nuclear reactor
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What is a nuclear reactor?
A nuclear reactor initiates and controls a self-sustaining series of nuclear fissions. During nuclear fission, a heavy atomic nucleus splits, releasing energy to power systems.
Preventive measures, like multiple safety systems, reports, inspections, operating rules, and qualification tests, ensure reactors operate safely. Mitigating measures lessen adverse consequences.
Structural components, such as the reactor vessel, hold the system together. Control rods regulate power and shut down the reactor in emergencies. Safety rods quickly terminate the chain reaction.
Nuclear reactors produce steam for electricity, study materials, and create radionuclides for various applications. Though beneficial, nuclear power raises concerns about safety, cost, waste disposal, and potential for weapons development.
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What is the main purpose of a nuclear reactor?
The main purpose of a nuclear reactor is to produce steam or hot gases that supply heat directly or drive turbines to generate electricity. Nuclear reactors rely on nuclear fission, where a heavy atomic nucleus splits into smaller fragments, releasing energy. This energy is then harnessed to create a continuous, self-sustaining series of fissions known as a chain reaction. These reactors are designed to maintain the chain reaction at a controlled level, unlike atomic bombs, where the reaction increases intensely. Besides generating power, nuclear reactors are also used to study material structures, produce radionuclides, and power systems like cardiac pacemakers.
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How does a nuclear reactor produce energy?
A nuclear reactor produces energy by initiating and controlling a sustained nuclear fission chain reaction, where the nucleus of an atom splits into two smaller nuclei, releasing a large amount of energy. This energy is used to heat water, producing steam that drives turbines connected to generators, ultimately producing electricity. The heat generated by reactors can also be used directly or to propel submarines and surface vessels.
The reactor's design incorporates preventive and mitigating safety measures to prevent the release of radioactive isotopes, such as fission products and heavy elements like plutonium.
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What are the main parts inside a nuclear reactor?
Nuclear reactors have various designs, but most share basic components. The core, a central region, houses the fuel, cladding, coolant, and moderator. Structural components, like the reactor vessel, hold the system together.
Coolant systems extract heat from nuclear fission and convert it to power. Control rods regulate the reactor's power, start-up, and shut-down. Safety systems and preventive measures ensure that reactors operate safely.
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What is nuclear fission, and how is it used in reactors?
Nuclear fission is the splitting of a heavy atomic nucleus, such as uranium or plutonium, into two smaller nuclei of roughly equal mass, a process that releases a large amount of energy. This can occur spontaneously or be induced by bombarding the nucleus with particles like neutrons.
In a nuclear reactor, the neutrons released during fission can initiate fission in nearby nuclei, creating a chain reaction. When this chain reaction is controlled, the heat produced can generate steam or hot gases, which either provide heat directly or drive turbines to generate electricity for society's benefit.
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How do people make nuclear reactors safe to operate?
To ensure the safe operation of nuclear reactors, a combination of preventive and mitigating measures are employed.
Preventive measures include:
* Strict design and operating protocols
* Functionally independent, inherently redundant, and diverse safety systems
* Regular reports and inspections
* High standards of quality assurance
* Adherence to design requirements, such as a negative power-reactivity coefficient and safety rods that can be injected under all circumstancesMitigating measures, or safety systems, include:
* Safety rod systems to quickly shut down the reactor
* Containment structures to prevent the release of radioactive materials
* Emergency core-cooling systems to cool the core and fuel region
* Emergency power systems to supply electrical power to support systemsThese measures aim to minimize risks, with regulatory agencies like the Nuclear Regulatory Commission (NRC) overseeing and judging their effectiveness.
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nuclear reactor, any of a class of devices that can initiate and control a self-sustaining series of nuclear fissions. Nuclear reactors are used as research tools, as systems for producing radioactive isotopes, and most prominently as energy sources for nuclear power plants.
Principles of operation
Nuclear reactors operate on the principle of nuclear fission, the process in which a heavy atomic nucleus splits into two smaller fragments. The nuclear fragments are in very excited states and emit neutrons, other subatomic particles, and photons. The emitted neutrons may then cause new fissions, which in turn yield more neutrons, and so forth. Such a continuous self-sustaining series of fissions constitutes a fission chain reaction. A large amount of energy is released in this process, and this energy is the basis of nuclear power systems.
In an atomic bomb the chain reaction is designed to increase in intensity until much of the material has fissioned. This increase is very rapid and produces the extremely prompt, tremendously energetic explosions characteristic of such bombs. In a nuclear reactor the chain reaction is maintained at a controlled, nearly constant level. Nuclear reactors are so designed that they cannot explode like atomic bombs.
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Most of the energy of fission—approximately 85 percent of it—is released within a very short time after the process has occurred. The remainder of the energy produced as a result of a fission event comes from the radioactive decay of fission products, which are fission fragments after they have emitted neutrons. Radioactive decay is the process by which an atom reaches a more stable state; the decay process continues even after fissioning has ceased, and its energy must be dealt with in any proper reactor design.
Chain reaction and criticality
The course of a chain reaction is determined by the probability that a neutron released in fission will cause a subsequent fission. If the neutron population in a reactor decreases over a given period of time, the rate of fission will decrease and ultimately drop to zero. In this case the reactor will be in what is known as a subcritical state. If over the course of time the neutron population is sustained at a constant rate, the fission rate will remain steady, and the reactor will be in what is called a critical state. Finally, if the neutron population increases over time, the fission rate and power will increase, and the reactor will be in a supercritical state.
Before a reactor is started up, the neutron population is near zero. During reactor start-up, operators remove control rods from the core in order to promote fissioning in the reactor core, effectively putting the reactor temporarily into a supercritical state. When the reactor approaches its nominal power level, the operators partially reinsert the control rods, balancing out the neutron population over time. At this point the reactor is maintained in a critical state, or what is known as steady-state operation. When a reactor is to be shut down, operators fully insert the control rods, inhibiting fission from occurring and forcing the reactor to go into a subcritical state.
Reactor control
A commonly used parameter in the nuclear industry is reactivity, which is a measure of the state of a reactor in relation to where it would be if it were in a critical state. Reactivity is positive when a reactor is supercritical, zero at criticality, and negative when the reactor is subcritical. Reactivity may be controlled in various ways: by adding or removing fuel, by altering the ratio of neutrons that leak out of the system to those that are kept in the system, or by changing the amount of absorber that competes with the fuel for neutrons. In the latter method the neutron population in the reactor is controlled by varying the absorbers, which are commonly in the form of movable control rods (though in a less commonly used design, operators can change the concentration of absorber in the reactor coolant). Changes of neutron leakage, on the other hand, are often automatic. For example, an increase of power will cause a reactor’s coolant to reduce in density and possibly boil. This decrease in coolant density will increase neutron leakage out of the system and thus reduce reactivity—a process known as negative-reactivity feedback. Neutron leakage and other mechanisms of negative-reactivity feedback are vital aspects of safe reactor design.
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A typical fission interaction takes place on the order of one picosecond (10−12 second). This extremely fast rate does not allow enough time for a reactor operator to observe the system’s state and respond appropriately. Fortunately, reactor control is aided by the presence of so-called delayed neutrons, which are neutrons emitted by fission products some time after fission has occurred. The concentration of delayed neutrons at any one time (more commonly referred to as the effective delayed neutron fraction) is less than 1 percent of all neutrons in the reactor. However, even this small percentage is sufficient to facilitate the monitoring and control of changes in the system and to regulate an operating reactor safely.