Nuclear Power Plants and Control of Chain Reactions
Chain reactions naturally give rise to reaction rates that grow (or shrink) exponentially, whereas a nuclear power reactor needs to be able to hold the reaction rate reasonably constant. To maintain this control, the chain reaction criticality must have a slow enough time-scale to permit intervention by additional effects (e.g., mechanical control rods or thermal expansion). Consequently, all nuclear power reactors (even fast-neutron reactors) rely on delayed neutrons for their criticality. An operating nuclear power reactor fluctuates between being slightly subcritical and slightly delayed-supercritical, but must always remain below prompt-critical.
It is impossible for a nuclear power plant to undergo a nuclear chain reaction that results in an explosion of power comparable with a nuclear weapon, but even low-powered explosions due to uncontrolled chain reactions, that would be considered "fizzles" in a bomb, may still cause considerable damage and meltdown in a reactor. For example, the Chernobyl disaster involved a runaway chain reaction but the result was a low-powered steam explosion from the relatively small release of heat, as compared with a bomb. However, the reactor complex was destroyed by the heat, as well as by ordinary burning of the graphite exposed to air. Such steam explosions would be typical of the very diffuse assembly of materials in a nuclear reactor, even under the worst conditions.
In addition, other steps can be taken for safety. For example, power plants licensed in the United States require a negative void coefficient of reactivity (this means that if water is removed from the reactor core, the nuclear reaction will tend to shut down, not increase). This eliminates the possibility of the type of accident that occurred at Chernobyl (which was due to a positive void coefficient). However, nuclear reactors are still capable of causing smaller explosions even after complete shutdown, such as was the case of the Fukushima Daiichi nuclear disaster. In such cases, residual decay heat from the core may cause high temperatures if there is loss of coolant flow, even a day after the chain reaction has been shut down (see SCRAM). This may cause a chemical reaction between water and fuel that produces hydrogen gas which can explode after mixing with air, with severe contamination consequences, since fuel rod material may still be exposed to the atmosphere from this process. However, such explosions do not happen during a chain reaction, but rather as a result of energy from radioactive beta decay, after the fission chain reaction has been stopped.
Read more about this topic: Nuclear Chain Reaction
Famous quotes containing the words nuclear power, nuclear, power, plants, control, chain and/or reactions:
“The emotional security and political stability in this country entitle us to be a nuclear power.”
—Ronald, Sir Mason (b. 1930)
“Language is as real, as tangible, in our lives as streets, pipelines, telephone switchboards, microwaves, radioactivity, cloning laboratories, nuclear power stations.”
—Adrienne Rich (b. 1929)
“I spoke at a woman’s club in Philadelphia yesterday and a young lady said to me afterwards, “Well, that sounds very nice, but don’t you think it is better to be the power behind the throne?” I answered that I had not had much experience with thrones, but a woman who has been on a throne, and who is now behind it, seems to prefer to be on the throne.”
—Anna Howard Shaw (1847–1919)
“From the time the Englishman’s bones harden into bones at all, he makes his skeleton a flagstaff, and he early plants his feet like one who is to walk the world and the decks of all the seas.”
—Willa Cather (1876–1947)
“The awareness that health is dependent upon habits that we control makes us the first generation in history that to a large extent determines its own destiny.”
—Jimmy Carter (James Earl Carter, Jr.)
“It is the future that creates his present.
All is an interminable chain of longing.”
—Robert Frost (1874–1963)
“We have all had the experience of finding that our reactions and perhaps even our deeds have denied beliefs we thought were ours.”
—James Baldwin (1924–1987)