2. Physical and chemical properties of UF6

error due to the aerosol scattering is negligible, and errors may arise when the wavelengths

In this chapter, a system consists of UV-DIAL and Raman lidar for real-time actively remote monitoring uranium-enrichment plants has been proposed [9]. Because of the fast reaction of UF6 with water vapor in the atmosphere (see Figure 1), simultaneous measurement of UF6 and HF may decrease the measurement's uncertainty and improve the sensitivity. A green targeting laser diode can be used to visualize and help the operator to determine where is aiming with invisible UV laser beam during the screening. The system comprises a frequencyquadrupled, compact, pulsed Nd:YAG laser at 10 Hz repetition rate for the off-wavelength at 266 nm and a frequency-doubled Nd:YAG-pumped Coumarin 450 using a Blazed grating mounted in Littrow configuration operating in the frequency doubled for the on-wavelength at 245 nm. The grating is invariably used to allow tuning of the laser across the wide gain bandwidth of the laser. Raman scattering measurements of HF at 297.3 nm with Raman frequency shift 3959 cm<sup>1</sup> are helpful to identify HF as a probe for real-time detection and

It should be mentioned that the absorption spectroscopy is also utilized for remotely detection of HF at 1.28 μm [12]. However, it has a drawback of inability to distinguish HF and also limitation due to the strong absorption of water vapor in the region which hampers the measurements, especially at relatively low HF concentrations. Hence, because of the very weakness of Raman scattering of water [10], Raman lidar may be a versatile technique to study HF in the wet environment. Moreover, Raman signal and thereby SNR can be significantly enhanced using UV light especially in the solar blind ultraviolet (200 –310 nm). We believe combined UV-DIAL and Raman lidars is a promising and reliable real-time tool for measuring

Figure 1. Varying UF6 and HF concentration in the moisture atmosphere versus time [11].

are significantly separated.

92 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

localization of toxic UF6 leaks.

Uranium hexafluoride can be a solid, liquid, or gas, depending on its temperature and pressure. The phase diagram of UF6 is schematically shown in Figure 2, which presents different physical forms of UF6 as a function of temperature and pressure [13]. At atmospheric pressure (14.7 psia), UF6 is a solid below a temperature of 134F (56.6C) and a gas at temperatures above 134F. Liquid UF6 is formed only at temperatures greater than 147.2F (64.02C) and at pressures greater than 1.5 times atmospheric pressure (~22 psia). At atmospheric pressure, solid UF6 will transform directly to UF6 gas (sublimation) when the temperature is raised to 134F, without going through a liquid phase. Detail description on its physical properties can be found in Ref. [14]. All three phases, solid, liquid, and gas, coexist at 64.02C (147.2F, the triple point) and P = 1.497 atm. Only the gaseous phase exists above 446F (230.2C, the critical temperature), at which critical pressure is 45.5 atm.

Uranium hexafluoride is the substance most suitable for use in the fuel cycle facilities processes because of its exotic physical properties. Since fluorine exists naturally in only one isotopic form (F-19), the physical processes widely used for enrichment of U-235, such as diffusion, centrifugation, and molecular laser isotope separation (MLIS), increase only the concentrations of uranium isotopes. The process of fluorination of uranium dioxide to produce UF6 can be subdivided into the following chemical reactions:

Figure 2. UF6 phase diagram, showing relationship between pressure, temperature, and physical form [13].

$$\rm U\_3O\_8 + 2H\_2 \to 3UO\_2 + 2H\_2O \tag{1}$$

$$\text{UO}\_2 + 4\text{HF} \to \text{UF}\_4 + 2\text{H}\_2\text{O} \tag{2}$$

$$\text{UF}\_4 + \text{F}\_2 \to \text{UF}\_6 \tag{3}$$

UF6 þ H2O ! UOF4 þ 2HF (10.a)

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UOF4 þ H2O ! UO2F2 þ 2HF þ heat (10.b)

The UF6 hydrolysis reaction releases approximately 58 kJ/g mole of H2O, which heats the plume and causes plume rise that afterward slowly sink to the ground. This hydrolysis reaction plays an important role in determining the fate of UF6 that is released into the atmosphere. If a release occurs inside a building, this fog may impair escape from the release area or may difficult planned emergency actions. A dense fog was observed, for example, at the Hanau conversion plant, in 1987, during a UF6 release from an autoclave [17]. It has been reported that UO2F2 concentrations as low as 1 g/m3 are visible, and visibility is less than 90 cm

Remotely Monitoring Uranium-Enrichment Plants with Detection of Gaseous Uranium Hexafluoride and HF Using…

