Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3) - 2

to me in manuscript, leads me to expect that the element uranium may be
turned into a new and important source of energy in the immediate
future. Certain aspects of the situation which has arisen seem to call
for watchfulness and, if necessary, quick action on the part of the
Administration. I believe therefore that it is my duty to bring to your
attention the following facts and recommendations:
In the course of the last four months it has been made probable—through
the work of Joliot in France as well as Fermi and Szilard in
America—that it may become possible to set up a nuclear chain reaction
in a large mass of uranium, by which vast amounts of power and large
quantities of new radium-like elements would be generated. Now it
appears almost certain that this could be achieved in the immediate
future.
This new phenomenon would also lead to the construction of bombs, and it
is conceivable—though much less certain—that extremely powerful bombs of
a new type may thus be constructed. A single bomb of this type, carried
by boat and exploded in a port, might very well destroy the whole port
together with some of the surrounding territory. However, such bombs
might very well prove to be too heavy for transportation by air.
[Illustration: _Albert Einstein_]
The United States has only very poor ores of uranium in moderate
quantities. There is some good ore in Canada and the former
Czechoslovakia, while the most important source of uranium is Belgian
Congo.
In view of this situation you may think it desirable to have some
permanent contact maintained between the Administration and the group
of physicists working on chain reactions in America. One possible way
of achieving this might be for you to entrust with this task a person
who has your confidence and who could perhaps serve in an inofficial
capacity. His task might comprise the following:
a) to approach Government Departments, keep them informed of the
further development, and put forward recommendations for Government
action, giving particular attention to the problem of securing a
supply of uranium ore for the United States;
b) to speed up the experimental work, which is at present being
carried on within the limits of the budgets of University
laboratories, by providing funds, if such funds be required, through
his contacts with private persons who are willing to make
contributions for this cause, and perhaps also by obtaining the
co-operation of industrial laboratories which have the necessary
equipment.
I understand that Germany has actually stopped the sale of uranium
from the Czechoslovakian mines which she has taken over. That she
should have taken such early action might perhaps be understood on the
ground that the son of the German Under-Secretary of State, von
Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where
some of the American work on uranium is now being repeated.
Yours very truly,
[Illustration: /signed/]
(Albert Einstein)
As the quantity of uranium within which the fission chain reaction was
initiated grew larger, more and more of the neutrons produced found a
mark and the fission reaction would die out more and more slowly.
Finally, at some particular size—the “critical size”—the fission
reaction did not die at all, but maintained itself, with enough of the
neutrons produced finding their mark to keep the nuclear reaction
proceeding at a steady rate. At any greater size the nuclear reaction
would accelerate and there would be an explosion.
It wasn’t even necessary to send neutrons into the uranium to start the
process. In 1941 the Russian physicist Georgii Nikolaevich Flerov
(1913- ) found that every once in a while a uranium atom would
undergo fission without the introduction of a neutron. Occasionally the
random quivering of a nucleus would bring about a shape that the nuclear
interaction could not bring back to normal and the nucleus would then
break apart. In a gram of ordinary uranium, there is a nucleus
undergoing such “spontaneous fission” every 2 minutes on the average.
Therefore, enough uranium need only be brought together to surpass
critical size and it will explode within seconds, for the first nucleus
that undergoes spontaneous fission will start the chain reaction.
First estimates made it seem that the quantity of uranium needed to
reach critical size was extraordinarily great. Fully 99.3% of the metal
is uranium-238, however, and, as soon as fission was discovered, Bohr
pointed out that there were theoretical reasons for supposing that it
was the uranium-235 isotope (making up only 0.7% of the whole) that was
the one undergoing fission. Investigation proved him right. Indeed, the
uranium-238 nucleus tended to absorb slow neutrons without fission, and
to go on to beta-particle production that formed isotopes of neptunium
and plutonium. In this way uranium-238 actually interfered with the
chain reaction.
