Figure 1. A pressurized water reactor with manage rods checked out sticking the end of the head of the reactor.

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A regulate rod is a machine that is offered to absorb neutrons so that the atom chain reaction ensuing within the reactor core deserve to be slowed under or stopped fully by inserting the rods further, or accelerated by removing lock slightly.<2> Essentially, regulate rods provide real-time manage of the fission process, ensuring that it remains active while avoiding it from accelerating out the control.

Uranium-235 fission publication 2.5 neutron on average, yet only one neutron is essential to sustain the nuclear chain reaction at a secure rate. The control rods absorb these extra neutrons and can be used to readjust the strength output the the reactor. When inserted the typical amount, their position is at criticality and the strength output stays the same.<2> If the rod is driven in the variety of neutrons decreases in addition to the strength output and also the reactor is below criticality. The contrary is true if the rods are pulled the end slightly as fission goes beyond criticality. This is displayed in number 2.

Figure 2. A schematic showing exactly how reactor strength output transforms with exactly how much the control rods (shown in green) room inserted. On the left, the control rods space inserted more than usual, reduce the strength output that the reactor. On the right, the manage rods are inserted less than usual, increasing the strength output.<3>

## Design

Control rods room rods, plates, or tube containing a material (like boron or hafnium) that absorbs neutrons.<4> several of these regulate rods, around the size of fuel rods, are spaced evenly and connected top top one end by a steel bracket known as a spider. Typical reactors can contain approximately 50 of this clusters with 20 individual manage rods in every cluster.<5>

The capacity of a manage rod to absorb neutron to manage the fission chain reaction needs a an option of material that has high neutron-absorbing abilities. The measure up of exactly how well a product absorbs neutron is recognized as its spirit absorption cross section, or $sigma_a$, measured in barns (equal come 10-28 square meters).<5> typically speaking, regulate rods room made utilizing cadmium, hafnium, or enriched boron.<2>

Along through the an option in material, the mechanical properties and also cost are essential when designing a regulate rod. For example, boron-10 is one of the ideal neutron absorbers, yet it is not appropriate for building control rods as it is very brittle. Additionally, herbal boron must be enriched to have reasonable absorption levels, i beg your pardon becomes very expensive.<5>

## Safety Precautions

Control rods are crucial safety functions of reactors as they allow the user to manage the output of the reactor. In some types of reactors, many notably CANDU reactors, the control rods are hosted up by electromagnets. This means that if over there is some kind of strength failure or ns of signal the regulate rods are immediately released and also fall right into the reactor core since of gravity.<6> This stays clear of the fission reaction indigenous continuing and also acts together the primary shutdown mechanism in instance of emergency. This dropping movement can additionally be induced manually if the machinery holding the rods up fails in part way. When the control rods space dropped right into the reactor, it is a procedure known together scramming.

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<7> In boil water reactors, the style is various as the control rods have to be pushed up right into the reactor since they are located on the bottom of the reactor in the design.

## Authors and Editors

Bethel Afework, Ethan Boechler, Jordan Hanania, Kailyn Stenhouse, Luisa Vargas Suarez, Jason DonevLast updated: October 18, 2021Get Citation