 It should be obvious that the key to successful operation of a nuclear power plant is controlling the neutronary production factor K. A rough schematic of a fission reactor is shown in the picture on the left. The uranium fuel is usually in the form of rods and is usually enriched in uranium-235. Between and surrounding the uranium fuel rods is something that will slow down the neutrons. This is because the low-energy neutrons have the highest probability for initiating a fission reaction. This material is called the moderator because it moderates the energy of the neutrons. The most common material used for the moderator is water. Water works well as a moderator because the neutrons have almost exactly the same mass as the protons that make up the hydrogen component of the water. The similar masses mean that it is possible for the neutron to lose all of its energy in a single head-on collision with a proton. Perhaps the most important part of the reactor are the control rods. These are rods made of materials that have a particularly high probability for absorbing neutrons. Some examples of such materials include cadmium, indium or silver. When the control rods are inserted into the reactor between the fuel elements, they can absorb the neutrons before they are able to reach another fuel rod and initiate another fission reaction. When the control rods are inserted all the way into the reactor, the fission chain reaction will slow down and stop. When the control rods are pulled out, the fission chain reaction will be possible and the fission rate will increase. Hence the typical control sequence for a reactor will go something like this. First you pull the rods out to give k greater than 1 and the power output of the reactor will increase. Once you reach the desired power level, you push the rods in until you reach the so-called criticality condition of k equals 1. The reactor will now be in a steady state of constant power. To alter the power output up or down, you move the rods out or in respectively until you reach the desired power before readjusting the rods to bring k back to a value of 1 again. When you want to turn the reactor off, you push the rods all the way in so that k is less than 1 and the fission rate dies down. It is important to note that the products of a fission reaction are radioactive nuclei that have half-lives and emit alpha, beta and gamma radiation. So even if you put the control rods in and turn the fission reaction off, the nuclear fuel will contain these radioactive fuel products and the decay processes will produce residual heat in the fuel long after the fission process stops. The ability to control this residual heat is very important. A failure of the backup diesel power generators after the earthquake and tsunami in 2011 resulted in a loss of cooling power at the Fukushima power plants in Japan. Even though the reactors successfully shut down following the earthquake, the build-up of heat in the reactors after the cooling pumps failed eventually resulted in the fuel melting and a major accident. It is worth noting an important subtle point about reactor control. To be able to control the neutron reproduction factor, it must be possible to move the control rods in and out on a time scale that is comparable to the rate at which the neutron reproduction factor changes. Fortunately, it turns out that some of the neutrons emitted following a fission reaction arise from the subsequent beta decays of the fission fragments. This means that their emission is delayed by some seconds to minutes, a time scale comparable to the rate at which the control rods can be moved. If not for these delayed neutrons, a nuclear fission reactor would be almost impossible to control.