Basic Principles of Fission Reactors

320 Pages·1961·24.59 MB·English

by  W. R. Harper

Basic Principles of
Fission Reactors
W. R. HARPER, Ph.D., F. Inst. P.
1961
INTERSCIENCE PUBLISHERS INC., NEW YORK
Interscience Publishers Ltd., London
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Copyright © 1961 by W. R. Harper
Library of Congress Catalog Card Number 61-9061
Interscience Publishers, Inc.
250 Fifth Avenue, New York 1, N.Y.
For Great Britain and Northern Ireland:
Interscience Publishers Ltd.
88/90, Chancery Lane, London, W.C.2.
PRINTED IN THE NETHERLANDS
BY N.V. DIJKSTRA'S DRUKKERIJ, VOORHEEN BOEKDRUKKERIJ GEBR. HOITSEMA, GRONINGEN
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Contents
Preface vii
1. The Basis of Design 1
2. The West Stands Experiment 15
3. The Industrial Power Reactor 31
4. Fissile Fuel 56
(\ The Fuel Element 77
6; Neutron Source and Neutron Sink 97
W. The Fast Reactor 117
8. The Moderator 143
9. The Homogeneous Moderated Reactor 161
10. The Heterogeneous Reactor 186
11. The Influence of Purpose on Design 199
'Si Control of Radioactivity 215
T3. Radiation Protection 231
1i. Removal of Heat 241
fReactor with Varying Fux 256
Operating a Reactor 277
Appendices 297
Bibliogr9aphy 307
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Index9 309
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PREFACE
This book takes the reader to about the stage at which detailed
reactor design begins, without assuming any previous knowledge
of nuclear technology. It does, however, assume an appropriate
background of physics and mathematics (and some chemistry).
Though intended as an introduction to a postgraduate course, it
should not be too advanced for the young student well prepared
to proceed to a higher education in science or engineering, and it
also provides a coherent account of reactors for those professionally
qualified in neighbouring fields.
Without sweeping generalizations, reactor theory is difficult
to follow at a first meeting. The mathematical treatment has
therefore been chosen to clarify basic principles; for example,
the diffusion of slow neutrons into a fuel lump is considered for
spherical symmetry in order to avoid the use of Bessel functions,
though it is cylindrical symmetry that is important in practice.
Mathematical proofs that serve only an immediate purpose, and
can be ignored without detracting from the subsequent logical
development of the book, are marked with a star in the margin.
They can 9well be omitted at a first reading, continuing reading
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at the dou9ble star.
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Only su45ch nuclear physics as is necessary for an understanding
of reactor04s is given, and this is not introduced until it is required,
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so that it 5is first encountered in some reactor context. The physics
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not being0 segregated in special chapters will make it necessary
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to use the3 index for cross-referencing, and it has been prepared
p.
with this idn mind. The diagrams should be regarded as part of
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the tex7/t, and are meant to be studied.
The way i02n which the underlying physical principles are trans-
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lated intoet/ mathematical form and then embodied by the engineer
in workingn equipment (with an eye on economics) is emphasized
throughoudle.t the book. In an overall view, these different aspects of
the designan problem cannot be separated.
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The presedl.nt state of reactor technology cannot be understood
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viii
PREFACE
without reference to its origins, and the reactors described in
this book are all of historical importance, selected to illustrate
the different parts of the text. Particulars of these reactors are
given in the Appendices.
It is easier to explain why a bomb explodes than how to design
a power reactor so as to be safe, and it is instructive to compare
the two cases. The principle of the bomb is, therefore, described in
relation to the problem of taming a reactor. The dangers inherent
in the peaceful uses of nuclear energy are frankly faced, so that
the reader may learn how they can be properly circumvented.
The first two chapters introduce important ideas in nuclear
engineering that will be new to the reader, so that a nuclear power
station can be described early in the book. This enables the
detailed and quantitative discussions of the later chapters to be
fitted into a proper background. The reactor itself is the centre-
piece of the book. Attendant subjects such as radiation protection,
health physics, waste disposal and economic implications are
included s9o as to put the reactor in perspective, but are not
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pursued f9urther.
