Nuclear fission control rods

The energy produced in a nuclear reactor vessel is the result of a nuclear fission (atom splitting) process that occurs when sufficient nuclear material is brought together (critical mass). Under these circumstances, a chain reaction occurs and an external supply of neutrons is not required. A nuclear fuel control rod system raises or lowers the nuclear fuel (which is contained within fuel rods) inside the reactor vessel. 

In nuclear reactors control rods control the amoimt of fission that can occur. When the control rods are inserted into the fuel assembly, they absorb neutrons, preventing them from inducing fission in the fuel rods. 

The turbine and generator components of a nuclear power plant have exact counterparts in power plants fueled by fossil fuels. The uniqueness of the nuclear power plant lies in its core. The core is a nuclear reactor where fission takes place under conditions that keep the reactor operating just below the critical level. The core contains three parts fuel rods, moderators, and control rods. These components act on the flow of neutrons within the core, as shown in Figure 22-13. The fate of neutrons must be controlled carefully. Fission must be sustained at a steady rate that produces sufficient energy to mn a generator, but the rate must not be allowed to increase and destroy the reactor. 

Cadmium is a neutron absorber, making it useful as control rods in nuclear reactors. The rods are raised to activate the reactor and then lowered into the reactor to absorb neutrons that halt the fission reaction. 

Hafnium has a great affinity for absorbing slow neutrons. This attribute, along with its strength and resistance to corrosion, makes it superior to cadmium, which is also used for making control rods for nuclear reactors. This use is of particular importance for the type of nuclear reactors used aboard submarines. By moving the control rods in and out of a nuclear reactor, the fission chain reaction can be controlled as the neutrons are absorbed in the metal of the rods. The drawback to hafnium control rods is their expense it costs approximately one million dollars for several dozen rods for use in a single nuclear reactor. 

Erbium has limited commercial use, but it is used as an alloy metal for vanadium to make it easier to work and to form spring steel. The oxide of erbium is pink, which is used to color glass and to make lasers that will operate at normal room temperatures. It has hmited use as control rods in nuclear fission reactors. 

Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fission chain reaction that results in a rapid explosion with tremendous energy and becomes a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these fuels are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass... 

Constructive Interference when waves combine to reinforce each other Control to remove or account for the effect of a variable in an experiment Control Rod rods used in nuclear reactor to absorb neutrons and control fission of radioactive fuel... 

A fission reactor contains three components nuclear fuel rods, control rods, and a liquid (usually water) to transfer the heat created by fission from the reactor to the turbine. The nuclear fuel is primarily uranium-238 plus about 3 percent uranium-235. Because the uranium-235 atoms are so highly diluted with... 

The fission neutrons at birth have energies of approximately 1 to 2 MeV In a thermal reactor the neutron energy is rapidly reduced through collisions with light nuclei to thermal (—.02 to 1 eV), to promote for more efficient capture. Besides the nuclear fuel, there are many other materials in the reactor core also competing for the neutrons, including the moderator (the material used to slow down or thermalize the neutrons), fertile nuclides that produce additional fissile material (discussed in a later section), neutron poisons present in control rods, the coolant, fuel element cladding, and other structural materials. 

When controlled carefully, fission can be used to produce electricity. In nuclear power plants, moderators are used to slow down the neutrons produced during nuclear fission. It is common to have moderators made from graphite or with heavy water, 2H20. Control rods are also placed in the nuclear reactor to absorb neutrons and slow down the rate at which the fission takes place. 

The term eff — 1 is called excess reactivity, and /eff — l)/ eff is called reactivity. Because the fissile material is continuously used up by fission and because the fission products absorb neutrons, a certain excess reactivity is necessary to operate a nuclear reactor. This excess reactivity is compensated by control rods that absorb the excess neutrons. These control rods contain materials of high neutron absorption cross section, such as boron, cadmium or rare-earth elements. The excess reactivity can also be balanced by addition to the coolant of neutron-absorbing substances such as boric acid. 

