creating two smaller nuclei and two or three free neutrons travelling at high speed. This process creates heat and it is this heat that reactors are designed to utilise to generate electricity.
2.2 The Chain Reaction
The amount of heat given out from a single atom being split is of course very small. The energy needed to run a 100 watt light bulb is equivalent to the energy produced by the splitting of about three million million atoms every second. This high number of atoms is, however, only equivalent to about a one thousandth millionth of a gram of uranium. What is needed therefore, in order to generate signficant amounts of heat, is a reaction which involves the splitting of a vast number of uranium atoms every second. This can be achieved by utilising the free neutron particles, that are produced in the fission process, to initiate fission in other uranium atoms which in turn produce further neutrons, and so on. We have what is called a ‘chain reaction'. Nuclear reactors are designed so that when they operate at steady power the neutrons produced in one generation of fissions are just enough to initiate the same number of fission processes in the next generation, taking into account losses of neutrons from the reactor core.
2.3 Moderation
The neutrons produced in the fission process leave the newly split atom at high speed, too fast to cause fission in other uranium-235 atoms. If these neutrons are to be used to initiate new fissions they must be slowed down, or 'moderated'. In order to achieve this, material is introduced into the core of the nuclear reactor which will slow down the neutrons by absorbing some of their energy but which will not actually capture them and so prevent them from causing further fissions. Such a material is referred to as a 'moderator'. A simple analogy would be to imagine a billiard ball bouncing off the cushions of a billiard table. With each rebound a little energy will be given up to the cushion and the ball will slow down a little. If it were not for friction, several rebounds would be needed to slow the ball down significantly. In the same way, fast neutrons need to make several collisions with the atoms in the moderator to slow them down sufficiently to have any chance of causing further fissions. This is an important safety feature since it means that if the moderator is removed for any reason the chain reaction will stop since it cannot be kept going by fast neutrons alone. A number of different materials are used as moderators in different designs of reactors. In the Magnox and Advanced Gas Cooled reactors, graphite is used. In the Canadian CANDU reactors, heavy water is used, that is water formed from oxygen and a heavy type of hydrogen known as deuterium. The boiling water and pressurised water reactors (BWR & PWR) use normal water both as the moderator and to remove heat from the reactor core. In the Russian RBMK design a combination of graphite and water is used. This can, under some circumstances, have serious safety implications and was a major contributor to the Chernobyl accident.
2.4 Control
The overall rate at which fission reactions take place, and thus the rate at which the reactor produces energy,
depends on the number of free neutrons present. This can be controlled by inserting or removing control rods into or out of the core. Control rods are made of materials which will absorb neutrons without causing fission.
If control rods are lowered into the nuclear reactor's core they rob the reaction of neutrons and hence the reaction rate slows down. Removing the control rods increases the reaction rate. Reactors have large numbers of control rods (typically 100's) and are designed to ensure that only a fraction of the total number of control rods need to be inserted in order to shut the chain reaction down completely. Altering the reactor power can be achieved by varying the height of a number of partially inserted control rods. Dropping all the control rods into the core shuts down the reactor very quickly. If for any reason the rods would not go in then there are alternative means to shut the reactor down. In a PWR this is usually by injecting boric acid, a neutron absorber, into the primary circuit water.
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