- "Spliting the atom" redirects here. For the EP of
the same name by Massive Attack, see
Splitting the Atom.
In
nuclear physics and
nuclear chemistry,
nuclear
fission is a
nuclear
reaction in which the
nucleus of
an
atom splits into smaller parts, often
producing free
neutrons and lighter
nuclei, which may eventually produce
photons (in the form of
gamma
rays). Fission of heavy elements is an
exothermic reaction which can release
large amounts of
energy both as
electromagnetic radiation and as
kinetic energy of the fragments
(
heating the bulk material where fission takes
place). For fission to produce energy, the total
binding energy of the
resulting elements has to be higher than that of the starting
element. Fission is a form of
nuclear transmutation because the
resulting fragments are not the same
element as the original atom.
Nuclear fission produces
energy for
nuclear power and to drive the explosion of
nuclear weapons. Both uses are made
possible because certain substances called
nuclear fuels undergo fission when struck by
free neutrons and in turn generate neutrons when they break apart.
This makes possible a self-sustaining
chain reaction that releases energy at a
controlled rate in a
nuclear reactor
or at a very rapid uncontrolled rate in a
nuclear weapon.
The amount of
free energy
contained in nuclear fuel is millions of times the amount of free
energy contained in a similar mass of chemical fuel such as
gasoline, making nuclear fission a very
tempting source of energy; however, the products of nuclear fission
are
radioactive and remain so for
significant amounts of time, giving rise to a
nuclear waste problem. Concerns over
nuclear waste accumulation and
over the
destructive potential of
nuclear weapons may counterbalance
the desirable qualities of
fission as an
energy source, and give rise to ongoing
political debate over
nuclear power.
Physical overview
Mechanics
A visual representation of an induced nuclear fission event where a
slow-moving neutron is absorbed by the nucleus of a uranium-235
atom, which fissions into two fast-moving lighter elements (fission
products) and additional neutrons.
Most of the energy released is in the form of the kinetic
velocities of the fission products and the neutrons.
Also shown is the capture of a neutron by uranium-238 to
become uranium-239.
Nuclear fission can occur without
neutron
bombardment, as a type of
radioactive
decay. This type of fission (called
spontaneous fission) is rare except in a
few heavy isotopes. In engineered nuclear devices, essentially all
nuclear fission occurs as a "
nuclear
reaction"—a bombardment-driven process that results from the
collision of two subatomic particles. In nuclear reactions, a
subatomic particle collides with an atomic nucleus and causes
changes to it. Nuclear reactions are thus driven by the mechanics
of bombardment, not by the relatively constant
exponential decay and
half-life characteristic of spontaneous
radioactive processes.
A great many nuclear reactions are known. Nuclear fission differs
importantly from other types of nuclear reactions in that it can be
amplified and sometimes controlled via a nuclear
chain reaction. In such a reaction, free
neutrons released by each fission event can
trigger yet more events, which in turn release more neutrons and
cause more fissions.
The
chemical element isotopes that can sustain a fission chain reaction
are called
nuclear fuels, and are said
to be
fissile. The most common nuclear fuels
are
235U (the isotope of
uranium with an
atomic mass of 235 and of use in nuclear
reactors) and
239Pu (the
isotope of
plutonium with an
atomic mass of 239). These fuels break apart
into a bimodal range of chemical elements with atomic masses
centering near 95 and 135
u (
fission products). Most nuclear fuels
undergo
spontaneous fission only
very slowly, decaying instead mainly via an
alpha/
beta
decay chain over periods of
millennia to
eon. In
a
nuclear reactor or nuclear weapon,
the overwhelming majority of fission events are induced by
bombardment with another particle, a neutron, which is itself
produced by prior fission events.
Energetics
Typical fission events release about two hundred million
eV of energy for each fission event. By
contrast, most
chemical oxidation reactions (such as burning
coal or
TNT) release at
most a few
eV per event, so nuclear
fuel contains at least ten million times more usable energy than
does chemical fuel. The energy of nuclear fission is released as
kinetic energy of the fission
products and fragments, and as
electromagnetic radiation in the
form of
gamma rays; in a nuclear reactor,
the energy is converted to
heat as the
particles and gamma rays collide with the atoms that make up the
reactor and its
working fluid, usually
water or occasionally
heavy water.