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I am gr45eatly indebted to several experimental establishments
of the A04tomic Energy Commission of the U.S.A., to Atomic
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Energy 5of Canada Ltd. and to the United Kingdom Atomic
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Energy 0Authority, for generous help in providing me with a
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number o3f the illustrations which appear in this book. I am also
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indebted dfor other illustrations to authors and publishers who
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have allow7/ed me to make use of their copyright. Detailed acknowl-
edgemen02ts are made in the text, but special thanks are due to
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D. Van et/Nostrand Company Inc., the McGraw-Hill Book Company
Inc. annd the Pergamon Press. I have made considerable use of
diagramsdle. from papers published in the proceedings of the Geneva
Conferencanes on the Peaceful Uses of Atomic Energy.
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CHAPTER 1
The Basis of Design
In a coal-fired electrical generating station it is oxygen in air
that keeps the fuel burning; in a nuclear generating station it
is neutrons. The smallest unit of length used by the engineer is
a microinch, but a cubic microinch of the air that enters the
furnaces of a generating station nevertheless contains about 85
oxygen molecules: the molecules are very small. They are, actually,
a lot smaller than this figure would indicate since they occupy
rather less than a thousandth of the volume taken up by the gas,
spreading through the greater volume because of their thermal
agitation. This thermal motion would enable them to escape from
a solid enclosure were it not for the tight packing of the molecules
in a solid, which normally leaves no interstices of sufficient size
for the molecules of a gas to penetrate.
The mass of a neutron is slightly greater than the mass of a
hydrogen atom, and neutrons can reach thermal equilibrium by
collision with the molecules of ordinary matter, so neutrons can
constitute a kind of gas. A neutron gas, however, has very different
properties9 from an ordinary gas: to a first approximation, the
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neutrons 9can pass right through the walls of an enclosure. Though
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the mass 45of a neutron is comparable with the mass of a gas
molecule,04 its size is to be compared, not with that of the entire
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molecule,5 but with that of the atomic nucleus, and this is very
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much less0. Neutrons do not interact with the extranuclear parts
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of an atom3 because the forces that reign there are electrical,
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and the ndeutron has no charge. The only forces that influence
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neutrons 7/(apart from gravity, the effect of which is difficult to
detect) ar02e the forces that hold a nucleus together, and their
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effect fallet/s off so rapidly with distance as to become negligible
just ountside the nucleus. Thus, if a neutron is travelling towards
the wall odle.f a solid container, the only obstacles it 'sees' are the
tiny nucleani of the atoms of which the wall is composed, and their
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target aredl.a does not amount in all to more than a small fraction
of the areha of the wall, unless the wall is thick. A neutron gas,
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2
BASIC PRINCIPLES OF FISSION REACTORS
then, consisting of particles of mass similar to that of the molecules
of a material gas, will, if endowed with similar thermal agitation,
escape at once from a container that would hold the material
gas; furthermore it can occupy the same space as a solid body
without being prevented thereby from behaving like a gas.
Looking further than this simple picture, we must allow for
the fact that encounters with nuclei, though rare by comparison
with molecular-gas collisions, do occur and lead to four kinds of
events, three of which have a special and individual importance
in nuclear reactors. These events are absorption followed by
fission, capture (absorption without fission), and elastic and
inelastic scattering.
