What is a Reactor?
Atom and Nucleus
If you take a pair of metal hemispheres and slam them together very fast face to face, one
of two things may happen. You may get a loud clunk. Or you, the hemispheres and
everything else in the vicinity may be almost instantly vaporized in a burst of incredible
heat. If the latter happens, you can be sure that the metal was a particular kind of
uranium, not that the confirmation will do you much good.
What has vaporized you is raw energy, released from the innermost structure of the
uranium. The energy in the interior of uranium was revealed to the world on 6 August
1945, in the sky above Hiroshima, Japan. Never has a source of energy made a more
horrifying debut. Yet, paradoxically, the most overpowering energy humanity has learned
to release comes from the tiniest reservoir we have yet learned to tap: the nucleus of an
atom.
What is an 'atom'? And what is its 'nucleus'? Suppose you take a lump of lead, and cut it
into smaller and smaller pieces. When the pieces are so small that your knife is too
clumsy, switch to an imaginary knife and keep cutting. Ultimately the pieces will get so
small that if you cut any more you will not get two pieces of lead: the next cut will
change the identity of what you are cutting. The smallest piece which is still lead is called
an atom of lead.
The word 'atom' means 'indivisible'. You cannot divide an atom of lead and still get lead.
But you can divide the atom and get smaller pieces which are no longer lead. If you start
to dismantle an atom, the first part you remove is called an 'electron'. Until this stage you
have been able to cut without encountering any conspicuous electrical effects; but the
electron is negatively charged, and the remainder of the atom is left positively charged.
The parts of the atom have become 'ions': a negative ion (the electron) and a positive ion
(the remainder of the atom). Each time you remove another electron you leave still more
positive charge on the remainder. It becomes doubly 'ionized', triply 'ionized', and so on.
Since negative and positive charges attract each other, it is harder and harder to prise off
successive electrons.
Suppose, however, that you manage to remove all the electrons. (For most atoms this is
in practice very difficult.) What you have left is the innermost heart of the atom: the
nucleus. This is where all the positive charges are. Furthermore, you would now find an
enormously increased difficulty in cutting any more. Surprisingly enough, although the 14
nucleus contains only positive charges (which repel each other), its constituent pieces
cling together with a loyalty which makes the outer electrons look frankly promiscuous.
There used to be much popular talk of 'splitting the atom', but the problem was rather one
of splitting the nucleus of the atom. 'Atomic' bombs should have been called 'nuclear'
bombs: for the shattering energy they released came from the rupturing of nuclei (one
nucleus, two or more nuclei).
Uranium
What makes uranium so dramatically different from other substances? To appreciate its
unique characteristics we must first consider some basic nuclear physics: that is, wha t
nuclei consist of and how they behave. An atom is made of electrons around a nucleus; a
nucleus in turn is made up of 'protons' and 'neutrons'. A proton has a positive charge; a
neutron has no electrical charge and is 'neutral'. At first it seems difficult to understand
how a nucleus stays together at all. The positive charges of the protons ought to push
them violently apart. But within the compact volume of the nucleus a new kind of force
comes into effect: an immensely powerful short-range attractive force acting equally
between protons and neutrons - which, from this point of view, are all 'nucleons'. The
short-range nuclear force holds them together, against the repulsive effect of the protons'
positive charges. In this way the neutrons act as 'nuclear cement'.
However, in a nucleus which contains 92 protons - that is, a nucleus of uranium - the
repulsive force among the protons is on the verge of overcoming the nuclear force. If
there are as many as 146 neutrons also present, the nucleus can remain intact - barely.
This form of uranium, containing in all 238 nucleons, is called uranium-238 or 238/92U.
For reasons that need not concern us here, involving the grouping and compatibility of
nucleons, the next most probable arrangement is a uranium nucleus containing three
fewer neutrons: uranium-235, 235/92U. Atoms with these lighter nuclei make up about
0.7 per cent of naturally-occurring uranium. (If nuclei have the same number of protons,
they are nuclei of the same chemical 'element': thus, every nucleus with 92 protons is the
nucleus of an atom of uranium. Atoms whose nuclei have the same number of protons but
different numbers of neutrons are called 'isotopes' of the element: for instance, uranium-
238 and uranium-235 are isotopes of uranium.) The uranium-235 nucleus has a property
unique among all the more than 200 types of nuclei found in nature in significant quantity
before 1942. The uranium-235 nucleus is already under near-disruptive internal stress; a
stray neutron blundering into it can rupture it completely.
Radioactivity Produces Radiation
When a stray neutron hits a uranium-235 nucleus, the result is a 'compound nucleus' of
uranium-236. It is called a compound nucleus because it does not last long. The energy
added by the neutron - even a 'slow' one - overcomes the precarious stability of the
nucleus and, almost instantly, it flies apart. The rupture of a uranium-236 compound
nucleus usually results in about two fifths of the nucleus flying off in one direction and 15
about three fifths in the opposite direction, with perhaps two or three odd neutrons also
shooting out. The flying fragments burst out with so much energy that a subsequent tally
of masses reveals a shortage: some of the mass of the original nucleus has been converted
into energy. This is the source of the enormous energies released in such nuclear events.
For example, one common subdivision results in one chunk of 38 protons and 52
neutrons, another of 54 protons and 89 neutrons, and 3 odd neutrons: making up, of
course, 236 nuc leons in all. The chunk containing 38 protons is a nucleus of strontium;
since it contains in all 90 nucleons it is the notorious strontium-90. The chunk containing
54 protons is a nucleus of the inert gas xenon; since it contains in all 143 nucleons it is
xenon-143.
Such a complete rupture of a nucleus is called a 'fission', by analogy with the biological
term for the division of a growing cell. More precisely it is called 'nuclear fission'. When
it is provoked by the impact of an additional neutron it is called 'induced fission'. such is
the case with uranium-235 just described. Some very heavy nuclei are so unstable that
they may rupture even without being struck by a neutron; such a rupture is called
'spontaneous fission'. Fission, whether induced or spontaneous, is the most violent kind of
breakdown that a nucleus can experience. But there are others. A nucleus of uranium-
238, for instance, while not being so near to rupture as its lighter relative, is still under
severe stress: so much so that sooner or later it is likely to squirt out a lump made up of
two protons and two neutrons. Since this makes a larger proportional reduction in the
proton contingent than in the neutron contingent, the remaining nucleus, now containing
only 90 protons and 144 neutrons, is slightly less stressed. (It is a nucleus of the metal
thorium- 234.) The lump or 'particle' squirted out is identical in every respect to an
ordinary helium nucleus; but since it emerges with considerable velocity, and ploughs a
furrow through whatever slows it down, it is given a special name: it is an 'alpha particle'.
Most nuclei with at least 83 protons undergo a breakdown this violent; they are called
'alpha-emitters'.
The balance between protons and neutrons in thorium-234, while more satisfactory, is far
from ideal. In effect, by emitting an alpha particle, the nucleus has over-adjusted. This
leads to a yet more delicate form of breakdown. Out of the nucleus containing 90 protons
and 144 neutrons there suddenly squirts an electron. It is identical in every respect to the
electrons outside the nucleus; but since it emerges with considerable velocity it too is
given a special name: it is a 'beta particle'. The nucleus which remains now contains one
more positive charge than it had. But since an electron is very much less massive than a
nucleon, there are the same number of nucleons as before: 234. A neutron has apparently
turned into a proton. The nucleus now contains 91 protons and 143 neutrons. it is a
nucleus of protactinium-234. Like thorium-234, protactinium-234 is also a 'beta emitter';
- when it emits a beta particle it becomes uranium-234, which is an alpha emitter. So,
leapfrogging down by alternate alpha and beta emission, the nucleus alters itself until it
has only 82 protons and 124 neutrons, and is at last stable: a nucleus of lead-206.
