|Fig. 1: Nautilus being launched. (Source: Wikimedia Commons)|
Nuclear propulsion, unlike the conventional propulsion, uses nuclear power as source of propulsion. Due to its high-power density  compare to conventional propulsion but high maintenance cost, applications of nuclear propulsion has been extensively researched and developed by military.  Reactors were developed and still being developed for many applications; however, nuclear submarines remains the most successful application of nuclear propulsion. And all this, started with the first ever built nuclear submarine, the Nautilus. (Fig.1)
Sharing names with Nautilus from the famous book by Jules Verne, 20,000 Leagues under the sea, Nautilus the nuclear submarine was first planned by Admiral Hymann G. Rickover. Known as also the Father of the Nuclear Navy, he is remembered as being abrasive even to his friends. However, when it came to his job, he was very intent.  He and his team adopted the pressurized water reactor (PWR) proposal by Westinghouse and succeeded in building the S1W reactor.  In 1951, construction of the Nautilus was authorized by the congress and finished on 1955. Noticeable feat of Nautilus include; non-stop submerging for 2,100 km. This was the longest and fastest ever achieved with a submarine at the time.  Nautilus then was decommissioned in 1930 and now it lives in Submarine Force Library and Museum in Groton, Connecticut.
Submarines are the most effective application for Nuclear propulsion, because of two major reasons. First, they do not require air for energy generation. Second, they travel a long distance without refueling. The reactor that was installed on Nautilus is S2W reactor. S2W is not a random name. Each letter has a meaning to it. S stands for the reactor being installed to a submarine, 2 stands for being the second generation core and W is for Westinghouse, the contract designer.
S2W reactor was a PWR type reactor. PWR type reactor consists of three loops; primary, secondary and tertiary loop. All three loops are independent and the working fluids do not mix among them. Primary loop is where the actual fission reaction happens. At the secondary loop, thermal energy generated at the primary loop is transform to mechanical energy. This mechanical energy is then finally changed to electrical energy. Lastly, at the tertiary loop heat created is dumped into environment. This environment is usually sea for submarines. S2W operated on uranium core and produced 10 MW of power. 
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