Quantification of Lanthanides in a Molten Salt Reactor Surrogate Off-Gas Stream Using Laser-Induced Breakdown Spectroscopy

Abstract

To enable the deployment of molten salt reactor technology, the development of off-gas treatment systems and advanced monitoring tools capable of operating with high temperatures and radiation fields while delivering near real-time information is necessary. This study aims to fulfill this requirement and proposes laser-induced breakdown spectroscopy (LIBS) for monitoring molten salt aerosol streams. A sheath gas measuring method was developed to protect optical elements from aerosol particles and to ensure a relatively constant aerosol stream for measurement. An aqueous system was studied to demonstrate the utility of LIBS for monitoring possible fission products in an aerosol stream: Gd, Nd, and Sm up to 2000 parts per million (ppm). A calibration model was built using partial least squares (PLS) regression with five, six, and nine latent variables for Gd, Nd, and Sm, respectively. This calibration model successfully estimated the concentrations of three test samples, which were validated with inductively charged plasma optical emission spectroscopy measurements at a 99.9% confidence interval. To enhance these models, a genetic algorithm was used to filter the spectra before entering the PLS model, thereby limiting the spectral features being regressed to those with greater correlations to concentration. This allowed for the number of latent variables used in the PLS models to be reduced to four, three, and three for Gd, Nd, and Sm, respectively. Lastly, the genetic algorithm-filtered PLS models were used to predict the concentrations of the aerosol stream on a real-time dataset and resulted in a 73%, 18%, and 25% improvement in root mean squared error of prediction compared to the original PLS models developed.

Monitoring Noble Gases (Xe and Kr) and Aerosols (Cs and Rb) in a Molten Salt Reactor Surrogate Off-Gas Stream Using Laser-Induced Breakdown Spectroscopy (LIBS)

“…The aerosol generation and measurement assemblies were placed into a LIBSCAN-150 system (Applied Photonics), for more information on the measurement assemblies the reader is referred to a previous study. 8 This 1064 nm neodymium-doped yttrium aluminum garnet (Nd: YAG) laser has a ∼5 ns pulse length with an energy of 161 ± 2.25 mJ and a 500 µm spot size. 15 The plasma light was measured using a Catalina Scientific Instruments EMU-120/ 65 echelle spectrograph outfitted with a Raptor Falcon Blue electron multiplying charge-coupled device (EMCCD) camera (λ/FWHM ≈ 12, 000).…”

“…These filters were developed using a genetic algorithm designed for feature selection, which was described in a previous study. 8 The wavelength filter removes spectral information that does not benefit the model's predictive capability. The wavelength trimming serves the same purpose to a lesser extent by only allowing a specified wavelength range to be modeled by the PLS algorithm.…”

“…7 Depending on the requirements, optical spectroscopic methods can be selected for monitoring molecular fingerprints (e.g., Raman for vibrational modes) or quantization of elemental abundance (e.g., laser-induced breakdown spectroscopy). 7,8 Optical spectroscopy is complementary to the gamma spectrometry traditionally used for monitoring nuclear facilities and can aid when high background levels may interfere with radiation measurements.…”

“…Laser-induced breakdown spectroscopy (LIBS) is a powerful technique capable of monitoring most elements in the periodic table, both stable and radioactive, as well as some molecular signals. [7][8][9][10][11] In a recent study, Namitha et al explored LIBS for detection of gaseous Xe and Kr in the headspace of a sodium-cooled fast breeder reactor to detect fuel pin rupture. 11 Univariate calibration curves were built with calibration samples ranging from 60 parts per billion (ppb) to 3 parts per million (ppm) and from 40 ppb to 2 ppm for Xe and Kr, respectively, in Ar.…”

“…Previously, a LIBS measurement system was shown to have the ability to simultaneously monitor multiple aerosol lanthanides (Gd, Nd, and Sm) in real time using chemometric regression models. 8 However, there is still a need to show this system's ability to monitor noble gases and aerosol species concurrently.…”