On the way, the nucleus regularly finds itself, after emitting an alpha or beta particle, still
unduly agitated or 'excited'. To settle itself down it gives off a burst of energy in a form 16
closely akin to ordinary light, but much more energetic, and invisible. This burst of
energy is called a 'gamma ray'. It is identical in every respect to the well-known 'X-ray',
except that an X-ray comes from the electron layers outside the nucleus, whereas a
gamma ray comes from inside the nucleus.
Consider also the nucleus of strontium-90, one of the two large fragments formed by the
induced fission of uranium 235 in the earlier example. The strontium-90 nucleus, a
'fission product', has a disproportionately large number of neutrons for its protons,
coming as it does from a much heavier nucleus which requires more 'cement'.
Accordingly, the strontium-90 nucleus is also a beta emitter. Sooner or later it squirts out
a high-velocity electron - a beta particle - and one of its neutrons is replaced by a proton.
It becomes a nucleus of yttrium-90, another beta emitter, which by the same process
becomes a nucleus of zirconium-90, which is stable. Beta emissions from fission product
nuclei are often followed by one or more gamma rays.
There are thus four ways in which a nucleus can alter itself: fission, alpha emission, beta
emission, and gamma emission. From a lump of material containing such unstable nuclei
the emissions from these activities shoot out radially in all directions: the lump is said to
be 'radioactive', and the emissions - neutrons, alpha and beta particles, and gamma rays -
are called 'radiation'. A collection of nuclei which shoots out one such emission per
second is said to exhibit one 'becquerel' (Bq) of radioactivity, after Henri Becquerel, who
first discovered the phenomenon of radioactivity in 1896. The becquerel is now the
accepted international unit of radioactivity, but you will still see an older unit, the 'curie'.
A curie of radioactive material shoots out not one but 37 000 000 000 emissions per
second. This is the radioactivity of one gram of radium, one of the first substances known
to be radioactive, which was discovered by Marie Curie. A curie is a lot of radioactivity;
you will also encounter metric subdivisions - the millicurie, microcurie, nanocurie and
picocurie, in descending steps of 1000 as usual. You can see that one nanocurie equals 37
becquerels of radioactivity. You should note also that 'radioactivity' produces 'radiation';
the two terms are not interchangeable, although even official statements sometimes use
'radioactivity' when they mean 'radiation' and vice versa.
In a radioactive substance it is impossible to tell whether a particular nucleus is on the
point of radioactive breakdown, or 'decay'. Nonetheless, in a sufficiently large sample of
any particular radioactive nuclear species, or 'radioisotope', a certain fraction of the nuclei
always decay in a quite regular length of time. For instance, if you start with 1000 nuclei
of strontium-90, 28 years later 500 will have decayed and you will have 500 left. After a
further 28 years, 250 of the remaining 500 will have decayed, and you will have 250 left.
And so on: however much you start with, 28 years later half will have decayed and only
half will be left. Obviously the corresponding radioactivity will also have fallen by one
half. For strontium 90 the period of 28 years is called its 'half-life'. Each radio-isotope has
a half-life for each form of radioactivity it exhibits: in each case the half-life is the time
during which half the nuclei in a sample decay, and the corresponding radioactivity falls
to half the initial level. Half-lives of different radio-isotopes range from fractions of
millionths of a second to millions of years.17
The Effects of Radiation
Unless radioactive decay takes place in a vacuum, the radiation emitted must pass
through the surrounding substance. The consequences depend on the substance, on the
type of radiation, on its energy and on its intensity. An alpha particle, made up of four
nucleons with two positive charges, interacts vigorously with surrounding atoms, tearing
off electrons and knocking nuclei out of place. In doing so, the alpha particle quickly
gives up its own energy, travelling only a short distance but doing enormous damage
along its path. Most alpha radiation is stopped within the thickness of a single sheet of
paper. A beta particle, much less massive and with only one negative charge, disturbs and
dislodges neighbouring electrons, but loses its energy less swiftly and therefore travels
somewhat farther than an alpha particle. Most beta radiation is stopped within the
thickness of a thin sheet of metal. A gamma ray, with no electrical charge, loses its
energy much more gradually, and can travel a long distance, causing a relatively small
amount of disturbance at any particular point on its path. A neutron, also without
electrical charge, is likewise free to travel a long distance, and is slowed down mainly by
direct collision with nuclei. Gamma or neutron radiation can penetrate more than a metre
of concrete.
Dislodging an electron from an atom makes the atom an ion: so emissions from nuclei are
'ionizing radiation'. When ionizing radiation passes through a material it causes changes
in the structure of the material - sometimes temporary, sometimes permanent, sometimes
useful, sometimes harmful. The effects of ionizing radiation depend roughly on how
much energy the radiation releases into a given amount of material - the more energy, the
more disruption. The original unit of radiation exposure was the 'roentgen', named after
Wilhelm Roentgen, discoverer of X-rays.
The effects of ionizing radiation become particularly important if the radiation is passing
through living matter; the delicate molecular arrangements of living matter can be easily
upset by radiation. There are several units used to measure radiation effects on living
matter. Until recently the most common have been the 'radiation absorbed dose', or 'rad',
and the 'roentgen equivalent man', or 'rem'; the new international standard units are the
'gray' (Gy), equal to 100 rads, and the 'sievert' (Sv), equal to 100 rem. The rem and
sievert allow for the greater severity of alpha or neutron damage for equivalent energy
delivery. For beta and gamma radiation one gray is about the same as one sievert; for
neutrons and alpha particles one gray may be up to 20 sieverts, depending on the energy
of the particles.
The question of the biological effects of radiation is surrounded by controversy. But it is
known that a dose of perhaps 400 rem of radiation over the whole body will kill half the
adult human beings exposed to it; and very much smaller doses will produce cell damage
that may lead to leukaemia and other kinds of cancer. Furthermore, radiation damage to
the complex molecules in the reproductive cells which contain the hereditary information
may produce mutant offspring. Even a single gamma ray can disrupt a gene; it may 18
produce unforeseeable effects if this particular gene should be in a reproductive cell
which subsequently helps to form a child.
A more detailed discussion of radiation biology is given in Appendix B. Suffice it to say
here that the danger of radiation to living matter seems to increase in direct proportion to
the amount of radiation exposure, beginning from the very lowest doses. There does not
appear to be a threshold dose - that is, one below which damage does not occur. We are
already subjected to continual radiation from the natural radioactive substances in our
surroundings, and from cosmic rays. Any human activity which tends to add further
sources of radiation to our surroundings must be potentially harmful. Just how harmful -
and in return for what benefits - is still under debate; this book is intended to make one
aspect of the debate more intelligible, whatever your criteria.
The Chain Reaction
In a lump of uranium there are always a few stray neutrons, produced either by
spontaneous fission or by cosmic rays. Suppose that one of these stray neutrons induces a
nucleus of uranium 235 to undergo fission. As well as the two fission products, there
shoot outward perhaps two or three high-energy neutrons. (The chances are better than 99
to 1 that these neutrons will emerge virtually at the instant of fission: 'pr ompt' neutrons.
But there is a slight chance that a neutron will not emerge until some seconds later: a
'delayed' neutron. As we shall see, delayed neutrons are of considerable importance.)
There are three possibilities open to the high-energy neutrons from fission. A neutron
may reach the surface of the material and escape. It may strike another nucleus and be
absorbed without causing any immediate breakdown. Or - most importantly - it may
strike another nucleus and, in turn, cause this nucleus to rupture. The chances of a
neutron causing such an induced fission depend on the neutron's energy and on the
nucleus it strikes. A fast neutron, fresh from an earlier fission, tends to go right through a
nucleus so fast that nothing happens to the nucleus. Once in a while a fast neutron will
rupture a nucleus; indeed only a fast neutron can rupture a nucleus of uranium-238.
