Nanosecond Laser-Induced Breakdown Spectroscopy for Novel Nuclear Diagnostics

Abstract

The success of nuclear energy is inherently tied to the ability to implement effective nuclear instrumentation. Developing instrumentation is challenging because of the large scale of deployment, limited workforce, and the advent of novel reactor designs. Laser-induced breakdown spectroscopy (LIBS), a technique where laser pulses produce microplasmas that emit material-characteristic light, may enable the development of novel reactor diagnostics. The work performed in this thesis aimed to demonstrate the utility of LIBS for operational monitoring and safeguards of advanced reactors.

Within nuclear safeguards, reliable uranium enrichment assay is paramount to ensuring compliance with nuclear arms treaties. LIBS represents a promising alternative to current measurement techniques, as it is compatible with the gaseous uranium hexafluoride (UF6) typically found in enrichment facilities. Studies of the spectral behavior of the UF6 LIBS plasma excited by various lasers are presented in the 646 nm spectral region, which is promising for enrichment measurements. Results show that, while this region is not impacted by the excitation of higher energy features at increased pressures and laser energies, best practice is still to perform measurements at low energies and ambient pressures. While the different lasers produced similar results, the 1064 nm, nanosecond Nd:YAG laser holds a slight advantage in terms of SBR, electron number density, and plasma temperature. Evidence is presented for the self-absorption of the prominent U I 646.49 nm line suggested for enrichment measurement.

LIBS has also been proposed as a diagnostic for safeguarding molten salt reactors (MSRs). When combined with laser-induced fluorescence (LIF), where the excitation wavelength of the light source is tuned to a resonant transition of the analyte, a potential exists to rapidly collect characteristic data from harsh environments of MSRs. The feasibility of such measurements was investigated using neodymium chloride as a surrogate for salt matrices used in MSRs. A measurement scheme for the detection of Nd using LIF is demonstrated, and the sensitivity of LIF is compared with that of resonant LIBS. The approach has the potential to avoid the issue of splashing usually associated with LIBS of liquid samples.

The compatibility of LIBS with fluid analytes also implies the potential for monitoring high-temperature gas reactors (HTGRs). A previous study demonstrated the feasibility of using LIBS for the detection of Xe in ambient He, mimicking a n HTGR environment. Here, I show that the sensitivity can be significantly improved through double-pulse LIBS. In the experiment, an SBR enhancement factor of up to 14 was achieved, lowering the limit of detection to the ppb range.

For both MSRs and HTGRs, the implementation of laser instrumentation will depend on the ability to make measurements in high radiation environments. To address the associated difficulties, the final study in this work presents a framework for quantifying the effects of radiation damage of glass components on the collected LIBS spectra. By examining the changes in spectrally-dependent linear absorption of common glasses after exposure to radiation, the predicted attenuation effects are used to calculate line ratio changes, the effects on Boltzmann plot analysis, and changes to sensitivity.