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Accelerators and Reactors (file A06)

by Ludwik Kowalski, Sylvie Leray and David Whittal

The purpose of this section is to quickly review those nuclear energy concepts and vocabulary which are necessary to understand the newly conceived waste destroying systems. An accelerator is an electrical machine which speeds up charged particles. It can be a circular path device, such as a cyclotron, or a linear path machine, such as a linac. The two most important characteristics of an accelerator are the final energy of its particles, for example, protons of 1000 MeV*, and the maximum beam current, for example, 10 milliampers. Nuclear reactors are devices in which fission is used to generate heat; the heat is then used to produce electricity. This is accomplished in a chain reaction, as explained later in this section. The possibility of a chain reaction was experimentally confirmed in 1942 by Fermi. Ten years later electricity was already generated in reactors on a very small scale. The current status of nuclear energy has been outlined in the introduction.

Nuclear physicists refer to energies of individual particles in terms
of units called MeV. One Joule is equal to 6.25*1012 MeV,
one calorie is 4.18 Joules.

A nuclear power plant does not differ significantly from a traditional fossil fuel plant in terms of how mechanical work is used to produce electricity; the work is done by compressed steam flowing through a turbine. In both cases steam is produced by heating water but the ways of obtaining heat are very different. In a traditional plant this is done through chemical oxidation (burning of coal, gas or oil) while in a nuclear plant this is commonly done through fission of 235U nuclei. This process of " nuclear burning" is commonly known as a chain reaction; it is schematically illustrated in Figure 2 . White circles, on that figure, represent fissioning nuclei, arrows represent neutrons, and black circles represent nuclei which absorb neutrons without fissioning. The neutrons cause fission in other nuclei, these nuclei emit neutrons leading to additional fission events, to more neutrons, to additional fission events, etc. Each fission fragment is at once absorbed into the surrounding and its energy becomes heat. A nuclear chain reaction, as ordinary burning, can proceed in three different ways: it can extinguish itself, it can become explosive or it can remain steady.

click to see Figure 2 (use the back button to return later).

A system in which the burning of fissionable fuel leads to a progressive self-extinction is called subcritical while a system in which the rate of burning is steady is called critical. If the average number of neutrons per fission event (a white circle) is 2.5 then the net average outcome is 2.5-1= 1.5 neutrons; one neutron is replaced, on the average, by 1.5 neutrons. The net outcome of a non-fission event (black circle) is 0-1=-1; one neutron is absorbed without a replacement. The average outcome of a general collision (either black or white circle) must thus be a number, k, whose value is between -1 and +1.5, depending on the relative probabilities of non-fissioning and fissioning events. These probabilities, in turn, depend on the composition of the fuel, on its arrangement within the reactor core, and on the non-fissionable material present. In a critical nuclear system k is unity while in a subcritical system k is less than one.

Contemporary reactors are designed to operate as critical systems. To illustrate the difference between the critical and subcritical systems consider a situation in which a chain reaction is started by introducing 1000 neutrons into a large core. In a critical system the reaction is going to be self-sustained because, on the average, each of the absorbed neutrons is replaced through fission. In a subcritical system, on the other hand, there are only partial replacements of the absorbed neutrons and the chain reaction will eventually extinguish itself. This is similar to what can happen to a population of rats started by 500 male-female pairs of parents. In this analogy, the value of k is the ratio of the average number of births in one generation over the average number of births in the previous generation. A system is subcritical (declining population) when k<1, critical (steady population) when k=1 and explosive when k>1. A subcritical system in which k is only slightly smaller than unity will generate many more neutrons than were initially present. For example, every bunch of 1000 neutrons injected into a subcritical system may be able to generate a self-extinguishing chain reaction releasing 10000 neutrons. This means that for every neutron entering the system ten are available for various tasks.

The core of a nuclear reactor consists of many fuel elements separated by large distances in water. Elements are fabricated from a mixture of two isotopes of uranium: the thermally fissionable 235U (less than 4%) and non fissionable 238U. Neutrons produced in a fission event occurring in one element usually escape that element and travel through water before causing fission in another element. This slows them down; slow neutrons are more likely to cause fission than fast neutrons released in a fission event. The process of slowing down neutrons is called moderation; the technical name of the medium which separates fuel elements, and which slows neutrons, is "moderator". In today's reactors water serves two functions: it moderates neutrons and it removes heat from the assemblies of fuel elements.

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