However, if a neutron ricochets among other nuclei, bouncing off each and giving up its
energy bit by bit, it soon slows down until it is just jostling with the shared heat-energy of
the rest of the material. It is then a 'thermal neutron'. A thermal neutron takes much
longer to go through a nucleus, and is thus much more likely to rupture a nucleus of
uranium 235 than is a fast neutron.
If, in a lump of uranium-235, one nucleus undergoes fission, the neutrons it releases may
strike other nuclei, causing more fissions and releasing more neutrons, If there are
enough uranium-235 nuclei sufficiently close together, the spreading disruption
multiplies with astonishing rapidity: more and more neutrons, more and more ruptured
nuclei, their fragments flying, more and more energy: a 'chain reaction'. If there is enough
uranium- 235, packed closely together for long enough, and if the chain reaction is out of
control, the result is a nuclear explosion: an 'atomic bomb'. Slamming two suitable
hemispheres of uranium-235 together at a very high velocity will indeed create a nuclear
explosion; but there are other, much more efficient, techniques - and materials.19
Needless to say, as soon as an appropriate arrangement of appropriate material became
possible it was tried out - on 16 July 1945, at the top of a tall tower in the desert near
Alamogordo, New Mexico: the world's first nuclear explosion, code-named Trinity.
Within three weeks an atomic bomb made of uranium-235 devastated Hiroshima. But,
seemingly, one 'doomsday weapon' was not enough, and uranium-235 was not the only
nucleus that could be used. A neutron can penetrate a nucleus of uranium 238 without
rupturing it. If this happens, the resulting neutron-heavy nucleus soon emits a beta
particle, and then another, to become a nucleus of plutonium-239. Like uranium-235 and
only a few other isotopes - all at present very rare - plutonium-239 is 'fissile': that is, it
can undergo a chain reaction of successive fissions, as the Trinity test demonstrated. On 9
August 1945 such a chain reaction obliterated Nagasaki.
The Nuclear Reactor
If chain reactions in uranium-235 and plutonium-239 could be used only in weapons, the
situation would already be sufficiently complicated. But, more than two years before
enough pure fissile material of either kind had been accumulated to make a weapon, it
was found to be possible to control a chain reaction: to have it maintain itself without
multiplying out of control. Indeed it was by this means that the plutonium was produced
for the Alamogordo and Nagasaki bombs. The arrangement used to create and control a
sustained nuclear chain reaction is called a 'nuclear reactor'.
The difference between an uncontrolled and a controlled chain reaction is profound. An
arrangement of fissile nuclei which is to undergo an uncontrolled chain reaction - a
nuclear explosion - must be sudden and final. An arrangement of fissile nuclei which is to
sustain a continuing controlled chain reaction must be much more carefully organized.
Curiously enough it takes many more nuclei - that is, much more material - to build a
reactor than it takes to set off an explosion. This is of course partly because an explosion
requires comparatively pure fissile material; in a reactor the fissile material is
comparatively dilute, and there is accordingly much more material in all. But there must
also be many more fissile nuclei themselves. The reason for this is the role played by the
all-important neutrons.
If a chain reaction is to be self-sustaining, it must keep itself supplied with neutrons.
Consider the following typical sequence. A neutron plunges into a nucleus of uranium-
235. The nucleus ruptures, as well as two fission-product nuclei it also shoots out three
neutrons. One of these three goes out through the surface of the lump of uranium and is
lost. Another is absorbed by a nucleus of uranium-238, which begins its two-stage change
into plutonium-239, but does not rupture. This leaves one neutron. If this third neutron
now plunges into another uranium-235 nucleus and ruptures it, the process can continue;
otherwise the chain reaction is snuffed out.
At any instant, inside the lump of uranium, there must be the right number of neutrons of
the proper energy to propagate the chain. In effect, for a sustained chain reaction, each
neutron which is lost by causing a fission must be replaced by exactly one neutron which
does likewise. The system then has a 'reproduction factor' of 1. When this condition is 20
achieved the system is said to be 'critical', and the situation is called 'criticality'. Despite a
common misconception to the contrary, 'criticality' is not here used to imply 'danger'.
You say that a nuclear system 'goes critical' just as you say that a car engine 'starts'. If on
average each neutron lost when it causes a fission is replaced by more than one which
also causes fission, the reaction 'runs away'; the reproduction factor is greater than 1, and
the system is 'divergent'. If on average each neutron so lost is replaced by fewer than one
which causes fission, the reaction will stop; the reproduction factor is less than 1. This is
why a piece of uranium below a certain minimum size cannot under normal conditions
sustain a chain reaction: there is too much surface through which neutrons can leak out.
Moderators
The basic requirements for a continuing controlled chain reaction are therefore, first, a
collection of fissile nuclei appropriately distributed in space; and, second, a
self-replenishing supply of neutrons of just sufficient numbers and energy to keep the
chain reaction going. In natural uranium, only 0.7 per cent of the nuclei are fissile
uranium-235. These fissile nuclei, only seven out of every thousand, are not sufficiently
close together to keep up a chain reaction; too many neutrons are absorbed by the heavier
uranium-238 nuclei, without causing fission. To improve the prospects for a sustained
chain reaction it is necessary either to increase the proportion of uranium-235 relative to
uranium-238; or to slow down the neutrons to thermal energies, at which they are much
more readily absorbed by uranium-235; or to do both.
As we shall see, increasing the proportion of fissile uranium-235 - so called 'enrichment'
of the uranium - is a complex and expensive process. But even a small increase, say from
0.7 per cent to 2 or 3 per cent, makes a marked difference, provided that the neutrons
from fission are slowed down. This can be done by means of a material with light nuclei -
a 'moderator'. A fast neutron striking a light nucleus in the moderator gives up a fraction
of its energy, and after a few such collisions has slowed to thermal energy. The best
moderators are the lightest nuclei: those of hydrogen. Ordinary water, containing two
hydrogen atoms per molecule, is a satisfactory moderator. But ordinary hydrogen nuclei
absorb neutrons, Better still is a rarer form of hydrogen nucleus: the proton-plus-neutron
form called 'heavy hydrogen' or 'deuterium'. If two atoms of heavy hydrogen combine
with an atom of oxygen, the result is a molecule of 'heavy water' or deuterium oxide
(sometimes written D2O), which is much the best moderator of fast neutrons.
One other substance is widely used as a moderator: carbon, in the form of graphite. A
carbon nucleus - six protons and six neutrons is much more massive than either form of
hydrogen nucleus, and is therefore not such a good moderator, But graphite is less
expensive than heavy water; furthermore it is a solid, which can be structurally useful in a
reactor.21
Reactor Design and Operation
To set up a nuclear reactor you proceed as follows. You take a good many pieces of
material containing uranium 235 - usually uranium metal or oxide, natural or enriched:
the 'fuel'. (You can also use plutonium-239, although this - as we shall see - involves
some difficulties.) For a large reactor you need many tonnes of fuel, much more than
enough to achieve criticality. One obvious reason for the extra fuel is to enable you to
operate the reactor for some time before replacing the fuel. Other reasons will become
clear in a moment.
You seal the pieces of fuel into casings called 'cladding', to support the fuel and to
confine the fission products that will be produced. You position the assemblies of sealed
fuel, called 'fuel elements', supporting them as necessary; remember that they may be
very heavy indeed. You intersperse the fuel elements with moderator, to slow down the
neutrons, and with neutron absorber, to enable you to control the chain reaction. You also
include measuring instruments to tell you what is going on inside the reactor. You need to
know, in particular, the temperature and the concentration of neutrons at various places
inside the reactor.