In any quantity of uranium, the more uranium-235 present and the less
uranium-238, the more easily the chain reaction would proceed and the
lower the critical size needed. Vast efforts were therefore made to
separate the 2 isotopes and prepare uranium with a higher than normal
concentration of uranium-235 (“enriched uranium”).
Of course, there was no great desire for a fearful explosion to get out
of hand while the chain reaction was being studied. Before any bomb
could be constructed, the mechanism of the chain reaction would have to
be studied. Could a chain reaction capable of producing energy (for
useful purposes as well as for bombs) be established? To test this, a
quantity of uranium was gathered in the hope that a _controlled_ chain
reaction of uranium fission could be established. For that purpose,
control rods of a substance that would easily absorb neutrons and slow
the chain reaction were used. The metal, cadmium, served admirably for
this purpose.
Then, too, the neutrons released by fission were pretty energetic. They
tended to travel too far too soon and get outside the lump of uranium
too easily. To produce a chain reaction that could be studied with some
safety, the presence of a moderator was needed. This was a supply of
small nuclei that did not absorb neutrons readily, but absorbed some of
the energy of collision and slowed down any neutron that struck it.
Nuclei such as hydrogen-2, beryllium-9, or carbon-12 were useful
moderators. When the neutrons produced by fission were slowed, they
travelled a smaller distance before being absorbed in their turn and the
critical size would again be reduced.
Toward the end of 1942 the initial stage of the project reached a
climax. Blocks of graphite containing uranium metal and uranium oxide
were piled up in huge quantities (enriched uranium was not yet
available) in order to approach critical size. This took place under the
stands of a football stadium at the University of Chicago, with Enrico
Fermi (who had come to the United States in 1938) in charge.[1]
The large structure was called an “atomic pile” at first because of the
blocks of graphite being piled up. The proper name for such a device,
and the one that was eventually adopted, was, however, “nuclear
reactor”.
On December 2, 1942, calculations showed that the nuclear reactor was
large enough to have reached critical size. The only thing preventing
the chain reaction from sustaining itself was the cadmium rods that were
inserted here and there in the pile and that were soaking up neutrons.
[Illustration: _Cutaway model of the West Stands of Stagg Field showing
the first pile in the squash court beneath it._]
[Illustration: _The exterior of the building._]
[Illustration: _Graphite layers form the base of the pile, left. On the
right is the seventh layer of graphite and edges of the sixth layer
containing 3¼-inch pseudospheres of black uranium oxide. Beginning with
layer 6, alternate courses of graphite containing uranium metal and/or
uranium oxide fuel were separated by layers of solid graphite blocks._]
[Illustration: _Tenth layer of graphite blocks containing pseudospheres
of black and brown uranium oxide. The brown briquets, slightly richer in
uranium, were concentrated in the central area. On the right is the
nineteenth layer of graphite covering layer 18 containing slugs of
uranium oxide._]
One by one the cadmium rods were pulled out. The number of uranium atoms
undergoing fission each second rose and, finally, at 3:45 p.m., the
uranium fission became self-sustaining. It kept going on its own (with
the cadmium rods ready to be pushed in if it looked as though it were
getting out of hand—something calculations showed was not likely).
News of this success was announced to Washington by a cautious telephone
call from Arthur Holly Compton (1892-1962) to James Bryant Conant
(1893- ). “The Italian navigator has landed in the new world”, said
Compton. Conant asked, “How were the natives?”, and the answer was,
“Very friendly”.
This was the day and moment when the world entered the “nuclear age”.
For the first time, mankind had constructed a device in which the
nuclear energy being given off was greater than the energy poured in.
Mankind had tapped the reservoirs of nuclear energy and could put it to
use. Had Rutherford lived but 6 more years, he would have seen how wrong
he was to think it could never be done.
The people of earth remained unaware of what had taken place in Chicago
and physicists continued to work toward the development of the nuclear
bomb.
