Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3) - 1

Worlds Within Worlds:
The Story of Nuclear Energy
Volume 3
Nuclear Fission · Nuclear Fusion · Beyond Fusion

by Isaac Asimov

United States Atomic Energy Commission
Office of Information Services
Library of Congress Catalog Card Number 75-189477
1972
_Nothing in the history of mankind has opened our eyes to the
possibilities of science as has the development of atomic power. In the
last 200 years, people have seen the coming of the steam engine, the
steamboat, the railroad locomotive, the automobile, the airplane, radio,
motion pictures, television, the machine age in general. Yet none of it
seemed quite so fantastic, quite so unbelievable, as what man has done
since 1939 with the atom ... there seem to be almost no limits to what
may lie ahead: inexhaustible energy, new worlds, ever-widening knowledge
of the physical universe._
Isaac Asimov
[Illustration: Photograph of night sky]


Nuclear energy is playing a vital role in the life of every man, woman,
and child in the United States today. In the years ahead it will affect
increasingly all the peoples of the earth. It is essential that all
Americans gain an understanding of this vital force if they are to
discharge thoughtfully their responsibilities as citizens and if they
are to realize fully the myriad benefits that nuclear energy offers
them.
The United States Atomic Energy Commission provides this booklet to help
you achieve such understanding.


UNITED STATES ATOMIC ENERGY COMMISSION
Dr. James R. Schlesinger, Chairman
James T. Ramey
Dr. Clarence E. Larson
William O. Doub
Dr. Dixy Lee Ray
[Illustration: Isaac Asimov]


ISAAC ASIMOV received his academic degrees from Columbia University and
is Associate Professor of Biochemistry at the Boston University School
of Medicine. He is a prolific author who has written over 100 books in
the past 18 years, including about 20 science fiction works, and books
for children. His many excellent science books for the public cover
subjects in mathematics, physics, astronomy, chemistry, and biology,
such as _The Genetic Code_, _Inside the Atom_, _Building Blocks of the
Universe_, _Understanding Physics_, _The New Intelligent Man’s Guide to
Science_, and _Asimov’s Biographical Encyclopedia of Science and
Technology_. In 1965 Dr. Asimov received the James T. Grady Award of the
American Chemical Society for his major contribution in reporting
science progress to the public.
[Illustration: Photograph of night sky]
CONTENTS

VOLUME 1
Introduction 5
Atomic Weights 6
Electricity 11
Units of Electricity 11
Cathode Rays 13
Radioactivity 17
The Structure of the Atom 25
Atomic Numbers 30
Isotopes 35
Energy 47
The Law of Conservation of Energy 47
Chemical Energy 50
Electrons and Energy 54
The Energy of the Sun 55
The Energy of Radioactivity 57

VOLUME 2
Mass and Energy 69
The Structure of the Nucleus 75
The Proton 75
The Proton-Electron Theory 76
Protons in Nuclei 80
Nuclear Bombardment 82
Particle Accelerators 86
The Neutron 92
Nuclear Spin 92
Discovery of the Neutron 95
The Proton-Neutron Theory 98
The Nuclear Interaction 101
Neutron Bombardment 107

VOLUME 3
Nuclear Fission 117
New Elements 117
The Discovery of Fission 122
The Nuclear Chain Reaction 127
The Nuclear Bomb 131
Nuclear Reactors 141
Nuclear Fusion 146
The Energy of the Sun 146
Thermonuclear Bombs 148
Controlled Fusion 150
Beyond Fusion 158
Antimatter 158
The Unknown 163
Reading List 165
[Illustration: _Enrico Fermi (left) and Niels Bohr discuss physics as
they stroll along the Appian Way outside Rome in 1931._]