The hydrolysis reaction is very fast and is limited by the availability of water. To hydrolyze 1000 kg of UF6, 100 kg of water is required; at 25�C and 70% relative humidity, this amount of water is contained in 6000 m<sup>3</sup> of air. Following a large-scale release of UF6 outside, the dispersion is governed by meteorological conditions. The plume could still contain unhydrolyzed UF6 even after traveling a distance of several 100 meters. In other words, although the hydrolysis reaction of UF6 is fast, after escaping of UF6 into the atmosphere, besides HF and UO2F2, UF6 may also be found in the atmosphere. Only escaping a few 100 g of UF6 into the atmosphere will raise the formation toxic and opaque cloud of uranyl fluoride [19]. UO2F2 is a particulate that is very soluble in the lungs, and the uranium acts as a heavy metal poison that can affect the kidneys. HF is an acid vapor that can cause acid burns on the skin or lungs. In the

It should be emphasized that enriched UF6 cannot be directly used in reactors, as it does not withstand high temperatures or pressures. It is therefore converted into UO2. Fuel pellets are formed by pressing UO2, which is sintered (baked) at temperatures of over 1400�C to achieve high density and stability. The pellets are cylindrical and are typically 8–15 mm in diameter and 10–15 mm long. They are packed in long metal tubes to form fuel rods, which are grouped in "fuel assemblies" for introduction into a reactor. The spent fuel contains uranium (96%), plutonium (1%), and high-level waste products (3%). The uranium with less than 1% fissile U-235 and the plutonium can be reused. Some countries chemically reprocess usable uranium and plutonium to separate them from unusable waste. Recovered uranium from reprocessing can be returned to the conversion plant, converted to UF6, and subsequently re-enriched. Recovered plutonium, mixed with uranium, can be used to fabricate mixed oxide fuel (MOX).

A schematic diagram of the combination of DIAL and Raman lidar is shown in Figure 3. The description of our DIAL system is provided in details in the earlier papers [8]. The Nd:YAG laser is a Q-switched with pulse repetition rate of 10 Hz and pulsewidth of 10 ns with an output power of several 100 mJ per pulse at the fundamental wavelength, 1064 nm. In the laser unit, the 1064-nm laser beam is sent through the frequency doubling and quadrupling harmonic

[18]. Fog can also occur in unconfined areas if the humidity is high.

event of an accidental release, its toxicity level can be reached in minutes.

3. Materials and methods

The density of UF6 changes with temperature and for the solid phase can be described by the following equation [15]:

$$
\rho\_s = 5194 - 5.168T \text{ kg/m}^3 \tag{4}
$$

for 0�C ≤ T ≤ 64�C. In the liquid state, its density varies in a nonlinear fashion and can be summarized by the following equation [15]:

$$
\rho\_l = 1670 + 152.03(T\_c - T)^{0.5} \text{ kg/m}^3 \tag{5}
$$

where Tc = 230.2�C, which is accurate close to triple point (T = 64.02�C, p = 1.497 atm), and

$$
\rho\_l = 2084.3 - 3.1T + 371(T\_c - T)^{0.3045} \text{ kg/m}^3 \tag{6}
$$

which is more accurate close to the critical point (T = 230�C, p = 45.5 atm). Note that T in Eqs. (4)–(6) is measured in �C.

In the vapor phase, the density of UF6 can be described according to an equation which is similar in form to the ideal gas law [15]:

$$\rho\_v = \frac{4291p}{T\left(1 - 1.3769 \times 10^6 p/T^3\right)} \text{ kg/m}^3 \tag{7}$$

where p is in atm, and T is in K. In the range of 50–140�C, Eq. (8) gives density values [16], which is similar to those obtained using Eq. (7), but it is applicable over a wider temperature range and does not have a singularity limit, i.e.

$$
\rho\_v = \frac{4291p}{T} \left( 1 + 1.2328 \times 10^6 p/T^3 \right) \,\mathrm{kg/m^3} \tag{8}
$$

Comparing Eqs. (4) and (5), we can see that during the change from solid to liquid at 64.02�C, a volume expansion of 25.36% takes place (density changes from 4863.25 kg/m<sup>3</sup> (solid) to 3629.95 kg/m<sup>3</sup> (liquid)).

