US, Russia, and China are developing next generation nuclear submarine fleets, other countries are also mastering nuclear submarine technology

A nuclear submarine is a submarine powered by a nuclear reactor. The performance advantages of nuclear submarines over “conventional” (typically diesel-electric) submarines are considerable. Nuclear submarines are more powerful than diesel-powered submarines because nuclear reactors powering them can run for years without the need for refuelling. Nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently as is necessary for conventional submarines and thus enhance their survivability  .