When the element uranium is bombarded by neutrons, a unique reaction called fission takes place. The uranium nucleus breaks into two pieces, which fly apart with a large release of energy. In addition, several extra neutrons are emitted. These cause more uranium nuclei to split apart, which creates more energy and more neutrons in a so-called chain reaction process. In an atomic bomb, the chain reaction becomes an uncontrolled explosion. In a nuclear power plant, the chain reaction is maintained in a steady state by control rods which absorb extra neutrons. 

Safety features at a nuclear power plant include automatic shutdown of the fission process by insertion of control rods, emergency water cooling for the cote in case of pipeline breakage, and a concrete containment shell. It is impossible for a reactor to have a nuclear explosion because the fuel enrichment in a reactor is intentionally limited to about 3% uranium-235, while almost 100% pure uranium-235 is required for a bomb. The worst accident at a PWR would be a steam explosion, which could contaminate the inside of the containment shell. 

The source of energy in a nuclear reactor is a fission reaction in which neutrons collide with nuelei of uranium-235 or plutonium-239 (the fuel), causing them to split apart. The products of a fission reaction include not only energy but also new elements (known as fission products) and free neutrons. A constant and reliable flow of neutrons is insured in the reactor by a moderator, which slows down the speed of neutrons, and by control rods, which limit the number of neutrons available in the reactor and, hence, the rate at which fission can occur. In a nuclear weapon, the fission chain reaction, once triggered, proceeds at an exponentially increasing rate, resulting in an explosion in a nuclear reactor, it proceeds at a steady, controlled rate. Most commercial nuclear power plants are incapable of undergoing an explosive nuclear chain reaction, even should their safety systems fail this is not true of all research reactors (e.g., some breeder reactors). 

Gadolinium is used in control rods in nuclear power plants. Energy produced during nuclear fission is used to generate electricity. Nuclear fission is the process in which large atoms (usually uranium or plutonium) break apart, releasing energy. The smaller atoms produced are called fission products and are radioactive. 

In order to make electricity from nuclear fission, the fission reaction must be carefully controlled. To do that, the number of neutrons must also be kept under close control. Hafnium has the ability to absorb ( soak up ) neutrons very easily. It is used in rods that control how fast a fission reaction takes place. This property is one of the few ways in which hafnium differs from zirconium. Although hafnium is very good at... 

Another potential use for holmium is a result of its very unusual and strong magnetic properties. It has been used in alloys with other metals to produce some of the strongest magnetic fields ever produced. Holmium also has some limited use in the manufacture of control rods for nuclear power plants. Control rods limit the number of neutrons available to cause the fission of uranium in nuclear reactors, thus controlling the amount of energy produced in the plant. 

In a nuclear reactor, represented in Figure 12, the fuel rods are surrounded by a moderator. The moderator is a substance that slows down neutrons. Control rods are used to adjust the rate of the chain reactions. These rods absorb some of the free neutrons produced by fission. Moving these rods into and out of the reactor can control the number of neutrons that are available to continue the chain reaction. Chain reactions that occur in reactors can be very dangerous if they are not controlled. An example of the danger that nuclear reactors can create is the accident that happened at the Chernobyl reactor in the Ukraine in 1986. This accident occurred when technicians briefly removed most of the reactor s control rods during a safety test. However, most nuclear reactors have mechanisms that can prevent most accidents. 

The fission within a nuclear reactor is started by a neutron-emitting source and is stopped by positioning the control rods to absorb virtually all of the neutrons produced in the reaction. The reactor core contains a reflector that acts to reflect neutrons back into the core where they will react with the fuel rods. A coolant, usually water, circulates through the reactor core to carry off the heat generated by the nuclear fission reaction. The hot coolant heats water that is used to power stream-driven turbines which produce electrical power. 

Most commercial nuclear power plants in the United States are light water reactors, moderated and cooled by ordinary water. Figure 26-12 is a schematic diagram of a light water reactor plant. The reactor core at the left replaces the furnace in which coal, oil, or natural gas is burned in a fossil fuel plant. Such a fission reactor consists of five main components (1) fuel, (2) moderator, (3) control rods, (4) cooling system, and (5) shielding. 

Control rods Rods of materials such as cadmium or boron steel that act as neutron absorbers (not merely moderators), used in nuclear reactors to control neutron fluxes and therefore rates of fission. 

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