Nuclear Reactors

What is a nuclear reactor?
A nuclear reactor is a device in which the nuclear reaction, called “fission” is sustained and controlled in a self-supporting manner.

What do you mean by “fission”?
Generally speaking, fission means that the nucleus of an atom has been split into at least two other nuclei. Fission is typically initiated when a neutron interacts with a heavy atom, such as uranium. The neutron causes the uranium nucleus to split, releasing a great amount of energy, and producing two or three more neutrons. Because more neutrons are produced than are necessary for the reaction, it is easy to see how a self-sustaining reaction can take place . . . at least for as long as there are fissionable atoms available.

What kinds of atoms can be split apart by fission?
The most common are Uranium-233 (U-233), Uranium-235 (U-235) and Plutonium-239 (Pu-239).

Are all reactors alike?
No, they are not. Some are designed to take advantage of the heat that is produced in the reaction, and others are designed to take advantage of the neutrons that are produced. Although there are many varieties of nuclear reactors, they all have certain common features.

What are these features?
First, each must contain some sort of fissionable material, sometimes called “fuel”. They also contain some sort of “moderating material”, a “reflector” to conserve escaping neutrons, and some way of removing the heat that is generated. Finally, each is equipped with measuring and controlling instruments, and a variety of protective devices.

What kind of fuel is in a reactor?
The most common fuel is uranium. Typical fuel elements are made of uranium as it occurs naturally, which contains a small fraction of U-235. Other types contain uranium that has been “enriched” in U-235.

What does a “moderator” do?
It typically takes “slow” neutrons to cause U-235 atoms to fission. However, during the fission reaction, “fast” neutrons, meaning neutrons with high energies, are produced. So in order to keep the reaction going, a means of slowing down the neutrons so that they can be absorbed by another U-235 atom is necessary.

What are moderators made of?
Plain old water is the most common moderator. Other types are “heavy water”, which is water molecules that contain deuterium (rather than hydrogen), beryllium, and graphite (carbon).

You said that there must be some way of removing heat. How is that done?
The type of coolant used is specific to the type of reactor. For example, power reactors use water as a coolant, while breeder reactors, meaning those designed to produce additional fissionable material, use liquid sodium metal. Gas cooled reactors, on the other hand, use carbon dioxide or helium as a coolant.

Now, what about the reflectors?
Reflectors are used to make neutrons that are on their way out of the core of the reactor (i.e., the fuel source) head back into the core in order to produce additional fissions. In most cases, beryllium and carbon are used.

If the escaping neutrons are directed back to the reactor core, is there any way of slowing the reaction down or speeding it up?
Absolutely. Devices known as “control rods” are used for this. Control rods are made of materials that are good at “capturing” neutrons, thus removing them from further reaction with the fuel. As the control rods, which are interspersed with the fuel elements, are lowered from the core, the reaction increases, meaning more and more neutrons are produced, and more and more U-235 is fissioned. When the control rods are raised into the core, neutrons are captured and the reaction slows down.

You said previously that some reactors are designed to take advantage of the neutrons produced. What kinds of reactors are these?
These are often called “research reactors”. Research reactors can provide intense sources of neutrons that can be used for a variety of purposes, including the production of other radioactive elements, known as by-product materials, for commercial and medical uses. Relatively speaking, research reactors are small, compared to power reactors. Their power levels can range from as little as 10 milliwatts to a few megawatts. And the size of the fuel source is only a few kilograms. However, that fuel is often highly enriched fuel (i.e., 20% or more U-235, as compared to the natural abundance of less than one percent).

What about reactors designed to take advantage of heat?
These types of reactors are called “power reactors”. They use the heat produced during fission to create steam, which is used to drive a turbine generator. Most power reactors hold around 100 tons of fuel, that is enriched to between one (1) and five (5) percent. A typical power reactor has a power output of around 1,100 to 1,200 megawatts.

Are all power reactors the same?
No. Some are “boiling water” reactors, where the fuel sits, like a heating element, in a tank of water, and the steam that comes off the top of the water is used to turn the turbine generator. Others are “pressurized water reactors”, where the fuel sits inside of and heats up a closed water loop. The loop of hot water then runs through a steam generator, along side of a secondary loop of water that has not come in contact with the fuel. The water in the secondary loop then flashes to steam, which is then used to turn the turbine generator.

