Measurement of uranium and fission products in a spent fuel sample using fiber-optic laser-induced breakdown spectroscopy

Abstract

This study presents the development and application of a fiber-optic laser-induced breakdown spectroscopy (FO-LIBS) system designed for remote, in situ analysis of nuclear fuel debris at the Fukushima Daiichi Nuclear Power Station (FDNPS). To evaluate its performance in uranium detection and spatial mapping, a two-stage experimental approach was used. In the first stage, the FO-LIBS detection limit for uranium was determined in a laboratory environment using UO₂ samples with different concentrations. Based on prominent uranium emission lines and calibration restricted to a low-concentration linear region (<10 wt%), a detection limit of approximately 3.5–3.7 wt% was achieved, with regression coefficient R² = 0.95. The system's ability for two-dimensional elemental mapping was then demonstrated on a simulated debris sample (UO₂–Zr–SUS), and the LIBS spatial maps were validated against scanning electron microscopy-energy dispersive X-ray spectroscopy measurements. In the second stage, the FO-LIBS system was deployed for the first time in a hot cell under high radiation (230 mSv h⁻¹) to analyse an actual boiling water reactor's spent fuel sample, as an exemplar of FDNPS fuel debris. The system successfully identified the main fuel components (U and Zr) along with several long-lived fission products and related markers (analytical proxies) such as Sr, Cs, Mo, Ba, and Rb. The LIBS emission intensity of Ba, Rb, and Cs near the periphery region was slightly higher than at the center of the fuel. This indicates a higher concentration of these elements in that area.