What is the function of the control rods in a nuclear reactor? What substances are used to construct control rods?Why are these substances chosen?

Control rods in a nuclear reactor serve as crucial safety devices that regulate and control the rate of the nuclear fission chain reaction. They work by absorbing the neutrons produced during the fission process, which effectively reduces the number of free neutrons available to trigger further reactions. By adjusting the position and amount of control rods inside the reactor core, the reactor operators can control the power output of the reactor.

Control rods are typically constructed from materials that have a high affinity for capturing and absorbing neutrons. Some common materials used to make control rods include: 1. Silver-indium-cadmium alloy (Ag-In-Cd): This alloy consists of approximately 80% silver, 15% indium, and 5% cadmium. It is highly effective in capturing neutrons and has good mechanical properties. 2. Boron Carbide (B4C): Boron carbide is a highly effective neutron absorber due to the high cross-section of boron-10 isotope for neutron absorption. It also has the advantage of having high mechanical strength and good chemical stability. 3. Hafnium (Hf): Hafnium is a rare metal with a high neutron absorption cross-section. Owing to its properties, hafnium is often used in control rods in naval reactors.

These substances are chosen for control rods due to several reasons: 1. High neutron absorption: The primary requirement for a control rod is its ability to absorb neutrons effectively. The materials mentioned above have a high neutron absorption cross-section, which means they can efficiently capture and absorb neutrons, thus slowing down or stopping the chain reaction when necessary. 2. Mechanical properties: The control rods must be able to withstand the radiation and chemical environment in the reactor core. They should not corrode or degrade easily. Silver-indium-cadmium alloy, boron carbide, and hafnium possess suitable mechanical properties and chemical stability for this purpose. 3. Thermal properties: The control rods should have a low thermal expansion coefficient so that they do not deform during temperature fluctuations in the reactor. The materials used for control rods usually have a low thermal expansion coefficient, making them suitable for use in a reactor environment. As a result, the mentioned materials are used for control rods owing to their ability to absorb neutrons, their mechanical and thermal properties, and their chemical stability, which helps maintain safety and control in a nuclear reactor.