You are now ready to start up your reactor. Before start-up, with all the absorbers in the
interior of the reactor soaking up neutrons, the neutron density is so low it is difficult to
measure, unless you intentionally include a separate source of neutrons as a sort of
primer. A common form of absorber is a rod thrust through the interior of the assembly: a
'control rod'. Such a rod incorporates a material like boron, which absorbs neutrons like a
sponge. The rod may be made, for instance, of boron steel. So long as enough control
rods are in place no chain reaction is possible. To start up your reactor - to make a chain
reaction possible - you begin withdrawing control rods.
The region of the reactor in which the reaction takes place is called the core. You
withdraw control rods very slowly from the core, usually in short steps, in suitable
symmetry to maintain a more or less uniform build-up of neutron density inside the
reactor. In due course your reactor 'goes critical': a self-sustaining chain reaction is
established, in which each neutron lost by causing a fission is replaced by exactly one
neutron (either prompt or delayed) which does likewise. If the chain reaction could be
sustained by prompt neutrons alone it would be 'prompt critical', and difficult to control.
The dependence of the chain reaction on delayed neutrons allows you to adjust the
reaction-rate gradually instead of abruptly.
Taking absorber out of a stable chain reaction is called 'adding reactivity'; the neutron
density increases, and the rate of the reaction increases. But the build -up is gradual,
because some of the neutrons do not emerge immediately after fission. The smaller the
added reactivity, the longer is the time taken for the neutron density to increase by a
given proportion. This time is called the 'reactor period', and is a very important measure
of how well the reactor can be controlled. When a reactor has a short period it is liable to
be skittish. Of course inserting absorber - 'adding negative reactivity' - produces a reverse
effect. When the desired rate is established you reposition the absorbers to stabilize the 22
reaction at that rate.
To economize on neutrons you can surround the reactor core with a reflector to bounce
errant neutrons back into the reaction region. The best reflecting materials are the
moderator materials; in effect you can extend the volume of moderator beyond the region
of fuel elements. Since the presence or absence of reflector affects the neutron density in
the core, you can add reactivity by adding reflector, or vice versa; some reactor designs
utilize this effect for control purposes.
Before pulling the control rods far enough out to let your reactor go critical you must take
precautions against the radiation pouring out from the core. Neither alpha nor beta
particles will get beyond the fuel cladding (unless it leaks); but gamma rays and neutrons
can travel through metres of concrete and still be dangerous to living matter. Therefore,
you surround your reactor with enough concrete or other protective 'shielding' to cut
down the radiation outside to as low a level as you think advisable.
Xenon Poisoning
Normal start-up and shutdown of a reactor are both lengthy processes, and may take
many hours. If it is necessary to stop the chain reaction quickly, for instance in the event
of a malfunction, the emergency shutdown is called a 'scram'. If an operating reactor is
left to itself its reaction-rate will gradually dwindle, not necessarily because fissile nuclei
are being used up - in some reactors the number of fissile nuclei may even be increasing -
but also because of the build-up of fission products which absorb neutrons.
The most voracious of all is xenon-135. The consequent phenomenon, called xenon
poisoning, is an intriguing demonstration of the slightly surrealistic circumstances in
which reactors operate.
When you start up your reactor for the first time, the fuel contains no xenon-135. For
several hours after start-up, fission processes generate tellurium-135 and iodine-135,
which in turn generate xenon-135, which starts gobbling neutrons. Each xenon-135
nucleus which succeeds in capturing a neutron is thereby charged into xenon-136 - much
less voracious. The xenon-135 nuclei which fail to capture neutrons nonetheless undergo
beta decay into caesium-135, also much less voracious. Accordingly, after matters have
had a chance to settle down, as much xenon-135 is being lost as is being generated. There
is a certain average concentration of xenon-135 in the reactor core, which remains the
same as long as the chain reaction proceeds at the same rate. You budget for so many
neutrons lost to xenon-135, and operate accordingly. But when you change the reactionrate you upset the balance, and the consequences may be embarrassing.
Iodine-135 turns into xenon-135 with a half-life of 6.7 hours. Xenon-135 turns into
caesium-135 with a half-life of 9.2 hours - that is, slightly more slowly. Suppose you shut
down your reactor. The neutron flux falls to near zero; xenon-135 stops capturing
neutrons. From the moment of shutdown more xenon-135 is being generated than is
being lost: while your reactor is shut down, the amount of neutron absorber in its core is 23
steadily, surreptitiously increasing. If, several hours later, you try to start up your reactor
again, you may, even with the control rods completely out, be unable to add enough
reactivity to reach criticality. To be always able to start up your reactor at any time after
shutdown, you will find it necessary to include more fuel, or otherwise to arrange for an
excess of available reactivity over and above what normal operation needs. Apart from
the obvious cost of the extra fuel, this means that even in normal operation you must
leave some control rods partly inserted. It is not easy to do so without distorting the
uniform neutron density in the core, and producing a less than ideal pattern of chain
reaction. Reactor designers have to decide what sort of compromises they can best
achieve, to satisfy the conflicting requirements made necessary by phenomena like xenon
poisoning.
Refuelling
While you operate your reactor, changes take place in the fuel. The number of uranium-
235 nuclei dwindles gradually as they undergo fission. Some of the uranium-238 nuclei
capture neutrons and change to plutonium-239. Some of these plutonium-239 nuclei
undergo fission. Others capture additional neutrons and become plutonium-240,
plutonium-241 and other isotopes of elements heavier than uranium - 'transuranic
actinides'. Fission products are formed; most fission products are radioactive, and
undergo radioactive changes into more stable nuclei, some very rapidly, others very
slowly. Fission products also capture neutrons. The composition of the reactor fuel grows
increasingly complex as the chain reaction proceeds; it becomes more and more difficult
to keep track of all the competing processes taking place. Some of the fission products
are gaseous, like krypton and xenon; these gaseous fission products build up inside the
fuel, exerting pressure and trying to leak out. The intense neutron flux plays havoc with
the crystal structure of the fuel, the cladding, and possibly the moderator, knocking the
nuclei out of place and setting up stresses and strains in the material. Soon or later, it is
necessary to take out used fuel and replace it.
There is an assortment of different procedures for 'refuelling' or 'recharging' a reactor.
Some designs can be refuelled while the reactor is in operation, replacing one or more
fuel elements at a time: 'on-load refuelling'. Other designs are shut down for refuelling,
and perhaps one-third of the core is replaced at one time: 'off-load refuelling'. All
refuelling procedures must be carried out with extreme care because of the intense
radioactivity of the fission products in the reactor core and in the used 'spent' or
'irradiated' fuel.
Power from a Reactor
If you set up a reactor on a sufficiently large scale, and let the chain reaction run fast
enough, the energy released by the rupturing uranium-235 (and plutonium-239) nuclei
makes the whole assembly hot - potentially very hot indeed. Complete fission of all the
nuclei in a kilogram of uranium-235 would release a total energy of about one million
kilowatt-days - that is, as much heat as would be given off by one million one-bar electric
fires operating for one 24-hour day. That is a lot of heat. Accordingly, the fuel in a 24
reactor must be arranged so that the heat is given off gradually enough to keep
temperatures manageable. The amount of heat given off per unit volume in a reactor core
is called the 'power density'. It may be anything up to several hundred kilowatts of heat
per litre; if such an outpouring of energy is not to melt - and indeed boil - the whole
aggregation of material, it must be efficiently removed.
Atom and Nucleus
If you take a pair of metal hemispheres and slam them together very fast face to face, one
of two things may happen. You may get a loud clunk. Or you, the hemispheres and
everything else in the vicinity may be almost instantly vaporized in a burst of incredible
heat. If the latter happens, you can be sure that the metal was a particular kind of
uranium, not that the confirmation will do you much good.
What has vaporized you is raw energy, released from the innermost structure of the
uranium. The energy in the interior of uranium was revealed to the world on 6 August
1945, in the sky above Hiroshima, Japan. Never has a source of energy made a more
horrifying debut. Yet, paradoxically, the most overpowering energy humanity has learned
to release comes from the tiniest reservoir we have yet learned to tap: the nucleus of an
atom.