NUCLEAR FISSION

New Elements
In 1934 Enrico Fermi began his first experiments involving the
bombardment of uranium with neutrons—experiments that were to change the
face of the world.
Fermi had found that slow neutrons, which had very little energy, were
easily absorbed by atomic nuclei—more easily than fast neutrons were
absorbed, and certainly more easily than charged particles were.
Often what happened was that the neutron was simply absorbed by the
nucleus. Since the neutron has a mass number of 1 and an atomic number
of 0 (because it is uncharged), a nucleus that absorbs a neutron remains
an isotope of the same element, but increases its mass number.
For instance, suppose that neutrons are used to bombard hydrogen-1,
which then captures one of the neutrons. From a single proton, it will
become a proton plus a neutron; from hydrogen-1, it will become
hydrogen-2. A new nucleus formed in this way will be at a higher energy
and that energy is emitted in the form of a gamma ray.
Sometimes the more massive isotope that is formed through neutron
absorption is stable, as hydrogen-2 is. Sometimes it is not, but is
radioactive instead. Because it has added a neutron, it has too many
neutrons for stability. The best way of adjusting the matter is to emit
a beta particle (electron). This converts one of the neutrons into a
proton. The mass number stays the same but the atomic number increases
by one.
The element rhodium, for example, which has an atomic number of 45, has
only 1 stable isotope, with a mass number of 103. If rhodium-103 (45
protons, 58 neutrons) absorbs a neutron, it becomes rhodium-104 (45
protons, 59 neutrons), which is not stable. Rhodium-104 emits a beta
particle, changing a neutron to a proton so that the nuclear combination
becomes 46 protons and 58 neutrons. This is palladium-104, which is
stable.
[Illustration: _Fermi’s laboratory in Rome in 1930._]
As another example, indium-115 (49 protons, 66 neutrons) absorbs a
neutron and becomes indium-116 (49 protons, 67 neutrons), which gives
off a beta particle and becomes tin-116 (50 protons, 66 neutrons), which
is stable.
There are over 100 isotopes that will absorb neutrons and end by
becoming an isotope of an element one higher in the atomic number scale.
Fermi observed a number of these cases.
Having done so, he was bound to ask what would happen if uranium were
bombarded with neutrons. Would its isotopes also be raised in atomic
number—in this case from 92 to 93? If that were so it would be very
exciting, for uranium had the highest atomic number in the entire scale.
Nobody had ever discovered any sample of element number 93, but perhaps
it could be formed in the laboratory.
In 1934, therefore, Fermi bombarded uranium with neutrons in the hope of
obtaining atoms of element 93. Neutrons were absorbed and whatever was
formed did give off beta particles, so element 93 should be there.
However, four different kinds of beta particles (different in their
energy content) were given off and the matter grew very confusing. Fermi
could not definitely identify the presence of atoms of element 93 and
neither could anyone else for several years. Other things turned up,
however, which were even more significant.
Before going on to these other things, however, it should be mentioned
that undoubtedly element 93 was formed even though Fermi couldn’t
clearly demonstrate the fact. In 1939 the American physicists Edwin
Mattison McMillan (1907- ) and Philip Hauge Abelson (1913- ),
after bombarding uranium atoms with slow neutrons, were able to identify