When a
uranium nucleus fissions into two
daughter nuclei fragments, an energy of ~200 MeV is released.
For uranium-235 (total mean fission energy 202.5 MeV), typically
~169 MeV appears as the
kinetic
energy of the daughter nuclei, which fly apart at about 3% of
the speed of light, due to
Coulomb
repulsion. Also, an average of 2.5 neutrons are emitted
with a kinetic energy of ~2 MeV each (total of 4.8 MeV). The
fission reaction also releases ~7 MeV in prompt
gamma ray photon.
This energy amounts to about 181 MeV, or ~ 89% of the total energy.
The remaining ~ 11% is released in beta decays which have various
half-lives, but begin as a process in the fission products
immediately; and in delayed gamma emissions associated with these
beta decays. For example, in uranium-235 this delayed energy is
divided into about 6.5 MeV in betas, 8.8 MeV in
antineutrinos (released at the same time as the
betas), and finally, an additional 6.3 MeV in delayed gamma
emission from the excited beta-decay products (for a mean total of
~10 gamma ray emissions per fission, in all).
The 8.8 MeV / 202.5 MeV = 4.3% of the energy which is released as
antineutrinos is not captured by the reactor material as heat, and
escapes directly through all materials (including the Earth) at
nearly the speed of light, and into interplanetary space. Almost
all of the remaining radiation is converted to heat, either in the
reactor core or its shielding.
Some processes involving neutrons are notable for absorbing or
finally yielding energy—for example neutron kinetic energy does not
yield heat immediately if the neutron is captured by a uranium-238
atom to breed plutonium-239, but this energy is emitted if the
plutonium-239 is later fissioned. On the other hand, so called
"delayed neutrons" emitted as radioactive decay products with
half-lives up to a minute, from fission-daughters, are very
important to reactor control because they give a characteristic
"reaction" time for the total nuclear reaction to double in size,
if the reaction is run in a "
delayed-critical" zone which
deliberately relies on these neutrons for a supercritical
chain-reaction (one in which each fission cycle yields more
neutrons than it absorbs). Without their existence, the nuclear
chain-reaction would be
prompt
critical and increase in size faster than it could be
controlled by human intervention. In this case, the first
experimental atomic reactors would have run away to a dangerous and
messy "prompt critical reaction" before their operators could have
shut them down. If these neutrons are captured without producing
fissions, they produce heat as well.
Product nuclei and binding energy
In fission there is a preference to yield fragments with even
proton numbers, which is called the odd-even effect on the
fragments charge distribution. However, no odd-even effect is
observed on fragment
mass number distribution.
This result is attributed to
nucleon pair
breaking.
In nuclear fission events the nuclei may break into any combination
of lighter nuclei, but the most common event is not fission to
equal mass nuclei of about mass 120; the most common event
(depending on isotope and process) is a slightly unequal fission in
which one daughter nucleus has a mass of about 90 to
100
u and the other the remaining 130 to
140
u. Unequal fissions are energetically
more favorable because this allows one product to be closer to the
energetic minimum near mass 60
u (only a
quarter of the average fissionable mass), while the other nucleus
with mass 135
u is still not far out of the
range of the most tightly bound nuclei (another statement of this,
is that the atomic
binding energy
curve is slightly steeper to the left of mass
120
u than to the right of it).
Origin of the active energy and the curve of binding
energy
Nuclear fission of heavy elements produces energy because the
specific
binding energy (binding
energy per mass) of intermediate-mass nuclei with
atomic numbers and
atomic masses close to
62Ni and
56Fe is greater than the nucleon-specific binding energy
of very heavy nuclei, so that energy is released when heavy nuclei
are broken apart.
The total rest masses of the fission products (
Mp)
from a single reaction is less than the mass of the original fuel
nucleus (
M). The excess mass
Δm =
M –
Mp
is the
invariant mass of the energy
that is released as
photons (
gamma rays) and kinetic energy of the fission
fragments, according to the
mass-energy equivalence formula
E =
mc².The variation in specific
binding energy with
atomic number is
due to the interplay of the two fundamental
forces acting on the component
nucleons (
protons and
neutrons) that make up the nucleus. Nuclei are bound
by an attractive
strong
interaction between nucleons, which overcomes the
electrostatic repulsion between
protons. However, the strong nuclear force acts only over extremely
short ranges, since it follows a
Yukawa
potential. For this reason large nuclei are less tightly bound
per unit mass than small nuclei, and breaking a very large nucleus
into two or more intermediate-sized nuclei releases energy.