Enriched uranium was successfully prepared. Critical sizes were brought
low enough to make a nuclear bomb small enough to be carried by plane to
some target. Suppose one had 2 slabs of enriched uranium, each below
critical size, but which were above critical size if combined. And
suppose an explosive device were added that, at some desired moment,
could be set off in such a way that it would drive 1 slab of enriched
uranium against the other. There would be an instant explosion of
devastating power. Or suppose the enriched uranium were arranged in
loosely packed pieces to begin with so that the flying neutrons were in
open air too often to maintain the chain reaction. A properly arranged
explosion might compress the uranium into a dense ball. Neutron
absorption would become more efficient and again, an explosion would
follow.
[Illustration: _Nuclear Fission of Uranium: A neutron hits the nucleus
of an atom of uranium. The neutron splits the nucleus into two parts and
creates huge amounts of energy in the form of heat. At the same time
other neutrons are released from the splitting nucleus and these
continue the fission process in a chain reaction._]
On July 16, 1945, a device that would result in a nuclear explosion was
set up near Alamogordo, New Mexico, with nervous physicists watching
from a safe distance. It worked perfectly; the explosion was tremendous.
By that time Nazi Germany had been defeated, but Japan was still
fighting. Two more devices were prepared. After a warning, one was
exploded over the Japanese city of Hiroshima on August 6, 1945, and the
other over Nagasaki 2 days later. The Japanese government surrendered
and World War II came to an end.
It was with the blast over Hiroshima that the world came to know it was
in the nuclear age and that the ferocious weapon of the nuclear bomb
existed. (The popular name for it at the time was “atomic bomb” or
“A-bomb”.)
During the war, German scientists may have been trying to develop a
nuclear bomb, but, if so, they had not yet succeeded at the time Germany
met its final defeat. Soviet physicists, under Igor Vasilievich
Kurchatov (1903-1960), were also working on the problem. The dislocation
of the war, which inflicted much more damage on the Soviet Union than on
the United States, kept the Soviet effort from succeeding while it was
on. However, since the Soviets were among the victors, they were able to
continue after the war.
In 1949 the Soviets exploded their first nuclear bomb. In 1952 the
British did the same; in 1960, the French; and in 1964, the Chinese.
Although many nuclear bombs have been exploded for test purposes, the
two over Hiroshima and Nagasaki have been the only ones used in time of
war.
Nor need nuclear bombs be considered as having destructive potential
only. There is the possibility that, with proper precautions, they might
be used to make excavations, blast out harbors or canals, break up
underground rock formations to recover oil or other resources, and in
other ways do the work of chemical explosives with far greater speed and
economy. It has even been suggested that a series of nuclear bomb
explosions might be used to hurl space vehicles forward in voyages away
from earth.

Nuclear Reactors
The development of the nuclear chain reaction was not in the direction
of bombs only. Nuclear reactors designed for the controlled production
of useful energy multiplied in number and in efficiency since Fermi’s
first “pile”. Many nations now possess them, and they are used for a
variety of purposes.[2]
[Illustration: _The USS_ Nautilus, _the world’s first nuclear powered
submarine, in New York harbor_.]
In 1954 the first nuclear submarine the USS _Nautilus_ was launched by
the United States. Its power was obtained entirely from a nuclear
reactor, and it was not necessary for it to rise to the surface at short
intervals in order to recharge its batteries. Nuclear submarines have
crossed the Arctic Ocean under the ice cover, and have circumnavigated
the globe without surfacing.
In 1959 both the Soviet Union and the United States launched
nuclear-powered surface vessels. The Soviet ship was the icebreaker,
_Lenin_, and the American ship was a merchant vessel, the NS _Savannah_.
In the 1950s nuclear reactors were also used as the source of power for
the production of electricity for civilian use. The Soviet Union built a
small station of this sort in 1954, which had a capacity of 5,000
kilowatts. The British built one of 92,000 kilowatt capacity, which they
called Calder Hall. The first American nuclear reactor for civilian use
began operation at Shippingport, Pennsylvania, in 1958. It was the first
really full-scale civilian nuclear power plant in the world.