What is an 'atom'? And what is its 'nucleus'? Suppose you take a lump of lead, and cut it
into smaller and smaller pieces. When the pieces are so small that your knife is too
clumsy, switch to an imaginary knife and keep cutting. Ultimately the pieces will get so
small that if you cut any more you will not get two pieces of lead: the next cut will
change the identity of what you are cutting. The smallest piece which is still lead is called
an atom of lead.
The word 'atom' means 'indivisible'. You cannot divide an atom of lead and still get lead.
But you can divide the atom and get smaller pieces which are no longer lead. If you start
to dismantle an atom, the first part you remove is called an 'electron'. Until this stage you
have been able to cut without encountering any conspicuous electrical effects; but the
electron is negatively charged, and the remainder of the atom is left positively charged.
The parts of the atom have become 'ions': a negative ion (the electron) and a positive ion
(the remainder of the atom). Each time you remove another electron you leave still more
positive charge on the remainder. It becomes doubly 'ionized', triply 'ionized', and so on.
Since negative and positive charges attract each other, it is harder and harder to prise off
successive electrons.
Suppose, however, that you manage to remove all the electrons. (For most atoms this is
in practice very difficult.) What you have left is the innermost heart of the atom: the
nucleus. This is where all the positive charges are. Furthermore, you would now find an
enormously increased difficulty in cutting any more. Surprisingly enough, although the 14
nucleus contains only positive charges (which repel each other), its constituent pieces
cling together with a loyalty which makes the outer electrons look frankly promiscuous.
There used to be much popular talk of 'splitting the atom', but the problem was rather one
of splitting the nucleus of the atom. 'Atomic' bombs should have been called 'nuclear'
bombs: for the shattering energy they released came from the rupturing of nuclei (one
nucleus, two or more nuclei).
Uranium
What makes uranium so dramatically different from other substances? To appreciate its
unique characteristics we must first consider some basic nuclear physics: that is, wha t
nuclei consist of and how they behave. An atom is made of electrons around a nucleus; a
nucleus in turn is made up of 'protons' and 'neutrons'. A proton has a positive charge; a
neutron has no electrical charge and is 'neutral'. At first it seems difficult to understand
how a nucleus stays together at all. The positive charges of the protons ought to push
them violently apart. But within the compact volume of the nucleus a new kind of force
comes into effect: an immensely powerful short-range attractive force acting equally
between protons and neutrons - which, from this point of view, are all 'nucleons'. The
short-range nuclear force holds them together, against the repulsive effect of the protons'
positive charges. In this way the neutrons act as 'nuclear cement'.
However, in a nucleus which contains 92 protons - that is, a nucleus of uranium - the
repulsive force among the protons is on the verge of overcoming the nuclear force. If
there are as many as 146 neutrons also present, the nucleus can remain intact - barely.
This form of uranium, containing in all 238 nucleons, is called uranium-238 or 238/92U.
For reasons that need not concern us here, involving the grouping and compatibility of
nucleons, the next most probable arrangement is a uranium nucleus containing three
fewer neutrons: uranium-235, 235/92U. Atoms with these lighter nuclei make up about
0.7 per cent of naturally-occurring uranium. (If nuclei have the same number of protons,
they are nuclei of the same chemical 'element': thus, every nucleus with 92 protons is the
nucleus of an atom of uranium. Atoms whose nuclei have the same number of protons but
different numbers of neutrons are called 'isotopes' of the element: for instance, uranium-
238 and uranium-235 are isotopes of uranium.) The uranium-235 nucleus has a property
unique among all the more than 200 types of nuclei found in nature in significant quantity
before 1942. The uranium-235 nucleus is already under near-disruptive internal stress; a
stray neutron blundering into it can rupture it completely.
Radioactivity Produces Radiation
When a stray neutron hits a uranium-235 nucleus, the result is a 'compound nucleus' of
uranium-236. It is called a compound nucleus because it does not last long. The energy
added by the neutron - even a 'slow' one - overcomes the precarious stability of the
nucleus and, almost instantly, it flies apart. The rupture of a uranium-236 compound
nucleus usually results in about two fifths of the nucleus flying off in one direction and 15
about three fifths in the opposite direction, with perhaps two or three odd neutrons also
shooting out. The flying fragments burst out with so much energy that a subsequent tally
of masses reveals a shortage: some of the mass of the original nucleus has been converted
into energy. This is the source of the enormous energies released in such nuclear events.
For example, one common subdivision results in one chunk of 38 protons and 52
neutrons, another of 54 protons and 89 neutrons, and 3 odd neutrons: making up, of
course, 236 nuc leons in all. The chunk containing 38 protons is a nucleus of strontium;
since it contains in all 90 nucleons it is the notorious strontium-90. The chunk containing
54 protons is a nucleus of the inert gas xenon; since it contains in all 143 nucleons it is
xenon-143.
Such a complete rupture of a nucleus is called a 'fission', by analogy with the biological
term for the division of a growing cell. More precisely it is called 'nuclear fission'. When
it is provoked by the impact of an additional neutron it is called 'induced fission'. such is
the case with uranium-235 just described. Some very heavy nuclei are so unstable that
they may rupture even without being struck by a neutron; such a rupture is called
'spontaneous fission'. Fission, whether induced or spontaneous, is the most violent kind of
breakdown that a nucleus can experience. But there are others. A nucleus of uranium-
238, for instance, while not being so near to rupture as its lighter relative, is still under
severe stress: so much so that sooner or later it is likely to squirt out a lump made up of
two protons and two neutrons. Since this makes a larger proportional reduction in the
proton contingent than in the neutron contingent, the remaining nucleus, now containing
only 90 protons and 144 neutrons, is slightly less stressed. (It is a nucleus of the metal
thorium- 234.) The lump or 'particle' squirted out is identical in every respect to an
ordinary helium nucleus; but since it emerges with considerable velocity, and ploughs a
furrow through whatever slows it down, it is given a special name: it is an 'alpha particle'.
Most nuclei with at least 83 protons undergo a breakdown this violent; they are called
'alpha-emitters'.
The balance between protons and neutrons in thorium-234, while more satisfactory, is far
from ideal. In effect, by emitting an alpha particle, the nucleus has over-adjusted. This
leads to a yet more delicate form of breakdown. Out of the nucleus containing 90 protons
and 144 neutrons there suddenly squirts an electron. It is identical in every respect to the
electrons outside the nucleus; but since it emerges with considerable velocity it too is
given a special name: it is a 'beta particle'. The nucleus which remains now contains one
more positive charge than it had. But since an electron is very much less massive than a
nucleon, there are the same number of nucleons as before: 234. A neutron has apparently
turned into a proton. The nucleus now contains 91 protons and 143 neutrons. it is a
nucleus of protactinium-234. Like thorium-234, protactinium-234 is also a 'beta emitter';
- when it emits a beta particle it becomes uranium-234, which is an alpha emitter. So,
leapfrogging down by alternate alpha and beta emission, the nucleus alters itself until it
has only 82 protons and 124 neutrons, and is at last stable: a nucleus of lead-206.
On the way, the nucleus regularly finds itself, after emitting an alpha or beta particle, still
unduly agitated or 'excited'. To settle itself down it gives off a burst of energy in a form 16
closely akin to ordinary light, but much more energetic, and invisible. This burst of
energy is called a 'gamma ray'. It is identical in every respect to the well-known 'X-ray',
except that an X-ray comes from the electron layers outside the nucleus, whereas a
gamma ray comes from inside the nucleus.
Consider also the nucleus of strontium-90, one of the two large fragments formed by the
induced fission of uranium 235 in the earlier example. The strontium-90 nucleus, a
'fission product', has a disproportionately large number of neutrons for its protons,
coming as it does from a much heavier nucleus which requires more 'cement'.
