July 22, 2008
Exclusive: The Anatomy of a Civil Nuclear Reactor
Last time, we discussed the basic components of a nuclear reaction, and a few introductory concepts about nuclear history, nuclear reactors, and nuclear weapons. In this installment, we'll explore the basic anatomy of a nuclear reactor, types of civil nuclear reactors, and the nuclear enrichment cycle.
All nuclear reactors are not created equally. Aside from the basic issue of energy output, there are a number of factors that combine to determine a reactor's classification. Reactors can be classified by the type of fuel they use, or by the substance they use to moderate the nuclear reactor. They can be classified by coolant type, or by design generation, fuel phase, or application. Not all reactors are alike; different designs are used for different purposes, and have different capabilities. A detailed description of each and every combination could easily fill a textbook, but all nuclear reactors share basic components in common.
With few exceptions, electricity is produced by generators attached to turbines. When a turbine spins, the attached generator creates usable energy by forcing the interaction of opposing electromagnetic fields. A civil nuclear fission reactor facilitates this by providing the heat for a standard steam power plant. Once the steam reaches optimal conditions, it is then allowed to pass through the turbine from a high pressure area to a low pressure area, spinning the turbine in the process. This process generates electricity. Steam power can use any number of different fuels, such as coal, natural gas, or petroleum - any heat source that can sufficiently heat water into superheated steam. In comparison to these other fuel types, a nuclear reactor is remarkably efficient.
Civil nuclear reactors require fissile material. This material can take several forms. Many reactors use low-enriched Uranium. Low-enriched Uranium consists of fuel rods in which the concentration of U-235 is between three and twenty percent of the total uranium content. This is the minimum U-235 content necessary to create a self-sustaining reaction. This minimum U-235 content requirement is known as critical mass. This is the minimum amount necessary for a continued, controlled chain reaction. This controlled chain reaction produces the heat necessary to heat the water in a pressurized water reactor, which in turn provides the heat for the attached steam turbine system. With the exception of heat-exchanging pipes, in which heat from the nuclear reactor cycle is transferred through proximity with water from the steam system, the two systems operate separately. The radiation from the reactor remains within the reactor vessel, and with proper maintenance and monitoring, radiation is contained entirely within the reactor building.
As complex as a nuclear reactor may seem, it consists of a few simple components: fissile material (usually some type of Uranium), assembled within a reactor vessel and arranged in order to generate a controlled fission reaction used to produce heat. It also includes a cooling infrastructure, a system for altering the position of the fuel rods (for the purpose of controlling the rate of nuclear reactions), and a system for moderating the flow of neutrons caused by the reaction itself.
Various reactor designs use different types of fuel. Some reactors, such as the Canadian Deuterium Uranium (CANDU) reactor type, are actually able of using naturally-occurring Uranium to generate a fission reaction. As mentioned previously, many reactors use fuel rods, bundles, or pebbles composed of enriched Uranium. Some reactors use Plutonium, an element created as a by-product of some Uranium reactors; however, Plutonium is more commonly used in nuclear weapons. Not unlike the differences between diesel and gasoline automobile engines, different fuels necessitate different reactor configurations, and these configurations have different technical properties. In particular, some reactors can create fissile material capable of being used in a nuclear weapon (highly enriched Uranium, or Plutonium), and some can't.
The nuclear enrichment cycle itself begins with mining. Uranium occurs naturally in extremely low concentrations in soil, but is most commonly used from any of a small number of locations in which higher concentrations of Uranium are found. Once the Uranium is mined, it then undergoes a chemical leaching process that produces a concentrated substance known as yellowcake. Yellowcake is radioactive, and its content is between 60 and 70% pure Uranium. This substance then undergoes several chemical processes, after which point it is heated to around 150° Fahrenheit to create Uranium hexafluoride gas. Uranium hexafluoride is highly corrosive and reactive, and requires special handling and specially engineered equipment in order to be safely used. It is at this point that the actual process of enrichment begins.
Although there are multiple enrichment methods, the most common employs centrifuges. In an enrichment centrifuge, the Uranium hexafluoride gas is pumped into the centrifuge vessel, then further heated and spun at high speeds. While the heavier U-238 isotopes collect at the bottom of the centrifuge to form depleted Uranium, the lighter U-235 isotopes are then pumped out of tubes at the top of the centrifuge. Depleted Uranium has various military and civil applications, most notably in armor-piercing ammunition and tank armor; however, its use is controversial, as Uranium exposure is believed by some to cause health problems. This process must be repeated many times in a large assembly of centrifuges known as a cascade in order to produce enriched Uranium, and even more times in order to produce weapons-grade, highly enriched Uranium. This enrichment process is the basis of most civil and military nuclear programs.
The next installment will address the military applications of nuclear technology.