Sparge Sampling of Molten Salts for Online Monitoring via Laser-Induced Breakdown Spectroscopy

Sparge Sampling of Molten Salts for Online Monitoring via Laser-Induced Breakdown Spectroscopy

Introduction

To meet the growing need for safe and efficient energy sources, many countries are turning to nuclear power, and advanced reactor designs are of particular interest. One category of advanced reactors that has received growing attention is molten salt reactors. Molten salt reactors employ a high-temperature molten salt as the primary coolant, which improves the efficiency of electricity production and reduces risks associated with traditional reactors, including high pressures and the possibility of reactor meltdown. Some molten salt reactor designs also use a molten salt as liquid fuel by dissolving fissile material (e.g., uranium trichloride, uranium tetrafluoride) into the primary salt. In addition to their use in advanced reactor designs, molten salts are also used to electrochemically reprocess spent nuclear fuel and are being considered as a blanket material for fusion reactors to breed future fuel (e.g., tritium). Molten salts also have non-nuclear applications, including solar energy and chemical processing. Despite the growing interest in molten salts, there are barriers to their successful implementation in nuclear energy infrastructure. Namely, in situ analytical methods are needed to monitor the composition of molten salts and radionuclide transport in real time by tracking the ingrowth of transmuted elements, fission products, and contaminants from corrosion and ingression of air and moisture. In addition to monitoring the salt itself, the off-gas stream of a molten salt reactor must be monitored. This off-gas stream would include fission gases that are released into the headspace, other volatile species, and aerosols that are generated from the salt due to bubbling and agitation. Thus, the analytical methods must be able to handle a mixed-phase stream (i.e., gases and aerosols) that may be corrosive and contain radioactive material.

One technique that is well suited to handle the challenges of monitoring molten salt systems is laser-induced breakdown spectroscopy. In laser-induced breakdown spectroscopy, a high-powered pulsed laser is focused to a point on or within the sample; the coupling of the laser energy into the sample results in rapid heating, leading to ablation of the sample and formation of a microplasma containing excited atoms from the sample. As the plasma cools, these atoms emit photons with characteristic wavelengths. With sufficient spectrometer resolution, isotopes of both light

Aerosol Generation

Transport and heavy elements can be differentiated, including isotopes of interest in molten salt reactors-especially hydrogen and uranium. Laser-induced breakdown spectroscopy is a popular technique for elemental analysis of complex samples because it can detect elements from across the periodic table in a wide range of concentrations with little-to-no sample preparation. Laser-induced breakdown spectroscopy is compatible with solid, liquid, gas, or mixed-phase (aerosol) samples. These traits have made laser-induced breakdown spectroscopy an attractive technique for in situ measurements in nuclear applications as well as a strong candidate for analysis of molten salts.

Examples in literature demonstrate laser-induced breakdown spectroscopy for analyzing molten salts, although many of these are proof-of-principle studies. Contaminants have been detected at micrograms per gram levels in frozen salt samples, but this is not amenable to real-time monitoring. Similar sensitivity has been achieved by directly firing the laser onto the surface of the molten salt; however, this resulted in issues associated with splashing, variable surface location relative to the optical focal point of the instrument, formation of oxidation layers on the surface, and plasma quenching. Due to the complications reported, aerosolization has been proposed as a potential real-time sampling approach. Previous studies have explored surrogate systems (i.e., aqueous aerosols and noble gases) to characterize the aerosols and demonstrate the capabilities of laser-induced breakdown spectroscopy for monitoring aerosol-bearing off-gas streams. An aerosol sampling approach using a Collison nebulizer was successfully applied to sample molten lithium chloride-potassium chloride eutectic spiked with uranium or cerium by Williams and Phongikaroon, who reported limits of detection of six hundred fifty micrograms per gram for uranium and one hundred forty-eight micrograms per gram for cerium. More recently, a sparging approach was investigated by Andrews et al. that did not require a nebulizer, preheated gases, or trace heating between aerosol generation and laser-induced breakdown spectroscopy measurement. However, optimization of the sampling parameters and evaluation of the quantitative