Fission
Metallic uranium from naturally occurring ore contains two *
isotopes, that is, two different kinds of atom having the same
chemical properties because the parts of the atoms outside the
nucleus are identical; they have different nuclear properties
because t9he one kind of nucleus, 238U, contains three more neutrons
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in its stru9cture than the other, 235U, though both contain the
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same num45ber of protons. If they did not the total nuclear electrical
charges w04ould be different, and the outer parts of the atoms
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could not5 be built in the same way; in other words they could
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not both h0ave the same chemical properties and would therefore
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be differe3nt chemical elements. Less than 1 per cent of the natural
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isotopic mdixture is 235U, but it is only the occurrence of this
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particular7/ isotope that has made nuclear power production
technolog02ically possible. This is because the absorption of a
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neutron bet/y 235U can provoke a form of atomic disintegration called
fission, inn which the nucleus splits into roughly two halves, and
because tdle.he fission process involves the liberation of a relatively
large amoanunt of energy, and finally because more neutrons are
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thrown oudl.t during fission than are required to provoke it. Hence
one fissiohn can cause another, and another, and yet another
and a chap://in-reaction can occur — always supposing that
* The 0htt.0058 per cent of 234U is of no concern to us.
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THE BASIS OF DESIGN
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neutron wastage is not too great. Neutrons are ejected during
fission with speeds much greater than thermal speeds; these fast
neutrons are much less efficient than thermal neutrons in provoking
fission in 235U.
Neutron Capture
A neutron ejected from a 235U fission in a block of natural
uranium has a very poor change of causing another fission in
235U: it is much more likely to be absorbed in a 238U nucleus
without causing fission. A 239U nucleus is formed instead, which,
though unstable, does not eject a neutron. For this reason, no
chain-reaction can be built up in a simple block of natural uranium.
Different elements absorb neutrons to a greatly differing
extent, the extent depending also on the speed of the neutron,
particularly for intermediate and slow neutrons. At intermediate
speeds absorption peaks occur which may be high and sharp.
For slow neutrons it is a general rule that the absorption coeffi-
cient is inversely proportional to the speed of the neutron, in^ther
words pro9portional to the time spent in the neighbourhood of
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the nucle9us.
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To avoid u45ndue wastage of neutrons in a reactor, only elements
which hav04e small absorption coefficients, for example aluminium,
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can beuse5d for structural purposes insidftthe actiyejpart^Elements
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like boron0, on the other hand, which absorb neutrons strongly,
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are valua3ble for control purposes: the build-up of the chain-
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reaction ids progressively discouraged as boron-steel rods are
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pushed in7/to a reactor.
It freq02uently happens that a nucleus, after capturing a neutron,
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and thereet/by becoming another isotope of the same element, is
then radionactive. The new isotope decays into another element
which madle.y again be radioactive, in which case the process continues
until a staanble element is formed. The radiations that are emitted
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during thedl. decay are dangerous to health, may be a nuisance in
other wayhs, and their emission can last for a considerable time.
This calls p://for special precautions in the handling of reactor com-
ponents whtthich have been exposed to the neutron flux. The effect
is turned T / to advantage, however, in the preparation of radioactive
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4
BASIC PRINCIPLES OF FISSION REACTORS
isotopes, useful in industry and medicine, by irradiating the ap-
propriate element inside a reactor.
Elastic Scattering
Neutrons colliding with nuclei may behave much as two billiard
balls do, no energy being lost and momentum being shared, thus
resulting in a slowing-down of the neutron. It is this process that
enables a self-sustaining chain-reaction to be established in
natural uranium by incorporating what is known as a moderator,
Fig. 1.1. Schematic arrangement of pile.
commonly graphite. The active core of the reactor then consists
of uranium 'diluted' with graphite. A common arrangement is to
insert long thin uranium rods into an array of holes left in a large
assembly of graphite blocks, as in Fig. 1.1, such a reactor having
originally been called a 'pile'* from the manner of building it.
The dimensions of the array are so chosen that a (fast) fission
neutron from 235U has a good chance of escaping from the rod in
which it is liberated before being captured by 238U: the rods must
not be too9 thick. The neutron must then pass through sufficient
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moderato9r to be slowed down past the speed at which absorption
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* In 45British terminology the word pile is reserved for a natural uranium
reactor;04 in American terminology no distinction is made between pile
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and react5or, and the word pile is going out of use.
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