A Process Description of the Nuclear Generation of Electrical Power

Introduction

Nuclear power can be concisely explained as the use of controlled nuclear fission to produce energy.  The electricity generated by a nuclear reactor is safe, inexpensive, scalable, and harmless to the environment when proper safety and training guidelines are followed. 

Nuclear power provides one-fifth of the power produced in the United States (Energy Information Administration [EIA], 2008, p. 276) and over one-third of the power produced in the European Union (World Nuclear Organisation, 2009).  The plants themselves are, with the exception of the plants used by the Navy to power their submarines and aircraft carriers, operated and maintained by private companies and overseen by the Nuclear Regulatory Commission.

A full description of the generation of nuclear power or the operation of a power plant is beyond the scope of this article.  Instead, this document will provide a general overview of the most common components and characteristics of power plants and describe how they generate electricity.  This document presupposes knowledge of basic physics, chemistry, electrical principles, and thermodynamics, although this knowledge is not essential to comprehension.

Nuclear power is complex only in its specifics; its general principles, based on technology hundreds of years old at this point, can be comprehended easily.  The reactor core functions just like the boiler of an old steam power plant in a locomotive, old ship, or factories during the industrial revolution.  The core indirectly heats water to boiling, which produces steam that turns a turbine.  The turbine is coupled to a generator, which generates electricity.

Background

Although the first atom was “split” in 1917, the era of nuclear power did not truly begin until 1942, when a group of distinguished scientists, headed by Enrico Fermi, built the Chicago Pile-1, the first artificial nuclear reactor, on a racquetball court at the University of Chicago (National Park Service, 2003).  The first tentative steps toward producing electrical power from nuclear reactions were made in the nineteen-fifties, with the EBR-I in Idaho (generating 100 kilowatts) from 1951 to 1955; the USSR’s Obninsk plant (5 megawatts) in 1954; and the United Kingdom’s Calder Hall power station (50MW initially) in 1956 (“Nuclear power,” 2010).

Around the same time, the United States Navy began the construction of its first nuclear-powered surface ships and submarines.  The requirement of diesel fuel for these ships had long impaired their abilities, severely limiting the amount of time that submarines could remain submerged and the time that both types of ship could remain at sea.  With over two hundred and twenty nuclear-powered vessels and no reactor accidents, the Navy remains the largest single operator of nuclear power plants in the world, as well as one of the very safest (“Hyman G. Rickover”, 2010).

The adoption of nuclear power has been retarded, first due to the skepticism of political figures such as Harry S. Truman (“Nuclear power,” 2010), and later due to lowered energy demands and public skepticism, which intensified after prominent incidents at Three Mile Island and Chernobyl (“Anti-nuclear movement in the United States,” 2010).  Nevertheless, nuclear power remains an appealing and arguably sustainable source of energy until more renewable, more ecologically friendly, and less complex technologies such as wind and solar power can be deployed in large-scale configurations.

Components and Systems

Systems

Conventional nuclear power plants can be subdivided into three major systems:

• Reactor core – The reactor core consists of a large metal tank with thick metal walls, designed to withstand extremes of heat and pressure.  Within the core is a mass of fissile material, commonly Uranium-235.  Control rods are inserted into and withdrawn from the core to control the power of the reactions taking place within.  The control rods are made from a “poisonous” substance, referring to the ability of the material to absorb the neutrons ejected by the fission process, greatly decreasing the number of fission reactions taking place.
• Primary coolant system – The primary coolant system pumps a coolant (usually, and for the purposes of this document, water) through the core (to absorb the heat within the core), transfers it to the heat exchanger (where the heat is passed to the secondary coolant system), then pumps the water back to the core.  This water is pumped at high pressure to resist boiling.  Boiling would result in the core being exposed to air, which is an extremely poor conductor of heat, particularly when compared to water.  The core, unable to shed the heat generated by the fission reactions, would melt (meltdown).  There are frequently two or more loops in the primary system, each feeding its own secondary coolant system, which can operate in isolation from each other while sharing a core. The current load requirements of the power plant determine the number of active loops in the primary coolant system.
• Secondary coolant system – The secondary coolant system pumps water through the heat exchanger (where it absorbs heat from the primary coolant) and to the turbines (where the water flashes to steam and turns the turbine, driving the generator and generating electricity).  The steam is cooled and condenses to its liquid phase in a chamber called a condenser, whereupon it is pumped back to the heat exchanger to continue the cycle.

Isolation of Radioactive Components

The power plant is constructed to contain radiation.  Water itself cannot become radioactive, but corrosion and rust processes (minimized in the primary coolant system through the use of alkaline chemicals to maintain a basic pH) can result in the primary coolant containing radioactive particles.  The separation of the primary coolant system from the secondary system (as well as the constant filtration of both coolants) minimizes the possibility of transfer of radioactive particles into an environment where they can harm humans or the environment.

In addition, the sequestration of the reactor core and primary system means that the most common tasks of plant maintenance can be safely undertaken while the plant is critical.  Issues affecting the safety of the plant as a whole, or with maintenance of the primary coolant system or core, require that the plant be shut down completely.

