Question

in nuclear reactor chain reaction is controlled by

Answer 1

There are numerous ways to control the chain reaction in a nuclear reactor, some natural based on the physics of the reactor and others by engineered control systems.  Basically, to control the chain reaction, you have to control the number of neutrons (for light water reactors, the number of thermal neutrons).  This can be done with neutron absorbing control rods or neutron absorbing chemicals added to the coolant.  It is also a natural affect that occurs when the coolant/moderator heats up and becomes less dense.  It doesn't, as suggested in another answer, have anything to do with prompt and delayed neutrons.  Delayed neutrons do allow engineered systems to act to control the reactor but ALL neutrons, prompt and delayed, are moderated (slowed down or thermalized).  As described in another answer, in the fission process, there are two types of neutrons generated.  These are called prompt and delayed neutrons.  Prompt neutrons are generated in the actual fission event and appear within 10E-14 seconds of the fission.  This is too fast for any control system to respond.  A reactor that becomes critical or supercritical on prompt neutrons alone is said to be prompt critical or prompt supercritical (Chernobyl went prompt supercritical and that ultimately let to it's destruction).  Delayed neutrons are produced as a result of the radioactive decay of fission products (called delayed neutron precursors) and appear from a fraction of a second to about a minute after the fission event.  In reactor physics we refer to the amount of delayed neutrons as the delayed neutron fraction.  It is the delayed neutron fraction that allows us to control reactor power and determines how fast various transients and accidents proceed.  The idea is that the reactor is taken almost critical with prompt neutrons and is then taken the rest of the way to critical with delayed neutrons.

I hope you will like this answer and please mark me as a brain list


There are numerous ways to control the chain reaction in a nuclear reactor, some natural based on the physics of the reactor and others by engineered control systems.  Basically, to control the chain reaction, you have to control the number of neutrons (for light water reactors, the number of thermal neutrons).  This can be done with neutron absorbing control rods or neutron absorbing chemicals added to the coolant.  It is also a natural affect that occurs when the coolant/moderator heats up and becomes less dense.  It doesn't, as suggested in another answer, have anything to do with prompt and delayed neutrons.  Delayed neutrons do allow engineered systems to act to control the reactor but ALL neutrons, prompt and delayed, are moderated (slowed down or thermalized).  As described in another answer, in the fission process, there are two types of neutrons generated.  These are called prompt and delayed neutrons.  Prompt neutrons are generated in the actual fission event and appear within 10E-14 seconds of the fission.  This is too fast for any control system to respond.  A reactor that becomes critical or supercritical on prompt neutrons alone is said to be prompt critical or prompt supercritical (Chernobyl went prompt supercritical and that ultimately let to it's destruction).  Delayed neutrons are produced as a result of the radioactive decay of fission products (called delayed neutron precursors) and appear from a fraction of a second to about a minute after the fission event.  In reactor physics we refer to the amount of delayed neutrons as the delayed neutron fraction.  It is the delayed neutron fraction that allows us to control reactor power and determines how fast various transients and accidents proceed.  The idea is that the reactor is taken almost critical with prompt neutrons and is then taken the rest of the way to critical with delayed neutrons.

Answer 2

Nuclear reactors  have control rods, which you stick down into the fuel to absorb neutrons, to slow the reaction down, or withdraw to speed the reaction up