Jong-Il Yun

Highly sensitive analysis of boron and lithium in aqueous solution using dual-pulse laser-induced breakdown spectroscopy.

We have applied a dual-pulse laser-induced breakdown spectroscopy (DP-LIBS) to sensitively detect concentrations of boron and lithium in aqueous solution. Sequential laser pulses from two separate Q-switched Nd:YAG lasers at 532 nm wavelength have been employed to generate laser-induced plasma on a water jet. For achieving sensitive elemental detection, the optimal timing between two laser pulses was investigated. The optimum time delay between two laser pulses for the B atomic emission lines was found to be less than 3 μs and approximately 10 μs for the Li atomic emission line. Under these optimized conditions, the detection limit was attained in the range of 0.8 ppm for boron and 0.8 ppb for lithium. In particular, the sensitivity for detecting boron by excitation of laminar liquid jet was found to be excellent by nearly 2 orders of magnitude compared with 80 ppm reported in the literature. These sensitivities of laser-induced breakdown spectroscopy are very practical for the online elemental analysis of boric acid and lithium hydroxide serving as neutron absorber and pH controller in the primary coolant water of pressurized water reactors, respectively.

Effect of reduction on the stability of Pu(VI) hydrolysis species

Abstract The chemical speciation of aqueous plutonium species has been performed by spectrophotometry. The main focus was concentrated on the effect of Pu(V) produced by the reduction of Pu(VI) on the stability of the Pu(VI) hydrolysis species. In order to detect the trace amounts of PuO2+ ions, a liquid waveguide capillary cell with an optical path length of 100 cm was connected to a spectrophotometer. As a consequence of the improved detection sensitivity, an absorption band at 569 nm being indicative of Pu(V) was observed within one day after the sample preparation from the pure Pu(VI) solution. The first hydrolysis species was (PuO2)2(OH)22+, at plutonium concentrations of 0.1–0.12 mM and at weak acidic conditions (pH 5–6). The formation constant of (PuO2)2(OH)22+ was determined to be log*β′22=−7.34±0.22 at 0.01 M NaClO4. This dinuclear hydroxide species was unstable and disappeared within one month after the sample preparation at the present experimental condition. In the neutral pH range, the second hydrolysis species was formed and remained for over one year. The samples showed different behavior for the reduction of Pu(VI). The time dependent distribution of plutonium species and their characteristic features in the absorption spectra are reported.

Analysis of oxidation behavior of Ni-base superalloys by laser-induced breakdown spectroscopy

The application of laser-induced breakdown spectroscopy (LIBS) to elemental depth profile analysis of high temperature oxidation behavior of Ni-based superalloys is presented. An oxidation test of two Ni-base superalloys, Alloy 617 and Haynes 230, to be considered as the most promising candidates for very high temperature reactor (VHTR), was carried out in dry air at 900 °C. A duplex external oxide layer of MnCr2O4 and Cr2O3 with internal Al2O3 oxides was mainly formed in both alloys. In addition, in Alloy 617, Ni and Ti enriched oxides were observed at the surface unlike in Haynes 230, and Alloy 617 was more susceptible to intergranular oxidation. Generally, the oxidation of Alloy 617 was more extensive than that of Haynes 230, i.e. Alloy 617 formed a thicker oxide layer of ∼8 μm, compared with Haynes 230 with an oxide layer of ∼5 μm in thickness after 1000 h. Depth profiles obtained by LIBS are found to be in broad agreement with those obtained by established techniques such as X-ray diffraction (XRD), scanning electron microscopy coupled to energy dispersive X-ray spectrometry (SEM-EDS), and secondary ion mass spectrometry (SIMS).

ASSESSMENT OF WIND CHARACTERISTICS AND ATMOSPHERIC DISPERSION MODELING OF 137Cs ON THE BARAKAH NPP AREA IN THE UAE

This paper presents the results of an analysis of wind characteristics and atmosphere dispersion modeling that are based on computational simulation and part of a preliminary study evaluating environmental radiation monitoring system (ERMS) positions within the Barakah nuclear power plant (BNPP). The return period of extreme wind speed was estimated using the Weibull distribution over the life time of the BNPP. In the annual meteorological modeling, the winds from the north and west accounted for more than 90 % of the wind directions. Seasonal effects were not represented. However, a discrepancy in the tendency between daytime and nighttime was observed. Six variations of cesium-137 ( 137 Cs) dispersion test were simulated under severe accident condition. The 137 Cs dispersion was strongly influenced by the direction and speed of the main wind. A virtual receptor was set and calculated for observation of the 137 Cs movement and accumulation. The results of the surface roughness effect demonstrated that the deposition of 137 Cs was affected by surface condition. The results of these studies offer useful information for developing environmental radiation monitoring systems (ERMSs) for the BNPP and can be used to assess the environmental effects of new nuclear power plant.

