Why we need Nuclear Submarine??
If you take a nuclear power plant, and you shrink it, and you put it inside a submarine, you get a nuclear submarine. Nuclear subs have two big advantages over "normal" submarines. A normal submarine has either a diesel or a gasoline engine. It also has a huge bank of batteries. The sub can't run its engine underwater – the engine needs air to run. So when it is underwater, the sub uses its batteries to power an electric motor. Once the batteries get low, the sub has to come up to the surface. It runs its engines long enough to recharge the batteries, and then goes underwater again. This is a big problem, because it’s hard to hide a sub when it’s up on the surface running a big diesel engine. The other problem is that the big diesel engine needs thousands and thousands of gallons of fuel, and the fuel takes up lots of space in the sub. Since a nuclear power plant doesn't "burn" anything, it doesn't need a supply of air like a diesel engine does. That means that the nuclear reactor in a nuclear sub can run constantly. The sub can stay underwater for months at a time if it wants to. It never has to come to the surface for air. This is a huge advantage, because the sub can go all over the world undetected.
The other advantage of nuclear power is that the uranium fuel is long lasting. A nuclear sub can go for a decade or more without refueling. The only reason it needs ever come to port is to replace the crew and load up a new supply of food for them. For this same reason, modern aircraft carriers use nuclear power. They can stay at sea without having to worry about refueling.