When gaseous UF6 escapes into the moist atmosphere, it rapidly reacts and hydrolyzes with the ambient water vapor to form hydrogen fluoride (HF) gas and uranyl fluoride (UO2F2) particles with diameters of a few microns, both of which are very toxic. This exothermic reaction can be written in the following forms:

$$\text{UF}\_6 + 2\text{H}\_2\text{O} \rightarrow \text{UO}\_2\text{F}\_2 + 4\text{HF} + \text{heat} \tag{9}$$

or

Remotely Monitoring Uranium-Enrichment Plants with Detection of Gaseous Uranium Hexafluoride and HF Using… http://dx.doi.org/10.5772/intechopen.73356 95

$$\text{UF}\_6 + \text{H}\_2\text{O} \rightarrow \text{UOF}\_4 + 2\text{HF} \tag{10.a}$$

$$\text{UOF}\_4 + \text{H}\_2\text{O} \rightarrow \text{UO}\_2\text{F}\_2 + 2\text{HF} + \text{heat} \tag{10.b}$$

The UF6 hydrolysis reaction releases approximately 58 kJ/g mole of H2O, which heats the plume and causes plume rise that afterward slowly sink to the ground. This hydrolysis reaction plays an important role in determining the fate of UF6 that is released into the atmosphere. If a release occurs inside a building, this fog may impair escape from the release area or may difficult planned emergency actions. A dense fog was observed, for example, at the Hanau conversion plant, in 1987, during a UF6 release from an autoclave [17]. It has been reported that UO2F2 concentrations as low as 1 g/m3 are visible, and visibility is less than 90 cm [18]. Fog can also occur in unconfined areas if the humidity is high.

The hydrolysis reaction is very fast and is limited by the availability of water. To hydrolyze 1000 kg of UF6, 100 kg of water is required; at 25�C and 70% relative humidity, this amount of water is contained in 6000 m<sup>3</sup> of air. Following a large-scale release of UF6 outside, the dispersion is governed by meteorological conditions. The plume could still contain unhydrolyzed UF6 even after traveling a distance of several 100 meters. In other words, although the hydrolysis reaction of UF6 is fast, after escaping of UF6 into the atmosphere, besides HF and UO2F2, UF6 may also be found in the atmosphere. Only escaping a few 100 g of UF6 into the atmosphere will raise the formation toxic and opaque cloud of uranyl fluoride [19]. UO2F2 is a particulate that is very soluble in the lungs, and the uranium acts as a heavy metal poison that can affect the kidneys. HF is an acid vapor that can cause acid burns on the skin or lungs. In the event of an accidental release, its toxicity level can be reached in minutes.

It should be emphasized that enriched UF6 cannot be directly used in reactors, as it does not withstand high temperatures or pressures. It is therefore converted into UO2. Fuel pellets are formed by pressing UO2, which is sintered (baked) at temperatures of over 1400�C to achieve high density and stability. The pellets are cylindrical and are typically 8–15 mm in diameter and 10–15 mm long. They are packed in long metal tubes to form fuel rods, which are grouped in "fuel assemblies" for introduction into a reactor. The spent fuel contains uranium (96%), plutonium (1%), and high-level waste products (3%). The uranium with less than 1% fissile U-235 and the plutonium can be reused. Some countries chemically reprocess usable uranium and plutonium to separate them from unusable waste. Recovered uranium from reprocessing can be returned to the conversion plant, converted to UF6, and subsequently re-enriched. Recovered plutonium, mixed with uranium, can be used to fabricate mixed oxide fuel (MOX).

#### 3. Materials and methods

U3O8 þ 2H2 ! 3UO2 þ 2H2O (1)

UO2 þ 4HF ! UF4 þ 2H2O (2)

<sup>ρ</sup><sup>s</sup> <sup>¼</sup> <sup>5194</sup> � <sup>5</sup>:168<sup>T</sup> kg=m3 (4)

<sup>ρ</sup><sup>l</sup> <sup>¼</sup> <sup>1670</sup> <sup>þ</sup> <sup>152</sup>:03ð Þ Tc � <sup>T</sup> <sup>0</sup>:<sup>5</sup> kg=m3 (5)

<sup>ρ</sup><sup>l</sup> <sup>¼</sup> <sup>2084</sup>:<sup>3</sup> � <sup>3</sup>:1<sup>T</sup> <sup>þ</sup> <sup>371</sup>ð Þ Tc � <sup>T</sup> <sup>0</sup>:<sup>3045</sup> kg=m3 (6)

<sup>p</sup>=T<sup>3</sup> kg=m3 (7)

p=T<sup>3</sup> kg=m<sup>3</sup> (8)

UF6 þ 2H2O ! UO2F2 þ 4HF þ heat (9)

The density of UF6 changes with temperature and for the solid phase can be described by the

for 0�C ≤ T ≤ 64�C. In the liquid state, its density varies in a nonlinear fashion and can be

where Tc = 230.2�C, which is accurate close to triple point (T = 64.02�C, p = 1.497 atm), and

which is more accurate close to the critical point (T = 230�C, p = 45.5 atm). Note that T in