element 93. Since uranium had originally been named for the planet,
Uranus, the new element beyond uranium was eventually named for the
planet Neptune, which lay beyond Uranus. Element 93 is therefore called
“neptunium”.
[Illustration: _Lise Meitner_]
[Illustration: _Emilio Segrè_]
[Illustration: _Edwin M. McMillan_]
[Illustration: _Otto R. Frisch_]
[Illustration: _Glenn T. Seaborg_]
[Illustration: _Philip H. Abelson_]
What happened was exactly what was expected. Uranium-238 (92 protons,
146 neutrons) added a neutron to become uranium-239 (92 protons, 147
neutrons), which emitted a beta particle to become neptunium-239 (93
protons, 146 neutrons).
In fact, neptunium-239 also emitted a beta particle so it ought to
become an isotope of an element even higher in the atomic number scale.
This one, element 94, was named “plutonium” after Pluto, the planet
beyond Neptune. The isotope, plutonium-239, formed from neptunium-239,
was only feebly radioactive, however, and it was not clearly identified
until 1941.
The actual discovery of the element plutonium came the year before,
however, when neptunium-238 was formed. It emitted a beta particle and
became plutonium-238, an isotope that was radioactive enough to be
easily detected and identified by Glenn Theodore Seaborg (1912- ),
and his co-workers, who completed McMillan’s experiments when he was
called away to other defense research.
Neptunium and plutonium were the first “transuranium elements” to be
produced in the laboratory, but they weren’t the last. Over the next 30
years, isotopes were formed that contained more and more protons in the
nucleus and therefore had higher and higher atomic numbers. At the
moment of writing, isotopes of every element up to and including element
105 have been formed.
A number of these new elements have been named for some of the
scientists important in the history of nuclear research. Element 96 is
“curium”, named for Pierre and Marie Curie; element 99 is “einsteinium”
for Albert Einstein; and element 100 is “fermium” for Enrico Fermi.
Element 101 is “mendelevium” for the Russian chemist Dmitri Mendeléev,
who early in 1869 was the first to arrange the elements in a reasonable
and useful order. Element 103 is “lawrencium” for Ernest O. Lawrence.
“Rutherfordium” for Ernest Rutherford has been proposed for element 104.
And “hahnium” for Otto Hahn (1879-1968), a German physical chemist whose
contribution we will come to shortly, has been proposed for element 105.
Neptunium, however, was not the first new element to be created in the
laboratory. In the early 1930s, there were still 2 elements with fairly
low atomic numbers that had never been discovered. These were the
elements with atomic numbers 43 and 61.
In 1937, though, molybdenum (atomic number 42) had been bombarded with
neutrons in Lawrence’s laboratory in the United States. It might contain
small quantities of element 43 as a result. The Italian physicist Emilio
Segrè (1905- ), who had worked with Fermi, obtained a sample of the
bombarded molybdenum and indeed obtained indications of the presence of
element 43. It was the first new element to be manufactured by
artificial means and was called “technetium” from the Greek word for
“artificial”.
The technetium isotope that was formed was radioactive. Indeed, all the
technetium isotopes are radioactive. Element 61, discovered in 1945 and
named “promethium”, also has no stable isotopes. Technetium and
promethium are the only elements with atomic numbers less than 84 that
do not have even a single stable isotope.