Because of the short range of the strong binding force, large
nuclei must contain proportionally more neutrons than do light
elements, which are most stable with a 1–1 ratio of protons and
neutrons. Extra neutrons stabilize heavy elements because they add
to strong-force binding without adding to proton-proton repulsion.
Fission products have, on average, about the same ratio of neutrons
and protons as their parent nucleus, and are therefore usually
unstable because they have proportionally too many neutrons
compared to stable isotopes of similar mass. This is the
fundamental cause of the problem of
radioactive high
level waste from nuclear reactors. Fission products tend to be
beta emitters,
emitting fast-moving
electrons to conserve
electric charge as excess neutrons convert
to protons inside the nucleus of the fission product atoms.
Nuclear or "fissile" fuels
The most common nuclear fuels,
235U and
239Pu, are not major radiological hazards by themselves:
235U has a
half-life of
approximately 700 million years, and although
239Pu
has a half-life of only about 24,000 years, it is a pure
alpha particle emitter and hence not
particularly dangerous unless ingested. Once a
fuel element has been used, the remaining fuel
material is intimately mixed with highly radioactive fission
products that emit energetic
beta
particles and
gamma rays. It is a
well known fact that the nucleus is one ten thousandth of an atom,
which causes nuclear fission.
Some fission products have half-lives as
short as seconds; others have half-lives of tens of thousands of
years, requiring long-term storage in facilities such as Yucca
Mountain nuclear waste repository
until the fission products decay into
non-radioactive stable isotopes.
Chain reactions
Several heavy elements, such as
uranium,
thorium, and
plutonium, undergo both
spontaneous fission, a form of
radioactive decay and
induced
fission, a form of
nuclear
reaction. Elemental isotopes that undergo induced fission when
struck by a free
neutron are called
fissionable; isotopes that undergo fission when
struck by a
thermal, slow moving
neutron are also called
fissile. A few
particularly fissile and readily obtainable isotopes (notably
235U and
239Pu) are called
nuclear fuels because they can sustain a chain
reaction and can be obtained in large enough quantities to be
useful.
All fissionable and fissile isotopes undergo a small amount of
spontaneous fission which releases a few free neutrons into any
sample of nuclear fuel. Such neutrons would escape rapidly from the
fuel and become a
free neutron, with a
mean lifetime of about 15 minutes
before they decayed to
protons and
beta particles. However, neutrons almost
invariably impact and are absorbed by other nuclei in the vicinity
long before this happens (newly-created fission neutrons are moving
at about 7% of the speed of light, and even moderated neutrons are
moving at about 8 times the speed of sound). Some neutrons
will impact fuel nuclei and induce further fissions, releasing yet
more neutrons. If enough nuclear fuel is assembled into one place,
or if the escaping neutrons are sufficiently contained, then these
freshly generated neutrons outnumber the neutrons that escape from
the assembly, and a
sustained nuclear chain reaction will
take place.
An assembly that supports a sustained nuclear chain reaction is
called a
critical assembly
or, if the assembly is almost entirely made of a nuclear fuel, a
critical mass. The word
"critical" refers to a
cusp in
the behavior of the
differential
equation that governs the number of free neutrons present in
the fuel: if less than a critical mass is present, then the amount
of neutrons is determined by
radioactive decay, but if a critical mass
or more is present, then the amount of neutrons is controlled
instead by the physics of the chain reaction. The actual
mass of a
critical mass of nuclear fuel
depends strongly on the geometry and surrounding materials.
Not all fissionable isotopes can sustain a chain reaction. For
example,
238U, the most abundant form of uranium, is
fissionable but not fissile: it undergoes induced fission when
impacted by an energetic neutron with over 1 MeV of kinetic
energy. But too few of the neutrons produced by
238U
fission are energetic enough to induce further fissions in
238U, so no chain reaction is possible with this
isotope. Instead, bombarding
238U with slow neutrons
causes it to absorb them (becoming
239U) and decay by
beta emission to
239Np which
then decays again by the same process to
239Pu; that
process is used to manufacture
239Pu in
breeder reactors. In-situ plutonium
production also contributes to the neutron chain reaction in other
types of reactors after sufficient plutonium-239 has been produced,
since plutonium-239 is also a fissile element which serves as fuel.
