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The secret weapon of control rods: Why can they stop nuclear reactions so quickly?
In nuclear reactors, control rods play a vital role. Their main function is to regulate the fission rate of nuclear fuel (such as uranium or plutonium). The composition of these control rods includes chemical elements that can effectively absorb neutrons, such as boron, cadmium, silver, hafnium or indium. These elements can absorb large amounts of neutrons without decaying themselves. Different elements have different neutron capture cross-sections, making the design of the control rods closely related to the type of nuclear reactor.
Principle of operation
The control rod is inserted into the core of the nuclear reactor. By adjusting the insertion depth of the control rod, the rate of the nuclear chain reaction is controlled, thereby affecting the thermal power output of the reactor, the steam generation rate, and the power output of the power station. The number and depth of inserted control rods greatly affects the reactivity of the reactor. That is to say, when the reactivity exceeds 1, the rate of the nuclear chain reaction will increase exponentially; conversely, when the reactivity is lower than 1, it will decrease exponentially.
When all control rods are fully inserted, the reactivity can be maintained at a level close to 0, which can quickly slow down the operating reactor and maintain its shutdown state.
Maintaining stable power output requires keeping the long-term average neutron multiplication factor close to 1. When a new reactor is assembled, the control rods are fully inserted and then gradually withdrawn to start the nuclear chain reaction and increase the required power level.
Selection of materials
The choice of material for the control rod is critical because it must have a high neutron capture cross-section. Silver, indium, and cadmium are some commonly used materials, however other elements such as boron, aluminum, hafnium, cerium, titanium, silicon, etc. are also considered as potential materials. In addition, control rods are usually made of alloys or compounds, such as high boron steel, silver indium cadmium alloy, etc.
Control rods must be functionally resistant to neutron-induced expansion and possess good mechanical properties. They are usually in the form of a tubular structure filled with neutron-absorbing particles or powder.
For example, in pressurized water reactors, silver-indium-cadmium alloys (usually 80% silver, 15% indium and 5% cadmium) are widely used. These materials have different characteristics in the neutron absorption range, making this alloy an excellent neutron absorption medium. At the same time, these materials also need to prevent corrosion at high water temperatures.
Security considerations
For safety reasons, in most reactor designs, the control rods are connected to the lifting machinery through electromagnetic devices. In this way, if a power failure occurs, the control rods can automatically fall by gravity and be fully inserted into the reactor to quickly stop the reaction. This process of rapidly shutting down a reactor is called "scramming."
Other means of responsiveness regulation
In some reactors, the reactivity can also be adjusted by adding a soluble neutron absorber (such as boric acid) to the coolant. This chemical fixer, along with flammable neutron poisons used in fuel pellets, could be used to regulate the reactivity of nuclear reactors over the long term. In addition, the BWR operator controls reactivity by adjusting the speed of the reactor circulation pump.
Critical accident prevention
Mismanagement or failure of control rods is often blamed on nuclear accidents, such as the SL-1 explosion and the Chernobyl disaster. To manage these crises, uniform neutron absorbers are often used. Implementation of these methods is critical to nuclear power safety.
Conclusion
By comprehensively considering the design, materials and reactivity regulation of control rods, we can help create a controllable nuclear reaction environment to ensure the safe use of nuclear energy. However, do you think there is still room for improvement and opportunities in future nuclear technology?