In the vapor phase, the density of UF6 can be described according to an equation which is

where p is in atm, and T is in K. In the range of 50–140�C, Eq. (8) gives density values [16], which is similar to those obtained using Eq. (7), but it is applicable over a wider temperature

Comparing Eqs. (4) and (5), we can see that during the change from solid to liquid at 64.02�C, a volume expansion of 25.36% takes place (density changes from 4863.25 kg/m<sup>3</sup> (solid) to

When gaseous UF6 escapes into the moist atmosphere, it rapidly reacts and hydrolyzes with the ambient water vapor to form hydrogen fluoride (HF) gas and uranyl fluoride (UO2F2) particles with diameters of a few microns, both of which are very toxic. This exothermic

<sup>ρ</sup><sup>v</sup> <sup>¼</sup> <sup>4291</sup><sup>p</sup>

<sup>T</sup> <sup>1</sup> � <sup>1</sup>:<sup>3769</sup> � 106

<sup>T</sup> <sup>1</sup> <sup>þ</sup> <sup>1</sup>:<sup>2328</sup> � <sup>106</sup>

following equation [15]:

Eqs. (4)–(6) is measured in �C.

3629.95 kg/m<sup>3</sup> (liquid)).

or

similar in form to the ideal gas law [15]:

range and does not have a singularity limit, i.e.

reaction can be written in the following forms:

<sup>ρ</sup><sup>v</sup> <sup>¼</sup> <sup>4291</sup><sup>p</sup>

summarized by the following equation [15]:

94 Uranium - Safety, Resources, Separation and Thermodynamic Calculation

UF4 þ F2 ! UF6 (3)

A schematic diagram of the combination of DIAL and Raman lidar is shown in Figure 3. The description of our DIAL system is provided in details in the earlier papers [8]. The Nd:YAG laser is a Q-switched with pulse repetition rate of 10 Hz and pulsewidth of 10 ns with an output power of several 100 mJ per pulse at the fundamental wavelength, 1064 nm. In the laser unit, the 1064-nm laser beam is sent through the frequency doubling and quadrupling harmonic

diaphragm with a diameter D. An MgF2 plano-convex lens (L) after the diaphragm is used to provide a collimated beam. The collimated beam after passing through a monochromator is focused on the entrance pupil of the detector through a Lyot tunable birefringent or Fabry-Pérot interference filters (IF). The diaphragm is in the focal plane of the telescope and sets the field of view (FOV) of the receiver, FOV ≈ D/F. To obtain a required FOV, we select the diameter of the diaphragm for a given focal length of the telescope. The FOV of the receiver is usually set small (about 100 micro-radians full angle FOV) to reject the background noise and

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As mentioned above, along with the power of the laser, the size of the primary optics is an important factor in determining the effectiveness of the system. Since the returned signal from the far distance is relatively weak, to improve the SNR and increase the detection range, a reasonable FOV is essential to suppress the sky background light, especially during the daytime measurements under sunlit conditions. The smaller aperture optics is used to work in close ranges, for example, a few 100 m, where the returned signal is a larger fraction of the transmitted power. To address the minimum distance at which returning signals are completely in the instrument's FOV, the FOV of the instruments needs to be wide enough. The wide field angle of the receiver reduces the overlap range to several 100 meters. However, the increase in FOV increases the collection of unwanted solar background, resulting in unacceptably poor SNR during daytime measurements. One way to reduce the background solar radiation in the wide FOV lidars is working in the solar blind region. Moreover, by adding an AR coating, the overall transmission of the lenses can be increased. The UV folding mirrors also can reduce

Monochromator is used to separate the signals, and narrowband filters (IF) are used to extract the Raman and elastic signals at 266, 245, and 297.3 nm. Narrow bandpass filters designed for these center wavelengths allow isolation of the Raman line and increase the SNR by rejecting the out-of-band radiations. Some of the channels may use neutral density filters for reducing the signal intensity to a level that does not saturate the PMT. The high-resolution reflection grating of the monochromator diffracts the background noise as well and makes it possible to separate and block the majority of background noises with combination of a small diaphragm. Most detectors have a window or a lens between the active area of the detector and the energy source. The solid angle cone from which energy can reach the detector is determined by the distance of the detector surface from the front surface of the window or by curvature of the lens. Antireflection coatings are applied to the detector materials or, where applicable, windows and lenses to reduce the reflection losses. The acquisition system is based on a three-channel transient digitizer working in the photon counting mode for increased

The total photon counts received by the elastic-DIAL at the distance R from both the aerosol

and molecular backscattering species, Nsig(R), is given by the general lidar equation:

achieve acceptable SNR.

the nonresonant background noise.

sensitivity at low signal levels.

