From the earliest times, physical human power (oars) and wind energy (sails) were used to propel ships. It was not until the first half of the 19th century that mechanical propulsion was introduced. Different types of main propulsion systems were built and gradually improved: the steam engine, the steam turbine and the diesel internal combustion engine. Developments in the period after the Second World War led to the construction and trial of new ship power plants and, above all, to the use of nuclear energy for ship propulsion. The nuclear ships and vessels that exist today offer very serious advantages and are capable of achievements that cannot be obtained with other propulsion systems. The most important component of a nuclear power plant is the reactor. Its central part, called the core, contains nuclear fuel such as uranium 235, an isotope characterised by the fact that under certain conditions the nuclei of its atoms undergo fission and a very large amount of energy is released simultaneously. The fission reaction is a chain reaction - the initiation of one reaction leads to further reactions. This results in a self-sustaining reaction that can be controlled. 
[001] Simplified diagram of a ship's nuclear power station: 1. reactor, 2. heat exchanger, 3. main circuit pump, 4. primary circuit fluid, 5. reactor containment, 6. safety container, 7. primary circuit cover, 8. secondary circuit steam, 9. steam turbine, 10. condenser, 11. pump, 12. secondary circuit condensate, 13. transmission, 14. ship propeller The shipboard nuclear power station, shown simplified in [001], consists of a primary circuit and a secondary circuit. In the primary circuit, water flows under high pressure to cool the reactor, that is, it receives the heat generated in its core and transfers this heat in a heat exchanger to the secondary circuit. The water is then pumped back into the reactor. The secondary circuit water is turned into steam. The steam is fed to the main turbine, driving the ship's propeller through a mechanical or electrical transmission.  [002] Comparison of conventional ship's power station to nuclear power station: 1. steam turbine, 2. steam boilers, 3. steam turbine, 4. safety container with reactor and primary circuit When comparing with a conventional ship's power station [002], it appears that the primary circuit acts as a steam boiler (the reactor of the type described can itself be compared to the furnace of a boiler) and the secondary circuit in general corresponds to the system normally used on turbine ships. When the reactor is in operation, dangerous radiation occurs. The reactor itself is surrounded by a shield and, together with other primary-circuit equipment, is sealed in a safety container or in a separate compartment, outside of which there is a second shield. The secondary circuit of a nuclear power plant is free of radioactivity under normal operating conditions. The power stations installed on most of the nuclear ships and vessels constructed to date are built and operated in this way. They have proven to be reliable in function and safe to operate. All steam and motor engines consume large amounts of fuel and must be refueled frequently, which limits the sailing area and makes the ships dependent on supply bases. This difficulty completely falls away on a nuclear vessel, as a single load of nuclear fuel is sufficient for a minimum of several years of sailing. Although in today's designs the weight of a nuclear power plant is greater than that of an ordinary power plant of the same power, the sum of the weights of the power plant and the fuel supply for one longer voyage is eminently less for a nuclear ship. For this reason, a nuclear ship can carry more cargo. 
[003] Nuclear icebreaker Lenin launched in 1957. When steam boilers and internal combustion engines are operating, air is needed to burn the fuel, whereas a nuclear reactor operates on a different basis, taking no air at all and emitting no exhaust. It was this feature, together with other advantages, that first led to the installation of nuclear power stations on submarines. Nuclear propulsion is most suitable for bulk cargoes, like oil tankers, high-speed general cargo vessels, such as refrigerators, passenger ships and special vessels like icebreakers or whaling bases. This type of propulsion is proving particularly useful for various types of warships and is being considered in the design of underwater research vessels. 
[004] The captain's bridge of the NS Savannah ship. The crew numbered 110. The ship could take on board 60 passengers and 9300 t of cargo. The first vessel equipped with nuclear propulsion was the US submarine Nautilus, which completed trials in 1955 and later famously passed under the ice of the North Pole. The first nuclear ships used for peacetime purposes were the icebreaker Lenin and the passenger-commercial ship Savannah. 
[005] Soviet Typhoon-class submarines in their homeport. A nuclear ship, like any other ship, is subject to various types of accidents, such as collision, running aground, fire and sinking. In the case of a nuclear vessel, the consequences of such a failure would be incomparably more dangerous than for ships with an ordinary power plant. If radioactive substances were to escape from the reactor in the aftermath of an accident, not only would the crew and the immediate surroundings of the ship be endangered, but there would also be severe contamination of the water and air. For this reason, a special design of the reactor and the ship's hull protection is necessary. Currently, excessive safeguards are deliberately used, which increases costs and weight. During the normal operation of a nuclear power plant, waste is generated which must be disposed of safely. Gaseous wastes are released into the air after dilution and are not hazardous to the environment. Solid and liquid waste, on the other hand, is stored on board the ship until it arrives in port. Once ashore, nuclear ship waste is processed like other waste from nuclear power plants and laboratories and prepared for disposal after packaging in sealed and durable tanks. These tanks are then stored deep underground or at sea, in well-defined locations. The storage of nuclear reaction waste is still an important topic in the discussions of international committees. • • •
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