194
K. A. S. Fessler et al.
uranium holdup can: (1) affect criticality measurements for enriched material, (2)
cause operational problems such as plugging flow paths or preventing valves from
closing completely, and (3) results in what is known as “hidden inventory” in the
system where uranium is not mass balanced from the feedstock to the product. Since
the gas cell holds UF 6 for such a short time, and at room temperature, corrosion
of stainless steel is not an issue [16]. Institutional knowledge has also shown ZnSe
windows and Kalrez elastomers are resistant to UF 6 at low temperatures, and thus
far, no corrosion or degradation of any cell materials has been observed.
In infrared absorption spectroscopy, photons from a light source (e.g., the quantum
cascade laser) are absorbed by the molecule into discrete energy levels determined
by the molecular geometry. Uranium hexafluoride has an octahedral geometry and
displays infrared active molecular vibrations for the F 1u symmetry, which corresponds the third, ν 3 , and fourth, ν 4 , fundamental frequencies, as well as a multitude
of combination bands and overtones [17]. The ν 3 (625 cm
−1 ) and ν 4 (186 cm
−1 )
fundamental frequencies are associated with the uranium hexafluoride molecule
asymmetric stretching and asymmetric bending, respectively. Because of the lack of
compact, tunable lasers in the mid-infrared spectral region where the ν 3 (625 cm
−1 )
and ν 4 (186 cm
−1 ) fundamental frequencies lie, the high performance infrared method
uses the ν 2 + ν 3 uranium hexafluoride combination absorption band at approximately
1157 cm
−1 , which is a combination motion of the ν 2 stretch and the ν 3 asymmetric
stretch. Preliminary tests used a laser centered at the ν 1 + ν 3 absorption band around
1291 cm
−1 , however the ν 2 + ν 3 band was determined to be the optimal vibrational
frequency for the intended application due to the spectral region having fewer potential interferents from common processing impurities and the two combination bands
exhibiting similar absorption efficiencies. The infrared isotopic effect is dependent
on the elements involved in the vibrational motion contributing to the vibrational
energy level. Vibrational frequencies decrease with increasing mass, which causes
the uranium hexafluoride molecule with uranium-238 to shift in vibrational frequency
to a higher frequency by approximately 0.65 cm
−1 from the uranium-235 uranium
hexafluoride molecule.
Despite the high resolution capability of the quantum cascade laser (0.0005 cm
−1 )
being three orders of magnitude better than the 0.65 cm
−1 shift observed for the
infrared absorption of uranium-238 uranium hexafluoride molecule from that of
uranium-235, the spectra recorded using the high performance infrared system did not
show two resolved absorption bands. Only one broad absorption band was detected
and is a result of peak broadening from rotational hot-bands contributions. Instead
of detecting two distinct vibrational bands and determining the isotopic content from
the ratio of peak intensities, the high resolution of the quantum cascade laser was
used to precisely measure the frequency of the molecular shift. Using a principal
component regression model, the measured frequency of the molecular shift can be
used to predict the isotopic content. As previously stated, the uranium hexafluoride
isotopic absorbance is modeled as a linear combination of the three spectral features:
absorbance due to uranium-235, absorbance due to uranium-238, and background
absorbance.
K. A. S. Fessler et al.
uranium holdup can: (1) affect criticality measurements for enriched material, (2)
cause operational problems such as plugging flow paths or preventing valves from
closing completely, and (3) results in what is known as “hidden inventory” in the
system where uranium is not mass balanced from the feedstock to the product. Since
the gas cell holds UF 6 for such a short time, and at room temperature, corrosion
of stainless steel is not an issue [16]. Institutional knowledge has also shown ZnSe
windows and Kalrez elastomers are resistant to UF 6 at low temperatures, and thus
far, no corrosion or degradation of any cell materials has been observed.
In infrared absorption spectroscopy, photons from a light source (e.g., the quantum
cascade laser) are absorbed by the molecule into discrete energy levels determined
by the molecular geometry. Uranium hexafluoride has an octahedral geometry and
displays infrared active molecular vibrations for the F 1u symmetry, which corresponds the third, ν 3 , and fourth, ν 4 , fundamental frequencies, as well as a multitude
of combination bands and overtones [17]. The ν 3 (625 cm
−1 ) and ν 4 (186 cm
−1 )
fundamental frequencies are associated with the uranium hexafluoride molecule
asymmetric stretching and asymmetric bending, respectively. Because of the lack of
compact, tunable lasers in the mid-infrared spectral region where the ν 3 (625 cm
−1 )
and ν 4 (186 cm
−1 ) fundamental frequencies lie, the high performance infrared method
uses the ν 2 + ν 3 uranium hexafluoride combination absorption band at approximately
1157 cm
−1 , which is a combination motion of the ν 2 stretch and the ν 3 asymmetric
stretch. Preliminary tests used a laser centered at the ν 1 + ν 3 absorption band around
1291 cm
−1 , however the ν 2 + ν 3 band was determined to be the optimal vibrational
frequency for the intended application due to the spectral region having fewer potential interferents from common processing impurities and the two combination bands
exhibiting similar absorption efficiencies. The infrared isotopic effect is dependent
on the elements involved in the vibrational motion contributing to the vibrational
energy level. Vibrational frequencies decrease with increasing mass, which causes
the uranium hexafluoride molecule with uranium-238 to shift in vibrational frequency
to a higher frequency by approximately 0.65 cm
−1 from the uranium-235 uranium
hexafluoride molecule.
Despite the high resolution capability of the quantum cascade laser (0.0005 cm
−1 )
being three orders of magnitude better than the 0.65 cm
−1 shift observed for the
infrared absorption of uranium-238 uranium hexafluoride molecule from that of
uranium-235, the spectra recorded using the high performance infrared system did not
show two resolved absorption bands. Only one broad absorption band was detected
and is a result of peak broadening from rotational hot-bands contributions. Instead
of detecting two distinct vibrational bands and determining the isotopic content from
the ratio of peak intensities, the high resolution of the quantum cascade laser was
used to precisely measure the frequency of the molecular shift. Using a principal
component regression model, the measured frequency of the molecular shift can be
used to predict the isotopic content. As previously stated, the uranium hexafluoride
isotopic absorbance is modeled as a linear combination of the three spectral features:
absorbance due to uranium-235, absorbance due to uranium-238, and background
absorbance.