4. Results and discussion

Figure 3. Schematic diagram of the combined differential absorption/Raman lidars for simultaneously remote detection of UF6 and HF.

crystal modules to deliver a laser beam at 266 nm (4th harmonic) for the off-wavelength. A practical reason for choosing fourth harmonic of Nd:YAG laser as off-wavelength is its reliability, compactness, long lifetime, and low running costs. However, its main drawback is the strong absorption by ozone at 266 nm [20].

A frequency-tripled Nd:YAG-pumped Coumarin 450 dye laser using a Littrow grating mounting operates in the frequency doubled mode for the on-wavelength, 245 nm. The grating is invariably used to allow tuning the laser across the wide gain bandwidth of the laser, the biggest advantage of dye lasers. Use of a diffraction grating alone as a wavelength selector (with suitable beam expanding optics allowing utilization of a large area of the grating surface) renders a spectral width of 0.01 nm. To reduce linewidth, an intracavity etalon is often included in the optical path. Use of an etalon along with a diffraction grating can render spectral widths as low as 0.0005 nm [21]. Coumarin 450 dye lasers with the spectral emission 427–488 nm is considered because of tunability, compactness, output stability, design simplicity, and good beam quality. Coumarin 450 has an absorption peak at 366 nm and an emission peak at 440 nm.

The laser output radiations are expanded by a beam expander (BE). The expanded beams are folded 90 by UV-enhanced aluminum-coated mirrors which have a good reflectivity in the region of 250–300 nm (R > 86%) and subsequently steered them toward the UF6 and hydrolyzed products plume released into the atmosphere. After interaction with particles and molecules of the atmosphere and plume, the elastic and inelastic backscattered radiations at 266, 245, and 297.3 nm are collected by a Newtonian-type telescope. It has an aspheric primary mirror with the focal length F. A secondary flat mirror reflects the converging light through a diaphragm with a diameter D. An MgF2 plano-convex lens (L) after the diaphragm is used to provide a collimated beam. The collimated beam after passing through a monochromator is focused on the entrance pupil of the detector through a Lyot tunable birefringent or Fabry-Pérot interference filters (IF). The diaphragm is in the focal plane of the telescope and sets the field of view (FOV) of the receiver, FOV ≈ D/F. To obtain a required FOV, we select the diameter of the diaphragm for a given focal length of the telescope. The FOV of the receiver is usually set small (about 100 micro-radians full angle FOV) to reject the background noise and achieve acceptable SNR.

As mentioned above, along with the power of the laser, the size of the primary optics is an important factor in determining the effectiveness of the system. Since the returned signal from the far distance is relatively weak, to improve the SNR and increase the detection range, a reasonable FOV is essential to suppress the sky background light, especially during the daytime measurements under sunlit conditions. The smaller aperture optics is used to work in close ranges, for example, a few 100 m, where the returned signal is a larger fraction of the transmitted power. To address the minimum distance at which returning signals are completely in the instrument's FOV, the FOV of the instruments needs to be wide enough. The wide field angle of the receiver reduces the overlap range to several 100 meters. However, the increase in FOV increases the collection of unwanted solar background, resulting in unacceptably poor SNR during daytime measurements. One way to reduce the background solar radiation in the wide FOV lidars is working in the solar blind region. Moreover, by adding an AR coating, the overall transmission of the lenses can be increased. The UV folding mirrors also can reduce the nonresonant background noise.

Monochromator is used to separate the signals, and narrowband filters (IF) are used to extract the Raman and elastic signals at 266, 245, and 297.3 nm. Narrow bandpass filters designed for these center wavelengths allow isolation of the Raman line and increase the SNR by rejecting the out-of-band radiations. Some of the channels may use neutral density filters for reducing the signal intensity to a level that does not saturate the PMT. The high-resolution reflection grating of the monochromator diffracts the background noise as well and makes it possible to separate and block the majority of background noises with combination of a small diaphragm. Most detectors have a window or a lens between the active area of the detector and the energy source. The solid angle cone from which energy can reach the detector is determined by the distance of the detector surface from the front surface of the window or by curvature of the lens. Antireflection coatings are applied to the detector materials or, where applicable, windows and lenses to reduce the reflection losses. The acquisition system is based on a three-channel transient digitizer working in the photon counting mode for increased sensitivity at low signal levels.
