THE NUCLEAR REACTOR
The basic components of a reactor for controlled release of nuclear energy are shown.
The core consists of the fuel and moderator, and the control rods are arranged for easy
movement in and out of the core.
A nuclear reactor is a unit constructed to enclose all the equipment and material necessary to produce and control the process of nuclear fission. There are different types of reactors. The power reactor is used to generate heat for conversion into steam. A research or experimental reactor is operated as a source of neutrons for producing radioactive isotopes or for performing neutron diffraction studies of materials. Both must contain certain basic components.
The nuclear fuel may be either uranium or plutonium. In the case of uranium it may be in the form of natural uranium containing 0.7 percent U-235 and 99.3 percent U-238. It may be natural uranium metal that is enriched with fissionable U-235. It also may be pure U-235 that has been completely separated from its more abundant isotope U-238.
Compounds of uranium are also used as fuel. These include uranium oxide, uranyl sulfate, and uranyl nitrate. Depending on the construction and purpose of the reactor, the uranium fuel is made into a number of different elements. Solid uranium fuels are used in the shape of rods, slugs, cylinders, flat plates, curved plates, and pellets. Plutonium, separated by special chemical means, is sometimes used in the form of a pure metal.
The second essential part of a nuclear reactor is the moderator material. It must be relatively light in mass and must not absorb neutrons. Fast neutrons, produced by fission, are slowed by a series of collisions with the nuclei of the moderator atoms. A light nucleus is most effective because its weight is relatively close to that of a neutron. Energy lost by the neutrons in collision is absorbed by the nuclei of the moderator. Among the moderators that have been used in nuclear reactors are carbon in the form of graphite, the light metal beryllium, heavy water (having a deuterium nucleus), and ordinary water.
One other essential part of the reactor is the coolant. The main coolant is a liquid or gas that is pumped or blown through the reactor core to remove heat given off mainly by the fuel. If this circulation is not maintained, the temperature of the fuel assembly can become high enough to melt the fuel elements.
The commonest method of controlling fission is by the insertion and withdrawal of a neutron-absorbing material such as a cadmium rod. As the rod is moved into the reactor, more and more neutrons are absorbed, and the fission reaction is slowed down. As it is withdrawn, and less and less of its surface is exposed for neutron absorption, the reaction rate increases. Safety, or shut-off, rods may also be provided in a reactor. These rods are made from boron or other good neutron absorbers. Any number of them can be inserted into the reactor simultaneously to bring the fission reaction to a complete stop.
A layer of material called the reflector surrounds the core containing the fuel and moderator. The reflector may be ordinary water, heavy water, graphite, or beryllium. The purpose of the reflector is to reflect escaping neutrons back into the core. In turn the reflector is surrounded by a thermal shield. The shield reflects neutrons and absorbs some radiation that is produced by the fission reaction. The thermal shield, as its name indicates, also absorbs heat created by radiation.
Finally, all internal parts of the reactor that have been described are surrounded by a biological shield. The biological shield stops radiation and neutrons that pass through the thermal shield. It is made of concrete, which may contain a heavy material such as iron and steel punchings. The biological shield is usually many feet in thickness. Its main purpose is to protect personnel working near the reactor.
Research and test reactors. There is a large variety of reactors with different design features for special uses. One type is the tank reactor. This reactor system has a grid of fuel elements enclosed in a reactor tank. The tank is usually installed in a concrete radiation shield but may be installed in a pool.
The core of the tank reactor is an assembly of aluminum-clad fuel plates usually made of an enriched uranium-aluminum alloy. The plates are placed in a fixed position in the closed reactor tank. Beryllium is often used as the primary reflector. The reactor is cooled and moderated by either ordinary water or heavy water. The heavy water usually permits lower fuel consumption and provides more uniform neutron beams from the reactor.
