Ikuo Wakaida

Development of a fiber-coupled laser-induced breakdown spectroscopy instrument for analysis of underwater debris in a nuclear reactor core

To inspect the post-accident nuclear core reactor of the TEPCO Fukushima Daiichi nuclear power plant (F1-NPP), a transportable fiber-coupled laser-induced breakdown spectroscopy (LIBS) instrument has been developed. The developed LIBS instrument was designed to analyze underwater samples in a high-radiation field by single-pulse breakdown with gas flow or double-pulse breakdown. To check the feasibility of the assembled fiber-coupled LIBS instrument for the analysis of debris material (mixture of the fuel core, fuel cladding, construction material and so on) in the F1-NPP, we investigated the influence of the radiation dose on the optical transmittance of the laser delivery fiber, compared data quality among various LIBS techniques for an underwater sample and studied the feasibility of the fiber-coupled LIBS system in an analysis of the underwater sample of the simulated debris in F1-NPP. In a feasible study conducted by using simulated debris, which was a mixture of CeO2 (surrogate of UO2), ZrO2 and Fe, we selected atomic lines suitable for the analysis of materials, and prepared calibration curves for the component elements. The feasible study has guaranteed that the developed fiber-coupled LIBS system is applicable for analyzing the debris materials in the F1-NPP.

Enhancement of intensity in microwave-assisted laser-induced breakdown spectroscopy for remote analysis of nuclear fuel recycling

An enhancement of emission intensity from a laser ablation plume, obtained by coupling a pulsed microwave using a simple wire antenna, is demonstrated to compensate the sensitivity reduction of a high resolution spectrometer that is required for nuclear fuel analysis. A gadolinium oxide sample was irradiated with 2.45 GHz, 250 W microwave pulse, and passed through a loop antenna. As a result, up to 50-fold enhancement of the emission signal was achieved for gadolinium ions. The enhancement enabled us to measure the mass concentration of europium per gadolinium, ranging from 5% to 100 ppm, and based on the extrapolation of the calibration curve the detection limit for microwave-assisted laser induced breakdown spectroscopy (MA-LIBS) was estimated to be 40 ppm. This offers a flexible and compact system of MA-LIBS for nuclear fuel analysis.

Development of RIMS apparatus for isotope analysis of calcium in nuclear waste materials

We have developed a frequency stabilized laser system for RIMS analysis of a long-lived radionuclide of calcium (41 Ca) in concrete waste materials. A stable reference laser has been developed using magnetically induced circular dichroism of atomic rubidium. The frequency stability was estimated to be 50 kHz over 8-h period from beat frequency measurement. Furthermore, a computer-controlled fringe offset lock system has been developed to transfer the high frequency stability to the other lasers for three-step excitation of atomic calcium. By using the system precise frequency tuning of 1 MHz error over 1 GHz scan was realized. The overall frequency tuning precision was also confirmed by a multistep fluorescence and resonance ionization spectroscopy of calcium.

Effect of liquid-sheet thickness on detection sensitivity for laser-induced breakdown spectroscopy of aqueous solution

For aqueous-solution-based elemental analysis, we used a thin liquid sheet (μm-scale thickness) in laser-induced breakdown spectroscopy with nanosecond laser pulses. Laser-induced plasma is emitted by focusing a pulsed Nd:YAG laser (1064 nm) on a 5- to 80-μm-thick liquid sheet in air. To optimize the conditions for detecting elements, we studied how the signal-to-background ratio (SBR) for Hα Balmer and Na-neutral emission lines depends on the liquid-sheet thickness. The SBR of the Hα Balmer and Na-neutral lines was maximized for a sheet thickness of ~20 μm at the laser energy of 100 mJ. The hydrodynamics of liquid flow induced by the laser pulse was analyzed by laser flash shadowgraph imaging. Time-resolved observation of the hydrodynamics and plasma emission suggests that the dependence of the SBR on the liquid-sheet thickness is correlated with the volume of flowing liquid that interacts with the laser pulses.

