A Nuclear Power Plant
Courtesy of R2 Controls
Nuclear reactors produce energy through a controlled fission chain reaction (see The First Chain Reaction). While most reactors generate electric power, some can also produce plutonium for weapons and reactor fuel. Power reactors use the heat from fission to produce steam, which turns turbines to generate electricity. In this respect they are similar to plants fueled by coal and natural gas. The components common to all nuclear reactors include a fuel assembly, control rods, a coolant, a pressure vessel, a containment structure, and an external cooling facility.
The speed of the neutrons in the chain reaction determines the reactor type (Figure 1). Thermal reactors use slow neutrons to maintain the reaction. These reactors require a moderator to reduce the speed of neutrons produced by fission. Fast neutron reactors, also known as fast breeder reactors (FBR), use high speed, unmoderated neutrons to sustain the chain reaction.
Figure 1 – Types of Nuclear Reactors
Thermal reactors operate on the principle that uranium-235 undergoes fission more readily with slow neutrons than with fast ones. Light water (H2O), heavy water (D2O), and carbon in the form of graphite are the most common moderators. Since slow neutron reactors are highly efficient in producing fission in uranium-235, they use fuel assemblies containing either natural uranium (0.7% U-235) or slightly enriched uranium (0.9 to 2.0% U-235) fuel. Rods composed of neutron-absorbing material such as cadmium or boron are inserted into the fuel assembly. The position of these control rods in the reactor core determines the rate of the fission chain reaction. The coolant is a liquid or gas that removes the heat from the core and produces steam to drive the turbines. In reactors using either light water or heavy water, the coolant also serves as the moderator. Reactors employing gaseous coolants (CO2 or He) use graphite as the moderator. The pressure vessel, made of heavy-duty steel, holds the reactor core containing the fuel assembly, control rods, moderator, and coolant. The containment structure, composed of thick concrete and steel, inhibits the release of radiation in case of an accident and also secures components of the reactor from potential intruders. Finally, the most obvious components of many nuclear power plants are the cooling towers, the external components, which provide cool water for condensing the steam to water for recycling into the containment structure. Cooling towers are also employed with coal and natural gas plants.
Figure 2 – The Fate of Plutonium in a Thermal Reactor
The power output or capacity of a reactor used to generate electricity is measured in megawatts of electricity, MW(e). However, due to the inefficiency of converting heat into electricity, this represents only about one third of the total thermal energy, MW(t), produced by the reactor. Plutonium production is related to MW(t). A production reactor operating at 100 MW(t) can produce 100 grams of plutonium per day or enough for one weapon every two months.
Another important property of a reactor is its capacity factor. This is the ratio of its actual output of electricity for a period of time to its output if it had been operated at its full capacity. The capacity factor is affected by the time required for maintenance and repair and for the removal and replacement of fuel assemblies. The average capacity factor for U.S. reactors has increased from 50% in the early 1970s to over 90% today. This increase in production from existing reactors has kept electricity affordable.
Figure 3 – Pressurized Water Reactor
Figure 4 – Boiling Water Reactor
(1) the core inside the reactor vessel creates heat, (2) a steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat, (3) the steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line, and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity.
Courtesy of the U.S. Nuclear Regulatory Commission
Heavy water reactors use D2O as the coolant/moderator, allowing natural, unenriched uranium to be used as the fuel. This is possible because D2O absorbs fewer neutrons than H2O. The heat transfer system is similar to that of the PWR, with the steam generator located within the containment structure. The heavy water reactor shown in Figure 5, known as CANDU, was developed in Canada and sold globally. The cost advantage in fuel is offset by the expense of producing D2O by a chemical exchange process or electrolysis. The individual fuel assemblies of a heavy water reactor can be replaced without shutting down the reactor, thus eliminating the down time involved with refueling a light water reactor. However, spent fuel produced by a heavy water reactor contains more plutonium and tritium than that from light water reactors. This, coupled with the difficulty in monitoring a continuously fueled reactor, causes concerns about the proliferation of nuclear weapons. Thus, heavy water is classified as a "sensitive material" because a nation possessing it can produce plutonium directly from natural uranium, eliminating the need for uranium enrichment.
Figure 5 – Heavy Water Reactor
Courtesy of Atomic Energy of Canada Limited
Another type of thermal reactor is graphite moderated and gas cooled. Twenty-six Magnox reactors (Figure 6), employing pressurized carbon dioxide as the coolant, were built in the United Kingdom but are currently being phased out of service. These gas cooled reactors have the same advantages as the heavy water reactors in that they can use natural uranium fuel and be fueled continuously.
