ORNL-5388 - the Molten Salt Energy Technologies Web Site

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electric field or an infrared laser pulse to pull the electrons off the atoms. Once
the atoms are ionized, they can be separated from the neutral atoms in the beam by
the use of electric or magnetic fields, or both.
The major limiting factor in the above process is the density of atoms in the
uranium "ribbon." There is an upper limit on the density and therefore on the rate
of production of enriched uranium, because both excitation energy and ionic charge are
very easily transferred to other atoms in collisions. Such collisions must be kept to
a minimum if a high selectivity is to be obtained.
Other technical difficulties in the 'development of the process are:
a. The corrosiveness of the uranium vapor.
b. The presence of thermally excited or ionized atoms of 235U in the uranium vapor
(at 2500°K, 45% of 23% atoms are not in the ground state).
c. The potential for self lasing of the uranium vapor.
d. Thermal ionization of 238U will seriously degrade the selectivity and thus
e.
limit the enrichment.
Lasers combining high energy density, rapid pulse repetition rate, high tuning
precision, and long-term stability and reliability must be developed.
Laser Enrichment with molecule^.^^ Gaseous UF6 is used in all proposed schemes
for molecular enrichment, since this is the only compound of uranium with a sizable
vapor pressure at reasonable temperatures. Because the molecule contains zeven atoms
and exhibits a high degree of symmetry, it produces a complicated spectrum of
vibrational and rotational excitations. The most interesting vibrational modes from
the point of view of laser excitations are those which involve motion of the uranium
atom and which therefore produce an oscillating electric dipole moment. Only these
modes are'likely to produce transitions from the ground state when excited by electromagnet
ic energy.
The low energies associated with these transiFions lead to two serious
problems for laser enrichment in UF6. The first problem is the creation of an
infrared laser with the correct frequency. The second problem is related to the
high occupation numbers of the low-energy vibrational states at temperatures where
UF6 has a high vapor pressure. Because so many low-lying states are occupied, it
is impossible to find a single excitation frequency that will be absorbed by most
of the molecules. The presence of these so-called "hot bands" reduces the efficiency
of the process very drastically.
The second problem is easily solved, at least in principle, if warm UF6 gas
is passed through a supersonic nozzle. The effect of the expansion is to convert
most of the kinetic energy of random motion of the gas in the reservoir into kinetic
energy of translational motion of the gas in the nozzle. As the gas accelerates
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