Total angular momenta of even-parity autoionizing levels and odd-parity high-lying levels of atomic uranium

Using three-step resonance ionization spectroscopy, over 200 even-parity autoionizing (AI) levels of atomic uranium, including Rydberg series converging to the second lowest ionic level (6L 11/2o), were observed in the 49 930–51 200 cm−1 energy range. Total angular momenta (J values) of these levels were determined by a polarization combination method as well as a method based on the J-momentum selection rule. Using the AI levels of which J values were determined unambiguously, unique J values were also assigned for about 70 high-lying odd-parity levels. The observed J-dependence on autoionization linewidth is interpreted as being due to a centrifugal potential barrier.

Development of radiation-resistant optical fiber for application to observation and laser spectroscopy under high radiation dose

In the Fukushima Daiichi Nuclear Power Station, it is necessary to survey the locations and conditions of fuel debris inside reactor pressure vessels or primary containment vessels under water and radiation environment in preparation for removing fuel debris. An optical fiber is well known for features such as signal transmission, light weight, superior insulation performance, water resistance and electromagnetic noise resistance. These features allow the optical fiber to simplify the instrumentation systems for in-vessel inspection, as long as provide that the optical fiber can be used under high radiation dose environment. The radiation resistance of an optical fiber was improved by increasing the amount of hydroxyl up to 1000 ppm in pure silica fiber. The improved optical fibers were irradiated with γ-ray up to 1 × 106 Gy using a 60Co source. They indicated a large peak around 600 nm and a peak tail from ultraviolet region, but no large absorption in infrared region except a hydroxyl absorption peak of 945 nm. We have confirmed that the optical fiber containing 1000 ppm hydroxyl has enough radiation resistance for radiation-induced transmission losses, and the infrared imaging is effective for observation under high radiation doses.

Ablation‐initiated Isotope‐selective Atomic Absorption Spectroscopy of Lanthanide Elements

For remote isotope analysis of low‐decontaminated trans‐uranium (TRU) fuel, absorption spectroscopy has been applied to a laser‐ablated plume of lanthanide elements. To improve isotopic selectivity and detection sensitivity of the ablated species, various experimental conditions were optimized. Isotope‐selective absorption spectra were measured by observing the slow component of the plume produced under low‐pressure rare‐gas ambient. The measured minimum line width of about 0.9 GHz was close to the Doppler width of the Gd atomic transition at room temperature. The relaxation rate of high‐lying metastable state was found to be higher than that of the ground state, which suggests that higher analytical sensitivity can be obtained using low‐lying state transition. Under helium gas environment, Doppler splitting was caused from particle motion. This effect was considered for optimization for isotope selection and analysis. Some analytical performances of this method were determined under optimum conditions an...

Isotope Shift Measurement for High-Lying Energy Levels of Atomic Uranium by Resonance Ionization Mass Spectrometry

Employing three-step resonance ionization mass spectrometry, isotope shift measurements of atomic uranium have been performed in the 32000–36000 cm -1 and 49950–50500 cm -1 regions. On the basis of the measured isotope shifts of several autoionizing Rydberg levels, isotopic difference in the ionization potential has been determined to be +445 mK (mK=10 -3 cm -1 ).

Determination of Ionization Potential of Calcium by High-Resolution Resonance Ionization Spectroscopy

High-resolution resonance ionization spectroscopy has been utilized to determine a precise ionization potential of Ca. Three-step resonance excitation with single-mode extended-cavity diode lasers populates long and unperturbed Rydberg series of 4snp ( 1 P 1 ) and 4snf ( 1 F 3 ) states in the range of n =20–150. Using an extended Ritz formula for quantum defects, the series convergence limit has been determined to be 49305.9240(20) cm -1 with the accuracy improved one order of magnitude higher than previously reported ones.

Development of Frequency Stabilized Diode Laser Based on a Spectroscopic Study of Magnetically Induced Circular Dichroism of Atomic Rubidium

We developed a frequency-stabilized diode laser using magnetically induced circular dichroism. From a spectroscopic study of the Rb-D2 line, the most stable Doppler-broadened and Doppler-free lines were found to be the transitions from the Fg=2 ground state of 85Rb and the crossover resonance from the Fg=3 ground state to the Fe=3 and 4 excited states of 85Rb, respectively. Furthermore, we measured the fluctuation of a laser locked to the Doppler-free line to be ±50 kHz/8 h and thus revealed the applicability to a long-term frequency stabilization. The laser developed can be used as a reference laser in a frequency stabilization system for multiple tunable lasers for resonance ionization mass spectrometry (RIMS) analysis.