The Discovery of Fission
But let us get back to the bombardment of uranium with neutrons research
that Fermi had begun. After he had reported his work, other physicists
repeated it and also got a variety of beta particles and were also
unable to decide what was going on.
[Illustration: _Lise Meitner and Otto Hahn in their laboratory in the
1930s._]
One way to tackle the problem was to add to the system some stable
element that was chemically similar to the tiny traces of radioactive
isotopes that might be produced through the bombardment of uranium.
Afterwards the stable element could probably be separated out of the
mixture and the trace of radioactivity would, it was hoped, be carried
along with it. The stable element would be a “carrier”.
Among those working on the problem were Otto Hahn and his Austrian
co-worker, the physicist Lise Meitner (1878-1968). Among the potential
carriers they added to the system was the element, barium, which has an
atomic number of 56. They found that a considerable quantity of the
radioactivity did indeed accompany the barium when they separated that
element out of the system.
A natural conclusion was that the isotopes producing the radioactivity
belonged to an element that was chemically very similar to barium.
Suspicion fell at once on radium (atomic number 88), which was very like
barium indeed as far as chemical properties were concerned.
Lise Meitner, who was Jewish, found it difficult to work in Germany,
however, for it was then under the rule of the strongly anti-Semitic
Nazi regime. In March 1938 Germany occupied Austria, which became part
of the German realm. Meitner was no longer protected by her Austrian
citizenship and had to flee the country and go to Stockholm, Sweden.
Hahn remained in Germany and continued working on the problem with the
German physical chemist Fritz Strassman (1902- ).
Although the supposed radium, which possessed the radioactivity, was
very like barium in chemical properties, the two were not entirely
identical. There were ways of separating them, and Hahn and Strassman
busied themselves in trying to accomplish this in order to isolate the
radioactive isotopes, concentrate them, and study them in detail. Over
and over again, however, they failed to separate the barium and the
supposed radium.
Slowly, it began to seem to Hahn that the failure to separate the barium
and the radioactivity meant that the isotopes to which the radioactivity
belonged had to be so much like barium as to be nothing else _but_
barium. He hesitated to say so, however, because it seemed unbelievable.
If the radioactive isotopes included radium, that was conceivable.
Radium had an atomic number of 88, only four less than uranium’s 92. You
could imagine that a neutron being absorbed by a uranium nucleus might
make the latter so unstable as to cause it to emit 2 alpha particles and
become radium. Barium, however, had an atomic number of 56, only a
little over half that of uranium. How could a uranium nucleus be made to
turn into a barium nucleus unless it more or less broke in half? Nothing
like that had ever been observed before and Hahn hesitated to suggest
it.
While he was nerving himself to do so, however, Lise Meitner, in
Stockholm, receiving reports of what was being done in Hahn’s laboratory
and thinking about it, decided that unheard-of or not, there was only
one explanation. The uranium nucleus _was_ breaking in half.
Actually, when one stopped to think of it (after getting over the
initial shock) it wasn’t so unbelievable at that. The nuclear force is
so short-range, it barely reaches from end to end of a large nucleus
like that of uranium. Left to itself, it holds together most of the
time, but with the added energy of an entering neutron, we might imagine
shock waves going through it and turning the nucleus into something like
a quivering drop of liquid. Sometimes the uranium nucleus recovers,
keeps the neutron, and then goes on to beta-particle emission. And
sometimes the nucleus stretches to the point where the nuclear force
doesn’t quite hold it together. It becomes a dumbbell shape and then the
electromagnetic repulsion of the two halves (both positively charged)
breaks it apart altogether.
It doesn’t break into equal halves. Nor does it always break at exactly
the same place, so that there were a number of different fragments
possible (which was why there was so much confusion). Still, one of the
more common ways in which it might break would be into barium and
krypton. (Their respective atomic numbers, 56 and 36, would add up to
92.)
Meitner and her nephew, Otto Robert Frisch (1904- ), who was in
Copenhagen, Denmark, prepared a paper suggesting that this was what was
happening. It was published in January 1939. Frisch passed it on to the
Danish physicist Niels Bohr (1885-1962) with whom he was working. The
American biologist William Archibald Arnold (1904- ), who was also
working in Copenhagen at the time, suggested that the splitting of the
uranium nucleus into halves be called “fission”, the term used for the
division-in-two of living cells. The name stuck.
In January 1939, just about the time Meitner and Frisch’s paper was
published, Bohr had arrived in the United States to attend a conference
of physicists. He carried the news of fission with him. The other
physicists attending the conference heard the news and in a high state
of excitement at once set about studying the problem. Within a matter of
weeks, the fact of uranium fission was confirmed over and over.
One striking fact about uranium fission was the large amount of energy
it released. In general, when a very massive nucleus is converted to a
less massive one, energy is released because of the change in the mass
defect, as Aston had shown in the 1920s. When the uranium nucleus breaks
down through the ordinary radioactive processes to become a less massive
lead nucleus, energy is given off accordingly. When, however, it breaks
in two to become the much less massive nuclei of barium and krypton (or
others in that neighborhood) much more energy is given off.
It quickly turned out that uranium fission gave off something like ten
times as much nuclear energy per nucleus than did any other nuclear
reaction known at the time.
Even so, the quantity of energy released by uranium fission was only a
tiny fraction of the energy that went into the preparation of the
neutrons used to bring about the fission, if each neutron that struck a
uranium atom brought about a single fission of that 1 atom.
Under those conditions, Rutherford’s suspicion that mankind would never
be able to tap nuclear energy probably still remained true. (He had been
dead for 2 years at the time of the discovery of fission.)
However, those were not the conditions.

