Boron has the property of absorbing neutrons without re-emitting any. When the control blades are fully inserted, they absorb so many neutrons from the uranium that there are not enough to allow a chain reaction to continue.
To put the reactor into operation, the control blades are raised very slowly. As fewer and fewer neutrons are absorbed, more and more neutrons are available to cause the splitting of uranium nuclei, until finally enough neutrons are available to sustain a chain reaction. Nuclear Fission Creates Heat The main job of a reactor is to house and control nuclear fission —a process where atoms split and release energy.
Fission and Fusion: What is the Difference? There are two types of light-water reactors operating in America. Pressurized water reactors. Graphic by Sarah Harman U. Department of Energy.
The core water cycles back to the reactor to be reheated and the process is repeated. Boiling Water Reactors. These fast neutrons are slowed through the use of a moderator such as heavy water and ordinary water. Some reactors use graphite as a moderator, but this design has several problems. Once the fast neutrons have been slowed, they are more likely to produce further nuclear fissions or be absorbed by the control rod.
This is called a chain reaction. The fissioning of atoms in the chain reaction also releases a large amount of energy as heat. The generated heat is removed from the reactor by a circulating fluid, typically water. This heat can then be used to generate steam, which drives turbines for electricity production.
In order to ensure the nuclear reaction takes place at the right speed, reactors have systems that accelerate, slow or shut down the nuclear reaction, and the heat it produces. This is normally done with control rods, which typically are made out of neutron-absorbing materials such as silver and boron.
Two examples of nuclear fissioning of uranium, the most commonly used fuel in nuclear reactors.
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