Fiber-optic LIBS using nitrogen gas emission for hardness assessment of simulated fuel debris

We report a feasibility study on surface hardness assessment of spatially inhomogeneous radioactive materials using fiber-optic laser-induced breakdown spectroscopy (FO-LIBS) with externally supplied nitrogen gas. Conventional LIBS-based hardness assessment relies on emission lines from material components, which may not be consistently available across heterogeneous samples such as nuclear fuel debris. To address this challenge, we propose the use of plasma temperature derived from atomic nitrogen emissions in the purge gas as a compositionally independent spectroscopic basis available at every measurement location, regardless of local elemental composition. The method was demonstrated using simulated fuel debris containing uranium, zirconium, and iron. Plasma temperatures derived from atomic nitrogen lines were higher for the harder zirconium-alloy phase than for the softer uranium-rich phase, in agreement with Vickers hardness measurements and with the conventional method using uranium emission lines. These results support the feasibility of the purge gas approach as an alternative indicator of relative hardness. Because the spectroscopic signal is supplied by the purge gas rather than by the sample itself, the approach is potentially useful for remote hardness discrimination in heterogeneous materials where suitable sample-derived emission lines are not consistently available.