1
Signatures of a Clandestine Uranium
Enrichment Facility
Taylor G. Duffin

Abstract—The Joint Comprehensive Plan of Action effectively
eliminates the plutonium route for Iran to build a nuclear
weapon; however, the stored centrifuges leave open the
possibility for a clandestine uranium enrichment facility leading
to a bomb. Consequently there is renewed interest in all possible
technologies that could detect such a facility. Signatures of
enrichment by centrifuge and laser isotope separation are
considered. LIBS, LAARS, LIDAR, Luminescence, mass
spectrometry, and cavity ringdown spectrometry are compared
as detection methods for uranyl fluoride. Based on differing
detection capabilities and the tradeoff between sensitivity and
standoff distance a combined detection approach should be
implemented.
I. INTRODUCTION
P
erhaps the easiest pathway to a nuclear weapon is
through highly enriched uranium. The plans for a guntype nuclear device utilizing weapons-grade uranium are
easily available. The simplicity of such a design allows
confidence in weapon functionality even without testing the
weapon. Additionally there is much less gamma radiation in
uranium weapon material than from a plutonium weapon
making illicit material much easier to handle and transport.
The creation of plutonium in a reactor and subsequent
chemical separations are highly regulated internationally and
have no civilian dual-use as opposed to simple uranium
enrichment which is used for civilian power reactors. Any
form of uranium enrichment in significant quantities could be
spawned from a legitimate nuclear power program to produce
weapons material. With the implementation of the Joint
Comprehensive Plan of Action in Iran, virtually all routes
toward a plutonium weapon have been closed. However Iran
still maintains a large stock of stored centrifuges, which could
be used to enrich uranium for a weapon. Consequently
detection of a clandestine uranium enrichment facility is of
great interest.
This paper was submitted on December 7th, 2015 for Nuclear Engineering
597A course entitled “Detector and Source Technologies for Nuclear
Security”. This course is a part of the Nuclear Security Education Program
which is sponsored by the National Nuclear Security Administration (NNSA)
through its Global Threat Reduction Initiative (GTRI).
T. G. Duffin is a graduate student in the Department of Mechanical and
Nuclear Engineering at Penn State University. Intense Laser Laboratory,
Hammond Building, Pennsylvania State University, University Park, PA
16802 USA (email: [email protected])
II. PATHWAYS TO ENRICHMENT
Uranium enrichment can be done in a multitude of ways
including the calutron, gaseous diffusion, gas centrifuge, and
laser isotope separation. Of these only diffusion and
centrifuges have been run at a commercial scale. Uranium
hexafluoride gas (UF6) is the form of uranium used in both of
these methods.
Gaseous diffusion was the first generation separation
technology and is being phased out in both the United States
and France in favor of the less energy intensive gas centrifuge
method. In gaseous diffusion pressurized UF6 is passed
through many porous membranes. The lighter U-235
molecules diffuse faster through these membranes and so the
gas passing through is slightly enriched [1].
Gas centrifuge technology sends the UF6 gas into a series
of centrifuges consisting of a vacuum tube and a rotor. As the
rotors spin at high speed the heavier U-238 molecules forced
towards the outer edge of the cylinder. With a thermal gradient
applied to the cylinder, convection brings the lighter U-235
center stream to the top and the outer U-238 stream is pulled
off the bottom. With many units inline effective separation
occurs as the tops stream is fed to the next centrifuge while the
bottom is recycled to the previous one [1].
Laser separation has distinct advantages over both
diffusion and centrifuges in lower energy and capitol costs.
Laser separation can be done by both atomic and molecular
methods; however as processes already using UF6 fit better
into the current fuel cycle molecular methods have been
favored. The main molecular laser process to enrich uranium
is SILEX (separation of isotopes by laser excitation), which is
the closest to commercial development. This process exploits
the photo-dissociation of UF6 to solid UF5+. The UF6 steam is
exposed to a CO2 laser operating at a wavelength of 10.8 μm
and optically amplified to 16 μm. The molecular bond to one
of the fluorine atoms bonded to U-235 is broken. The ionized
UF5+ is then electrically drawn to a collection plate while the
remaining UF6 containing U-238 is unaffected leading to
isotopic separation [1].
The two most concerning methods of enrichment for Iran
are centrifuge technology since they already possess many
centrifuges and from laser isotope separation since it is easy to
conceal especially with lower energy demand.
2
III. DETECTABLE SIGNATURES
Gaseous diffusion, gas centrifuges and the SILEX process
all rely on a UF6 input stream. As UF6 is released it will
hydrolyze rapidly in the air creating both uranyl fluoride
(UO2F2) particulates and gaseous hydrofluoric acid (HF)
according to the following chemical reactions.
UF6 + 2H2 O → UO2 F2 + 4HF
UF6 + H2 O → UOF4 + 2HF
UOF4 + H2 O → UO2 F2 + 2HF
(1)
(2)
(3)
Both uranyl fluoride and HF can be detected by a variety
of means. The particulates appear as a white aerosol cloud,
which rises from the site of a leak and then disperses
throughout the environment. The size of these particles range
from 0.05-0.08 microns and agglomerate to form particles
sized on the order of 1 micron [2]. The concentrations of the
produced particles vary based on the relative humidity at the
time of the release. If aerosol formation occurs under
stoichiometric conditions then the atom number densities are:
NHF ~ 2.4x1020 m-3, NUO2F2 ~ 0.7x1020 m-3 [3].