The world appeared to have far greater sources of energy than had been
expected. The “fossil fuels”—coal, oil and natural gas—were being used
at such a rate that many speculated that the gas and oil would be gone
in decades and the coal in centuries. Was it possible that uranium might
now serve as a new source that would last indefinitely?
It was rather disappointing that it was uranium-235 which underwent
fission, because that isotope made up only 0.7% of the uranium that
existed. If uranium-235 were all we had and all we ever could have, the
energy supply of the world would still be rather too limited.
There were other possible “nuclear fuels”, however. There was
plutonium-239, which would also fission under neutron bombardment. It
had an ordinary half-life (for a radioactive change in which it gave off
alpha particles) of 24,300 years, which is long enough to make it easy
to handle.
But how can plutonium-239 be formed in sufficient quantities to be
useful? After all, it doesn’t occur in nature. Surprisingly, that turned
out to be easy. Uranium-238 atoms will absorb many of the neutrons that
are constantly leaking out of the reactor and will become first
neptunium-239 and then plutonium-239. The plutonium, being a different
element from the uranium, can be separated from uranium and obtained in
useful quantities.
Such a device is called a “breeder reactor” because it breeds fuel.
Indeed, it can be so designed to produce more plutonium-239 than the
uranium-235 it uses up, so that you actually end up with more nuclear
fuel than you started with. In this way, all the uranium on earth (and
not just uranium-235) can be considered potential nuclear fuel.
[Illustration: _The Shippingport Atomic Power Station, the first
full-scale, nuclear-electric station built exclusively for civilian
needs, provides electricity for the homes and factories of the greater
Pittsburgh area. The pressurized-water reactor, which now has a
90,000-net-electrical-kilowatt capacity, began commercial operation in
1957. The reactor is in the large building in the center._]
[Illustration: _The lights of downtown Pittsburgh._]
The first breeder reactor was completed at Arco, Idaho, in August 1951,
and on December 20 produced the very first electricity on earth to come
from nuclear power. Nevertheless, breeder reactors for commercial use
are still a matter for the future.[3]
Another isotope capable of fissioning under neutron bombardment is
uranium-233. It does not occur in nature, but was formed in the
laboratory by Seaborg and others in 1942. It has a half-life of 162,000
years. It can be formed from naturally occurring thorium-232.
Thorium-232 will absorb a neutron to become thorium-233. Then 2 beta
particles are given off so that the thorium-233 becomes first
protactinium-233 and then uranium-233.
If a thorium shell surrounds a nuclear reactor, fissionable uranium-233
is formed within it and is easily separated from the thorium. In this
way, thorium is also added to the list of earth’s potential nuclear
fuels.[4]
If all the uranium and thorium in the earth’s crust (including the thin
scattering of those elements through granite, for instance) were
available for use, we might get up to 100 times as much energy from it
as from all the coal and oil on the planet. Unfortunately, it is very
unlikely that we will ever be able to make use of all the uranium and
thorium. It is widely and thinly spread through the crustal rocks and
much of it could not be extracted without using up more energy than
would be supplied by it once isolated.
Another problem rests with the nature of the fission reaction. When the
uranium-235 nucleus (or plutonium-239 or uranium-233) undergoes fission,
it breaks up into any of a large number of middle-sized nuclei that are
radioactive—much more intensely radioactive than the original fuel. (It
was from among these “fission products” that isotopes of element 61 were
first obtained in 1945. Coming from the nuclear fire, it reminded its
discoverers of Prometheus, who stole fire from the sun in the Greek
myths, and so it was called “promethium”.)
The fission products still contain energy and some of them can be used
in lightweight “nuclear batteries”. Such nuclear batteries were first
built in 1954. Some batteries, using plutonium-238 rather than fission
products, have been put to use in powering artificial satellites over
long periods.
