Nuclear fission

In physics, fission is a nuclear process in which a heavier unstable nucleus
divides or splits into two or more lighter nuclei, with the release of
substantial amounts of energy. When a free neutron of the proper energy is
captured by the nucleus of a fissionable atom, the resulting unstable
nucleus will split producing two or more fission products (atomic nuclei of
different elements formed from the protons and neutrons originally
comprising the nucleus before its fission), two or three free neutrons and a
tremendous amount of energy.

Atomic nuclei are made up of neutrons and protons. The number of protons is
equal to the atomic number, Z. The number of neutrons, N, is equal to the
difference between the atomic mass number, A, and the atomic number. There
are two sets of forces acting on these particles, ordinary electric Coulomb
forces of repulsion between the positive charges and the very short range
strong nuclear forces which bind the particles of a nucleus together. The

combined effects of these attractive and repulsive forces are such that only certain
combinations of neutrons and protons are stable. If the neutrons and protons are few
in number, stability occurs when their numbers are about equal. For larger nuclei, the
proportion of neutrons required for stability is greater. Finally, at the
end of the periodic table, where the number of protons is over 90 and the
number of neutrons nearly 150, there are no completely stable nuclei. (Some
of the heavy nuclei are almost stable as evidenced by very long half-lives.)
If an unstable nucleus is formed artificially by adding an extra neutron or
proton, eventually a change to a stable form occurs. This is not
accomplished by ejecting a proton or a neutron but by ejecting a positron or
an electron; within the nucleus a proton converts itself into a neutron and
positron (or a neutron converts itself into a proton and electron), and the
light charged particle is ejected. In other words, the mass number remains
the same but the atomic number changes. The stability conditions are not
very critical so that for a given mass number, i.e., given total number of
protons and neutrons, there may be several stable arrangements of protons
and neutrons (at most three or five) giving several isobars. For a given
atomic number, i.e., given number of protons, conditions can vary still more
widely so that some of the heavy elements have as many as ten or twelve
stable isotopes.

It is a general principle of physics that work must be done on a stable
system to break it up. Thus, if an assemblage of neutrons and protons is
stable, energy must be supplied to separate its constituent particles. If
energy and mass are equivalent then the total mass of a stable nucleus
should be less than the total mass of the separate protons and neutrons that
go to make it up. This mass difference, then, should be equivalent to the
energy required to disrupt the nucleus completely, which is called the
binding energy.

Most atomic nuclei can be penetrated by at least one type of atomic
projectile (or by gamma radiation). Any such penetration may result in a
nuclear rearrangement in the course of which a fundamental particle is
ejected or radiation is emitted or both. The resulting nucleus may be one of
the naturally available stable species, or - more likely - it may be an atom
of a different type which is radioactive, eventually changing to still a
different nucleus. This may in turn be radioactive and, if so, will again
decay. The process continues until all nuclei have changed to a stable type.
There are two respects in which these artificially radioactive substances
differ from the natural ones: many of them change by emitting positrons
(unknown in natural radio-activity) and very few of them emit alpha
particles. In every one of the cases where accurate measurements have been
made, the equivalence of mass and energy has been demonstrated and the
mass-energy total has remained constant. (Sometimes it is necessary to
invoke neutrinos to preserve mass-energy conservation.)

The principle of operation of both nuclear weapons and nuclear reactors is
that a nuclear chain reaction must occur. If one neutron causes a fission
that produces more than one new neutron, the number of fissions may increase
tremendously with the release of enormous amounts of energy. It is a
question of probabilities. There are four possible outcomes of a neutron
produced in the fission process:

  1. escape entirely from the fissionable material
  2. non-fission capture by fissionable material
  3. non-fission capture by nonfissionable impurities
  4. fission capture

If the loss of neutrons by the first three processes is less than the
surplus produced by the fourth, the chain reaction occurs; otherwise it does
not. This is often expressed numerically as the neutron multiplication
factor, k, where:

     k = f - l

With f being the average number of neutrons undergoing fission capture, and
l represents the average sum of neutron loss mechanisms 1, 2, and 3 above.
Any one of the first three loss processes may have such a high probability
in a given arrangement that the extra neutrons created by fission will be
insufficient to keep the reaction going. For example, should it turn out
that the non-fission capture by uranium has a much higher probability than
fission capture, there would presumably be no possibility of achieving a
chain reaction. If the number of neutrons causing fissions is decreasing
with time, the reactor is called subcritical. If the number is constant with
time, the reactor is called critical, and if the number is increasing with
time, it is called supercritical. The criticality state of the reactor is
represented by Keff (K-effective), where Keff is the number of neutrons in
one generation divided by the number of neutrons in the previous generation.

