Neutron Moderator

    

 A piece sample of graphite.

     In nuclear engineering, a neutron moderator is a medium that is used in order to reduce neutrons' speed, turning them into thermal neutrons capable of sustaining a nuclear chain reaction. The speed (as well as the energy of the neutrons) needs to be carefully monitored in order to make the processes as safe as possible and to obtain maximum efficiency. 

    

    Commonly used moderators include regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors).

    

    Neutrons are slowed down due to the interactions with the nuclei within the moderator. These are elastic interactions. The neutron's speed can be calculated and predicted by the number of interactions that take place in the moderator. This way, if the initial velocity is known, then you can calculate how many interactions are needed in order to reduce the neutron's speed to the desired one. 

    

    In a thermal nuclear reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits ("fissions") into two smaller atoms ("fission products"). The fission process for 235U nuclei yields two fission products: two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. The free neutrons are emitted with a kinetic energy of ~2 MeV each. Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become self sustaining — a chain reaction — under controlled conditions, thus liberating a tremendous amount of energy. 

    

    The probability of further fission events is determined by the fission cross section, which is dependent upon the speed (energy) of the incident neutrons. For thermal reactors, high-energy neutrons in the MeV-range are much less likely to cause further fission. (Note: It is not impossible for fast neutrons to cause fission, just much less likely.) The newly-released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down or "moderated", typically to speeds of a few kilometers per second, if they are to be likely to cause further fission in neighbouring 235U nuclei and hence continue the chain reaction. This speed happens to be equivalent to temperatures in the few hundred Celsius range. 

    

    In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully thermalised than others; for example, in a CANDU reactor nearly all fission reactions are produced by thermal neutrons, while in a pressurized water reactor (PWR) a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled supercritical water reactor (SCWR), the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor.

    

    Good moderators are also free of neutron-absorbing impurities such as boron. In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The German World War II nuclear program suffered a substantial setback when its inexpensive graphite moderators failed to work. At that time, most graphites were deposited on boron electrodes, and the German commercial graphite contained too much boron. Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators. In the U.S., Leo Szilard, a former chemical engineer, discovered the problem.