Detail the process of reactivity control in a micro reactor using control rods, focusing on the material properties of the rods and their impact on reactivity changes.

Reactivity control in a micro reactor, particularly using control rods, is essential for managing the nuclear chain reaction. Control rods are designed to absorb neutrons, thereby decreasing the reactor's reactivity and enabling operators to maintain a steady-state power level, shut down the reactor safely, and respond to unexpected changes in operating conditions. The effectiveness of control rods depends significantly on their material properties and their positioning within the reactor core. The primary function of a control rod is to reduce the neutron population in the reactor core, which in turn reduces the rate of fission. This is achieved through the selection of materials with high neutron absorption cross-sections. These materials readily absorb neutrons without undergoing fission themselves, and therefore do not contribute to the chain reaction. Common control rod materials include boron, cadmium, hafnium, and silver-indium-cadmium alloys. Boron, typically in the form of boron carbide (B4C), is often used in thermal reactors due to its high absorption cross-section for thermal neutrons and its cost-effectiveness and the ease with which it can be integrated into a fuel assembly. For example, in a small thermal micro reactor, control rods might be fabricated from B4C and inserted into specific locations within the fuel assembly to manage reactivity. Cadmium is another effective absorber, but it has the drawback of becoming activated by neutron capture which results in radioactive isotopes, and so its use may require further consideration for its management. Hafnium is a more expensive absorber but has excellent mechanical strength, h....