It is estimated that up to half of the power produced by a standard
"non-breeder" reactor is produced by the fission of plutonium-239
produced in place, over the total life-cycle of a fuel load.
Fissionable, non-fissile isotopes can be used as fission energy
source even without a chain reaction. Bombarding
238U
with fast neutrons induces fissions, releasing energy as long as
the external neutron source is present. This is an important effect
in all reactors where fast neutrons from the fissile isotope can
cause the fission of nearby
238U nuclei, which means
that some small part of the
238U is "burned-up" in all
nuclear fuels, especially in fast breeder reactors that operate
with higher-energy neutrons. That same fast-fission effect is used
to augment the energy released by modern
thermonuclear weapons, by jacketing the
weapon with
238U to react with neutrons released by
nuclear fusion at the center of the
device.
Fission reactors
Critical fission reactors are the most common type of
nuclear reactor. In a critical fission
reactor, neutrons produced by fission of fuel atoms are used to
induce yet more fissions, to sustain a controllable amount of
energy release. Devices that produce engineered but
non-self-sustaining fission reactions are
subcritical fission reactors.
Such devices use
radioactive decay
or
particle accelerators to
trigger fissions.
Critical fission reactors are built for three primary purposes,
which typically involve different engineering trade-offs to take
advantage of either the heat or the neutrons produced by the
fission chain reaction:
- power reactor are
intended to produce heat for nuclear power, either as part of a
generating station or a local
power system such as a nuclear
submarine.
- research reactors are
intended to produce neutrons and/or activate radioactive sources
for scientific, medical, engineering, or other research
purposes.
- breeder reactors are
intended to produce nuclear fuels in bulk from more abundant
isotopes. The better known fast breeder reactor makes
239Pu (a nuclear fuel) from the naturally very abundant
238U (not a nuclear fuel). Thermal breeder reactors
previously tested using 232Th to breed the fissile
isotope 233U continue to be studied and developed.
While, in principle, all fission reactors can act in all three
capacities, in practice the tasks lead to conflicting engineering
goals and most reactors have been built with only one of the above
tasks in mind.
(There are several early counter-examples,
such as the Hanford
N reactor
, now decommissioned). Power reactors
generally convert the kinetic energy of fission products into heat,
which is used to heat a
working fluid
and drive a
heat engine that generates
mechanical or electrical power. The working fluid is usually water
with a steam turbine, but some designs use other materials such as
gaseous
helium. Research reactors produce
neutrons that are used in various ways, with the heat of fission
being treated as an unavoidable waste product. Breeder reactors are
a specialized form of research reactor, with the caveat that the
sample being irradiated is usually the fuel itself, a mixture of
238U and
235U.For a more detailed description
of the physics and operating principles of critical fission
reactors, see
nuclear reactor
physics. For a description of their social, political, and
environmental aspects, see
nuclear
reactor.
Fission bombs
One class of
nuclear weapon, a
fission bomb (not to be confused with the
fusion bomb), otherwise known as an
atomic bomb or
atom bomb, is a fission reactor
designed to liberate as much energy as possible as rapidly as
possible, before the released energy causes the reactor to explode
(and the chain reaction to stop).
Development of nuclear weapons was the
motivation behind early research into nuclear fission: the Manhattan Project of the U.S. military during World War II carried out most of the early
scientific work on fission chain reactions, culminating in the
Trinity
test bomb and the Little Boy
and Fat
Man
bombs that were exploded over the cities Hiroshima, and Nagasaki, Japan
in August
1945.
Even the first fission bombs were thousands of times more
explosive than a comparable mass of
chemical explosive. For example, Little
Boy weighed a total of about four tons (of which 60 kg was
nuclear fuel) and was long; it also yielded an explosion equivalent
to about 15 kilotons of
TNT,
destroying a large part of the city of
Hiroshima. Modern nuclear weapons (which include a
thermonuclear
fusion as well as one or more fission
stages) are literally hundreds of times more energetic for their
weight than the first pure fission atomic bombs, so that a modern
single missile warhead bomb weighing less than 1/8th as much as
Little Boy (see for example
W88) has a yield of
475,000 tons of TNT, and could bring destruction to
10 times the city area.
