Introduction to Ships Powered by Nuclear Energy
The warships of many of the world’s navies are powered by nuclear energy, normally provided by a Pressurized Water Reactor. (PWR)
The heat generated in nuclear reactors is used to raise steam to drive steam turbines. The turbine can either drive the ship’s shafts through a gearbox, or use the electrical power to drive the propeller shaft.
Large submarines use nuclear energy because of its high power output coupled with the length of time between refueling. These submarines are able to cruise around under the world’s oceans without surfacing for many months.
Other naval ships such as cruisers and aircraft carriers are also powered by nuclear energy.
The use of nuclear energy to power merchant navy ships has not proven very successful, with only a handful being built with nuclear reactors. However there are numerous icebreakers operating in the Arctic using nuclear energy because of the high power output required to break through ice sometimes over a meter thick.
This is another article on the series of Nuclear Energy. This covers the use of nuclear energy to power warships and submarines. We shall examine the nuclear fuel enrichment required, the operation of the shipboard power plant, and the different methods of using this power to drive the propeller shafts.
We begin with a quick overview of nuclear energy, moving on to its application in powering the world’s warships and submarines.
Nuclear Energy Overview
Nuclear energy relies on the reactions of enriched uranium. In a PWR, the enriched uranium is placed in fuel rods which are bundled together to form the fuel assembly.
The fuel assembly is placed in the core of the reactor where the reactions take place. The speed and reactions are controlled by control rods and a moderator. In the core, uranium atoms are bombarded with free neutrons producing immense heat. This heat is transferred by a cooling medium to a heat exchanger where process water is converted to dry high pressure steam.
The steam is then used to drive a steam turbine, as in a normal steam plant.
Shipboard Nuclear Reactor Operation
The operation of a nuclear reactor to raise steam outlined above is similar to a shipboard one. The main differences being as follows,
- Type of Fuel Used
As we have seen the normal fuel used in the reactor is enriched uranium known as UO2, however the fuel used by the PWR for ships propulsion has a different form.
This fuel is still uranium based but alloyed with different percentages of aluminum or zirconium.
- Fuel Enrichment
The UO2 fuel is normally enriched with 3-5% of U235. The uranium alloy has been enriched to contain up to 90% U235, but is normally used at between 45-75% U235 enrichment in PWR for ships propulsion.
- Internal Neutron Shield
The internal neutron shield is designed to contain the aggressive neutrons movement occurring within the narrow compact pressure vessel containing the reactor core. The neutron shield is fabricated from a high mass/density material which provides a good source of neutron absorption. This prolongs the reactors core life by preventing the embrittlement of the steel of the pressure vessel, with some reactors being designed to last between 40 and 50 years.
- The Plant’s Compactness
The shipboard nuclear reactor is very compact compared to the normal power plant reactor.
This compact plant is normally about 4m high with a diameter of 2m. This makes these plants particularly suitable for submarines, where space is at a premium
- Power output
The power output from ships nuclear energy plants ranges from 200MWt for submarine propulsion to 300MWt for larger warships such as aircraft carriers.
Sketch of Nuclear Ships Steam and Elecyric Drive Operation