Figure 6 – Gas Cooled Reactor
(Courtesy Osterreichisches Ökologie-Institut)
The pebble bed represents a new design for a gas cooled reactor (Figure 7). It uses helium as the coolant and a fuel consisting of low enriched uranium dioxide coated with silicon carbide and pyrolitic carbon encased in small graphite spheres. The graphite serves as the moderator. Fresh fuel is added at the top of the reactor and the spent fuel removed from the bottom, allowing continuous operation. The helium coolant is maintained at high temperatures and pressures, which increases the efficiency of heat transfer and power production. South Africa is leading the effort to develop this reactor technology.
Figure 7 – Pebble Bed Reactor
Courtesy of the European Nuclear Society
Fast Neutron Reactors
Figure 8 – Fast Neutron Breeder Reactor
Courtesy of Nationmaster
An FBR is configured and operated to produce more fuel than it consumes. Fast neutrons are readily absorbed by fertile uranium-238, which then can undergo successive beta emissions to become fissile Pu-239. Thorium-232 is another fertile isotope that can absorb neutrons and produce fissile uranium-233 by beta emissions. These fissile isotopes can be reprocessed for nuclear reactor fuel or weapons. Because fast neutrons are not as efficient in producing fission as slow ones, FBRs require uranium oxide containing 20% U-235, plutonium oxide, or a mixture of these oxides, known as MOX, as fuel.
Originally FBRs were thought to be a means of extending global uranium resources by producing fissile Pu-239 or U-233 as reactor fuel. However, problems with reactor operations and material components combined with the discovery of new uranium deposits mean that FRBs are not economically competitive with existing thermal reactors. FBR research has produced technical advances but the limiting factor continues to be the price of FBR-produced reactor fuel versus the cost of uranium fuel. FBRs are more complex than other types of reactors and also raise concerns about the proliferation of plutonium for use in nuclear weapons.
Summary of Reactor Types
Table 1 summarizes the characteristics of the reactors discussed above.
Table 1 – Reactor Characteristics
*Percentage of U-235 isotope in the fuel compared to U-238 isotope. Natural uranium contains 0.7% U-235, slightly enriched uranium from 0.8 to 3.0% U-235, and low enriched uranium from 3.0 to 5.0% U-235.
Reactor Safety Concerns
Figure 9 - Chernobyl Nuclear Power Plant Following the 1986 Accident
Courtesy of the U.S. Department of Energy
Although the TMI accident was much smaller in scope and resulted in no deaths, it had a profound psychological impact on the public’s view of commercial nuclear power. The TMI plant was a pressurized light-water reactor, whereas Chernobyl had a reactor design similar to the one used for the reactors at Hanford, Washington, which produced plutonium for the United States nuclear weapons. Both accidents involved human error overriding safety features built into the reactor systems. Features of the new generations of reactors including passive safety systems and fewer pipes and valves are designed to minimize the potential for accidents and to limit the damaging effects should such an accident occur.
A Brief History of Nuclear Power Reactors
In addition to the reactors for generating electricity mentioned above, there are also 220 reactors powering ships and submarines. Another 284 reactors operating in 56 nations are used for research in a variety of areas. Approximately 20% of the electricity used in the United States is generated by 103 nuclear reactors, although no new reactors have been placed in operation since the 1970s. France leads the world in the generation of electricity with more than 70% coming from nuclear sources. France, Japan, Russia, and Great Britain reprocess spent reactor fuel from commercial reactors.
Interest in nuclear power has increased driven in large part by concerns over energy supplies and climate charge. Reactor designs have evolved from the first generation of reactors cited above to the generation II systems that included the PWR, BWR, and Magnox reactors of the 1970s and 1980s (Figure 10). Table 2 includes some of the designs for generation III and III+ reactors. These systems are touted to be more economical to build and operate, as well as containing improved passive safety features. Figures 11 and 12 show some of the advances in reactor design for the European Pressured Water Reactor and the Westinghouse AP1000 PWR.
Figure 10 – Evolution of Nuclear Reactors
Table 2 –Designs for Generation II and III+ Reactors
Courtesy of the U.S. Energy Information Administration
Figure 11 – European Pressured Water Reactor
Courtesy of Areva
Figure 12 – Design Features of the Westinghouse AP1000
Finally, designs for Generation IV reactors are being developed for deployment by 2030. These reactor designs will continue to address concerns associated with economics, safety, proliferation, waste, and reactor security.
Complete Bibliography on Nuclear Reactors from Alsos Digital Library for Nuclear Issues
|2005-2009 Kennesaw State University
Principal Investigator Laurence Peterson
Project Director Matthew Hermes