While the fundamental physics of the fission
chain reaction in a nuclear weapon is
similar to the physics of a controlled nuclear reactor, the two
types of device must be engineered quite differently (see
nuclear reactor physics). It is
impossible to convert a
nuclear
reactor to cause a true nuclear explosion , or for a nuclear
reactor to explode the way a nuclear explosive does, (though
partial fuel
meltdown and
steam explosions have occurred), and
difficult to extract useful power from a nuclear explosive (though
at least one
rocket propulsion system,
Project Orion,
was intended to work by exploding fission bombs behind a massively
padded vehicle).
The
strategic importance of nuclear
weapons is a major reason why the
technology of nuclear fission is politically
sensitive. Viable fission bomb designs are, arguably, within the
capabilities of bright undergraduates (see
John Aristotle Phillips) being
relatively simple from an engineering viewpoint. However, the
difficulty of obtaining fissile nuclear material to realize the
designs, is the key to the relative unavailability of nuclear
weapons to all but modern industrialized governments with special
programs to produced fissile materials (see
uranium enrichment and
nuclear fuel cycle).
History
Natural fission chain-reactors on Earth
Criticality in
nature
is uncommon. At three ore deposits at Oklo
in Gabon
, sixteen
sites (the so-called Oklo Fossil Reactors
) have been discovered at which self-sustaining
nuclear fission took place approximately 2 billion years
ago. Unknown until 1972 (but postulated by Paul
Kuroda in 1956), when French physicist Francis Perrin discovered the Oklo Fossil
Reactors
, it was realized that nature had beaten humans to
the punch. Large-scale natural uranium fission chain
reactions, moderated by normal water, had occurred far in the past
and would not be possible now. This ancient process was able to use
normal water as a moderator only because 2 billion years before the
present, natural uranium was richer in the shorter-lived fissile
isotope
235U (about 3%), than natural uranium available
today (which is only 0.7%, and must be enriched to 3% to be usable
in light-water reactors).
Artificial nuclear fission
New Zealander,
Ernest Rutherford
is credited with splitting the atom in 1917. His team in
Manchester, England bombarded nitrogen with naturally occurring
alpha particles from radioactive material and observed a proton
emitted with energy higher than the alpha particle. In 1932 his
students
John Cockcroft and
Ernest Walton, working under Rutherford's
direction, attempted to split the nucleus by entirely artificial
means, using a particle accelerator to bombard
lithium with protons, thereby producing two helium
nuclei.
After
English physicist James Chadwick
discovered the neutron in 1932, Enrico Fermi and his colleagues in Rome
studied the
results of bombarding uranium with neutrons in 1934. The first person who mentioned the idea of
nuclear fission in 1934 was
Ida
Noddack.
After the Fermi publication,
Lise
Meitner,
Otto Hahn and
Fritz Strassmann began performing similar
experiments in Germany. Meitner, an Austrian Jew, lost her
citizenship with the
Anschluss in 1938.
She fled and wound up in Sweden, but continued to collaborate by
mail and through meetings with Hahn in Sweden. By coincidence her
nephew
Otto Robert Frisch, also a
refugee, was also in Sweden when Meitner received a letter from
Hahn describing his chemical proof that some of the product of the
bombardment of uranium with neutrons, was
barium and not barium's much heavier chemical sister
element radium (barium's atomic weight is about 60% that of
uranium). Frisch was skeptical, but Meitner trusted Hahn's ability
as a chemist. Marie Curie had been separating barium from radium
for many years, and the techniques were well-known. According to
Frisch:
Was it a mistake? No, said Lise Meitner; Hahn was too
good a chemist for that. But how could barium be formed from
uranium? No larger fragments than protons or helium nuclei (alpha
particles) had ever been chipped away from nuclei, and to chip off
a large number not nearly enough energy was available. Nor was it
possible that the uranium nucleus could have been cleaved right
across. A nucleus was not like a brittle solid that can be cleaved
or broken; George Gamow had suggested
early on, and Bohr had given good
arguments that a nucleus was much more like a liquid drop. Perhaps
a drop could divide itself into two smaller drops in a more gradual
manner, by first becoming elongated, then constricted, and finally
being torn rather than broken in two? We knew that there were
strong forces that would resist such a process, just as the surface
tension of an ordinary liquid drop tends to resist its division
into two smaller ones. But nuclei differed from ordinary drops in
one important way: they were electrically charged, and that was
known to counteract the surface tension.
