By regulation, the design of the nuclear reactor must include stipulations for human operator error and equipment failure. Nuclear Plants in the western world use a Defense in Depth idea which is a system with numerous safety components, each with back-up and design to accommodate human error. The components include:

"Control of Radioactivity - This requires being able to control the neutron flux. If the neutron flux is decreased the radioactivity is decreased. The most common way to reduce the neutron flux is include neutron-absorbing control rods. These control rods can be partially inserted into the reactor core to reduce the reactions. The control rods are very important because the reaction could run out of control if fission events are extremely frequent. In modern nuclear power plants, the insertion of all the control rods into the reactor core occurs in a few seconds, thus halting the nuclear reaction as rapidly...
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