Large aerosols deposit by gravitational settling, and most
aerosols are efficiently scavenged from the atmosphere by
rain. However, fine and ultrafine aerosols can persist on time
scales of 4 to 40 days [4]. The size range of large aerosol
particles that settle to the ground is between 0.5 – 3.0 microns
[5].
There are concerns that even at large scale enrichment
facilities the total emissions of UF6 can be kept very low. As
most piping and the centrifuges operate below atmospheric
pressure, there is little leakage of the process gas to the
atmosphere [4]. Large accidents resulting in much higher
releases are seen as rare enough to not change the overall
detectable quantities in realistic timescales [6]. Some have
suggested that the detectable point will be at a clandestine
conversion facility that supplies the enrichment facility [4].
Under either analysis the potential signals of uranium
hexafluoride and its hydrolyzed products as well as their
detection methods remain the same.
Emission rates can be estimated by 1960’s U.S.
conversion facility cited by Albright and Barbour, which
leaked about 0.24 grams of uranium into the atmosphere for
every kilogram of uranium in UF6 produced [7]. If this is
scaled for a facility producing enough enriched material for a
weapon annually then the release is 3.8 kg-UF6 per year or
about 10 grams per day [4]. These concentrations will
decrease with distance from the facility based on diffusion and
wind conditions. The closer a detector can be to a facility the
higher the likelihood of detection. Consequently detection
technologies need to be extremely sensitive and be able to
obtain a high amount of signal to relative noise even at very
low concentrations.
IV. CANDIDATE TECHNOLOGIES
A. LIBS (Laser Induced Breakdown Spectroscopy)
The setup for LIBS is relatively simple. A pulsed laser
with a broad spectrum is directed at a target. The sample is
exposed to the laser pulse and a small amount of material (ng)
is ablated and forms a plasma that expands away from the
sample surface. The various elements in the plasma, which are
excited by the laser energy, relax to lower energy levels and
produce characteristic photoemissions. These are detected by a
spectrometer and subsequently can be identified. There is
characteristic isotopic shift for highly enriched versus natural
uranium with the proper experimental setup. The ablated
surface does not need to be prepared beforehand. A diagram of
a basic LIBS setup is shown in Figure 1.
Fig. 1. Schematic diagram of a simple LIBS system (a) laser source and
cooler; (b) pulsed laser head; (c) mirror; (d) focusing lens; (e) excitation
chamber; (f) sample; (g) collecting optics; (h) optical fiber; (i) detector trigger
signal; (j) wavelength selector; (k) detector array and (l) microcomputer. [8]
Limitations to LIBS include the fact that decent amount of
material is needed for a sequence of shots. Particles in an
environmental swipe or deposited from an air filter may not be
sufficient for multiple shots to improve the signal to noise
ratio (SNR). LIBS can also experience a lot of potential
interference by neighboring lines from different elements in
the sample. It is difficult to be quantitative with LIBS because
of these interferences.
B. LAARS (Laser Ablation, Absorption Ratio Spectrometry)
LAARS uses a laser ablating a surface similar to LIBS,
however this ablation is used more to vaporize and eject the
material rather than to excite it. The ablation still serves to
atomize the material regardless of initial composition. The
small ablation spot size also allows for spatial correlation on
the sample. At 15 μs after the laser ablation the plasma will
have cooled significantly to neutral atoms then two diode
lasers tuned to specific wavelengths are passed through
plasma. Photodetectors are inline with these lasers to measure
the amount of intensity absorbed by the plasma. The intensity
of the absorbance of the two beams is directly proportional to
the enrichment level. A schematic of this process is shown in
Figure 2.
3
There is little fluorescence response for solid-phase uranyl
fluoride fallout material. It needs to be dissolved in acid
solutions as the fluorine ions increase the fluorescence. This
requires additional sample preparation and the process is very
sensitive to the pH of the solution. Uranyl fluorescence can
also be analyzed in a solid glass-state, but this is impractical to
make from particulate samples [12].
Fig. 2. The Basic Components of the LAARS Instrument (Including a
Compact Nd:YAG Laser, Two External Cavity Diode Lasers, the LAARS
Sample Chamber, and the Absorbance Detectors. The diagram shows the
sample chamber configured to characterize planar samples.) [9]
This method eliminates much of the crosstalk between the
different isotope emissions in LIBS, which can skew the
enrichment level. Samples can be taken from collected aerosol
particles, swipe samples or adsorbed vapor. There is virtually
no sample preparation required. This technique can provide
uranium enrichment analysis to within ±0.1% accuracy. For
continuous monitoring aerosol particles can be captured by a
rotating drum impactor and be ready for ablation [9]. A
conceptual design for such an impactor is shown in Figure 3.