Critical Mass

The relative number of neutrons which escape from a quantity of uranium can
be minimized by changing the size and shape. In a sphere any surface effect
is proportional to the square of the radius, and any volume effect is
proportional to the cube of the radius. Now the escape of neutrons from a
quantity of uranium is a surface effect depending on the area of the
surface, but fission capture occurs throughout the material and is therefore
a volume effect. Consequently the greater the amount of uranium, the less
probable it is that neutron escape will predominate over fission capture and
prevent a chain reaction. Loss of neutrons by non-fission capture is a
volume effect like neutron production by fission capture, so that increase
in size makes no change in its relative importance.

The critical size of a device containing uranium is defined as the size for
which the production of free neutrons by fission is just equal to their loss
by escape and by non-fission capture. In other words, if the size is smaller
than critical, then by definition no chain reaction will sustain itself.

Moderators

Thermal neutrons have the highest probability of producing fission of U-235
but the neutrons emitted in the process of fission have high speeds (they
are not thermal). It is an oversimplification to say that the chain reaction
might maintain itself if more neutrons were created by fission than were
absorbed, because the probability both of fission capture and of non-fission
capture depends on the speed of the neutrons. Unfortunately, the speed at
which non-fission capture is most probable is intermediate between the
average speed of neutrons emitted in the fission process and the speed at
which fission capture is most probable.

For some years before the discovery of fission, the customary way of slowing
down neutrons was to cause them to pass through material of low atomic
weight, such as hydrogenous material. The process of slowing down or
moderation is simply one of elastic collisions between high speed particles
and particles practically at rest. The more nearly identical the masses of
neutron and struck particle, the greater the loss of kinetic energy by the
neutron. Therefore light elements are most effective as nuetron moderators.

It occurred to a number of physicists that it might be possible to mix
uranium with a moderator in such a way that the high speed fission neutrons,
after being ejected from uranium and before re-encountering uranium nuclei,
would have their speeds reduced below the speeds for which non-fission
capture is highly probable. The characteristics of a good moderator are that
it should be of low atomic weight and that it should have little or no
tendency to absorb neutrons. Lithium and boron are excluded on the latter
count. Helium is difficult to use because it is a gas and forms no
compounds. The choice of moderator therefore lay among hydrogen, deuterium,
beryllium, and carbon. Even now no one of these substances can be excluded
from the list of practical possibilities. It was Enrico Fermi and Leo
Szilard who first proposed the use of graphite (a form of carbon) as a
moderator for a chain reaction.

REDUCTION OF NON-FISSION CAPTURE BY ISOTOPE SEPARATION

An additional complication is that natural uranium contains three isotopes:
U-234, U-235, and U-238, present to the extent of approximately 0.006, 0.7,
and 99.3 per cent, respectively. We have already seen that the probabilities
of processes (2)and (4) are different for different isotopes. We have also
seen that the probabilities are different for neutrons of different energies.

For neutrons of certain intermediate speeds (corresponding to energies of a
few electron volts) U-238 has a large capture cross section for the
production of U-239 but not for fission. There is also a considerable
probability of inelastic (i.e., non-capture-producing) collisions between
high speed neutrons and U-238 nuclei. Thus the presence of the U-238 tends
both to reduce the speed of the fast neutrons and to effect the capture of
those of moderate speed. Although there may be some non-fission capture by
U-235, it is evident that if we can separate the U-235 from the U-238 and
discard the U-238, we can reduce non-fission capture and can thus promote
the chain reaction. In fact, the probability of fission of U-235 by high
speed neutrons may be great enough to make the use of a moderator
unnecessary once the U-238 has been removed. Unfortunately, U-235 is present
in natural uranium only to the extent of about one part in 140. Also, the
relatively small difference in mass between the two isotopes makes
separation difficult. Nevertheless, the possibility of separating U-235 was
recognized early on in the Manhattan Project as being of the greatest importance.

PRODUCTION AND PURIFICATION OF MATERIALS

It has been stated above that the cross section for capture of neutrons
varies greatly among different materials. In some it is very high compared
to the maximum fission cross section of uranium. If, then, we are to hope to
achieve a chain reaction, we must reduce effect (3) - non-fission capture by
impurities -to the point where it is not serious. This means very careful
purification of the uranium metal and very careful purification of the
moderator. Calculations show that the maximum per-missible concentrations of
many impurity elements are a few parts per million- in either the uranium or
the moderator. When it is recalled that up to 1940 the total amount of
uranium metal produced in this country was not more than a few grams and
even this was of doubtful purity, that the total amount of metallic
beryllium produced in this country was not more than a few pounds, that the
total amount of concentrated deuterium pro-duced was not more than a few
pounds, and that carbon had never been produced in quantity with anything
like the purity required of a moderator, it is clear that the problem of
producing and purifying materials was a major one.

Control - Weapons or Power?