Unfortunately, only a small proportion of the fission products can be
put to profitable use. Most must be disposed of. They are dangerous
because the radiations they give off are deadly and cannot be detected
by the ordinary senses. They are very difficult to dispose of safely,
and they must not be allowed to get into the environment, especially
since some of them remain dangerous for decades or even centuries.
[Illustration: _The Experimental Breeder Reactor No. 2 building complex
in Idaho. The reactor is in the dome-shaped structure._]


NUCLEAR FUSION

The Energy of the Sun
As it happens, though, nuclear fission is not the only route to useful
nuclear energy.
Aston’s studies in the 1920s had shown that it was the middle-sized
nuclei that were most tightly packed. Energy would be given off if
middle-sized nuclei were produced from either extreme. Not only would
energy be formed by the breakup of particularly massive nuclei through
fission, but also through the combination of small nuclei to form larger
ones (“nuclear fusion”).
In fact, from Aston’s studies it could be seen that, mass for mass,
nuclear fusion would produce far more energy than nuclear fission. This
was particularly true in the conversion of hydrogen to helium; that is,
the conversion of the individual protons of 4 separate hydrogen nuclei
into the 2-proton—2-neutron structure of the helium nucleus. A gram of
hydrogen, undergoing fusion to helium, would deliver some fifteen times
as much energy as a gram of uranium undergoing fission.
As early as 1920, the English astronomer Arthur Stanley Eddington
(1882-1944) had speculated that the sun’s energy might be derived from
the interaction of subatomic particles. Some sort of nuclear reaction
seemed, by then, to be the most reasonable way of accounting for the
vast energies constantly being produced by the sun.
The speculation became more plausible with each year. Eddington himself
studied the structure of stars, and by 1926 had produced convincing
theoretical reasons for supposing that the center of the sun was at
enormous densities and temperatures. A temperature of some 15,000,000 to
20,000,000°C seemed to characterize the sun’s center.
At such temperatures, atoms could not exist in earthly fashion. Held
together by the sun’s strong gravitational field, they collided with
such energy that all or almost all their electrons were stripped off,
and little more than bare nuclei were left. These bare nuclei could
approach each other much more closely than whole atoms could (which was
why the center of the sun was so much more dense than earthly matter
could be). The bare nuclei, smashing together at central-sun
temperatures, could cling together and form more complex nuclei. Nuclear
reactions brought about by such intense heat (millions of degrees) are
called “thermonuclear reactions”.
As the 1920s progressed further studies of the chemical structure of the
sun showed it to be even richer in hydrogen than had been thought. In
1929 the American astronomer Henry Norris Russell (1877-1957) reported
evidence that the sun was 60% hydrogen in volume. (Even this was too
conservative; 80% is considered more nearly correct now.) If the sun’s
energy were based on nuclear reactions at all, then it had to be the
result of hydrogen fusion. Nothing else was present in sufficient
quantity to be useful as a fuel.
More and more was learned about the exact manner in which nuclei
interacted and about the quantity of energy given off in particular
nuclear reactions. It became possible to calculate what might be going
on inside the sun by considering the densities and temperatures present,
the kind and number of different nuclei available, and the quantity of
energy that must be produced. In 1938 the German-American physicist Hans
Albrecht Bethe (1906- ) and the German astronomer Carl Friedrich von
Weizsäcker (1912- ) independently worked out the possible reactions,
and hydrogen fusion was shown to be a thoroughly practical way of
keeping the sun going.
Thanks to the high rate of energy production by thermonuclear reactions
and to the vast quantity of hydrogen in the sun, not only has it been
possible for the sun to have been radiating energy for the last
5,000,000,000 years or so, but it will continue to radiate energy in the
present fashion for at least 5,000,000,000 years into the future.
[Illustration: _Hans Bethe_]
Even so, the sheer quantity of what is going on in the sun is staggering
in earthly terms. In the sun 650,000,000 tons of hydrogen are converted
into helium every second, and in the process each second sees the
disappearance of 4,600,000 tons of mass.