The charge of a uranium nucleus, we found, was indeed
large enough to overcome the effect of the surface tension almost
completely; so the uranium nucleus might indeed resemble a very
wobbly unstable drop, ready to divide itself at the slightest
provocation, such as the impact of a single neutron. But there was
another problem. After separation, the two drops would be driven
apart by their mutual electric repulsion and would acquire high
speed and hence a very large energy, about 200 MeV in all;
where could that energy come from? ...Lise Meitner... worked out
that the two nuclei formed by the division of a uranium nucleus
together would be lighter than the original uranium nucleus by
about one-fifth the mass of a proton. Now whenever mass disappears
energy is created, according to Einstein's formula E=mc2, and one-fifth of a
proton mass was just equivalent to 200MeV. So here was the source
for that energy; it all fitted!
In December 1938, the German chemists Otto
Hahn and Fritz Strassmann sent
a manuscript to Naturwissenschaften reporting
they had detected the element barium after
bombarding uranium with neutrons; simultaneously, they communicated these
results to Lise Meitner. Meitner, and
her nephew Otto Robert Frisch,
correctly interpreted these results as being nuclear fission.
Frisch confirmed this experimentally on 13 January 1939. In 1944,
Hahn received the Nobel Prize
for Chemistry for the discovery of nuclear fission. Some
historians who have documented the history of the discovery of
nuclear fission believe Meitner should have been awarded the Nobel
Prize with Hahn.
Meitner’s
and Frisch’s interpretation of the work of Hahn and Strassmann
crossed the Atlantic Ocean with Niels
Bohr, who was to lecture at Princeton University
. Isidor Isaac
Rabi and Willis Lamb, two Columbia University physicists working
at Princeton, heard the news and carried it back to Columbia. Rabi
said he told Enrico Fermi; Fermi gave
credit to Lamb. Bohr soon thereafter went from Princeton to
Columbia to see Fermi. Not finding Fermi in his office, Bohr went
down to the cyclotron area and found Herbert L. Anderson. Bohr grabbed him by the
shoulder and said: “Young man, let me explain to you about
something new and exciting in physics.” It was clear to a number of
scientists at Columbia that they should try to detect the energy
released in the nuclear fission of uranium from neutron
bombardment. On 25 January 1939, a Columbia University
team conducted the first nuclear fission experiment in the United
States, which was done in the basement of Pupin Hall
; the members of the team were Herbert L. Anderson, Eugene T. Booth, John
R. Dunning, Enrico Fermi, G. Norris
Glasoe, and Francis G. Slack. The next day, the Fifth Washington
Conference on Theoretical Physics began in Washington,
D.C.
under the joint auspices of the George
Washington University
and the Carnegie Institution of
Washington. There, the news on nuclear fission was
spread even further, which fostered many more experimental
demonstrations.
Frédéric
Joliot-Curie's team in Paris discovered that secondary neutrons
are released during uranium fission, thus making a nuclear
chain-reaction feasible. The figure of about two neutrons being
emitted with nuclear fission of uranium was verified independently
by Leo Szilárd and Walter Henry Zinn. The number of neutrons
emitted with nuclear fission of 235U was then reported
at 3.5/fission, and later corrected to 2.6/fission by Frédéric Joliot-Curie,
Hans von Halban and Lew Kowarski.
The fission chain reaction
"Chain reactions" at that time were a
known phenomenon in chemistry, but the analogous process
in nuclear physics, using neutrons, had been foreseen as early as
1933 by Szilárd, although Szilárd at that time had no idea with
what materials the process might be initiated (Szilárd thought it
might be done with light neutron-rich elements). Szilárd, a
Hungarian born Jew, also fled mainland Europe after Hitler's rise,
eventually landing in the US.
With the news of fission neutrons from uranium fission, Szilárd
immediately understood the possibility of a nuclear chain reaction
using uranium. In the summer, Fermi and Szilard proposed the idea
of a nuclear reactor (pile) to
mediate this process. The pile would use natural uranium as fuel,
and graphite as the moderator of neutron energy (it had previously
been shown by Fermi that neutrons were far more effectively
captured by atoms if they were moving slowly, a process called
moderation when the neutrons were slowed after being
released from a fission event in a nuclear reactor).
