Boron and/or Cadmium control rods. They are inserted into the core to slow the reaction and withdrawn from the core to speed up the reaction. Both elements have a very high neutron capture crosssection, the more in the core the more excess neutrons they remove from the chain reaction.
No, the function of the control rods is to absorb surplus neutrons so that the chain reaction proceeds at a steady rate, and to compensate for the reducing reactivity of the reactor as the fuel is burned up over the refuelling cycle. They also have a very important safety function in shutting down the reactor fully when required, by inserting them fully, thus preventing any chain reaction from starting.
I found this: " The control rods, another important part of the reactor, regulate or control the speed of the nuclear chain reaction, by sliding up and down between the fuel rods or fuel assemblies in the reactor core. The control rods contain material such as cadmium and boron. Because of their atomic structure cadmium and boron absorb neutrons, but do not fission or split. Therefore, the control rods act like sponges that absorb extra neutrons." Here (you may have to copy and paste in two parts): http://www.aboutnuclear.org/view.cgi?fC=Electricity,Operation,Reactor,Control_Rods
You did not provide the list of "the following". However, the answer to the question is moderation. Moderation is the process whereby the neutron is slowed down in order to facilitate its subsequent capture by the nuclei of the fuel.
The material used to slow down high-speed neutrons in a reactor is called the moderator. The moderator in a pressurized water reactor is the water, which is the main coolant. Collisions between the neutron and hydrogen nuclei (protons) slow the neutron down (thermalizing it) and increasing the probability that it will be absorbed by another fissionable atom. That makes the chain go, it maintains the chain reaction. Slowing down neutrons does NOTslow down the process of nuclear fission. If anything, it maintains it or speeds it up because slower moving neutrons have a higher probability of being absorbed and continuing or building the chain reaction. A link is provided.
In an atomic bomb the time between fission generations is about 10ns and the entire reaction is over in 1us to 3us. In an atomic reactor the time between fission generations is about 10us because each neutron must collide about 1000 times with moderator atoms to slow down to "thermal" speeds.
Control rod act as brakes to reduce the reaction rate. However as nuclear reactors have so much excess reactivity, its like a car rolling downhill: remove the control rods a bit (release the brakes some) and the reaction rate increases.
No, the function of the control rods is to absorb surplus neutrons so that the chain reaction proceeds at a steady rate, and to compensate for the reducing reactivity of the reactor as the fuel is burned up over the refuelling cycle. They also have a very important safety function in shutting down the reactor fully when required, by inserting them fully, thus preventing any chain reaction from starting.
speed
I found this: " The control rods, another important part of the reactor, regulate or control the speed of the nuclear chain reaction, by sliding up and down between the fuel rods or fuel assemblies in the reactor core. The control rods contain material such as cadmium and boron. Because of their atomic structure cadmium and boron absorb neutrons, but do not fission or split. Therefore, the control rods act like sponges that absorb extra neutrons." Here (you may have to copy and paste in two parts): http://www.aboutnuclear.org/view.cgi?fC=Electricity,Operation,Reactor,Control_Rods
I think you are talking about nuclear reactors. PWR is a Pressurized Water Reactor, and is the basis for most power generators. BWR is a Boiling Water Reactor. As the water is a major moderating component, controlling the speed and intensity of the nuclear reaction, the steam bubbles in a BWR can present a problem.
Control rods need to contain material that is a strong absorber of neutrons, so that rapid shutdown can be achieved when necessary. Two such materials are Cadmium and Boron. Both have been used in different types of reactor. The elements may be used as alloys or as chemical compounds, and will be encased in stainless steel to avoid corrosion in the reactor.
The purpose of the iron in the reactor in the haber process is to speed up the reaction.
You did not provide the list of "the following". However, the answer to the question is moderation. Moderation is the process whereby the neutron is slowed down in order to facilitate its subsequent capture by the nuclei of the fuel.
Conventional nuclear power plants use critical mass in a way that is similar to how a fission bomb uses it, in a chain reaction. There are some very important differences, however. In a nuclear power plant, the reaction is controlled by a set of systems that moderate the amount and speed of neutrons, the temperature of the fuel, and so on. These prevent the reactor from getting too hot and melting down. This is not easy, but also not terribly hard. In the bomb, there is no provision for ongoing control of the chain reaction. Instead, there is a design that causes the critical mass to undergo the chain reaction while staying at critical mass for the process. This is really hard to do because as soon as the whole heats up a bit, it wants to fly apart, destroying the integrity of the critical mass. The design problems associated with maintaining critical mass were difficult enough that they were secret for a long time. In other words, in order for a critical mass to blow up as a bomb, it really has to be designed to be a bomb. And though a nuclear reactor can melt down and be very destructive in the process, and though the waste storage can be breached and be very destructive as a result, the nuclear explosion of a nuclear reactor is quite likely impossible. Explosions and meltdowns have happened, but the explosions are from steam or chemicals, rather than nuclear critical mass.
nuclear fuel (thorium, uranium, etc.), electron absorbing rod (krypton, boron, etc.), projectile (which give high speed to electron/proton),
One type (model) of reactor is the CANDU, or Canada (CAN) dueterium-uranium (DU)reactor. Ther are also differences between fusion and fission reactors, fast and thermal reactors (the differences bing in the speed of the neutrons used to initiate the chain reaction, usually fission), light or heavy water cooled or moderated reactors, and many other distinctions.
Moderator such as graphite or heavy water would reduce the speed of neutron there by making them as thermal neutrons. Such slow speed neutrons are needed to bring Uranium-235 into fission. Control rods such as Boron carbide or Cadmium are there to abosorb neutrons. So chain reaction of fission is ketp under control.