What about the reactors that use gas as a coolant?
In this case, the gas is heated by the fuel in a closed loop. The loop runs through and heats up a tank of water. The steam from that tank is then used to turn the turbine generator.

What kind of reactor was at Three Mile Island in 1979?
The reactor involved in that incident was one of two pressurized water reactors located just outside of Harrisburg, Pennsylvania. After some initiating events and some faulty decision-making on the part of the reactor operators, the core of the Three Mile Island reactor lost some of its water. As a result, the temperature inside the core increased, to the point where the fuel actually melted. At all times, however, the fuel remained within the reactor building. Unfortunately, gaseous radionuclides produced during the fission process escaped the reactor building, and were subsequently released to the atmosphere. However, the maximum radiation doses incurred by the population in the immediate vicinity of the plant were only a small fraction of natural background radiation doses.

What about the Chernobyl reactor?
This reactor, located in the former Soviet Union, used a graphite moderator, and water as a cooling mechanism. During a generator test that occurred in 1986, a series of delays and faulty decision-making resulted in power level and subsequent coolant level drops, along with increases in water temperature and steam production. Because the water volume was replaced by steam, this reactor type, which is not at all like the commercial power reactors in the United States, responded by increasing its power level. Although there was an order issued to quickly insert the controls rods (called a SCRAM), it took over 20 seconds for that to occur . . . too late to prevent a high power surge which melted the fuel. The hot fuel fragments were then injected into the cooling water, causing the power level to surge to over 100 times its rated maximum. This caused the reactor internals to explode through the roof.

These two events are awfully scary. Exactly how safe is nuclear power?
These two incidents were the result of unusual sequences of events and, in the case of the Chernobyl reactor, a difficult reactor design. These unfortunate and non-standard occurrences were unique to many, many years of safe reactor operation world-wide. As with any technology, we learn from our mistakes; thus the safety of nuclear power is even greater today than it was in, say, 1979.

Do you know of anywhere else I can get information about these incidents?
Sure.  There is a very good write-up about nuclear power plant safety on the internet. The address of that site is http://users.owt.com/smsrpm/nksafe.

If I compare nuclear power to other forms of power, like natural gas plants or coal-burning plants, is nuclear power a clean source of electrical energy?
Yes it is, for three very important reasons. First, the water discharged from a nuclear power plant during routine operations contains no harmful pollutants and easily meets regulatory standards for temperature designed to protect aquatic life. Second, nuclear plants do not burn fossil fuels. Therefore, they do not create acid rain, soot, urban smog, or carbon dioxide (a greenhouse gas). In addition, the air, water, and soil located near these plants are so clean that almost all have their own nature preserves housing a wide variety of plant and animal life. And third, from a radiation perspective, less radiation is received each year from living next door to a nuclear power than would be received in one round trip airline flight from New York to Los Angeles. When all of this is taken into account, it is clear that nuclear power is definitely a “clean” source of energy.

You say that radiation doses to people living near a nuclear power plant are low. How can I be sure?
Nuclear power plants are mandated, by law, to ensure all emissions are well below the levels that might result in radiation-related health effects. In fact, on average, these plants contribute less than 0.05% to the annual average background radiation exposure incurred by members of the population who reside right at the boundary fence. People who live within 50 miles of a U.S. nuclear power plant site receive less than one one-hundredth of a millirem from the plant.

How do you know?
Federal law requires the dose to populations living in the vicinity of all licensed nuclear power stations to be calculated once per year. These calculations are based upon the results of a comprehensive series of measurements made throughout the year, including measurements of radioactivity in air, water, vegetation, and direct exposure.

Can I see these estimates for the nuclear power plant in my neighborhood?
Yes you may. Copies of each facility’s environmental report for the year can sometimes be viewed at the visitor center for the site. If not there, or if there is no visitor center, you may read the reports in the U. S. Nuclear Regulatory Commission’s Public Document Room, or PDR. The PDR is located in Washington, D.C., but there are sometimes local PDRs set up in the vicinity of licensed facilities. You can find out by calling the information line at your local power plant.