Plutonium

Plutonium (chemical symbol Pu) is a chemical element with atomic number 94. In nature it has been detected in trace quantities, but only after it had been prepared in the laboratory by Glen Seaborg, Edwin McMillan, Joseph W. Kennedy, and Arthur C. Wahl in early 1941. All isotopes of the element are radioactive, they are α-emitters, except for  Pu-241, which is a β⁻-emitter.

Preparation

The very first preparation of plutonium was the 238 isotope. This was produced by colliding (in a cyclotron) uranium-238 (²³⁸U) with deuterons giving neptunium (²³⁸Np) and neutrons (n). The Np-isotope decays to plutonium, while radiating electrons (β⁻-particles):

${\begin{aligned}^{238}_{92}\mathrm {U} +\;_{1}^{2}\mathrm {H} \;&\longrightarrow \;_{93}^{238}\mathrm {Np} +2n\\^{238}_{93}\mathrm {Np} &\;\longrightarrow \;_{94}^{238}\mathrm {Pu} +\beta ^{-}\\^{238}_{94}\mathrm {Pu} &\;{\xrightarrow[{\mathrm {years} }]{50}}\;_{92}^{234}\mathrm {U} +_{2}^{4}\!\mathrm {He} \end{aligned}}$

The Pu-238 isotope has a half-life of 50 years.

The bulk production of the Pu-239 isotope occurs in a nuclear reactor in which uranium-238 captures slow neutrons and decays via neptunium:

${\begin{aligned}^{238}_{92}\mathrm {U} +n\;&\longrightarrow \;_{92}^{239}\mathrm {U} \\^{239}_{92}\mathrm {U} &\;{\xrightarrow[{\mathrm {mins} }]{23}}\;_{93}^{239}\mathrm {Np} +\beta ^{-}\\^{239}_{93}\mathrm {Np} &\;{\xrightarrow[{\mathrm {days} }]{2.3}}\;_{94}^{239}\mathrm {Pu} +\beta ^{-}\\^{239}_{94}\mathrm {Pu} &\;{\xrightarrow[{\mathrm {years} }]{24000}}\;_{92}^{235}\mathrm {U} +_{2}^{4}\!\mathrm {He} \\\end{aligned}}$

Capture of neutrons and the two following decays form a slow process. Even after a few days, the Pu-239 isotope is still mixed with considerable quantities of U-238, other components of the original materials, and fission products. When sufficient plutonium has been formed, the mixture is chemically separated and the plutonium can be crystallized out as the (chemically pure) Pu-239 metal. In general, this metal shows considerable contamination with the Pu-240 isotope.

Solid state

Solid plutonium exhibits six different allotropes (crystal structures), labeled α, β, γ, δ, δ', and ε. They exist at increasing temperatures. The allotropes show fairly large volume changes upon phase transitions, their densities ("specific weights") vary from 16.00 g/cm³ to 19.86 g/cm³. The α crystalline form exists at room temperatures. It is not a very good conductor of electricity, it has the highest electrical resistivity of any pure metallic element (1.46×10⁻⁶ Ω· m). Just as water, but unlike many other materials, plutonium becomes denser when it melts (at 639.4 °C, normal pressure).

Uses of isotopes

The fissionable isotope Pu-239 is used in nuclear weapons, while Pu-240 is useful as a fuel in nuclear reactors. "Weapons grade" has less than 7% Pu-240 and close to 93% Pu-239. Although Pu-240 is predominantly an α-radiator, it shows some spontaneous fission with accompanying emission of neutrons. When Pu-240 is compressed in a fission device, it emits sufficient neutrons to cause a premature chain reaction (predetonation), disrupting the core before it can give a full explosive yield. These characteristics, however, can be used productively in power reactors, which operate at a lower neutron flux density.

Plutonium fission devices must use implosion technology, as opposed to the simpler gun-type usable with uranium bombs that do not have the predetonation problem. The atomic bomb Fat Man that detonated over Nagasaki on August 9, 1945 had a Pu-239 core, as had the device (Trinity) tested in Jornada del Muerto (New Mexico) a few weeks earlier (on July 16); it was pretested because there was lower confidence in plutonium devices.

The gun-type design is obsolete, as implosion can be used with uranium devices as well, but plutonium offers the advantage of requiring a lighter fissionable mass than uranium. This is advantageous both in requiring less material, but also in reducing the overall weight of warheads, a major concern for missile delivery systems.

External link

Los Alamos National Lab