A history of the world's most dangerous element
by Jeremy Bernstein
Excerpt of Chapter IX
Plutonium is generated in a two-step process
that begins with the capture of a neutron by a uranium-238 nucleus.
But there will also be some uranium fission going on. This means
that fission fragments will be created, but there are also impurities
in the reactor fuel, elements such as boron. Plutonium decays by the
emission of alpha particles. When these alpha particles collide with
an element like boron, neutrons are produced. The question Seaborg
posed was, What concentration of these impurities could one toler-
ate so that the neutrons produced in such collisions did not generate
a fizzle? His calculations showed that the concentration of impurity
would have to be reduced to something like one part in 100 billion.
Both General Groves and Oppenheimer were so informed. To
make things worse, the British had realized the same thing, but
their calculations showed that the impurity concentration would
have to be reduced by another factor of 10. A discussion ensued of
how such purity could be achieved, and the conclusion was reached
only with great difficulty. Before anything was decided, samples of
reactor-produced plutonium began to arrive at Los Alamos. As I
explain shortly, these samples raised a new problem, which was of
such a serious character that it definitely ruled out a gun-assembly
plutonium weapon. The impurity question turned out to be, more
or less, irrelevant.
Meanwhile, William Zachariasen had begun the study of the structure of
plutonium and its compounds that he would pursue throughout the
war and, indeed, afterwards. In a report written in 1946, he noted,
"For the past three years within the plutonium project, I carried out
partial or complete crystal structure determinations of 140 different
compounds of plutonium, neptunium, uranium, thorium or rare
earth elements. My collaborator Dr. Rose Mooney made similar
determinations of an additional 20 compounds." Nearly all of the
plutonium compounds were studied using X-ray diffraction, and even
as late as 2005, Zachariasen's work on the structures of the oxides
of these elements in these three years represented more than half of
the total output of everyone else. As he later noted, "I remember
working like heil on New Year's Day and all holidays; often I worked
late for many, many hours to get the work done. I had a wonderful
One of Zachariasen's early discoveries was that plutonium
has "allotropes." Allotropes are different crystal structures of the same
element. The canonical example is carbon. Depending on how it has
been treated, carbon can manifest itself, for example, as graphite or
diamond. Allotropes are different from what we usually call "phases,"
which refer to whether the element is found in a liquid or a solid
state, for example. Nonetheless, you will frequently find the term
"phase" used for different allotropes. I also use it, from time to time,
since I can't think of a better term.
The first two allotropes of plutonium that Zachariasen found
were labeled with the Greek letters
α and δ.
allotropes by Greek letters in order of the increasing temperatures at
which the allotrope is question is stable.
For plutonium, stability is a relative concept since it does not take much
of a jar to cause an allotropic transformation. When he first discovered
these allotropes, Zachariasen did not know there were four more.
The full labeling is
α, β, γ, δ, δ' and ε.
We can worry about the rest
later and concentrate here on
α and δ.
The first thing to emphasize
is that an allotrope is not a property of a single atom. A plutonium
atom is a plutonium atom is a plutonium atom. If you have seen one,
you have seen them all. It is rather a property of the crystal structures
that can be built out of these atoms. It is these structures that are, or
are not, stable in a given temperature range.
Let's begin with the
It is stable up to a temperature of
122 °C. This means that it is stable at room temperature. Zachariasen
used X-ray diffraction to find the structure of its unit cell. While
crystals can be exceedingly complex and unique
they are nonetheless built out of a limited number of unit
In the case of snowflakes they are hexagonal.
, which you
turned out to be "monoclinic"
a crystal structure in which all of the axes in the unit cell
are not perpendicular to each other and may have different lengths.
The 16-atom unit cell for
perversely complicated. Moreover, it has less symmetry and hence
little plasticity or pliability. Thus if you tried to bend
metal, it would break like a piece of chalk. It behaves more like
a mineral than a metal. On the other hand, the
something else. It is stable between 317 and 453 °C and has a nice
symmetrie unit cell, what the crystallographers call a face-centered
. There are eight atoms in the corners and six in
the center of each face, making 4 in all. We can imagine displacing
this structure along a plane and preserving it. Indeed,
is as malleable as an ordinary metal, perfect for making into a bomb,
except for the fact that at lower temperatures it readily morphs into
presenting a much greater engineering challenge. The
densities are interesting. At 25 °C, the
density is 19.86 grams
per cubic centimeterâ
very dense indeed
while at 320 °C, the
density is 15.92 grams per cubic centimeter. The strange
results that Zachariasen first found for the densities are explained by
the mixture of different phases. Clearly, if you intend to use metallic
plutonium to make a bomb, you will be confronted with a very significant
metallurgical challenge. But worse is to come.
