A nuclear submarine is a submarine powered by a nuclear reactor, not necessarily one that is armed with nuclear missiles. Nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently, as is necessary for diesel-electric submarines. Furthermore, the power generated by a nuclear reactor allows nuclear submarines to operate at high speed for long periods, and the long interval between refuelling gives almost unlimited range. In September 2021 six countries' navies actively deployed the following classes of nuclear submarines:-

USA: “Los Angeles”, “Seawolf”, “Virginia” attack submarines and “Ohio” ballistic-missile submarines.

UK: “Trafalgar” and “Astute” attack submarines and “Vanguard” ballistic-missile submarines.

Russia: 4 classes of attack submarines, 5 classes of ballistic/cruise-missile submarines and 3 classes of special-purpose submarines

China: Types 091, 093 attack submarines and 092, 094 ballistic-missile submarines.

France: “Rubis” attack submarines, “Triomphant” ballistic-missile submarines (pictured).

India: Arihant ballistic missile submarine.

In addition, Brazil has such a submarine under development and Australia is about to acquire the technology. It is not known for certain whether any countries have conventionally powered submarines that are equipped with nuclear weapons, although Israel may be in this position.

More Info: en.wikipedia.org