Control rod assembly for VVER reactor. Absorber – boron carbide
Control rods are an important safety system of nuclear reactors. Their prompt action and prompt response of the reactor is indispencable. Control rods are used for maintaining the desired state of fission reactions within a nuclear reactor (i.e. subcritical state, critical state, power changes) They constitute a key component of an emergency shutdown system (SCRAM).
Control rods are rods, plates, or tubes containing a neutron absorbing material (material with high absorbtion cross-section for thermal neutron) such as boron, hafnium, cadmium, etc., used to control the power of a nuclear reactor. By absorbing neutrons, a control rod prevents the neutrons from causing further fissions.
Control rods usually constitute cluster control rod assemblies (PWR) and are inserted into guide thimbles within a nuclear fuel assembly. The absorbing material (e.g. pellets of Boron Carbide) is protected by the cladding usually made of stainless steel. They are grouped into groups (banks) and the movement occurs usually by the groups (banks). Typical total number of clusters is 70. This number is limited especially by number of penetrations of the reactor pressure vessel head.
A control rod is removed from or inserted into the reactor core in order to increase or decrease the reactivity of the reactor (increase or decrease the neutron flux). By the changes of the reactivity the changes of neutron power are performed. This in turn affects the thermal power of the reactor, the amount of steam produced, and hence the electricity generated.
In PWRs they are inserted from above, with the control rod drive mechanisms being mounted on the reactor pressure vessel head. Due to the necessity of a steam dryer above the core of a boiling water reactor, this design requires insertion of the control rods from underneath the core.
Load follow: grey rods and lead bank
Control Rods usage
- Reactor startup.
- Control of the reactor and power maneuvering.
- Axial offset control.
- Reactor shutdown.
- Emergency shutdown – SCRAM.
Grey control rods
Some nuclear power plants use load following. These plants have the capability to make power maneuvering between 30% and 100% of rated power, with a slope up to 5% of rated power per minute. They can respond very quickly to the grid requirements. In order to fulfill these requirements without introducing a large perturbation of the power distribution, special control rods have to be used. These control rods are called “grey” control rods. Grey control rods use a grey neutron absorber, which absorbs less neutrons than a “black” absorber. Consequently, they cause smaller depressions in the neutron flux and power in the vicinity of the rod.