The Nuclear Chain Reaction
Earlier in this history, we discussed chain reactions involving chemical
energy. A small bit of energy can ignite a chemical reaction that would
produce more than enough energy to ignite a neighboring section of the
system, which would in turn produce still more—and so on, and so on. In
this way the flame of a single match could start a fire in a leaf that
would burn down an entire forest, and the energy given off by the
burning forest would be enormously higher than the initial energy of the
match flame.
Might there not be such a thing as a “nuclear chain reaction”? Could one
initiate a nuclear reaction that would produce something that would
initiate more of the same that would produce something that would
initiate still more of the same and so on?
In that case, a nuclear reaction, once started, would continue of its
own accord, and in return for the trifling investment that would serve
to start it—a single neutron, perhaps—a vast amount of breakdowns would
result with the delivery of a vast amount of energy. Even if it were
necessary to expend quite a bit of energy to produce the 1 neutron that
would start the chain reaction, one would end with an enormous profit.
What’s more, since the nuclear reaction would spread from nucleus to
nucleus with millionths-of-a-second intervals, there would be, in a very
brief time, so many nuclei breaking down that there would be a vast
explosion. The explosion was sure to be millions of times as powerful as
ordinary chemical explosions involving the same quantity of exploding
material, since the latter used only the electromagnetic interaction,
while the former used the much stronger nuclear interaction.
The first to think seriously of such a nuclear chain reaction was the
Hungarian physicist Leo Szilard (1898-1964). He was working in Germany
in 1933 when Adolf Hitler came to power and, since he was Jewish, he
felt it would be wise to leave Germany. He went to Great Britain and
there, in 1934, he considered certain new types of nuclear reactions
that had been discovered.
In these, it sometimes happened that a fast neutron might strike a
nucleus with sufficient energy to cause it to emit 2 neutrons. In that
way the nucleus, absorbing 1 neutron and emitting 2, would become a
lighter isotope of the same element.
But what would happen if each of the 2 neutrons that emerged from the
original target nucleus struck new nuclei and forced the emission of a
pair of neutrons from each. There would now be a total of 4 neutrons
flying about and if each struck new nuclei there would next be 8
neutrons and so on. From the initial investment of a single neutron
there might soon be countless billions initiating nuclear reactions.
Szilard, fearing the inevitability of war and fearing further that the
brutal leaders of Germany might seek and use such a nuclear chain
reaction as a weapon in warfare, secretly applied for a patent on a
device intending to make use of such a nuclear chain reaction. He hoped
to turn it over to the British Government, which might then use its
possession as a way of restraining the Nazis and keeping the peace.
However, it wouldn’t have worked. It took the impact of a very energetic
neutron to bring about the emission of 2 neutrons. The neutrons that
then emerged from the nucleus simply didn’t have enough energy to keep
things going. (It was like trying to make wet wood catch fire.)
But what about uranium fission? Uranium fission was initiated by slow
neutrons. What if uranium fission also produced neutrons as well as
being initiated by a neutron? Would not the neutrons produced serve to
initiate new fissions that would produce new neutrons and so on
endlessly?
It seemed very likely that fission produced neutrons and indeed, Fermi,
at the conference where fission was first discussed, suggested it at
once. Massive nuclei possessed more neutrons per proton than less
massive ones did. If a massive nucleus was broken up into 2 considerably
less massive ones, there would be a surplus of neutrons. Suppose, for
instance, uranium-238 broke down into barium-138 and krypton-86.
Barium-138 contains 82 neutrons and krypton-86 50 neutrons for a total
of 132. The uranium-238 nucleus, however, contains 146 neutrons.
The uranium fission process was studied at once to see if neutrons were
actually given off and a number of different physicists, including
Szilard, found that they were.
Now Szilard was faced with a nuclear chain reaction he was certain would
work. Only slow neutrons were involved and the individual nuclear
breakdowns were far more energetic than anything else that had yet been
discovered. If a chain reaction could be started in a sizable piece of
uranium, unimaginable quantities of energy would be produced. Just 1
gram of uranium, undergoing complete fission, would deliver the energy
derived from the total burning of 3 tons of coal and would deliver that
energy in a tiny fraction of a second.
Szilard, who had come to the United States in 1937, clearly visualized
the tremendous explosive force of something that would have to be called
a “nuclear bomb”. Szilard dreaded the possibility that Hitler might
obtain the use of such a bomb through the agency of Germany’s nuclear
scientists.
Partly through Szilard’s efforts, physicists in the United States and in
other Western nations opposed to Hitler began a program of voluntary
secrecy in 1940, to avoid passing along any hints to Germany. What’s
more, Szilard enlisted the services of two other Hungarian refugees, the
physicists Eugene Paul Wigner (1902- ) and Edward Teller (1908- )
and all approached Einstein, who had also fled Germany and come to
America.
[Illustration: _Leo Szilard_]
[Illustration: _Eugene P. Wigner_]
Einstein was the most prestigious scientist then living and it was
thought a letter from him to the President of the United States would be
most persuasive. Einstein signed such a letter, which explained the
possibility of a nuclear bomb and urged that the United States not allow
a potential enemy to come into possession of it first.
Largely as a result of this letter, a huge research team was put
together in the United States, to which other Western nations also
contributed, with but one aim—to develop the nuclear bomb.

The Nuclear Bomb
Although the theory of the nuclear bomb seemed clear and simple, a great
many practical difficulties stood in the way. In the first place, if
only uranium atoms underwent fission a supply of uranium had at least to
be obtained in pure form, for if the neutrons struck nuclei of elements
other than uranium, they would simply be absorbed and removed from the
system, ending the possibility of a chain reaction. This alone was a
heavy task, since there had been so little use for uranium in quantity
that there was almost no supply in existence and no experience in how to
purify it.
Secondly, the supply of uranium might have to be a large one, for
neutrons didn’t necessarily enter the first uranium atom they
approached. They moved about here and there, making glancing collisions,
and travelling quite a distance, perhaps, before striking head-on and
entering a nucleus. If in that time they had passed outside the lump of
uranium, they were useless.
[Illustration: Franklin D. Roosevelt]
Albert Einstein
Old Grove Rd.
Nassau Point
Peconic, Long Island
August 2nd, 1939
F.D. Roosevelt,
President of the United States,
White House
Washington, D.C.
Sir:
Some recent work by E. Fermi and L. Szilard, which has been communicated