Laser-Induced Breakdown Spectroscopy: Application to Nuclear Waste Management

Laser-induced breakdown spectroscopy (LIBS) was applied to the analysis of simulant slurry samples used in the vitrification process of liquid radioactive wastes. The capability of direct analysis of slurry will significantly increase analytical throughput and will reduce waste generation in radiological analytical facilities, providing analyses suitable for waste acceptance and production records. LIBS as a remote and the real-time analytical tool has been evaluated for direct analysis of the Defense Waste Processing Facility (DWPF) Sludge Receipt and Adjustment Tank (SRAT) product. This paper examines the experimental conditions associated with the slurry measurement with LIBS to achieve good measurement precision. LIBS analysis was performed by two different detection systems: Czerny-Turner spectrometer coupled with intensified diode array detector (IDAD) and an Echelle spectrometer with intensified charge coupled device (ICCD). The Echelle detection system shows a high efficiency in simultaneous multi-element detection and determination of the physical quantities of the simulant. INTRODUCTION Laser induced breakdown spectroscopy (LIBS) is the spectroscopic method which analyzes the elemental composition in a laser-produced plasma plume which is produced by a focused high pulse energy laser beam. Strong electric field of laser pulse and pulse-duration induce the ablation, atomization, ionization and excitation of the sample. The spectral information which reflects the existence of the characteristic atom in the sample can be obtained from these optical emission spectra. LIBS is suitable for rapid on-line elemental analysis of any material phase and has proved its value in obtaining analytical atomic emission spectra directly from solid, liquid, and gaseous samples.[1-3] LIBS is a fast qualitative analysis technique that has been used on a variety of samples such as polluted soils, Mars surface, alloy, glass, explosive, tissues, etc. In some solid samples, LIBS has demonstrated an accuracy of 3-6% for elements with a concentration greater than 1 wt% and an accuracy of 5-10% or better for minor elements depending on their concentration.[4,5] Recently, LIBS technique has been popular because of its intrinsic advantages and the significant developments of instrument (broadband spectrometer with high resolution and intensified charge coupled device). Conjugation with the optical fiber, minimal sample preparation, and quick on-line elemental analysis are the distinguishable marks of the LIBS probe, which make it practicable to apply in the inaccessible place.[6,7] The typical laboratory analytical tools are generally unsuitable for field application. For example, ICP require the dissolution of sample to spray into a flame as a form of an aerosol. In the other hand, LIBS technique has ability to direct detection that can reduce time involved in sample pretreatment processes. Moreover, fast direct analysis and availability of optical fiber can avoid WM2009 Conference, March 1-5, 2009, Phoenix, AZ the risk of hazardous reagents and contamination of sample. LIBS technique can analyze conducting as well as non-conducting material because of the use of the laser beam. Vitrification of liquid radioactive wastes is an essential task in nuclear industries, facilitating the safe handling and long-term storage of radioactive waste. Analytic tools are necessary for analyzing the sludge prior to and during the vitrification process of the liquid radioactive wastes. The LIBS technique has been applied for the analysis of glass [8], glass melts [9,10] and glass batch [11]. The aim of this paper is to explore the possibility of applying LIBS as a remote and the near real-time analytical tool that can be used to analyze liquid radioactive wastes during the vitrification process using two detection systems. The main issues to be resolved with LIBS analysis of liquid samples are poor detection sensitivity and precision. Because water can quench the laser plasma and suppresses the LIBS signal, poor sensitivity may result. Large standard deviations for LIBS liquid data are due to the laser induced shock wave caused turbulence on the liquid surface. Slurry samples contain a large amount of water and large particle sizes. The effects of water content and particle sizes on LIBS measurement will also need to be studied to determine best data analysis method. To evaluate the figure of merit of direct slurry measurement with LIBS, two DWPF simulant slurry samples with different acid levels from SRNL were used in the study. The test results with these slurry samples showed that the splattering and surface cavitations result in large signal fluctuation. To improve LIBS’ reproducibility and detection limits for slurry measurements, various experimental parameters which can affect LIBS’ analytical figure of merit were studied. The study has shown that by using the appropriate slurry sample handling systems, optimum experimental parameters and some data processing techniques, reasonable accuracy and precision for the major elements can be achieved.[12] However, further work on improving signal sensitivity and data reproducibility is needed. The Echelle detection system providing broadband with high resolution is more suitable for detecting the multiple elements than the Czerny-Turner detection system for the application of the LIBS technique [11,13]. The work presents the results of LIBS analysis of simulant slurry samples using two different detection systems: Czerny-Turner spectrometer coupled with intensified diode array detector (IDAD) and an Echelle spectrometer with intensified charge coupled device (ICCD). 