![]() For example, boron-10 is one of the best neutron absorbers, however it is not ideal for building control rods as it is extremely brittle. Īlong with the choice in material, the mechanical properties and cost are important when designing a control rod. Generally speaking, control rods are made using cadmium, hafnium, or enriched boron. The measure of how well a material absorbs neutrons is known as its neutron absorption cross section, or \sigma_a, measured in barns (equal to 10 -28 square meters). The ability of a control rod to absorb neutrons to control the fission chain reaction requires a choice of material that has high neutron-absorbing abilities. Typical reactors can contain around 50 of these clusters with 20 individual control rods in each cluster. Several of these control rods, about the size of fuel rods, are spaced evenly and connected on one end by a metal bracket known as a spider. DesignĬontrol rods are rods, plates, or tubes containing a material (like boron or hafnium) that absorbs neutrons. On the right, the control rods are inserted less than usual, increasing the power output. On the left, the control rods are inserted more than usual, reducing the power output of the reactor. A schematic showing how reactor power output changes with how much the control rods (shown in green) are inserted. The opposite is true if the rods are pulled out slightly as fission goes beyond criticality. If the rod is pushed in the number of neutrons decreases along with the power output and the reactor is below criticality. When inserted the standard amount, their position is at criticality and the power output remains the same. The control rods absorb these extra neutrons and can be used to adjust the power output of the reactor. Uranium-235 fission releases 2.5 neutrons on average, but only one neutron is needed to sustain the nuclear chain reaction at a steady rate. Essentially, control rods provide real-time control of the fission process, ensuring that it remains active while preventing it from accelerating out of control. Ī control rod is a device that is used to absorb neutrons so that the nuclear chain reaction taking place within the reactor core can be slowed down or stopped completely by inserting the rods further, or accelerated by removing them slightly. ![]() A pressurized water reactor with control rods seen sticking out of the head of the reactor.
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