|Fig. 1: Nuclear submarine USS SeaWolf completed in 1995. (Source: Wikimedia Commons)|
The first utilization of nuclear energy was for use in nuclear weapons. This took place during World War II as there was a race to develop an atomic bomb which was ultimately used by the United States in Hiroshima and Nagasaki. Today, there are numerous applications for nuclear power and radiation besides weapons. Atomic energy is the source for more approximately 13% of the world's electricity production.  Atomic radiation has also proven useful in medical diagnostics and treatment.  The specific application that we will focus on in this report is that of nuclear propulsion.  Nuclear energy is appealing for use in propulsion (and similarly in other energy applications) because the energy sources are highly energy-dense, or said to have a low weight-to-capacity ratio. Harnessing atomic energy allows for source material to be of small mass yet still provide the voluminous amounts of energy essential for long journeys. Nuclear propulsion is used in applications such as Navy submarines (as seen in Fig. 1), icebreaker ships (Fig. 2), and is proposed for use in space probes such as manned mission to Mars. 
On journeys with limited space long travel times it is imperative that a low-mass fuel be used. For example, submarines may be underwater for weeks at a time, ice-breaker ships also make long journeys in extreme environments, and a journey to Mars would take on the order of 220 days to complete each leg.  In each of these cases nuclear energy is used because the fuel is lightweight and provides a large amount of energy.
|Fig. 2: The "NS 50 Let Pobedy" translated as "50 Years of Victory" is a Russian nuclear-powered icebreaker and is the largest such vessel in the world. (Source: Wikimedia Commons)|
The first historical application of nuclear fuel in propulsion (and the most widely used)is that in nuclear submarines. Immediately before submarines were nuclear they were powered by diesel fuel. This required that they be refueled regularly and they also generated lots of noise underwater making it easier for enemies to detect the sub. Nuclear submarines and ships essentially have a small-scale nuclear reactor on board the vessel that generates heat that is transduced into mechanical energy to propel the ship and electrical energy to power the on-board equipment.  These nuclear reactors operate in a comparable way to land-based reactors that generate electricity.
Icebreakers and merchant ships (as seen in Fig. 2) utilize the same propulsion techniques as nuclear submarines.  The Russian government is best known for icebreaker ships.  These ships are economically important for Russian to ship goods through the Arctic during winter months. They are also used for tourism and scientific expeditions. 
Nuclear energy has application in space and is proposed for use in propulsion. The electronics on current space probes are powered from radioactive materials but this is different from propulsion. From 2007-2008 NASA conducted a study to determine the necessary means to send a human to Mars.  In this study they suggested the use of enriched Uranium U-235 radioactively decay and produce heat to light LH2 (liquid hydrogen fuel) used to propel the craft.
|Fig. 3: Diagram of submarine nuclear reactor. (Source: Wikimedia Commons)|
Nuclear propulsion exists in many forms. The nuclear propulsion of a submarine is quite different from that of a space probe. Submarines and icebreakers have small nuclear reactors on board to generate heat which is converted into mechanical energy whereas nuclear powered space probes propel themselves by successive small nuclear explosions. 
Nuclear reactors are complicated machines. Building a land-based reactor is a non-trivial task and building one with space constraints for use use in a submarine or ice-breaker even more difficult. 
Highly enriched Uranium or Plutonium are the most commonly used fuel in submarines. In order to make the reactor operate a series of control rods lowers the Uranium or Plutonium into the reactor core. During nuclear fission Uranium breaks down and produces heat which creates steam in a heat exchanger and causes turbines to turn at high speeds (see Fig. 3). This energy turns the propellers. All the electronics are also powered by the nuclear reactor. Nuclear submarines are able to travel 100,000's km on just 4kg of Uranium fuel.
The primary safety concern that is always addressed on nuclear vessels is to ensure that everyone involved in the operation of the ship avoids nuclear radiation. As humans are on board these vessels, one would think that it would be best to keep the reactors as far from humans as possible. However, the radiation is inherent regardless of the reactor's location and thus protecting the reactor from catastrophic impact which could lead to the emission of heavy amounts of radiation is of more critical concern. On the nuclear submarine the reactor is not placed at the front or back of the ship because these are the locations most vulnerable to a collision. The nuclear reactor is actually placed in the center of the vessel and the lower level of the ship.  This keeps the reactor more isolated from vibrations or potential collisions the sub may have.
The most dangerous time for expose to unhealthy levels of radiation are likely during the fueling and decommissioning of nuclear vessels. Currently, nuclear submarines are able to operate for their entire lifespan without ever having to be refueled but large icebreakers need to be refueled every five to seven years.  During the extraction process vessels are brought to land-based port. The fuel is extracted, processing, and stored safely on land. Workers take precaution to avoid radiation exposure.
Not all submarines have been decommissioned on land. One particular example of decommissioning a submarine in water is that of the Russian K-27. This submarine was simply filled with furfuryl alcohol and bitumen in an attempt to prevent nuclear pollution and was towed and dumped in the Stepovogo Fjord of Norway.  Currently, the level of radionuclides in the region is not of concern but over time they may start to leak and measures may be taken to prevent harmful radioactive pollution. 
Harnessing the power of the atom has been a monumental technological advancement for mankind. Atoms are very energy-dense and atomic science and technology has provided us with the capability of extracting this energy from certain materials such as Uranium and Plutonium. The amount of energy per mass that can be utilized in these elements is orders of magnitude greater than anything ever before possible. Instead of using fossil fuels to power vessels such as submarines and ice-breaker merchant ships, nuclear energy can be used much more effectively. Nuclear propulsion is also ideal for use in space probes.
© Dustin Gerrard. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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