A nuclear reactor (or atomic reactor) is a facility in which certain nuclear reactions (usually nuclear fission processes) are carried out with high intensity (i.e., with a high-power density). Different applications are possible for this:
In a nuclear power plant, the nuclear reactions produce heat that is used to generate hot water vapor. (A boiling water reactor generates steam directly, while a pressurized water reactor works with liquid water and the steam is then produced in a separate steam generator.) The steam is used to drive a steam turbine and ultimately a generator for generating electrical energy. A nuclear power plant can contain several reactor blocks, which can usually be operated independently of one another.
The generated radioactive radiation is primarily used in research reactors. For example, intense neutron radiation is suitable for studying the atomic structure of solids.
Still, other reactors are mainly used to generate radioactive isotopes (radionuclides) B. for medicine (diagnostics and radiation therapy), various branches of scientific research, or in the industry (for example for the detection of material defects).
A number of reactors were and are mainly operated for breeding weapons-grade plutonium. Reactors used for commercial electricity generation can also be used to extract plutonium at the same time and were often used to camouflage nuclear weapons projects. (So far, no state has achieved nuclear weapons capability without first setting up a civilian nuclear program.)
Even with the transmutation in order to reduce the potential hazard of radioactive waste, some nuclear reactors can be used. The intensive neutron radiation in the reactor is used here to transform long-lived emitters into shorter-lived ones.
The term atomic reactor is synonymous with a nuclear reactor, just a little less precise: It is not clear that the decision processes take place in atomic nuclei and not, as in chemical processes, in the electron shells. In the early days of nuclear reactors, one also spoke of uranium machines.
Nuclear fission reactions always take place in today’s nuclear reactors. Nuclear fission releases a great deal of energy in the form of heat – orders of magnitude more than would be possible with the same amount of chemical fuel. In addition, a considerable amount of heat is also generated by the radioactive decay of the fission products. This decay heat represents one of the major safety problems of reactor operation.
It is conceivable that future nuclear reactors will be based on nuclear fusion (instead of nuclear fission). However, a nuclear fusion reactor is extremely difficult to implement; Despite decades of efforts at great expense, it has not yet been possible to maintain a nuclear fusion chain reaction for more than a few seconds. The rest of the article, therefore, deals exclusively with nuclear fission reactors, excluding exotic types that will remain of no technical importance for the foreseeable future.
The basic principle of the nuclear fission reactor
In order to be able to carry out nuclear fission at a high rate (i.e., with a much higher output than, for example, in an isotope battery), one needs a nuclear chain reaction. This can be achieved relatively easily in the nuclear fission of materials such as uranium or plutonium, because on the one hand, bombarding fissile atomic nuclei can trigger their fission and, on the other hand, the fission process itself releases several neutrons, which can then trigger further fission.
The prerequisite for the chain reaction is that the so-called criticality is achieved: For each nuclear fission that takes place, the released neutrons must on the average trigger at least one further nuclear fission. (The multiplication factor is then at least 1.) This requires that a sufficient amount of fissile material is concentrated in a sufficiently narrow space, i.e., the so-called critical mass is reached. Otherwise too many neutrons escape unused to the outside. Operation with a multiplication factor slightly below 1 is possible in principle if a strong external neutron source is available; however, this option has not yet been used.
The chain reaction can easily be started with the help of neutrons released spontaneously by radioactive decay as soon as criticality is reached. Therefore, no special neutron source is required to start a reactor.
In most reactors (only not in the case of so-called “fast” reactors) a moderator acts as a support, i. H. a material that slows down the neutrons released during fission (but if possible, without absorbing neutrons). Slow (“thermal”) neutrons can trigger further fission more effectively. In light water reactors and heavy water reactors, ordinary water (H 2 O) or heavy water (D 2O) as a moderator (and also to remove the generated heat). Heavy water favors the construction of smaller reactors, since it allows the critical mass to be kept smaller, but is not used in large-scale applications, if only because of its high production costs. Another disadvantage is that much more tritium is formed in heavy water – a highly radio-toxic element (which, however, is specifically generated in certain reactors). In some reactors, the moderator consists of other substances such as B. graphite.
In order to achieve operation over a long period of time with constant power, a delicate balance of the neutron flux must be maintained by a fast automatic control: For each nuclear fission that takes place, the released neutrons must on average trigger exactly one further nuclear fission. If it were even slightly more, the performance would be exponential increase and in a short time led to the destruction of the reactor (with possibly dramatic consequences). (An extremely intense explosion like an atomic bomb is not possible, however, since the chain reaction would be slowed down as a result of overheating before a substantial part of the fissile material can be converted.) Conversely, nuclear fission could quickly come to a standstill if a little too few neutrons Spark divisions. The power is regulated with the help of control rods that absorb neutrons and can be driven into the reactor core. You can e.g., B. contain cadmium. The more the nuclear fuel is consumed (“burned down”), the more the control rods need to be pulled out. To shut down the reactor, they are driven far in.
The criticality of the reactor is also influenced by the accumulation of fission products and their decay products. For such reasons, the critical properties of a reactor depend significantly on the precise operating conditions.
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