The rate of the fission chain reaction depends on the neutron population in the reactor core. One of the principal ways of controlling the reaction is therefore to use materials that absorb neutrons without undergoing fission. Such materials are known as neutron absorbers. Many different elements can be used for this purpose; boron and cadmium are among the most common.

Water with Dissolved Boric Acid

Many pressurised water reactors use boric acid dissolved in the reactor coolant as an additional means of controlling reactivity. Boron is a very effective neutron absorber. The boric acid concentration can be adjusted to provide relatively uniform reactivity control throughout the reactor core. During refueling and certain fuel-handling operations in PWRs, sufficiently borated water is maintained to ensure an adequate shutdown margin.

Control Rods

Video: Operating principle of control rods.

Control rods contain neutron-absorbing materials such as boron carbide, silver-indium-cadmium alloys or hafnium. They can be inserted into or withdrawn from the reactor core to control reactivity. Withdrawing the control rods reduces neutron absorption and increases reactivity, while inserting them increases neutron absorption and reduces reactivity. Control rods are therefore used to regulate reactor power and maintain the required operating state.

In VVER-440 reactors, some control assemblies consist of a fuel section and an absorber section located above it. When the absorber section is inserted into the active core, the fuel section is simultaneously displaced downwards out of the core. Moving the assembly in the opposite direction withdraws the absorber section and inserts the fuel section, thereby increasing reactivity.

Shut-off Rods

For rapid reactor shutdown, neutron-absorbing rods are rapidly inserted into the reactor core. They absorb neutrons and quickly suppress the fission chain reaction. In some reactor designs, the rods are held above the core by electromagnets and, when released, fall into the core under gravity. In other designs, they are actively driven or otherwise rapidly inserted into the core.

Absorbing Material Contained in the Fuel

Uranium fuel contains both 235U and 238U. Some neutrons are captured by 238U without causing fission, although this capture can ultimately lead to the production of fissile 239Pu. Fission also produces many different nuclides, some of which have very high neutron absorption cross-sections and therefore act as neutron poisons. Important examples include xenon-135 and samarium-149.

Neutron-absorbing materials may also be deliberately incorporated into the fuel as burnable absorbers. Gadolinium, for example, may be added to the fuel in the form of gadolinium oxide. As the fuel is irradiated, these absorbers gradually capture neutrons and are converted into isotopes with lower neutron absorption, reducing their effect over time. Burnable absorbers compensate for the excess reactivity of fresh fuel. As fuel burnup increases and the amount of fissile material decreases, the absorber is gradually depleted as well, helping to limit changes in reactivity during the fuel cycle.

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Some fission products with very high neutron absorption cross-sections are known as reactor poisons. One of the most important is xenon-135, which strongly affects reactor reactivity after changes in power and after shutdown.