What happens when boron absorbs a neutron

A Control rod or Control staff serves to regulate and shut down a nuclear reactor.

If a control rod 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 uncontrolled growth of the chain reaction in the reactor is prevented (see also criticality). In addition to other options for regulating a nuclear reactor, the output can also be regulated by moving the control rods more or less deeply into the reactor core. The deeper the control rods are inserted into the reactor core, the more neutrons are absorbed and the output of the reactor decreases. By fully retracting the control rods, the chain reaction can be completely stopped and the reactor switched off. In normal operation, some of the control rods present in a nuclear reactor are always located outside the reactor core in order to be able to shut down the reactor safely in an emergency. 'Switching off' initially only means interrupting the chain reaction. It does not mean that the reactor is no longer supplying heat.

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 via the control rod guide tubes. The control rods of a fuel assembly are individually built into one Control combined and several control elements to form a control element bank. As a rule, there are two of these I&C banks in a pressurized water reactor. One is used to regulate the reactor in the area of ​​rapid power changes, the other exclusively for the reactor emergency shutdown (RESA). In normal operation, however, the long-term power control takes place exclusively via the boric acid concentration in the primary circuit.

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 in, but between the fuel elements moved. 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 start-up and shutdown and for the reactor shutdown during normal operation. The reactor is controlled via the speed of the main coolant pumps.

By absorbing neutrons, the neutron absorbing material is consumed over time. In addition, helium and other gases are generated 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. The control rods are an indispensable part of nuclear reactors. Without them, the use of nuclear energy as it is taking place today would be inconceivable.

Category: Nuclear Technology