Accordingly, the strontium-90 nucleus is also a beta emitter. Sooner or later it squirts out
a high-velocity electron - a beta particle - and one of its neutrons is replaced by a proton.
It becomes a nucleus of yttrium-90, another beta emitter, which by the same process
becomes a nucleus of zirconium-90, which is stable. Beta emissions from fission product
nuclei are often followed by one or more gamma rays.
There are thus four ways in which a nucleus can alter itself: fission, alpha emission, beta
emission, and gamma emission. From a lump of material containing such unstable nuclei
the emissions from these activities shoot out radially in all directions: the lump is said to
be 'radioactive', and the emissions - neutrons, alpha and beta particles, and gamma rays -
are called 'radiation'. A collection of nuclei which shoots out one such emission per
second is said to exhibit one 'becquerel' (Bq) of radioactivity, after Henri Becquerel, who
first discovered the phenomenon of radioactivity in 1896. The becquerel is now the
accepted international unit of radioactivity, but you will still see an older unit, the 'curie'.
A curie of radioactive material shoots out not one but 37 000 000 000 emissions per
second. This is the radioactivity of one gram of radium, one of the first substances known
to be radioactive, which was discovered by Marie Curie. A curie is a lot of radioactivity;
you will also encounter metric subdivisions - the millicurie, microcurie, nanocurie and
picocurie, in descending steps of 1000 as usual. You can see that one nanocurie equals 37
becquerels of radioactivity. You should note also that 'radioactivity' produces 'radiation';
the two terms are not interchangeable, although even official statements sometimes use
'radioactivity' when they mean 'radiation' and vice versa.
In a radioactive substance it is impossible to tell whether a particular nucleus is on the
point of radioactive breakdown, or 'decay'. Nonetheless, in a sufficiently large sample of
any particular radioactive nuclear species, or 'radioisotope', a certain fraction of the nuclei
always decay in a quite regular length of time. For instance, if you start with 1000 nuclei
of strontium-90, 28 years later 500 will have decayed and you will have 500 left. After a
further 28 years, 250 of the remaining 500 will have decayed, and you will have 250 left.
And so on: however much you start with, 28 years later half will have decayed and only
half will be left. Obviously the corresponding radioactivity will also have fallen by one
half. For strontium 90 the period of 28 years is called its 'half-life'. Each radio-isotope has
a half-life for each form of radioactivity it exhibits: in each case the half-life is the time
during which half the nuclei in a sample decay, and the corresponding radioactivity falls
to half the initial level. Half-lives of different radio-isotopes range from fractions of
millionths of a second to millions of years.17
The Effects of Radiation
Unless radioactive decay takes place in a vacuum, the radiation emitted must pass
through the surrounding substance. The consequences depend on the substance, on the
type of radiation, on its energy and on its intensity. An alpha particle, made up of four
nucleons with two positive charges, interacts vigorously with surrounding atoms, tearing
off electrons and knocking nuclei out of place. In doing so, the alpha particle quickly
gives up its own energy, travelling only a short distance but doing enormous damage
along its path. Most alpha radiation is stopped within the thickness of a single sheet of
paper. A beta particle, much less massive and with only one negative charge, disturbs and
dislodges neighbouring electrons, but loses its energy less swiftly and therefore travels
somewhat farther than an alpha particle. Most beta radiation is stopped within the
thickness of a thin sheet of metal. A gamma ray, with no electrical charge, loses its
energy much more gradually, and can travel a long distance, causing a relatively small
amount of disturbance at any particular point on its path. A neutron, also without
electrical charge, is likewise free to travel a long distance, and is slowed down mainly by
direct collision with nuclei. Gamma or neutron radiation can penetrate more than a metre
of concrete.
Dislodging an electron from an atom makes the atom an ion: so emissions from nuclei are
'ionizing radiation'. When ionizing radiation passes through a material it causes changes
in the structure of the material - sometimes temporary, sometimes permanent, sometimes
useful, sometimes harmful. The effects of ionizing radiation depend roughly on how
much energy the radiation releases into a given amount of material - the more energy, the
more disruption. The original unit of radiation exposure was the 'roentgen', named after
Wilhelm Roentgen, discoverer of X-rays.
The effects of ionizing radiation become particularly important if the radiation is passing
through living matter; the delicate molecular arrangements of living matter can be easily
upset by radiation. There are several units used to measure radiation effects on living
matter. Until recently the most common have been the 'radiation absorbed dose', or 'rad',
and the 'roentgen equivalent man', or 'rem'; the new international standard units are the
'gray' (Gy), equal to 100 rads, and the 'sievert' (Sv), equal to 100 rem. The rem and
sievert allow for the greater severity of alpha or neutron damage for equivalent energy
delivery. For beta and gamma radiation one gray is about the same as one sievert; for
neutrons and alpha particles one gray may be up to 20 sieverts, depending on the energy
of the particles.
The question of the biological effects of radiation is surrounded by controversy. But it is
known that a dose of perhaps 400 rem of radiation over the whole body will kill half the
adult human beings exposed to it; and very much smaller doses will produce cell damage
that may lead to leukaemia and other kinds of cancer. Furthermore, radiation damage to
the complex molecules in the reproductive cells which contain the hereditary information
may produce mutant offspring. Even a single gamma ray can disrupt a gene; it may 18
produce unforeseeable effects if this particular gene should be in a reproductive cell
which subsequently helps to form a child.
A more detailed discussion of radiation biology is given in Appendix B. Suffice it to say
here that the danger of radiation to living matter seems to increase in direct proportion to
the amount of radiation exposure, beginning from the very lowest doses. There does not
appear to be a threshold dose - that is, one below which damage does not occur. We are
already subjected to continual radiation from the natural radioactive substances in our
surroundings, and from cosmic rays. Any human activity which tends to add further
sources of radiation to our surroundings must be potentially harmful. Just how harmful -
and in return for what benefits - is still under debate; this book is intended to make one
aspect of the debate more intelligible, whatever your criteria.
The Chain Reaction
In a lump of uranium there are always a few stray neutrons, produced either by
spontaneous fission or by cosmic rays. Suppose that one of these stray neutrons induces a
nucleus of uranium 235 to undergo fission. As well as the two fission products, there
shoot outward perhaps two or three high-energy neutrons. (The chances are better than 99
to 1 that these neutrons will emerge virtually at the instant of fission: 'pr ompt' neutrons.
But there is a slight chance that a neutron will not emerge until some seconds later: a
'delayed' neutron. As we shall see, delayed neutrons are of considerable importance.)
There are three possibilities open to the high-energy neutrons from fission. A neutron
may reach the surface of the material and escape. It may strike another nucleus and be
absorbed without causing any immediate breakdown. Or - most importantly - it may
strike another nucleus and, in turn, cause this nucleus to rupture. The chances of a
neutron causing such an induced fission depend on the neutron's energy and on the
nucleus it strikes. A fast neutron, fresh from an earlier fission, tends to go right through a
nucleus so fast that nothing happens to the nucleus. Once in a while a fast neutron will
rupture a nucleus; indeed only a fast neutron can rupture a nucleus of uranium-238.
However, if a neutron ricochets among other nuclei, bouncing off each and giving up its
energy bit by bit, it soon slows down until it is just jostling with the shared heat-energy of
the rest of the material. It is then a 'thermal neutron'. A thermal neutron takes much
longer to go through a nucleus, and is thus much more likely to rupture a nucleus of
uranium 235 than is a fast neutron.