DuPont had been contracted to construct the production reactors.
After Seaborg pointed out the impurity problem, there was some
reluctance to proceed. However, once General Groves had decided to
do something, it was next to impossible to stand in the way. Thus,
beginning in February 1943, construction started on a pilot project
located near Clinton, Tennessee what later became Oak Ridge. It
was designed to use the bismuth phosphate method of separation
that had been developed at the Met Lab. The Oak Ridge reactor
went critical in November and by April 1944, it was shipping grams
of plutonium to Los Alamos; but it soon became clear that a disaster
had occurred. To understand the issue let us review how plutonium
is produced in a reactor.
The basic fuel in these reactors was natural uranium, more than
99 percent uranium-238, the rest being mainly the fissile isotope
uranium-235. To enhance fission reactions, the neutrons created in
fission are slowed down by a moderator in this case, highly purified
graphite, the same moderator that Fermi had used in his reactor. But
some of the neutrons are absorbed by uranium-238 nuclei, producing
neptunium-239, which beta-decays to plutonium-239. To get
a substantial yield of plutonium-239, the reactor must be allowed
to run for a reasonable amount of time. The longer the reactor is
allowed to run before plutonium is separated from uranium, the
more plutonium you get. However, while plutonium-239 remains in
the reactor, it can absorb another neutron and become plutonium-240;
but this isotope of plutonium spontaneously fissions, producing
fast neutrons. There is now a balancing question, How much
plutonium-240 can you tolerate without producing a weapon that
The fact that plutonium-240 would be produced was already
known from the cyclotron production of plutonium. However,
there was so little material to work with that measurements of the
occurrence of this isotope were ambiguous. But now there were
gram quantities, and Emilio Segrè was given the job of measuring the rate
of spontaneous fission caused by the plutonium-240 in the sample
they had. By late spring, Segrè reported that the spontaneous fission
rate for this sample was at least five times as high as had been
observed for the cyclotron-produced plutonium. By July 4 it had
become clear that the gun-assembly method was not going to work
for plutonium. It was just too slow. Neutrons would trigger a chain
reaction before the material became supercritical. There was also a
spontaneous fission issue for uranium-238, but in a bomb like Little
Boy, some 90 percent of the material would be uranium-235,which
had a spontaneous fission rate that was some 1200 times lower. This
is why the gun-assembly method worked for uranium. There was
no realistic way of separating plutonium-239 from plutonium-240.
They differed by one mass unit, while uranium-235 and uranium-238
differed by three, which makes a huge difference when you are
trying to separate isotopes. My guess is that if the people working on
the bomb had not been persuaded that they were in a desperate race
with the Germans, and if General Groves had not shared this obsession,
the project might have stopped right there and then. As it was,
Oppenheimer got discouraged and considered resigning as directer
of Los Alamos. He didn't, but now the laboratory faced up to the
two problems: metallurgy and assembly. I will begin with metallurgy.
Enter into our story Cyril Stanley Smith
Smith was born in Birmingham, England, in 1903. He got a
degree in metallurgy from the University of Birmingham in 1924
and then a doctor of science from MIT in 1926. A year later, he
began working at the American Brass Company in Connecticut's
Naugatuck Valley. There he remained until the war, at which time
he went to work for the War Metallurgy Committee in Washington, D.C.
In February of 1943, while attending a meeting of
the American Institute of Mining, Metallurgical, and Petroleum
Engineers in New York, he was approached by the chemist Joseph
Kennedy, who was one of Seaborg's collaborators in the discovery of
plutonium and had been recruited to go to Los Alamos to head up
its newly formed chemistry department. It is not clear why Kennedy
contacted Smith in particular, although Smith had published a substantial
amount of work and held several patents. It is also not clear
what Kennedy could have told Smith about what would be going on
at Los Alamos because Smith had no clearance. However, he told him
enough, so that Smith saw going to Los Alamos as a way of escaping
a desk job in Washington for which, as he later recalled, he had a
Not long after Smith's encounter with Kennedy,
Oppenheimer had a recruiting talk with Smith on a park bench in
Washington. Oppenheimer was very good at this sort of thing. By
March of 1943, Smith was among the first group of scientists at Los
Alamos. He was put in charge of creating a metallurgy group, without
a clear idea of how big a job this was going to be. His first job was to
find metallurgists who were not otherwise engaged in the war effort.
This was not an easy task, but by 1945 when the war ended, he was
running a department with 115 people in it. One of the difficulties in
recruitment was that neither Smith nor anyone else, in the beginning,
knew what such a department was supposed to do.
