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Why are silver, cadmium and hafnium the star elements of control rods in nuclear power plants?
In the process of nuclear power generation, control rods play a vital role. They can effectively control the speed of nuclear fission to ensure the safe and stable operation of the reactor. This article explores why three elements, silver, cadmium and hafnium, are considered the star elements of control rods in nuclear power plants and the role they play in energy production.
Operation principle of control rod
Control rods are inserted into the core of a nuclear reactor to regulate the speed of the nuclear chain reaction. These control rods are made of chemical elements capable of absorbing large amounts of neutrons. Typical representatives of such elements are silver, cadmium and hafnium.
The insertion and removal of control rods directly affects the reactivity of the reactor and the heat energy and power it produces.
There are other reactor designs such as Boiling Water Reactor (BWR), Pressurized Water Reactor (PWR), etc., which use different control rod materials, depending on the energy distribution of their neutrons. When the reactor's reactivity calculation index exceeds 1, the speed of the nuclear chain reaction will increase rapidly with the time index; when the reactivity is below 1, the speed of the chain reaction will gradually decrease.
Why Silver, Cadmium and Hafnium?
These three elements make them the first choice for control rod materials due to their specific properties in neutron absorption. For example, silver-cadmium-nickel alloy (AIC) is widely used in many commercial pressurized water reactors. At the same time, cadmium has good mechanical strength and can maintain stability in high temperature environments, making it an excellent neutron absorbing material.
Hafnium's corrosion resistance and strength make it an important choice in reactors for some military uses.
Although silver, cadmium and hafnium provide excellent neutron capture properties, the availability and cost of each element affects its use in nuclear power generation. Take silver as an example. Although it performs well in neutron capture, its rarity limits its feasibility for large-scale applications.
Material desirability and evaluation
When selecting control rod materials, scientists will consider not only the efficiency of neutron capture, but also the material's heat resistance, machinability and future cost factors. Therefore, common synthetic combinations of elements, such as high boron steel or silver-cadmium alloys, have become the focus of scientists' research.
Materials with higher capture efficiency and durability often improve the overall safety of the reactor.
Given these factors, cobalt, nickel and other rare earth elements are also under future research and may be incorporated into control rod design materials. This not only improves availability, but also provides the nuclear energy industry with more options.
Safety and emergency measures
The safety of nuclear reactors relies heavily on the design of the control rods. The safety mechanisms of most modern reactors set up the control rods to automatically descend into the core to quickly stop the reaction in the event of a power failure. This feature is known in the nuclear energy community as "SCRAM" (Shutting Down Reactor Rapidly).
The rapid response capability of control rods is the basis for ensuring the safe operation of the reactor.
As far as the BWR is concerned, although its operating mode is different, once the need for emergency shutdown occurs, the specially designed hydraulic system will immediately place the control rods for safe shutdown, showing the important safety considerations in reactor design .
Conclusion
In summary, the three elements silver, cadmium and hafnium are important materials for control rods in nuclear power plants. Their excellent neutron absorption properties and other physical properties make them the preferred choice in reactor design. However, as we explore safer and more economical sources of energy, can we find other more superior and revolutionary materials to replace these traditional elements?