In August Hungarian-Jewish refugees Szilard, Teller and Wigner
thought that the Germans might make use of the fission chain
reaction. They decided to warn President Roosevelt of this possible
German menace, and persuaded German-Jewish refugee Albert Einstein to lend his name. The
Einstein–Szilárd
letter letter suggested the possibility of a uranium bomb
deliverable by ship, which would destroy "an entire harbor and much
of the surrounding countryside." The President received the letter
on 11 October 1939 — shortly after World War II began in
Europe, but two years before U.S. entry into it.
In England, James Chadwick proposed
an atomic bomb utilizing natural uranium, based on a paper by
Rudolf Peierls with the mass needed
for critical state being 30–40 tons. In America, J. Robert
Oppenheimer thought that a cube of uranium deuteride 10 cm on
a side (about 11 kg of uranium) might "blow itself to hell."
In this design it was still thought that a moderator would need to
be used for nuclear bomb fission (this turned out not to be the
case if the fissile isotope was separated).
In December, Heisenberg delivered a report to the German Ministry
of War on the possibility of a uranium bomb.
In Birmingham, England, Frisch teamed up with Rudolf Peierls, another German-Jewish
refugee. They had the idea of using a purified mass of the uranium
isotope (235U). They determined that an enriched
235U bomb could have a critical mass of only
600 grams, instead of tons, and that the resulting explosion
would be tremendous. (The amount actually turned out to be
15 kg, although several times this amount was used in the
actual uranium (Little
Boy
) bomb). In February 1940 they delivered the
Frisch–Peierls
memorandum. Ironically, they were still officially considered
"enemy aliens" at the time.
Glenn Seaborg, Joe Kennedy, Art
Wahl and Italian-Jewish refugee Emilio Segrè shortly discovered
239Pu in the decay products of 239U produced
by bombarding 238U with neutrons, and determined it to
be fissionable like 235U.
On June 28, 1941, the Office of Scientific Research and Development
was formed in the U.S. to mobilize scientific resources and apply
the results of research to national defense. In September, Fermi
assembled his first nuclear "pile" or reactor, in an attempt to
create a slow neutron induced chain reaction in uranium, but the
experiment failed for lack of proper materials, or not enough of
the materials which were available.
Producing a fission chain reaction in natural uranium fuel was
found to be far from trivial. Early nuclear reactors did not use
isotopically enriched uranium, and in consequence they were
required to use large quantities of highly purified graphite as
neutron moderation materials. Use of ordinary water (as opposed to
heavy water) in nuclear reactors
requires enriched fuel — the partial separation and relative
enrichment of the rare 235U isotope from the far more
common 238U isotope. Typically, reactors also require
inclusion of extremely chemically pure neutron moderator materials such as
deuterium (in heavy
water), helium, beryllium, or carbon, the latter usually as
graphite. (The high purity for carbon is
required because many chemical impurities such as the boron-10 component of natural boron, are very strong neutron absorbers and thus
poison the chain reaction.)
Production of such materials at industrial scale had to be solved
for nuclear power generation and weapons production to be
accomplished. Up to 1940, the total amount of uranium metal
produced in the USA was not more than a few grams, and even this
was of doubtful purity; of metallic beryllium not more than a few
kilograms; concentrated deuterium oxide (heavy water) not more than a few kilograms.
Finally, carbon had never been produced in quantity with anything
like the purity required of a moderator.
The problem of producing large amounts of high purity uranium was
solved by Frank Spedding using the
thermite process. Ames
Laboratory
was
established in 1942 to produce the large amounts of natural
(unenriched) uranium metal that would be necessary for the research
to come. The critical nuclear chain-reaction success
of the Chicago
Pile-1
(December 2, 1942) which used unenriched (natural)
uranium, like all of the atomic "piles" which produced the
plutonium for the atomic bomb, was also due specifically to
Szilard's realization that very pure graphite could be used for the
moderator of even natural uranium "piles". In wartime
Germany, failure to appreciate the qualities of very pure graphite
led to reactor designs dependent on heavy water, which in turn was
denied the Germans by Allied attacks in Norway, where heavy water was produced. These difficulties
prevented the Nazis from building a nuclear reactor capable of
criticality during the war.
For more detail on the early development of the first nuclear reactors and nuclear weapons, see Manhattan Project.
References
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