Uranium

Uranium is an element of the actinide series in the periodic table. It has atomic number 92. It
has six isotopes (U-233 through U-238). Uranium-238 and uranium-235 are the most usual.
All uranium isotopes are unstable; uranium is weakly radioactive, and it is subject to decay
as a result of alpha emission.

Refined uranium is a silver-white metal. Its density is higher than the density of lead by
about 70%. It is found naturally in low concentrations of a few parts per million in soil, rock
and water, and it is extracted from uranium-bearing minerals such as uraninite.

The naturally occurring isotopes of uranium are:

  • uranium-238 (99.27%)
  • uranium-235 (0.72%)
  • uranium-234 (0.0054%)

The half-life of uranium-238 is approximately 4.47 billion years and the half-life of uranium-235
is 704 million years. Thus it is possible to use them to determine an age for the Earth.

Many applications of uranium benefit from its nuclear properties. Uranium-235 is the only
fissile isotope in nature: it can sustain a nuclear fission chain reaction. Uranium-238 is
fissionable by fast neutrons and is fertile. This means that it can be used to make fissile
plutonium-239 in a nuclear reactor by means of transmutation.

The uranium-235 isotope is important for nuclear reactors and nuclear weapons. Because
the concentration of the isotope in natural uranium is very low, enrichment is necessary to
raise the concentration of fissile uranium-235 for nuclear weapons and the majority of
nuclear power plants. Although civilian power plants need uranium with a concentration of
2-5% U-235, nuclear weapons need a minimum of 80% U-235, and preferably more than
90%.

Enrichment

The separation of isotopes is a difficult process because different isotopes of the
same element have identical chemical properties. In this regard it is not possible to use everyday
chemical separation techniques. It is necessary to carry out separation on the basis of the
small mass difference between the isotopes.

There are two main commercial methods which are used for the enrichment of uranium:

  • Gas Centrifuge Process
  • This is the most usual technique for the enrichment of U-235 today. Uranium
    hexafluoride (UF6) in the gas phase is passed through many high speed centrifuges in
    a parallel formation. The heavier U-238 molecules move in the direction of the
    outside of the centrifuge cylinder. At the same time the lighter U-235 molecules
    assemble closer to the center.

  • Gas Diffusion Process
  • UF6 in the gas phase is passed through semi-permiable membranes. The difference
    in the mass between the U-235 and U-238 isotopes affects the relative rate of
    diffusion. It is possible to create a high level of separation as a result of a series of
    many stages.

Grades of Enrichment

Slightly Enriched Uranium (SEU) has a U-235 concentration of 0.9-2%.

Low Enriched Uranium (LEU) has a concentration of U-235 which is lower than 20%. To use in
nuclear power plants uranium is enriched to 2-5% U-235.

Highly Enriched Uranium (HEU) has a concentration of U-235 which is greater than 20%. The
uranium used in nuclear weapons is usually enriched to 80-90% or more (this is called
weapons-grade uranium).

Weapons-grade uranium metal was used in the American nuclear bomb called "Little Boy".
This was the first nuclear weapon to be employed in war and was dropped on Hiroshima in
1945.

High enrichment is important in order to reduce the necessary critical mass for a nuclear
weapon. It is possible to see the effect of uranium enrichment on critical mass in the graph.
As you see, the critical mass reduces when the enrichment increases. The critical mass
values for HEU are much lower than the critical mass values for LEU.