Thermonuclear Bombs
Could thermonuclear reactions be made to take place on earth? The
conditions that exist in the center of the sun would be extremely
difficult to duplicate on the earth, so there was a natural search for
any kind of nuclear fusion that would produce similar energies to those
going on in the sun but which would be easier to bring about.
There are 3 hydrogen isotopes known to exist. Ordinary hydrogen is
almost entirely hydrogen-1, with a nucleus made up of a single proton.
Small quantities of hydrogen-2 (deuterium) with a nucleus made up of a
proton plus a neutron also exist and such atoms are perfectly stable.
In 1934 Rutherford, along with the Australian physicist Marcus Laurence
Elwin Oliphant (1901- ) and the Austrian chemist Paul Harteck
(1902- ) sent hydrogen-2 nuclei flying into hydrogen-2 targets and
formed hydrogen-3 (also called “tritium” from the Greek word for
“third”) with a nucleus made up of a proton plus 2 neutrons. Hydrogen-3
is mildly radioactive.
Hydrogen-2 fuses to helium more easily than hydrogen-1 does and, all
things being equal, hydrogen-2 will do so at lower temperatures than
hydrogen-1. Hydrogen-3 requires lower temperatures still. But even for
hydrogen-3 it still takes millions of degrees.
Hydrogen-3, although the easiest to be forced to undergo fusion, exists
only in tiny quantities.
Hydrogen-2, therefore, is the one to pin hopes on especially in
conjunction with hydrogen-3. Only 1 atom out of every 6000 hydrogen
atoms is hydrogen-2, but that is enough. There exists a vast ocean on
earth that is made up almost entirely of water molecules and in each
water molecule 2 hydrogen atoms are present. Even if only 1 in 6000 of
these hydrogen atoms is deuterium that still means there are about
35,000 billion tons of deuterium in the ocean.
What’s more, it isn’t necessary to dig for that deuterium or to drill
for it. If ocean water is allowed to run through separation plants, the
deuterium can be extracted without very much trouble. In fact, for the
energy you could get out of it, deuterium from the oceans, extracted by
present methods and without allowing for future improvement, would be
only one-hundredth as expensive as coal.
The deuterium in the world’s ocean, if allowed to undergo fusion little
by little, would supply mankind with enough energy to keep us going at
the present rate for 500,000,000,000 years. To be sure, to make
deuterium fusion practical, it may be necessary to make use of rarer
substances such as the light metal lithium. This will place a sharper
limit on the energy supply but even if we are careful, fusion would
probably supply mankind with energy for as long as mankind will exist.
Then, too, there would seem to be no danger of hydrogen fusion plants
running out of control. Only small quantities of deuterium would be in
the process of fusion at any one time. If anything at all went wrong,
the deuterium supply could be automatically cut off and the fusion
process, with so little involved, would then stop instantly. Moreover,
there would be less reason to worry about atomic wastes, for the most
dangerous products—hydrogen-3 and neutrons—could be easily taken care
of.
It seems ideal, but there is a catch. However clear the theory, before a
fusion power station can be established some practical method must be
found to start the fusion process, which means finding some way for
attaining temperatures in the millions of degrees.
One method for obtaining the necessary temperature was known by 1945. An
exploding fission bomb would do it. If, somehow, the necessary
hydrogen-2 was combined with a fission bomb, the explosion would set off
a fusion reaction that would greatly multiply the energy released. You
would have in effect a “thermonuclear bomb”. (To the general public,
this was commonly known as a “hydrogen bomb” or an “H-bomb”.)
In 1952 the first fusion device was exploded by the United States in the
Marshall Islands. Within months, the Soviet Union had exploded one of
its own and in time thermonuclear bombs thousands of times as powerful
as the first fission bomb over Hiroshima were built and exploded.
All thermonuclear bombs have been exploded only for test purposes. Even
testing seems to be dangerous, however, at least if it is carried on in
the open atmosphere. The radioactivity liberated spreads over the world
and may do slow but cumulative damage.

Controlled Fusion