The power ratings of most research tank-type reactors range up to about 40,000 kilowatts. These reactors are used to study many nuclear problems. They are also employed in the manufacture of radioisotopes for medical and industrial purposes. New reactor designs, materials, and operating procedures are constantly being tested. This is done with test reactors. Those designed to check the irradiation of materials and components are usually called general-purpose test reactors. Those developed for a specific function are generally termed special test reactors.
Power reactors. A power reactor generates heat that is converted into steam. The steam can be used directly for power, as in a nuclear submarine. It can also be used to generate electric power for example, in a commercial nuclear power plant.
In 1954 the nuclear-powered submarine Nautilus was successfully launched. More submarines with atomic reactors followed. The United States Navy then began construction of nuclear-powered aircraft carriers and guided-missile cruisers. By the late 1970s the Navy had more than 75 nuclear-powered craft. A nuclear-powered merchant vessel, the Savannah, was launched in 1959. The success of the United States Navy nuclear submarine program was largely due to the efforts of Adm. H. G. Rickover.
The first commercial nuclear power plant that generated electricity in the United States was actually an outgrowth of the United States Navy reactor program. The pressurized water reactor (PWR) began production (capacity, 60,000 kilowatts) at Shippingport, Pa., in 1957. In this type of reactor, ordinary water functions both as moderator and coolant. The reactor vessel has steel walls several inches thick, and the reactor core is 3.7 meters (12 feet) high. The water enters the reactor vessel, rises through the core, and absorbs heat released by nuclear fission.
This water, under pressure of 140 kilograms per square centimeter (2,000 pounds per square inch) and at a temperature in excess of 260 C (500 F) travels through a closed coil system to a heat exchanger that also contains water. Heat is transferred from the hot reactor water through the coils to the water in the heat exchanger. The temperature and pressure of the reactor water fall. As relatively cool water, it passes out of the heat exchanger to pumps that return it to the nuclear reactor to be reheated and begin the cycle again. The reactor vessel, coolant pumps, heat exchangers, and piping are all enclosed in a concrete containment building.
The water originally in the exchanger absorbs the heat given up by the reactor water and is changed to steam. The steam is piped to a turbine. Steam pressure turns the turbine, which drives a generator. The steam then goes to a condenser, where it is changed back to water. Another cycle begins when the water returns to the heat exchanger. The electricity produced goes to an electric power station for distribution.
The pressurized water reactor is one of two basic light (or ordinary) water reactor (LWR) designs that have been approved and are in use or under construction in the United States. The other basic design is the boiling water reactor (BWR). The major difference between the PWR and the BWR is that the latter converts water to steam directly in the reactor core. The steam then turns the turbine, which drives the electricity generator. Thus the steam generator circuit of the PWR can be eliminated in the BWR.
Canada's commercial heavy water reactor (HWR), the CANDU (Canadian-Deuterium-Uranium) reactor, replaces ordinary water (H2O) with heavy water (D2O) in the pressurized loop to remove heat from the core. The deuterium in D2O is twice as heavy as the hydrogen in H2O. Since heavy water absorbs fewer neutrons than ordinary water, more thermal neutrons survive in the reactor, increasing the chance that they will hit the fissionable U-235. This permits the use of natural uranium fuel, which consists of (99.3 percent) U-238 and (0.7 percent) fissionable U-235. Because of this advantage, an HWR does not require expensive U-235 fuel enrichment.
Gas-cooled reactors (GCR) employ either carbon dioxide or helium as the coolant instead of water. Carbon dioxide is used in commercial nuclear plants in the United Kingdom and France. The use of helium is under development in the United States, where nuclear power plants supply about 18 percent of the electricity generated. More than 70 percent of the electricity in France comes from nuclear plants.
Breeder reactors. A breeder, or fast, reactor is designed to produce both power and new fuel at the same time. Breeder reactors do away with the moderator so that the neutrons retain higher velocity and kinetic energy. When these neutrons are captured by U-238, which is nonfissionable, they can convert it into a transuranic element, known as plutonium-239 (Pu-239), which is fissionable. This new fuel can be separated out after generation in a reactor for use as fuel in other reactors. Since U-238 is much more plentiful than naturally occurring U-235, the development of breeder reactors may bring a long-lasting nuclear fuel supply.