It was decided that the Los Alamos metallurgical group would
not work on plutonium until gram samples arrived from the reactors,
so they did various odd jobs, such as studying the properties of
compounds of uranium with hydrogen. One of the oddest arose
out of a request to take 620 pounds of gold and cast it into two
hemispheres. Later Smith found one of the hemispheres being used
as a doorstop. Once the plutonium began arriving at Los Alamos in
half-gram lots in March 1944, the work to make a usable metal of it
began in earnest. The first assumption was that its chemistry must be
like that of uranium, because by this time it was understood how to
make uranium into a metal: You began with uranium tetrafluoride
(UF4) and took advantage of the fact that if you heated it in the
presence of calcium (Ca), the calcium would be more attractive to
the fluorine than uranium would and you would induce the reaction
UF4 + 2Ca > U + 2CaF2, leaving uranium metal and calcium
difluoride. Calcium here has acted as what is called a reducing agent.
This sort of reaction was the way in which the micrograms of metallic
plutonium that Zachariasen had been using had been made. This
work was being done at the Met Lab by two young metallurgists
and Nick Dallas. At the end of 1943, Magel
and Dallas were producing one-gram buttons of pure uranium metal
from uranium fluoride. By early 1944, Oppenheimer had persuaded
the Met Lab to relinquish Magel and Dallas, who arrived in Smith's
group bearing their Met Lab equipment, which included a centrifuge.
They had performed the uranium reduction in a centrifuge, which
would then separate out the metal. They planned to do the same
thing for plutonium. Later it turned out that a better idea was to use
what was known as a stationary "bomb," a crucible specially lined so
that it could contain the plutonium compounds. But Magel liked the
thrill of the centrifuge; Smith referred to this approach as "excited,
energetic, but slightly slap-dash." Prior to the arrival of Magel and
Dallas, the Los Alamos people, using their experience with uranium,
tried to reduce plutonium trifluoride with calcium. They got what
has been described as a "grayish cokey mass containing no agglomerated
plutonium." Then Magel and Dallas got into the act.
Magel's 1995 description of what occurred may be in the
se non è vero è ben trovato
category, but it is very amusing to read.
The reduction of a gram quantity of plutonium was considered
a very big deal, because that amount of metal would allow much
improved measurements of many crucial material properties. The
reduction was supposed to take place on March 24, 1944, and
General Groves and several top administrators had been specially
invited to observe us as we did it.
Well, when does everything go wrong when you have a whole
lot of observers, right? So on the 23rd I said to Nick [Dallas], "Let's go
up to the lab and make the reduction tonight before all these people
get here." Nick agreed, and we carried out the reduction using the
hot-centered centrifuge bomb method. When it was done, we cut open
the bomb, dropped the little button of plutonium metal in a glass vial
and put it on Cyril Smith's desk with a note that read:
Here is your button of plutonium. We have gone to Santa Fe for the day.
Everyone was pretty mad at us and claimed that we had contaminated
the lathe and the back shop, when we opened the bomb
to retrieve the plutonium button. I don't believe that we had, but I
understood how they feit. In any case, once they had the button, they
immediately started measurements of the density and so forth ....
Magel and Dallas had produced the first sample
metallic plutonium that could be seen without the aid of a microscope.
It enabled measurements of the allotropic phases.
The thing to note here is that there are two
δ and δ',
in which the volume
when the temperature
is raised. This is totally counterintuitive and is another example of
just how bizarre an element plutonium is.
There were diagrams
during the war hut they were
rather rough-hewn. Most of the details were filled in after the war. To
give some idea, in 1958 a Russian chemist named Eugenii Makarov
published what became a standard text on the crystal chemistry of
uranium, thorium, plutonium, and neptunium. It was translated and
published in the United States the following year.
Even in 1958,
as their text makes clear, the crystal structure of the
unknown. The next year, Zachariasen showed that it had a similar
structure as the
During the war, anything that involved
simple scientific curiosity was put aside if it did not contribute to
making the bomb. One of the essential things that using these gram
samples of metallic plutonium enabled the Los Alamos people to do
was to measure the melting point of plutonium: the temperature at
which the metal melts. The early Met Lab experiments gave results
that seemed to be consistent (inconsistent? HJS) with the kind of temperatures one finds
for other metals. To give a few examples; iron melts at 1510 °C, while
steel melts at 1370 °C, copper at 1083 °C. On the other hand, it was
discovered that plutonium metal melts at 640 °C, an extraordinarily
low temperature, which you had better know if you are going to use
plutonium metal for something.
After making the first gram, Magel and Dallas made eight more
grams of the superpure plutonium that was thought to be required
for a gun-assembly weapon. But once it became clear that such a
weapon was impossible, superpure plutonium was no longer needed,
so they were out of a job. They decided to leave Los Alamos and join
a small Manhattan Project group at MIT.
© Jeremy Bernstein, 2007.