If, in a lump of uranium-235, one nucleus undergoes fission, the neutrons it releases may
strike other nuclei, causing more fissions and releasing more neutrons, If there are
enough uranium-235 nuclei sufficiently close together, the spreading disruption
multiplies with astonishing rapidity: more and more neutrons, more and more ruptured
nuclei, their fragments flying, more and more energy: a 'chain reaction'. If there is enough
uranium- 235, packed closely together for long enough, and if the chain reaction is out of
control, the result is a nuclear explosion: an 'atomic bomb'. Slamming two suitable
hemispheres of uranium-235 together at a very high velocity will indeed create a nuclear
explosion; but there are other, much more efficient, techniques - and materials.19
Needless to say, as soon as an appropriate arrangement of appropriate material became
possible it was tried out - on 16 July 1945, at the top of a tall tower in the desert near
Alamogordo, New Mexico: the world's first nuclear explosion, code-named Trinity.
Within three weeks an atomic bomb made of uranium-235 devastated Hiroshima. But,
seemingly, one 'doomsday weapon' was not enough, and uranium-235 was not the only
nucleus that could be used. A neutron can penetrate a nucleus of uranium 238 without
rupturing it. If this happens, the resulting neutron-heavy nucleus soon emits a beta
particle, and then another, to become a nucleus of plutonium-239. Like uranium-235 and
only a few other isotopes - all at present very rare - plutonium-239 is 'fissile': that is, it
can undergo a chain reaction of successive fissions, as the Trinity test demonstrated. On 9
August 1945 such a chain reaction obliterated Nagasaki.
The Nuclear Reactor
If chain reactions in uranium-235 and plutonium-239 could be used only in weapons, the
situation would already be sufficiently complicated. But, more than two years before
enough pure fissile material of either kind had been accumulated to make a weapon, it
was found to be possible to control a chain reaction: to have it maintain itself without
multiplying out of control. Indeed it was by this means that the plutonium was produced
for the Alamogordo and Nagasaki bombs. The arrangement used to create and control a
sustained nuclear chain reaction is called a 'nuclear reactor'.
The difference between an uncontrolled and a controlled chain reaction is profound. An
arrangement of fissile nuclei which is to undergo an uncontrolled chain reaction - a
nuclear explosion - must be sudden and final. An arrangement of fissile nuclei which is to
sustain a continuing controlled chain reaction must be much more carefully organized.
Curiously enough it takes many more nuclei - that is, much more material - to build a
reactor than it takes to set off an explosion. This is of course partly because an explosion
requires comparatively pure fissile material; in a reactor the fissile material is
comparatively dilute, and there is accordingly much more material in all. But there must
also be many more fissile nuclei themselves. The reason for this is the role played by the
all-important neutrons.
If a chain reaction is to be self-sustaining, it must keep itself supplied with neutrons.
Consider the following typical sequence. A neutron plunges into a nucleus of uranium-
235. The nucleus ruptures, as well as two fission-product nuclei it also shoots out three
neutrons. One of these three goes out through the surface of the lump of uranium and is
lost. Another is absorbed by a nucleus of uranium-238, which begins its two-stage change
into plutonium-239, but does not rupture. This leaves one neutron. If this third neutron
now plunges into another uranium-235 nucleus and ruptures it, the process can continue;
otherwise the chain reaction is snuffed out.
At any instant, inside the lump of uranium, there must be the right number of neutrons of
the proper energy to propagate the chain. In effect, for a sustained chain reaction, each
neutron which is lost by causing a fission must be replaced by exactly one neutron which
does likewise. The system then has a 'reproduction factor' of 1. When this condition is 20
achieved the system is said to be 'critical', and the situation is called 'criticality'. Despite a
common misconception to the contrary, 'criticality' is not here used to imply 'danger'.
You say that a nuclear system 'goes critical' just as you say that a car engine 'starts'. If on
average each neutron lost when it causes a fission is replaced by more than one which
also causes fission, the reaction 'runs away'; the reproduction factor is greater than 1, and
the system is 'divergent'. If on average each neutron so lost is replaced by fewer than one
which causes fission, the reaction will stop; the reproduction factor is less than 1. This is
why a piece of uranium below a certain minimum size cannot under normal conditions
sustain a chain reaction: there is too much surface through which neutrons can leak out.
Moderators
The basic requirements for a continuing controlled chain reaction are therefore, first, a
collection of fissile nuclei appropriately distributed in space; and, second, a
self-replenishing supply of neutrons of just sufficient numbers and energy to keep the
chain reaction going. In natural uranium, only 0.7 per cent of the nuclei are fissile
uranium-235. These fissile nuclei, only seven out of every thousand, are not sufficiently
close together to keep up a chain reaction; too many neutrons are absorbed by the heavier
uranium-238 nuclei, without causing fission. To improve the prospects for a sustained
chain reaction it is necessary either to increase the proportion of uranium-235 relative to
uranium-238; or to slow down the neutrons to thermal energies, at which they are much
more readily absorbed by uranium-235; or to do both.
As we shall see, increasing the proportion of fissile uranium-235 - so called 'enrichment'
of the uranium - is a complex and expensive process. But even a small increase, say from
0.7 per cent to 2 or 3 per cent, makes a marked difference, provided that the neutrons
from fission are slowed down. This can be done by means of a material with light nuclei -
a 'moderator'. A fast neutron striking a light nucleus in the moderator gives up a fraction
of its energy, and after a few such collisions has slowed to thermal energy. The best
moderators are the lightest nuclei: those of hydrogen. Ordinary water, containing two
hydrogen atoms per molecule, is a satisfactory moderator. But ordinary hydrogen nuclei
absorb neutrons, Better still is a rarer form of hydrogen nucleus: the proton-plus-neutron
form called 'heavy hydrogen' or 'deuterium'. If two atoms of heavy hydrogen combine
with an atom of oxygen, the result is a molecule of 'heavy water' or deuterium oxide
(sometimes written D2O), which is much the best moderator of fast neutrons.
One other substance is widely used as a moderator: carbon, in the form of graphite. A
carbon nucleus - six protons and six neutrons is much more massive than either form of
hydrogen nucleus, and is therefore not such a good moderator, But graphite is less
expensive than heavy water; furthermore it is a solid, which can be structurally useful in a
reactor.21
Reactor Design and Operation
To set up a nuclear reactor you proceed as follows. You take a good many pieces of
material containing uranium 235 - usually uranium metal or oxide, natural or enriched:
the 'fuel'. (You can also use plutonium-239, although this - as we shall see - involves
some difficulties.) For a large reactor you need many tonnes of fuel, much more than
enough to achieve criticality. One obvious reason for the extra fuel is to enable you to
operate the reactor for some time before replacing the fuel. Other reasons will become
clear in a moment.
You seal the pieces of fuel into casings called 'cladding', to support the fuel and to
confine the fission products that will be produced. You position the assemblies of sealed
fuel, called 'fuel elements', supporting them as necessary; remember that they may be
very heavy indeed. You intersperse the fuel elements with moderator, to slow down the
neutrons, and with neutron absorber, to enable you to control the chain reaction. You also
include measuring instruments to tell you what is going on inside the reactor. You need to
know, in particular, the temperature and the concentration of neutrons at various places
inside the reactor.
You are now ready to start up your reactor. Before start-up, with all the absorbers in the
interior of the reactor soaking up neutrons, the neutron density is so low it is difficult to
measure, unless you intentionally include a separate source of neutrons as a sort of
primer. A common form of absorber is a rod thrust through the interior of the assembly: a
'control rod'. Such a rod incorporates a material like boron, which absorbs neutrons like a
sponge. The rod may be made, for instance, of boron steel. So long as enough control
rods are in place no chain reaction is possible. To start up your reactor - to make a chain
reaction possible - you begin withdrawing control rods.
The region of the reactor in which the reaction takes place is called the core. You
withdraw control rods very slowly from the core, usually in short steps, in suitable
symmetry to maintain a more or less uniform build-up of neutron density inside the
reactor. In due course your reactor 'goes critical': a self-sustaining chain reaction is
established, in which each neutron lost by causing a fission is replaced by exactly one
neutron (either prompt or delayed) which does likewise. If the chain reaction could be
sustained by prompt neutrons alone it would be 'prompt critical', and difficult to control.
