Control rod
Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing many neutrons without themselves decaying.
When reactivity (as effective neutron multiplication factor) is above 1, the rate of the nuclear chain reaction increases exponentially over time.
Maintaining a constant power output requires keeping the long-term average neutron multiplication factor close to 1.
Control rods are partially removed from the core to allow the nuclear chain reaction to start up and increase to the desired power level.
In BWRs, due to the necessity of a steam dryer above the core, this design requires insertion of the control rods from beneath.
Other candidate elements include boron, cobalt, hafnium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.
[2] The material choice is influenced by the neutron energy in the reactor, their resistance to neutron-induced swelling, and the required mechanical and lifespan properties.
[5] Xenon is also a strong neutron absorber as a gas, and can be used for controlling and (emergency) stopping helium-cooled reactors, but does not function in cases of pressure loss, or as a burning protection gas together with argon around the vessel part especially in case of core catching reactors or if filled with sodium or lithium.
Silver-indium-cadmium alloys, generally 80% Ag, 15% In, and 5% Cd, are a common control rod material for pressurized water reactors.
Hafnium carbide can also be used as an insoluble material with a high melting point of 3890 °C and density higher than that of uranium dioxide for sinking, unmelted, through corium.
[10] A disadvantage is less titanium and oxide absorption, that other neutron absorbing elements do not react with the already high-melting point cladding materials and that just using the unseparated content with dysprosium inside of minerals like Keiviit Yb inside chromium, SiC or c11B15N tubes deliver superior price and absorption without swelling and outgassing.
An obvious explanation is resonance gamma rays increasing the fission and breeding ratio versus causing greater capture of uranium, and others over metastable conditions such as for isotope 235mU, which has a half-life of approximately 26 minutes.
Other means of controlling reactivity include (for PWR) a soluble neutron absorber (boric acid) added to the reactor coolant, allowing the complete extraction of the control rods during stationary power operation, ensuring an even power and flux distribution over the entire core.
In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage.
A notable exception to this fail-safe mode of operation is the BWR, which requires hydraulic insertion in the event of an emergency shut-down, using water from a special tank under high pressure.
Homogeneous neutron absorbers have often been used to manage criticality accidents which involve aqueous solutions of fissile metals.
In carbon dioxide-cooled reactors such as the AGR, if the solid control rods fail to arrest the nuclear reaction, nitrogen gas can be injected into the primary coolant cycle.