D. LIDAR
LIDAR uses a laser emission and its detected backscatter to
determine the presence and distance of a target. This is often
done with two different wavelengths that are closely spaced to
compensate for fluctuations in one wavelength. This is called
differential absorption LIDAR (DIAL). There is discussion of
using a combined system with both DIAL and Raman
scattering LIDAR. The DIAL system can provide information
on the UF6 concentration using the off- and on-wavelengths at
266 nm at 245 nm, respectively, whereas Raman scattering of
HF at 297.3 nm can identify and quantify HF [2]. A schematic
of the proposed combined DIAL/Raman LIDAR system is
shown in Figure 4.
Fig. 4. Schematic diagram of combined differential absorption-Raman
lidars for remote detection of UF6 [2].
Fig. 3. The Conceptual Rotating Drum Impactor [9]
A similar rotating drum collection method could also be
implemented with a LIBS system.
C. Luminescence
Laser-induced fluorescence (LIF) spectra of uranyl have a
characteristic structure consisting of 6–8 bands in the interval
from 470 to 630 nm with the most intense bands in the region
of 470–530 nm [10].
Most of the luminescence in acidic HF solutions of uranyl
fluorides arises from electronically excited UO2F2 (H2O)n.
Thus, luminescence detection of UO2F2 is potentially a rapid,
highly sensitive method for the detection of leaking UF6
cylinders, and the study of the fluorescence of UO2F2 is of
particular interest [11].
In Raman scattering, the frequency of the scattered radiation
is altered as a result of interaction of the photon with the
energy levels of the molecules. This is a characteristic energy
shift. Raman scattering is a weak process, and typically only 1
in 106 –108 photons undergo the Raman inelastic scattering
event. This drawback may be compensated with the use of a
high-power high-repetition-rate UV laser (on the order of
several tens of Hz), a telescope with a large area, a sensitive
PMT detector, and reducing background noise [2].
Raman is insensitive to water background. UV light in the
solar blind region (200-310 nm) can enhance the Raman signal
and improve SNR. SNR is variable between 1.5 to 120 based
on integration time and the field of view. The minimum
detectable concentration is 1.21 ppm at a range of 1000 m [2].
E. Mass Spectrometry
Mass spectrometry is a highly used method throughout all
of chemistry and its process details will not be described here.
It is a standard method used by the IAEA for environmental
samples. Typical environmental samples of uranium collected
under IAEA guidelines. The samples are loaded on a filament
and analysis is carried out by isotope-dilution (ID-TIMS) that
gives quantitative and isotopic composition information or by
4
mass spectrometry for isotopic data only. The detection limit
of the uranium signal was 107 atoms (~5x10-15 g); however
due to background and blank levels, the practical detection
limit is higher by two orders of magnitude. By a simpler
extraction method the detection limit is 5 ng/cm2 on the
surface or 40 ng/L [13].
F. Cavity Ring-Down Spectroscopy
Cavity ring-down creates a long path-length over which
light can travel by using a beam from a pulsed laser in a cavity
with high reflectivity mirrors at each end. Each pulse of the
laser enters the cavity and is reflected between the two
mirrors, the light intensity decaying in time due to absorption
and scattering by particles and gasses present between the
mirrors. Each time the laser is reflected from the back mirror,
a small fraction of light is transmitted through and is detected
by a photomultiplier tube, the signal decay is then analyzed
[13].
Cavity ring-down spectroscopy claims sensitivity down to
parts-per-trillion. A recent study uses Saturated Absorption
Cavity ring-down spectroscopy (SCAR) to reach parts-perquadrillion sensitivity on radiocarbon dioxide [15]. Cavity
ringdown can only detect in a limited bandwidth. It is difficult
to adjust to a different line for investigation as the cavity
length is also involved in the analysis. This is in contrast to a
method such as LIBS, which can detect many different
emission lines from the same sample. Additionally, the flow of
some neutral gas is needed in the cavity so that aerosols do not
deposit themselves onto the sensitive optics.
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V. METHOD COMPARISON
[12]
Most of these methods have excellent qualities in their
specific circumstances. All can detect down to low
concentrations. LAARS can detect down to femtograms U235. They claim uranium enrichment detection to within
±0.1% accuracy.
SCAR looks like the lowest in terms of minimum
detectable limit going down to part-per-quadrillion. It also has
a 30:1 signal-to-noise ratio.
LIDAR is excellent as a stand off technology. But it is
much less sensitive if used at long distances. The minimum
detectable concentration is 1.21 ppm at a range of 1000 m. A
shorter distance can increase detectable and it has a good SNR
if enough shots are taken.
[13]
VI. CONCLUSION
There is no perfect technology to detect a clandestine
enrichment facility. Many of these technologies would need to
be used in conjunction to identify such a facility. LIDAR is an
excellent method for measuring at standoff distance and the
only method truly able to do so. LAARS could be used in the
field if there was reasonable suspicion to collect
environmental samples. SCAR has the lowest minimum
detectable limit but would be difficult to make into a field
usable device. Luminescent methods are like traditional mass
spectrometry and take the analysis back to the lab with needed
sample preparation and so may not add much to current
techniques.
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