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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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