Plutonium

Plutonium is a trans-uranic actinide element with atomic number 94. It is a metal of silver
look that oxidizes when it is in contact with air. Here it forms a dark oxide coating. Plutonium
also produces a significant amount of heat from radioactive decay.

The most important isotope of plutonium is plutonium-239. It has a half-life of 24,100 years.
The plutonium-239 isotope can be used for nuclear weapons. Plutonium-239 is fissile which
means it can sustain a nuclear fission chain reaction. This leads to applications for nuclear
weapons and nuclear reactors.

Fission of a kilogram of plutonium-239 can produce an explosion which is equal to many
thousands of tons of TNT. This huge energy which can be released makes plutonium-239
useful for nuclear weapons. The nuclear properties of plutonium-239, and the ability to
produce large volumes of almost-pure Pu-239, led to its use in nuclear weapons and nuclear
power stations. Of all the nuclear fuels, Pu-239 has the smallest critical mass.

Normally Pu-239 is produced in nuclear reactors as a result of transmutation of the uranium-238
isotope, which is found in the nuclear fuel rods. U-238 is bombarded with neutrons and
undergoes neutron capture to form U-239. This decays into neptunium-239 and then it
decays to form Pu-239.

When Pu-239 is produced in nuclear reactors, other isotopes of plutonium are also formed.
The presence of a large amount of contaminating Pu-240 restricts the potential for use in a
nuclear bomb, because Pu-240 has a relatively high spontaneous fission rate. This raises the
neutron levels in the background and thus increases the risk of predetonation of the weapon
(it starts the nuclear fission chain reaction too early).

Plutonium is classified according to the percentage of the contaminating plutonium-240 that
it contains:

  • Super-grade 2-3%
  • Weapons-grade < 7%
  • Fuel-grade 7-18%
  • Reactor-grade > 18%

The production of plutonium in sufficient quantities for the first time was a big part of the
Manhattan Project during World War II. The first nuclear test conducted by the Americans
called "Trinity" (July 1945) used a plutonium-239 core. The atomic bomb dropped on
Nagasaki (August 1945) called "Fat Man" also had a core of plutonium-239. Radiation
experiments on humans to study plutonium were conducted without the agreement of
participants, and a number of criticality accidents, of which some were lethal, occurred
during and after the war. The disposal of plutonium waste from nuclear power plants and
disassembled nuclear weapons which were built during the Cold War is a concern in the
context of nuclear-proliferation and the environment.

The American atomic bomb "Fat Man" used a plutonium core which had only 0.9% Pu-240.
However, modern American nuclear weapons have weapons-grade plutonium with around
6.5% Pu-240. A lower content of Pu-240 is not necessary for a nuclear weapon to function
correctly and would increase the cost of production. Plutonium produced in nuclear reactors
can vary in composition, but the isotope profile is mostly similar. This table shows
representative plutonium compositions for both weapons-grade and reactor-grade plutonium:

Pu Grade % Pu-238 % Pu-239 % Pu-240 % Pu-241 % Pu-242
Weapons-grade 0.0 92.8 6.5 0.7 0.0
Reactor-grade 2.0 61.0 24.0 10.0 0.3

The following graph shows how the values for critical mass change for plutonium of different
compositions. The critical mass reduces as the concentration of Pu-239 is increased.

Metallurgy

Plutonium oxidizes easily and corrodes very quickly where there are even low levels of
moisture. It has six distinct phases (allotropes) in the solid form. This is more than any other
element. These allotropes, which are different structural modifications of an element, have
very similar internal energies. However they differ to a large degree in densities and crystal
structure. Plutonium is thus very sensitive with regard to changes in temperature, pressure,
or chemistry, and permits dramatic changes in volume after phase transitions from one
allotrope to another. The densities of the different phases differ from approximately 16 g cm-3
to 20 g cm-3.

Phase Density / g cm -3 Stability Range / °C
Alpha 19.84 Stable below 122
Beta 17.80 122-206
Gamma 17.20 224-300
Delta/delta prime 15.90 319-476
Epsilon 17.00 476-641 (melting point)
Liquid 16.65 641 - boiling point

At room temperature pure plutonium metal exists in a crystal structure called the alpha
phase. In the alpha phase the plutonium crystal structure is at its maximum density (19.8 g cm-3).
The atoms in the alpha phase are essentially covalently bonded (as opposed to
metallically bonded). This gives it physical properties which are more like a mineral than a
metal, making it hard, strong, and brittle. Normal metal fabrication techniques cannot be
used on the alpha phase. The alpha phase changes readily to the beta phase at temperatures
slightly higher than room temperature, becoming plastic and malleable.

The delta phase is the lowest density phase (15.9 g cm-3).In this phase plutonium is also
quite malleable, as well as the gamma phase. Delta phase plutonium has metallic properties
and is extremely ductile. It is strong and malleable like aluminum. This means that it is
simple to form and machine it. Although the delta phase unusually shrinks when it is heated,
the negative coefficient of expansion is not large. Plutonium in the delta phase is only
slightly stable. It usually collapses to the more dense alpha phase at very low pressures.

Because of the presence of so many different phases, it can be very difficult to machine
plutonium as it changes state very easily.

It is possible to stabilize plutonium in the delta phase at room temperature by means of
alloying it with certain metals such as gallium, aluminum, cerium, indium, scandium, and
americium at concentrations of a few mol % (% of atoms that are the alloying agent). This
permits its welding in weapons and stops low temperature phase changes after fabrication.
The 3% gallium alloy was used in the American "Fat Man" atomic bomb. The explosive shock
waves used to compress the plutonium core in a nuclear weapon will cause a phase
transition from delta to the alpha phase. This helps to achieve supercriticality.

Toxicity

Plutonium is a highly toxic heavy metal and actively emits alpha radiation. Ingested or
inhaled plutonium is extremely dangerous, because it irradiates internal organs with alpha
radiation. Ingestion or inhalation of large amounts can cause acute radiation poisoning and
death. When it is inhaled, plutonium can pass into the bloodstream. When it is in the
bloodstream, plutonium moves through the body and into the bones, liver, or other organs.
Generally plutonium that reaches body organs stays in the body during decades and
continues to expose the tissue around it to radiation. It thus may cause cancer. Isotopes of
plutonium accumulate in the bone marrow.