The use of Raman and TRLF spectroscopy for differentiating early stage alteration products of spent nuclear fuel

Abstract

Non-invasive techniques capable of distinguishing the alteration products of spent nuclear fuel during prolonged storage would be of great benefit when assessing the potential mobility of uranium from the fuel matrix. Two potential alteration products, becquerelite (Ca(UO2)6O4(OH)6·8(H2O)) and vandendriesscheite (Pb1.5(UO2)10O6(OH)11·11(H2O)), were characterised by multiple laser-wavelength Raman and time-resolved laser fluorescence spectroscopy (TRLFS). Chemical composition was confirmed by scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction. Well-defined Raman features were obtained, particularly with a 785 nm laser, allowing subtle differences between the two minerals to be observed. Differences were also found in the Raman spectra using lasers at 457, 532 and 633 nm, with the degree of luminescence depending on the metal cation present. Chemical quenching of luminescence in Raman data correlated with luminescence excitation and emission spectra results. Five and seven luminescence emission peaks were resolved for becquerelite and vandendriesscheite, respectively. The first reported luminescence excitation spectra were collected for each mineral but not all of the spectral ranges could be resolved into individual peaks. A luminescence decay lifetime of 5.5 ± 0.9 μs was obtained for vandendriesscheite. This study demonstrates that Raman spectroscopy and TRLFS are potentially valuable techniques for characterising uranyl oxy-hydroxides and could be used in conjunction with other methods, such as laser induced breakdown spectroscopy (LIBS), as part of a remote analysis package in nuclear waste management.