2. EXPERIMENTAL DETAILS 2.1 LIBS Systems Two different systems based on Czerny-Turner and Echelle spectrometers have been applied to record the LIBS spectra in this study (See Table 1 for the specification of these two detection systems). Figure 1 shows a general schematic diagram of the experimental setup used for recording LIBS spectra. A frequency-doubled, Q-switched Nd:YAG laser (Continuum Surelite I for the Czerny-Turner and Continuum Surelite III for the Echelle detection system) was incorporated into the LIBS system as an excitation source. The 532-nm laser light is focused onto the sample surface using an ultraviolet (UV) grade quartz lens of 300 or 500 mm focal length. Atomic emission from the laser-induced plasma was collected by an optical fiber bundle using a UV-grade quartz lens. The first detection system contains a Czerny-Turner spectrograph (SPEX 500M) with a grating of 2400 grooves/mm fitted with a 1024element intensified diode array detector (IDAD). The simultaneous spectral coverage of this detection system is about 20 nm with a linear dispersion of 19.5 pm/pixel. The Echelle spectrometer detection system is equipped with a 1024 x 1024 element intensified charge coupled device (ICCD). The simultaneous spectral coverage of this detection system is from 200-780 nm. The linear dispersion for the Echelle spectrometer system varies from 5 pm/pixel at 200 nm to 19 pm/pixel at 780 nm. Therefore, the Echelle spectrometer detection system provides a higher resolution capability at the UV-VIS region than the Czerny-Turner spectrometer system. A pulse generator is used to trigger and synchronize the detector with the laser operation and provides the desired gate delay and width for detection. Figure 2 shows a spectral comparison of the LIBS signal recorded by the Czerny-Turner spectrometer and the Echelle spectrometer, respectively. The Czerny-Turner spectrometer has a linear dispersion value greater than that WM2009 Conference, March 1-5, 2009, Phoenix, AZ of the Echelle spectrometer [11] at the observed spectral region and therefore, it is not sufficient to resolve the multiple spectral emission lines in the region shown in Figure 2. 2.2 Sample Preparation Sludge Receipt and Adjustment Tank (SRAT) slurry sample used in this study is mainly made up of 78.2% water (H2O), 5.8% ferric oxide (Fe2O3), 2.6% alumina (Al2O3), 3.6% sodium oxide (Na2O), and small quantities of oxides of carbon, silica, chromium, manganese, magnesium, etc. The slurry composition was chosen as a surrogate for the radioactive slurry that is input into the Savannah River Site’s Defense Wastes Processing Facility (DWPF) glass melter. For the vitrification process, the slurry sample was first acidified until the pH 6 using strong nitric acid. Glass frit containing SiO2 (70.0%), B2O3 (12.0%), Na2O (11.0%), Li2O (5.0%) and MgO (2.0%) was added into the acidified SRAT slurry. This mixture called as the slurry mix evaporator (SME) products is finally fed into the glass melter to make the solid stimulant low activity test reference material (LRM) glass. 3. RESULTS AND DISCUSSIONS 3.1 Characteristics of laser-induced plasma Echelle spectrometer provides large spectral coverage (200-780nm) with good spectral resolution, it is therefore used to characterize laser-induced plasma from slurry sample. The integrated emission line intensity in the optically thin plasma condition is given by ) ( ) / exp( T U kT E n g A hc B I j x j ji    (1) Here, n is the total number density of species x, T is the excitation temperature, U(T) is the atomic partition function,  is the transition wavelength (in m). h is Planck’s constant (in Js), c is the speed of light (in m/s), B is a constant factor, Aji is the transition probability (in s), the index j-th and i-th indicate the upper and lower atomic levels, and gj is the degeneracy of the upper atomic level. The excitation temperature can be calculated for the same species [14] by the natural logarithm of Equation (1): kT E T U n g A I j x j ji   ) ) ( ln( ln  (2) This method is known as Boltzmann plot and uses the slope of the dashed line (-1/kT), obtained from linear fitting, to yield the excitation temperature. The excitation temperature was also estimated from Fe I line intensities of SME products as a function of Fe concentrations. The average excitation temperature was 7137K. The error bars in the excitation temperature correspond to the standard deviation of ten measurements [15]. The spectral line wavelengths, energies of the upper levels, statistical weights, and transition probabilities for each element were obtained from NIST atomic spectral database [16]. Electron density can be estimated by the Stark broadened FWHM linewidth, based on the electron impact approximation, and corrected for quasi-static ion broadening [17]. Assuming the contribution from quasistatic ion broadening to be negligible, the ratio formulation for non-hydrogen-like emission lines is given by ) 10 w 2 ( n 16 e e Stark    (3) Here, ∆Stark is the Stark broadened FWHM linewidth in Angstrom, ne is the electron density in cm, and we is the reference electron impact parameter width in Angstrom. The constant (10) in cm is the reference electron density. Notably, Stark broadened FWHM (∆Stark) is the main contribution of Lorentz WM2009 Conference, March 1-5, 2009, Phoenix, AZ broadening FWHM linewidth (i.e. ∆Stark ≈ ΔλL) [18]. Considering the symmetry of the observed emission line centered at the peak value, the Fe I line at 381.584 nm was selected to extract the FWHM linewidth of the Lorentz component (ΔλL). A non-linear least-squares fitting of Voigt function to the observed experimental data points ,obtained from the SME product with 4.05% Fe concentration, was performed utilizing a Levenberg-Marquardt algorithm installed in Origin software version 7.0 (OriginLab Co., USA). The estimated electron density was 2.06  10 cm using the calculated Lorentz linewidth. Substituting the electron density in equation (4) yielded the ionization temperature [19,20] which was estimated as 7120K from Fe I 381.584 nm and Fe II 275.574 nm : ) kT E E V exp( gA gA h n ) kT m 2 ( 2 I I