Portable LIBS for In-Situ Identification of Nuclear Materials

Chemical analysis of nuclear materials is challenging, costly, and time-consuming. This is especially true at the Los Alamos National Laboratory (LANL) Plutonium Facility. Some of the limitations working in this facility include limited space for analytical instrumentations inside and outside our radiologically “hot” gloveboxes, material handling restrictions, difficulties in packaging and transporting samples to a separate analytical facility, and timely measurements and analysis. An in-situ analytical technique that could provide rapid analysis of the chemical composition of nuclear materials could save LANL considerable time and costs. Laser induced breakdown spectroscopy (LIBS) could potentially help with these efforts.

LIBS is a fast analytical technique that can quickly characterize the elemental composition of a target sample.

LIBS is a fast analytical technique that can quickly characterize the elemental composition of a target sample. It operates by producing a focused laser pulse that transforms sample surface (less than a microgram) into a plasma. Within that plasma, atoms and ions are excited to higher energy states. The excited species emit photons with characteristic wavelengths that are indicative of the chemical elements present in the sample—LIBS probes this “atomic fingerprint” of the target material. In order to extend its applications to the nuclear industry, we (and others) have been developing LIBS by targeting the detection of actinide materials and nuclear processes. In particular, we are employing recent advances in portable handheld LIBS instrumentation to explore the viability of this technique for in-situ nuclear material identification and manufacturing needs at the LANL Plutonium Facility. LIBS has the potential to decrease analysis times from many weeks to just a few minutes. In short, the characteristics that makes this technique desirable are: 

1. In-situ and near real-time measurements 
2. Rapid measurement time (seconds to minutes) 
3. Minimal sample destruction (ng–μg) 
4. Little to no sample preparation 
5. Ease of instrumentation set up 

Challenges

One of the challenges we hope to address with portable LIBS is the identification of legacy nuclear material, a need that is often encountered at the LANL Plutonium Facility. In many cases, legacy nuclear materials are poorly characterized and knowledge of the chemical composition is limited to the material’s known processing history. The use of portable handheld LIBS could provide a rapid assessment of these nuclear materials to determine if additional analytical actions are needed and to map a recovery or disposal path for them. The speed and efficiency with which these determinations can be made will allow for significant time and cost savings by reducing the need for a full chemical analysis by more sensitive and costly techniques.

Another major challenge we aim to address is in the application of LIBS to in-situ nuclear manufacturing. Our goal is to be able to develop a rapid in-situ LIBS analytical tool for the in-line application of nuclear manufacturing of plutonium metal and its associated alloys (Fig. 1). In several of these steps, we usually deploy a suite of off-line analytical techniques (e.g., inductively coupled plasma optical emission spectroscopy and X-ray fluorescence) to quantify the various elemental impurities or alloying constituents (e.g., gallium) to allow us to proceed to the next steps with confidence and to eventually certify the end product. 

Our goal is to develop an automated tool to give a binary go/no-go answer for the presence of impurities and to ultimately derive the impurity concentration with a known level of confidence.

Since the portable LIBS instrument has a low spectral resolution, we want to integrate LIBS with modeling and machine learning techniques to improve the accuracy of our data analyses. We will start by empirically “learning” the elemental plutonium spectrum from the measurements and then identify impurities in the LIBS spectrum. Our goal is to develop an automated tool to give a binary go/no-go answer for the presence of impurities and to ultimately derive the impurity concentration with a known level of confidence. The machine learning tools we are currently developing, based on convolutional neural networks, show promising results for identifying the presence of gallium impurities. The deployment of LIBS at some of these steps (Fig. 1) can potentially shorten the analytical characterization times. The development of portable LIBS with an integrated processing technique can potentially provide additional capabilities to our current suite of analytical tools. 

Deployment of portable LIBS inside radiologically hot gloveboxes 

In this research, we demonstrate the application of the portable LIBS at the LANL Plutonium Facility to help identify and characterize legacy nuclear materials and to show the feasibility of using this technology for current manufacturing needs. 

We used the commercial, off-the-shelf SciAps Z300 LIBS instrument inside a radiologically hot glovebox—because of its compact size and portability, this instrument can be easily set up in different glovebox locations. In the configuration presented here (Fig. 2), the instrument can either be transported to the sample, or vice-versa. The LIBS instrument is equipped with three spectrophotometers to provide a total continuous spectral range of 185–950 nm (180–365, 365–620, and 620–950 nm, individually). This instrument can detect all the naturally-occurring elements in the periodic table from hydrogen to uranium. In our current work, we have also been able to extend this to plutonium and americium. 

The observed complexities in the plutonium atomic emission spectra arise from numerous transitions between the quantum states of the atoms and ions (Fig. 3). The complex atomic structure of plutonium results in a very rich emission spectrum involving thousands of transitions involving the ground level and many of the excited levels. Even though the emission line spectrum is intricate and the instrument has low spectral resolution, peak fitting or other analysis can be used to deconvolve some of these emission lines. We are in the process of assigning these peaks. 

LIBS for nuclear material identification 

Elemental and chemical identification of legacy nuclear materials is one of the important applications we are exploring with portable LIBS. As we continue with the cleanup of nuclear materials stored at LANL, we want to be able to rapidly identify and confirm the chemical constituents of many legacy nuclear materials. 