The problems that have been discussed so far have to do merely with the
realization of the chain reaction. If such a reaction is going to be of use,
we must be able to control it. The problem of control is different depending
on whether we are interested in steady production of power or in an
explosion. In general, the steady production of atomic power requires a slow
-neutron-induced fission chain reaction occurring in a mixture or lattice of
uranium and moderator, while an atomic bomb requires a fast-neutron-induced
fission chain reaction in U-235 or Pu-239, although both slow- and
fast-neutron fission may contribute in each case. It seemed likely, even in
1940, that by using neutron absorbers a power chain reaction could be
controlled. It was also considered likely, though not certain, that such a
chain reaction would be self-limiting by virtue of the lower probability of
fission-producing capture when a higher temperature was reached.
Nevertheless, there was a possibility that a chain-reacting system might get
out of control, and it therefore seemed necessary to perform the
chain-reaction experiment in an uninhabited location

Up to this point we have been discussing how to produce and control a
nuclear chain reaction but not how to make use of it. The technological gap
between producing a controlled chain reaction and using it as a large-scale
power source or a explosive is comparable to the gap between the discovery
of fire and the manufacture of a steam locomotive.

Although production of power has never been the principal object of this
project, enough attention has been given to the matter to reveal the major
difficulty: the attainment of high-temperature operation. An effective heat
engine must not only develop heat but must develop heat at a high
temperature. To run a chain-reacting system at a high temperature and to
convert the heat generated to useful work is very much more difficult than
to run a chain-reacting system at a low temperature.

Of course, the proof that a chain reaction is possible does not itself
ensure that nuclear energy can be effective in a bomb. To have an effective
explosion it is necessary that the chain reaction build up extremely
rapidly; otherwise only a small amount of the nuclear energy will be
utilized before the bomb flies apart and the reaction stops. It is also
necessary that no premature explosion occur. This entire "detonation"
problem was and still remains one of the most difficult problems in
de-signing a high-efficiency atomic bomb.

Three ways of increasing the likelihood of a chain reaction have been
mentioned: use of a moderator; attainment of high purity of materials; use
of special material, either U-235 or Pu. The three procedures are not
mutually exclusive, and many schemes have been proposed for using small
amounts of separated U-235 or PU-239 in a lattice composed primarily of
ordinary uranium or uranium oxide and of a moderator or two different
moderators. Such proposed arrangements are usually called "enriched piles."

History

The process was discovered in 1939 by Otto Hahn, Lise Meitner and coworkers

The results of the bombardment of uranium by neutrons had proved interesting
and puzzling. First studied by Fermi and his colleagues in 1934, they were
not properly interpreted until several years later

On January 16, 1939, Niels Bohr of Copenhagen, Denmark, arrived in this
country to spend several months in Princeton, N. J., and was particularly
anxious to discuss some abstract problems with Albert Einstein. (Four years
later Bohr was to escape from Nazi-occupied Denmark in a small boat.) Just
before Bohr left Denmark two of his colleagues, O. R. Frisch and L. Meitner
(both refugees from Germany), had told him their guess that the absorption
of a neutron by a uranium nucleus sometimes caused that nucleus to split
into approximately equal parts with the release of enormous quantities of
energy, a process that soon began to be called nuclear "fission."

The occasion for this hypothesis was the important discovery of Otto Hahn
and Fritz Strassmann in Germany (published in Naturwissenschaften in early
January 1939) which proved that an isotope of barium was produced by neutron
bombardment of uranium. Immediately on arrival in the United States, Bohr
communicated this idea to his former student J. A. Wheeler and others at
Princeton University, and from them the news spread by word of mouth to
neighboring physicists including Enrico Fermi at Columbia University. As a
result of conversations among Fermi, J. R. Dunning, and G. B. Pegram, a
search was undertaken at Columbia for the heavy pulses of ionization that
would be expected from the flying fragments of the uranium nucleus. On
January 26, 1939, there was a conference on theoretical physics at
Washington, D. C., sponsored jointly by the George Washington University and
the Carnegie Institution of Washington.

Fermi left New York to attend this meeting before the Columbia fission
experiments had been tried. At the meeting Bohr and Fermi discussed the
problem of fission, and in particular Fermi mentioned the possibility that
neutrons might be emitted during the process. Although this was only a
guess, its implication of the possibility of a chain reaction was obvious. A
number of sensational articles were published in the press on this subject.
Before the meeting in Washington was over, several other experiments to
confirm fission had been initiated, and positive experimental confirmation
was reported from four laboratories (Columbia University, Carnegie
Institution of Washington, Johns Hopkins University, University of
California) in the February 15, 1939, issue of the Physical Review. By this
time Bohr had heard that similar experiments had been made in his laboratory
in Copenhagen about January 15. (Letter by Frisch to Nature dated January
16, 1939, and appearing in the February 18 issue.) FrŽdŽric Joliot in Paris
had also published his first results in the Comptes Rendus of January 30,
1939. From this time on there was a steady flow of papers on the subject of
fission, so that by the time (December 6, 1939) L. A. Turner of Princeton
wrote a review article on the subject in the Reviews of Modern Physics
nearly one hundred papers had appeared. Complete analysis and discussion of
these papers have appeared in Turner's article and elsewhere.