The Process of Power Generation

Core Physics and Heat Transfer

The operation of a power plant begins with circulating the primary coolant through the core.  Once the flow through the system is stable, the control rods are slowly withdrawn from the core.  The fuel within the core emits neutrons as it decays, which, without the inhibiting effect of the neutron poison in the control rods, are statistically far more likely to strike other fuel atoms.  The addition of the neutron makes the atom unstable; for instance, Uranium-235 decays into Thorium-231, which decays into Protactinium-231, and then generally to Actinium-227, Thorium-227, Radium-223, Radon-219, Polonium-215, Lead-211, Bismuth-211, Thallium-207, and then Lead-207.  At each step, the decay may be either Alpha decay (producing “alpha particles,” essentially Helium with great kinetic energy) or Beta decay (producing “beta particles,” or electrons emitted with high kinetic energy).  These particles are emitted in addition to the gamma rays (pure energy) that inevitably result from all radioactive decay as well as from collisions between particles.

The highly charged particles and gamma rays heat the water through the transference of kinetic energy.  Temperature is our measure of the average kinetic energy of the molecules of a substance.  Just like the interaction of billiard balls on a pool table, the collision of one highly charged particle with another transfers energy into the latter, which may then itself collide with other atoms or molecules.  Through this process, the average energy of the water molecules is increased, and therefore the temperature rises.

The Heat Exchanger and Transfer of Power

The heated water passes up and out of the core to the heat exchanger.  The heat exchanger is, in basic characteristics, little different from the radiator in a car or in an air conditioner. 

The speed of heat transfer depends on the time that the two materials are in contact as well as the surface area they share.  While the coolant must flow at high speed to prevent boiling in the core, the surface area between the two substances can be modified.  For this reason, air conditioners, radiators, and heat exchangers tend to share the same basic design: a tube or pipe through which one fluid flows, twisting and turning to maximize the length of the pipe while minimizing the space it occupies, comparatively narrow in diameter to increase the ratio of coolant that is flowing “in contact” with the transfer surface.  The pipes are joined to and pass through long, thin metal strips, which increase the surface area over which the heat transfer can occur.

The heat exchangers in radiators and air conditioners can be exposed to air.  In a nuclear power plant, however, it is the heat itself that we want to capture and force to perform useful work.  As a result, the pipes and metal strips are contained inside of a tank.  The heat exchanger is thus very similar to the reactor core, producing a great amount of heat that must be conveyed away from the heat exchanger chamber.

The Secondary Side of the Heat Exchanger

As in the reactor core, boiling in the heat exchanger is to be avoided.  Although damage is unlikely to result as the temperature of the primary coolant is far below the melting point of steel, boiling would severely impair efficiency.  Therefore, coolant flow in the secondary system is maintained at such a point that the water is heated to less than the point at which it would boil.  This is accomplished through varying the speed at which the secondary coolant is pumped through the heat exchanger.

Turning the Turbine

However, water is an inefficient way to turn a turbine: submarines and power plants could not operate the way an old-fashioned mill works.  There is a tremendous amount of energy within the water, which is best harnessed by depressurizing the water to the point where its temperature exceeds its boiling point, causing it to flash to steam.  Water, in its steam phase, occupies approximately one thousand times the volume that it occupies as a liquid.  The resulting power is what turns the turbines in a nuclear power plant.

The steam passes at supersonic speed through the blades of the turbine, which are great in number, tightly packed, and designed to draw the greatest amount of energy out of the steam.  The steam is allowed to drift through to the next stage of the secondary coolant system, the condenser, where the steam is suddenly and thoroughly cooled. 

The Condenser

The principles of heat transfer are applied again in the condenser, where a “tertiary coolant” is used to cool the steam coming from the turbine.  This coolant is seawater in Naval vessels and some land-based power plants, or freshwater in other land-based power plants.  The cool liquid is pumped in, cooling the pipes and contact areas within the condenser.  The steam hits these contact areas and condenses, much as moisture condenses on a cold glass on a hot, humid day.  Due to the differing volumes of a unit of steam as compared to a unit of water, the pressure in the condenser is lowered considerably in a very short amount of time.  This increases the pressure differential across the blades of the steam turbine, making it more effective.  Meanwhile, the secondary coolant system pumps collect the water from the condenser and propel it to the heat exchanger to continue the cycle.

Conclusion

The principles of the nuclear power plant are not very complex, and in fact bear great similarities to the steam plants developed during the Industrial Revolution.  The differences are mainly in the source of energy (nuclear power, which results both in its great efficiency and its potential dangers) and in the details, which have been engineered to extract the greatest amount of efficiency from the process.  Thermodynamically, the nuclear power plant is quite simple: it transfers the heat from its primary source (the fission reactions) to a primary coolant in the reactor core, transfers the heat from the primary coolant to the secondary coolant in the primary heat exchanger, transfers the kinetic energy from the secondary coolant to the turbine (which transfers that kinetic energy into electrical power with the generator), and transfers the remaining heat from the secondary coolant into a tertiary coolant in the condenser to achieve maximum efficiency.

Also noteworthy, though, is the complexity of the system that the nuclear reaction necessitates.  Reliable safeguards must exist to:

• prevent the core from boiling
• prevent transfers of fluid and possibly radioactive material from the primary to secondary systems and hence to the environment
• prevent damage to systems resulting from the extremes of temperature and pressure found in nuclear power plants
• protect both plant personnel and the world as a whole from the possible catastrophic effects of meltdown

In addition, nuclear power plants necessitate centralization of political power to ensure a consistent and dominant regulatory chain and provide vectors for military or attack.  From many standpoints, the long-term promise of technologies such as solar and wind power are immensely preferable to nuclear power.  The rates at which these technologies are currently progressing, however, make a single monolithic outlay for total adoption unappealing.  Interest in these alternatives to traditional and inherently centralized power is certain to grow, and will no doubt eventually replace nuclear power as the primary civil power schema.