Development of laser ablation absorption spectroscopy for nuclear fuel materials

Miyabe, Masabumi   ; Jung, K.; Oba, Masaki ; Akaoka, Katsuaki ; Wakaida, Ikuo  

Remote isotopic analysis for highly radioactive fuel debris containing various actinides, fission products and the other reactor materials is expected to be needed for the decommissioning of Fukushima Daiichi Nuclear Power Plant in Japan. This is because the information on the amount of fissile materials in the debris is of great importance for the purpose of safeguard verification and prevention of re-criticality accident. However, to decrease radiation dose to the analytical workers handling the debris, traditional analytical techniques such as radiometric and radiochemical methods are inadequate and an alternative technique is needed. For this reason, we are developing remote spectroscopic techniques (LIBS and LAAS) for debris analysis using laser ablation process. Laser ablation absorption spectroscopy (LAAS) is the technique by combining resonance absorption spectroscopy and laser ablation. In this technique for nuclear fuel materials, one uses the probe laser whose frequency is tuned to the resonance line of the specific isotope of U or Pu and determines the isotopic abundance from the measured absorbance. For realization of highly sensitive and isotope-selective analysis, it is necessary to optimize experimental conditions to decrease kinetic energy of the ablated species and the number of ionic species so as to reduce Doppler and Stark effects. For this optimization, we have studied the temporal and spatial variations in the density distribution under various background gas conditions. Using the optimum conditions, we have measured isotope-selective absorption spectra of U and Pu and evaluated several analytical performances of LAAS.