Control rod
A control rod , and control rod or control rod called the scheme and serves shutdown of a nuclear reactor .
It contains a material neutron absorbs strongly. When it is in the reactor core , it absorbs some of the neutrons released by the fission so that these are not available for further fission. In this way, the nuclear reaction rate of the chain reaction in the reactor is reduced (see also criticality ). Therefore, the output of the reactor - among other possibilities for regulation - can be regulated by moving the control rods more or less deeply into the reactor core.
By fully retracting the control rods, the chain reaction can be completely prevented, i.e. the reactor can be switched off. In normal operation, some of the existing control rods are always located outside the reactor core so that the reactor can be shut down safely in an emergency. Switching off , however, only means interrupting the chain reaction. It does not mean that the reactor is no longer supplying heat (see decay heat ).
Pressurized water reactor
In pressurized water reactors , the control rods usually consist of a steel cladding tube that is filled with a material that has a high absorption cross-section for thermal neutrons, usually cadmium or boron compounds . Several control rods can be inserted into each fuel assembly from above via the control rod guide tubes. The control rods of a fuel assembly are individually structurally combined to form a control element and several control elements are integrated into a control element bank . All bars of a control element bank are moved together as a whole, so they are each at the same entry depth. As a rule, there are two of these I&C banks in a pressurized water reactor. One is used to control the reactor in the area of rapid power changes, the other exclusively for the reactor shutdown . In normal operation, however, the long-term power control takes place exclusively via the boric acid concentration in the primary circuit in the pressurized water reactor .
Boiling water reactor
In the boiling water reactor there are no individual control rods, here the tubes with the neutron-absorbing material are assembled to form a control element with a cross-shaped cross-section. The tubes of a control element are enclosed by a common steel casing. In contrast to the pressurized water reactor, the control elements are not moved from above into , but from below between the fuel elements; there is one control element for every four fuel assemblies. Here, too, the control elements are grouped together to form banks. In contrast to the pressurized water reactor, the control rods are only used for starting up and shutting down the reactor and for switching off the reactor during normal operation. The long-term power regulation of the boiling water reactor takes place indirectly via the speed of the main coolant pumps, which influences the steam content in the reactor core and thus the reactivity (see vapor bubble coefficient ). The purpose of moving in from below is to compensate the steam content - which naturally increases from bottom to top in boiling water - with its reactivity-reducing effect, i.e. to make the power distribution more even in the vertical direction.
Other reactor types
In other types of reactors, control rods function in a similar way, but are often designed slightly differently. In the RBMK, for example, control rods for regulating the total output and for rapid shutdown are retracted from above, while additional control rods that can be retracted from below are used to balance the power distribution.
In the THTR-300 , a prototype of a high-temperature reactor , the control rods were only used for shutdown.
Wear of control rods
By absorbing neutrons, the neutron-absorbing material is consumed over time. In addition, helium and other gases are produced during absorption within the steel envelope . These gases can lead to high pressure within the steel envelope. For these reasons, control rods only have a limited service life and must be replaced after approx. 6 to 10 years.
literature
- Dieter Smidt: Reactor technology: an introduction to the design of nuclear power plants. 2nd edition Karlsruhe: G. Braun 1976. ISBN 3-7650-2018-4
- Albert Ziegler: Textbook of reactor technology . Berlin u. a .: Springer 1984. ISBN 3-540-13180-9