Inside a Nuclear Reactor

Nuclear energy is the largest producer of electric power without emitting (releasing) any significant pollution or greenhouse gases into the air. It gives us approximately 20 percent of our electricity each year. By comparison, the second-largest non-emitting producer is hydroelectric power (about 7 percent), followed by wind and solar power (each about 2 percent).

Since nothing is burned in the generation of nuclear energy, no harmful emissions are vented into the atmosphere. This is why nuclear power plants do not have the smokestacks common to fossil fuel generation facilities. Some nuclear power plants use large cooling towers to remove excess heat from cooling water before it is returned to the waterways. The discharge from these towers is water vapor, not smoke or radioactive matter.

So how is nuclear power produced, especially without creating air pollution?

A nuclear power plant produces electricity in much the same way as other electric power plants: Water is heated to produce steam, and the steam turns turbines that turn a generator shaft to make magnetism, which is what electricity is. The difference in a nuclear power plant is how the water is heated. Most power plants today use coal or natural gas to heat the water to become steam. In a nuclear power plant, the heat is created inside a reactor, where atoms are split to release their energy. This release produces great amounts of heat.

The design of a nuclear reactor is complex, but basically, each reactor has six main elements: (1) Fuel, (2) Control rods, (3) Coolant, (4) Moderator (5) Shield, (6) Reflector, and (7) Nuclear Vessel. Let's look at each part.

Fuel Fuel is needed in all energy-producing processes. For fuel to produce energy, it must be altered, which happens inside the reactor core in a nuclear power plant. In most U.S. reactors, the fuel consists of pellets of enriched uranium dioxide. The pellets are held inside long metal tubes called “fuel rods.” These fuel rods are put together, or “bundled,” to form the fuel assembly.

Control rods Control rods are metal tubes that are not filled with fuel. They absorb some of the neutrons released during fission. [link to glossary term] When the rods are right next to the fuel assembly, they absorb more neutrons. When they are farther away from the fuel assembly, they absorb fewer neutrons. The control rods are raised or lowered next to the fuel assembly to either slow down or speed up the release of energy from the splitting atoms.

Coolant The cooling process in a nuclear reactor is similar to the way a car radiator works to cool the engine. Because the fuel assembly gets very hot during a chain reaction, a coolant-usually water-is pumped through the reactor to carry the heat away. As with most power plants, two-thirds of the energy produced by a nuclear power plant goes into waste heat, and that heat is carried away from the plant in the coolant water (which remains uncontaminated by radioactivity).

Moderator When a neutron causes fission, fast neutrons are released. These fast neutrons need to be slowed down to lower energy levels. The material used to slow down the fast neutrons is called the “moderator.” Fast-moving neutrons strike the moderator material, which is not efficient at absorbing them, and slows them down. Water is an excellent moderator, because the water can also serve as a coolant.

Shield Nuclear fission results in the release of neutrons and several other by-products such as alpha rays, beta rays, gamma rays, and fast-moving neutrons. Radiation shielding is required to prevent this harmful radiation from leaving the reactor and affecting people and materials outside the reactor. Radiation shielding is made of concrete and steel.

Reflector Fast-moving neutrons are controlled with both a moderator and reflectors to keep them inside the reactor core so that a sustained and controlled chain reaction takes place. To reduce neutron loss, the inner surface of the reactor core is surrounded by a material to reflect these escaping neutrons back to the reactor core. This lining is known as “reflecting materials.” Various materials are used as reflecting material, including water and carbon.

Pressure Vessel The housing that contains all the components in the core is called the “vessel.” The vessel holds the coolant, provides a space for the rods, and acts as a buffer between the core and the environment outside the vessel. The material used to construct the vessel must be very strong so that it can withstanding great pressures; steel is commonly used for vessel construction.

Building a Nuclear Power Plant

Statistics show that U.S. commercial power plants have operated safely for more than a half-century. The success of nuclear power plants comes from continuing to apply technology advances to improve plant safety and health. It also results from the strict licensing, building standards, and regulations governing plant operations.

A nuclear power plant can only be built in the United States once the Nuclear Regulatory Commission (NRC) reviews both construction and operating plans. Before issuing a building permit and operating license for a nuclear plant, the NRC carefully reviews technical aspects of the proposed plan to ensure it will not present any undue risk to people, the environment, or national security interests.

In addition to NRC review, the complete licensing and constructino of a nuclear power plant requires a lengthy series of licenses and permits from Federal, State, and local government agencies. These permits and licenses determine where the plant can be lcoated, whether the power is really needed, and how construction will be carried out. They also ensure the protection of land, air, water, and local plant and animal life from pollution.

Nuclear power plants are designed and built to operate for at least 40 years. Laws allow that licenses can be renewed for an additional 20 years, but the plant must undergo a strict review process to ensure it is still meeting all safety criteria.

Handling Fuel

All operations involving radioactive materials-including nuclear power plants, hospitals, research centers, and industrial processes-create radioactive wastes that must be handled and disposed of safely.

Most used fuel from nuclear power plants is stored in forty feet deep pools of water near the reactor. The water cools the fuel rods to keep them from overheating, and it serves as an effective shield to protect workers from the radiation. The level of radiation begins declining immediately, and within 10 years it has decayed by 90 percent.

For long-term storage of used fuels, Congress passed the Nuclear Waste Policy Act of 1982 to create a national policy for the safe storage and disposal of high-level radioactive waste. The waste is shipped by truck or railroads to places specially designed to safely store the material. The NRC is responsible for licensing and regulating all commercial users and handlers of radioactive materials, including waste shippers and carriers. Please visit the section, “What Is Nuclear Waste?” to learn more about nuclear waste.