The dependence of the chain reaction on delayed neutrons allows you to adjust the
reaction-rate gradually instead of abruptly.
Taking absorber out of a stable chain reaction is called 'adding reactivity'; the neutron
density increases, and the rate of the reaction increases. But the build -up is gradual,
because some of the neutrons do not emerge immediately after fission. The smaller the
added reactivity, the longer is the time taken for the neutron density to increase by a
given proportion. This time is called the 'reactor period', and is a very important measure
of how well the reactor can be controlled. When a reactor has a short period it is liable to
be skittish. Of course inserting absorber - 'adding negative reactivity' - produces a reverse
effect. When the desired rate is established you reposition the absorbers to stabilize the 22
reaction at that rate.
To economize on neutrons you can surround the reactor core with a reflector to bounce
errant neutrons back into the reaction region. The best reflecting materials are the
moderator materials; in effect you can extend the volume of moderator beyond the region
of fuel elements. Since the presence or absence of reflector affects the neutron density in
the core, you can add reactivity by adding reflector, or vice versa; some reactor designs
utilize this effect for control purposes.
Before pulling the control rods far enough out to let your reactor go critical you must take
precautions against the radiation pouring out from the core. Neither alpha nor beta
particles will get beyond the fuel cladding (unless it leaks); but gamma rays and neutrons
can travel through metres of concrete and still be dangerous to living matter. Therefore,
you surround your reactor with enough concrete or other protective 'shielding' to cut
down the radiation outside to as low a level as you think advisable.
Xenon Poisoning
Normal start-up and shutdown of a reactor are both lengthy processes, and may take
many hours. If it is necessary to stop the chain reaction quickly, for instance in the event
of a malfunction, the emergency shutdown is called a 'scram'. If an operating reactor is
left to itself its reaction-rate will gradually dwindle, not necessarily because fissile nuclei
are being used up - in some reactors the number of fissile nuclei may even be increasing -
but also because of the build-up of fission products which absorb neutrons.
The most voracious of all is xenon-135. The consequent phenomenon, called xenon
poisoning, is an intriguing demonstration of the slightly surrealistic circumstances in
which reactors operate.
When you start up your reactor for the first time, the fuel contains no xenon-135. For
several hours after start-up, fission processes generate tellurium-135 and iodine-135,
which in turn generate xenon-135, which starts gobbling neutrons. Each xenon-135
nucleus which succeeds in capturing a neutron is thereby charged into xenon-136 - much
less voracious. The xenon-135 nuclei which fail to capture neutrons nonetheless undergo
beta decay into caesium-135, also much less voracious. Accordingly, after matters have
had a chance to settle down, as much xenon-135 is being lost as is being generated. There
is a certain average concentration of xenon-135 in the reactor core, which remains the
same as long as the chain reaction proceeds at the same rate. You budget for so many
neutrons lost to xenon-135, and operate accordingly. But when you change the reactionrate you upset the balance, and the consequences may be embarrassing.
Iodine-135 turns into xenon-135 with a half-life of 6.7 hours. Xenon-135 turns into
caesium-135 with a half-life of 9.2 hours - that is, slightly more slowly. Suppose you shut
down your reactor. The neutron flux falls to near zero; xenon-135 stops capturing
neutrons. From the moment of shutdown more xenon-135 is being generated than is
being lost: while your reactor is shut down, the amount of neutron absorber in its core is 23
steadily, surreptitiously increasing. If, several hours later, you try to start up your reactor
again, you may, even with the control rods completely out, be unable to add enough
reactivity to reach criticality. To be always able to start up your reactor at any time after
shutdown, you will find it necessary to include more fuel, or otherwise to arrange for an
excess of available reactivity over and above what normal operation needs. Apart from
the obvious cost of the extra fuel, this means that even in normal operation you must
leave some control rods partly inserted. It is not easy to do so without distorting the
uniform neutron density in the core, and producing a less than ideal pattern of chain
reaction. Reactor designers have to decide what sort of compromises they can best
achieve, to satisfy the conflicting requirements made necessary by phenomena like xenon
poisoning.
Refuelling
While you operate your reactor, changes take place in the fuel. The number of uranium-
235 nuclei dwindles gradually as they undergo fission. Some of the uranium-238 nuclei
capture neutrons and change to plutonium-239. Some of these plutonium-239 nuclei
undergo fission. Others capture additional neutrons and become plutonium-240,
plutonium-241 and other isotopes of elements heavier than uranium - 'transuranic
actinides'. Fission products are formed; most fission products are radioactive, and
undergo radioactive changes into more stable nuclei, some very rapidly, others very
slowly. Fission products also capture neutrons. The composition of the reactor fuel grows
increasingly complex as the chain reaction proceeds; it becomes more and more difficult
to keep track of all the competing processes taking place. Some of the fission products
are gaseous, like krypton and xenon; these gaseous fission products build up inside the
fuel, exerting pressure and trying to leak out. The intense neutron flux plays havoc with
the crystal structure of the fuel, the cladding, and possibly the moderator, knocking the
nuclei out of place and setting up stresses and strains in the material. Soon or later, it is
necessary to take out used fuel and replace it.
There is an assortment of different procedures for 'refuelling' or 'recharging' a reactor.
Some designs can be refuelled while the reactor is in operation, replacing one or more
fuel elements at a time: 'on-load refuelling'. Other designs are shut down for refuelling,
and perhaps one-third of the core is replaced at one time: 'off-load refuelling'. All
refuelling procedures must be carried out with extreme care because of the intense
radioactivity of the fission products in the reactor core and in the used 'spent' or
'irradiated' fuel.
Power from a Reactor
If you set up a reactor on a sufficiently large scale, and let the chain reaction run fast
enough, the energy released by the rupturing uranium-235 (and plutonium-239) nuclei
makes the whole assembly hot - potentially very hot indeed. Complete fission of all the
nuclei in a kilogram of uranium-235 would release a total energy of about one million
kilowatt-days - that is, as much heat as would be given off by one million one-bar electric
fires operating for one 24-hour day. That is a lot of heat. Accordingly, the fuel in a 24
reactor must be arranged so that the heat is given off gradually enough to keep
temperatures manageable. The amount of heat given off per unit volume in a reactor core
is called the 'power density'. It may be anything up to several hundred kilowatts of heat
per litre; if such an outpouring of energy is not to melt - and indeed boil - the whole
aggregation of material, it must be efficiently removed.
You remove the heat from the reactor by pumping a heat-absorbing fluid through the
core, past the hot fuel elements. The fluid can be a gas, such as air, carbon dioxide or
helium; or a liquid, such as water or molten metal. The choice of cooling fluid -'coolant'-
depends on how fast heat must be removed; on how expensive the fluid is; on how easy it
is to pump, and so on. The cooling system can be open-ended, passing ordinary air or
water directly through the core and back to the atmosphere or river; such an arrangement
has the virtue of simplicity, but may also have serious drawbacks, especially if fuel
cladding leaks. Alternatively the cooling system can be one or more closed circuits, in
which the same coolant passes through the core again and again, carrying heat out of the
core, discharging it outside the reactor, and then passing the rest of the way around the
circuit and back through the core again. If the cooling system is made up of closed
circuits, expensive or exotic coolants can be used, since they are confined in the system
instead of being lost. A closed circuit can also be pressurized, which will in most
instances dramatically improve efficiency of the coolant; a pressurized gas is denser and
can carry more heat per unit volume.
The cooling system, of whatever design, removes the heat from the reactor core; what
becomes of the heat thereafter depends on the reason for operating the reactor. The first
large reactors were operated exclusively to generate neutrons and turn uranium-238 into
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