As a test, we used the portable LIBS instrument to identify an unknown legacy inventory item (Fig. 4), believed to be the product of a salt-stripping process on a molten salt extraction residue. Based on the creation date, name, and visual appearance of the item, it was assumed that it was a product of a 30-year-old experimental research and developmental campaign to recover plutonium from chloride-based americium extraction salts. These mixtures typically contain sodium, potassium, and magnesium chloride salts, 3 wt.% of ²⁴¹Am (as AmCl₃), and 5–20 wt.% plutonium (as PuCl₃). The actinide isotopes are co-oxidized. 

The amount of entrained plutonium required that the salts were treated using a time-consuming aqueous recovery operation. An experimental effort was undertaken to develop an alternative pyrochemical-based process to separate the Pu and Am in the extraction salts, using the same furnaces that generated them. The experimental effort examined several possible routes for the selective removal of these actinides from the extraction salts: 

1. Co-reduction of the PuCl₃ and AmCl₃, followed by selective extraction of Am into an excess amount of Ca metal. 
2. Selective reduction of PuCl₃ by a sub-stoichiometric amount of Ca metal. 
3. Selective reduction of PuCl₃ using La metal. 

Since the acceptance criteria for the discard of pyrochemical residues must meet stringent requirements for the absence of potentially pyrophoric materials (such as calcium or its substitute aluminum), it was important to determine the elemental composition of the original, legacy inventory item. 

By interrogating freshly fractured surfaces of the legacy inventory item (Fig. 6) using LIBS we were able to rule out reaction route 3 for this product. Additionally, we also observed that the item was chemically heterogeneous (Fig. 4). While the overall chemical composition of the sample highlights its chemical constituents, the elemental distribution within the sample differs significantly. The Pu present is mainly located in the interior of the sample (Fig. 4 insert). It also contains a significant amount of Am. Therefore, through the use of our portable LIBS instrument, we were able to rule out one of the possible reaction routes and provide qualitative and localized elemental composition of this material.

LIBS for nuclear manufacturing monitoring 

Another goal is to develop a rapid in-situ LIBS analytical tool for monitoring nuclear manufacturing of plutonium metal and its associated alloys. One of the major challenges to achieving this goal is to be able to quantify key impurities and constituents (e.g., Ga) in bulk Pu metal. We are beginning to tackle this problem (Fig. 5). Ongoing LIBS measurements and analyses on other Pu-Ga alloy systems are being pursued to help determine the limit of detection for Ga and to build a suitable calibration data set to quantify its concentration. 

Summary 

We have demonstrated that a commercial off-the-shelf portable LIBS system has wide-ranging capabilities for elemental analysis of nuclear materials. Qualitative and potentially quantitative analyses of many impurities and constituents in plutonium metals, oxides, and other matrices are possible and should prove valuable to a number of ongoing LANL projects and programs. 

Dung Vu 

Dung Vu’s research interests involve the integration of chemistry, biology, and physics scientific fields. She has more than 25 years of experience in applying spectroscopic techniques to understanding the dynamics and function of biomolecules, synthesizing bionanomaterials, discovering disease-related biomarkers, developing biosensors, and characterizing actinide materials. Her current projects include applying and developing LIBS techniques to analyze nuclear materials for in-situ nuclear manufacturing processing and actinide material identification and elemental quantification.

Further reading 

1. R. Noll, et al., “LIBS analyses for industrial applications—an overview of developments from 2014 to 2018,” J. Anal. At. Spectrom., 2018. 33(6), 945. 
2. J.E. Barefield, et al., “Analysis and spectral assignments of mixed actinide oxide samples using laser-induced breakdown spectroscopy (LIBS),” Appl. Spectrosc., 2013, 67(4), 433. 
3. E. Garlea, et al., “Novel use of a hand-held laser induced breakdown spectroscopy instrument to monitor hydride corrosion in uranium,” Spectrochim. Acta Part B At. Spectrosc., 2019, 159, 105651. 
4. G.S. Senesi, R.S. Harmon, R.R. Hark, “Field-portable and handheld laser-induced breakdown spectroscopy: Historical review, current status and future prospects,” Spectrochim. Acta Part B At. Spectrosc., 2021, 175, 106013. 
5. B. Connors, A. Somers, D. Day, “Application of handheld laser-induced breakdown spectroscopy (LIBS) to geochemical analysis,” Appl Spectrosc, 2016. 70(5), 810. 
6. Y. LeCun, Y. Bengio, G. Hinton, “Deep learning,” Nature, 2015, 521(7553), 436. 
7. J. Blaise, M. Fred, R. Gutmacher, “The atomic spectrum of plutonium,” Technical report ANL-83-95, 1984, Argonne National Laboratory: Argonne, IL. 
8. K.W. Fife, M.H. West, “Pyrochemical investigations into recovering plutonium from americium extraction salt residues,” Technical report LA-10963-MS. 1987, Los Alamos National Laboratory: Los Alamos, NM. 

Acknowledgments 

Funding for this work is provided by the Materials Recovery and Recycle, Plutonium Sustainment, Advanced Simulation and Computing Programs. We thank Jeremy Mitchell and Clarissa Yablinsky for providing us with the 7 wt.% Ga-Pu alloy sample to perform LIBS measurements and analysis. We thank Brad DiGiovine for working with us to design a lightweight, compact, and stable LIBS instrument holder for use inside our radiologically hot gloveboxes. We thank Laura Worl, James Rubin, and Stephen Willson for initially recognizing the utility of LIBS for applications inside the LANL Plutonium Facility.