~3,500 km) SLBMs |
| Commissioned | August 2016 (Arihant) |
| Sister Boats | Arighat (2024), Aridhaman (2026), Arisudan (expected 2027) |
| Programme | Advanced Technology Vessel (ATV) |