The small EBR-1 breeder reactor first produced electric power in the United States in 1952. Other breeder reactors have since been developed in the United States, France, Germany, Italy, the United Kingdom, Japan, Russia, and India. The world's only operating commercial plant is the Super-Phoenix, a fast-breeder reactor in France.
Radiation Hazards
When nuclear fission of U-235 occurs, the atom may split in any of 30 or more ways and produce a total of about 200 fission products. These products are radioactive. They decay and release beta and gamma radiation. Beta rays can penetrate a short distance into the human body. Gamma rays have great penetrating powers and can pass through the human body with relatively little absorption. Exposure to such radiation can be injurious to the human body, and repeated exposures have a cumulative effect. In excessive amounts it can produce cataracts or forms of cancer. It can also damage reproductive cells and cause mutation of genes, producing possible physical defects in future generations.
In the operation of nuclear reactors, great care is exercised to protect personnel in the area and to safeguard instruments that are sensitive. The thermal shield and biological shield are examples of this built-in protection. Precise tools have been designed to handle radioactive material by remote control. When objects or areas have been irradiated, they are decontaminated by special methods. Radiation-resistant clothing has been designed for those who work in dangerous areas.
The Nuclear Fuel Cycle
The nuclear fuel cycle refers to the overall scheme in which nuclear fuel is mined, enriched, fabricated into fuel assemblies, used in a reactor, and then reprocessed. Reprocessed fuel material may follow one of three routes: return of material to the reactor, return of the material to the reactor after it undergoes enrichment, or temporary storage as waste material.
One form of reactor fuel is natural uranium. It is mined as an ore in the usual manner. To produce fissionable material the ore is first crushed and ground to a powder. The powder is mixed with water to form a slurry, which is dissolved in acid. Addition of barium carbonate precipitates radium and other metallic impurities. The uranium solution remaining is mixed with hydrogen peroxide, and uranium oxide is precipitated. The oxide is separated by filtration. It is dissolved in nitric acid, purified with ether, and precipitated as ammonium diuranate (NH4U2O7), a bright yellow solid. The ammonium diuranate is heated in an electric furnace and passed over hydrogen fluoride to produce solid uranium tetrafluoride. This compound is reduced at a high temperature to produce pure uranium metal. The metal is machined, enclosed in aluminum containers, and is then ready for use in a reactor.
In a second method of uranium ore treatment, natural uranium is changed to a compound known as uranium hexafluoride. This compound, though corrosive and very active, is the only compound of uranium that is gaseous at moderate temperatures (around 60 C, or 140 F). Uranium hexafluoride is treated in several ways to separate the U-235 isotope from the U-238 isotope. It can be separated by gaseous diffusion through porous barriers. The compound uranium hexafluoride is most suitable because fluorine has only one isotope. Thus the course of diffusion is determined by the uranium and is not influenced by the fluorine. The process consists of passing the gas through barriers that contain billions of holes smaller than two millionths of an inch. The U-235 passes through the barriers more rapidly and goes on to the next higher stage for further concentration. The weaker portion is recycled through a lower stage. The U-235 isotope is gradually separated and concentrated. This method is utilized by the light-water reactor industry in the United States. The enriched material is converted chemically into uranium dioxide and fabricated into pellets for use in the reactor.
After plutonium is produced in a reactor, it must be separated from all other fission products that are present. This is a hazardous procedure because of radiation. The irradiated uranium slugs containing plutonium go to a primary separating plant. From this separation come four liquids, including one of impure plutonium. The plutonium solution is separated chemically and converted to pure plutonium metal. This plutonium may be used as reactor fuel and thus renew the fuel cycle. In every ton of slugs there are only a few ounces of plutonium.