Formation, stability and structural characterization of ternary MgUO2(CO3)32− and Mg2UO2(CO3)3(aq) complexes

Abstract The formation of ternary Mg-UO2-CO3 complexes under weakly alkaline pH conditions was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) and compared to Ca-UO2-CO3 complexes. The presence of two different Mg-UO2-CO3 complexes was identified by means of two distinct fluorescence lifetimes of 17±2 ns and 51±2 ns derived from the multi-exponential decay of the fluorescence signal. Slope analysis in terms of fluorescence intensity coupled with fluorescence intensity factor as a function of log [Mg(II)] was conducted for the identification of the Mg-UO2-CO3 complexes forming. For the first time, the formation of both MgUO2(CO3)32− and Mg2UO2(CO3)3(aq) species was confirmed and the corresponding equilibrium constants were determined as log β0113=25.8±0.3 and log β0213=27.1±0.6, respectively. Complementarily, fundamental structural information for both Ca-UO2-CO3 and Mg-UO2-CO3 complexes was gained by extended EXAFS revealing very similar structures between these two species, except for the clearly shorter U-Mg distance (3.83 Å) compared with U-Ca distance (4.15 Å). These results confirmed the inner-sphere character of the Ca/Mg-UO2-CO3 complexes. The formation constants determined for MgUO2(CO3)32− and Mg2UO2(CO3)3(aq) species indicate that ternary Mg-UO2-CO3 complexes contribute to the relevant uranium species in carbonate saturated solutions under neutral to weakly alkaline pH conditions in the presence of Mg(II) ions, which will induce notable influences on the U(VI) chemical species under seawater conditions.

Sorption of cobalt(II) on soil: effects of birnessite and humic acid

To study the sorption behavior of Co(II) on soil and soil components such as birnessite, humic acid (HA) and their mixture, a series of experiment were conducted using the batch equilibrium technique on parameters such as equilibrium time, ionic strength, solution pH, and temperature. The soil samples collected from location near radioactive waste repository in Korea were used and birnessite was synthesized using a method by McKenzie for experiment. The experimental results indicate that Co sorption on soil, birnessite and soil with birnessite are strongly affected by the pH of solution. Typical for metal sorption to soils, the fraction of Co adsorbed increased as a function of pH at the experimental conditions. For sorption isotherm, the Freundlich equation provides a good fit for sorption on soil and soil with birnessite. Adsorption of HA on birnessite decreased with increase of pH, with a sharp decrease at pH 5–6. From Co sorption experiment in a ternary system of Co, birnessite, and HA, the presence of HA enhanced Co adsorption at pH below 6.5 and reduced the Co sorption at the intermediate and high pH.

Spectroscopic study on the mononuclear hydrolysis species of Pu(VI) under oxidation conditions

Abstract The formation constants of mononuclear hydrolysis species of Pu(VI) were determined by absorption spectroscopy under carefully designed experimental conditions. In order to exclude the formation of polynuclear hydrolysis species, the concentration of plutonium was diluted below 50 μM. To maintain the oxidation state of Pu(VI), NaOCl was applied as an oxidant. The trace amount of Pu(V) was monitored by the measurement of redox potential and absorption spectroscopy by using a liquid waveguide capillary cell with the optical path length of 100 cm. The determined molar absorption coefficients (ε, M−1 cm−1) were 272±26 and 436±33 for PuO2(OH)+ and PuO2(OH)2(aq) species, respectively. The formation constants of mononuclear hydrolysis species of Pu(VI) at ionic strength of 0.01 M NaClO4 were determined as follows: logߙ*β′1 ߙ(for PuO2(OH)+) = −5.8±0.3, logߙ*β′2 ߙ(for PuO2(OH)2(aq)) = −13.4±0.2 and logߙ*β′3ߙ(for PuO2(OH)3−) = −24.3±0.8.

EFFECT OF CARBONATE ON THE SOLUBILITY OF NEPTUNIUM IN NATURAL GRANITIC GROUNDWATER

This study investigates the solubility of neptunium (Np) in the deep natural groundwater of the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT). According to a Pourbaix diagram (pH-Eh diagram) that was calculated using the geochemical modeling program PHREEQC 2.0, the redox potential and the carbonate ion concentration both control the solubility of neptunium. The carbonate effect becomes pronounced when the total carbonate concentration is higher than 1.5 × 10 -2 M at E h = -200 mV and the pH value is 10. Given the assumption that the solubility-limiting stable solid phase is Np(OH)₄(am) under the reducing condition relevant to KURT, the soluble neptunium concentrations were in the range of 1 × 10 -9 M to 3 × 10 -9 M under natural groundwater conditions. However, the solubility of neptunium, which was calculated with the formation constants of neptunium complexes selected in an OECD-NEA TDB review, strongly deviates from the value measured in natural groundwater. Thus, it is highly recommended that a prediction of neptunium solubility is based on the formation constants of ternary Np(IV) hydroxo-carbonato complexes, even though the presence of those complexes is deficient in terms of the characterization of neptunium species. Based on a comparison of the measurements and calculations of geochemical modeling, the formation constants for the “upper limit” of the Np(IV) hydroxo-carbonato complexes, namely Np(OH)y(CO₃)z 4-y-2z , were appraised as follows: log K˚ 122 = -3.0±0.5 for Np(OH)₂(CO₃)₂ 2- , log K˚ 131 =-5.0±0.5 for Np(OH)₃(CO₃)-, and log K˚ 141 = -6.0±0.5 for Np(OH)₄(CO₃) 2- .

Redox behaviors of Fe(II/III) and U(IV/VI) studied in synthetic water and KURT groundwater by potentiometry and spectroscopy

The redox behaviors of Fe(II/III) and U(IV/VI) in both synthetic samples and natural groundwater were investigated with potentiometry, UV/VIS absorption spectroscopy, and time-resolved laser fluorescence spectroscopy. Total dissolved Fe(II/III) concentration along with presence of mixed redox couples of Fe2+/Fe3+ and Fe2+/Fe2O3(s) were revealed to be the major factors influencing on the redox potentials. Considerable discrepancies between redox potentials obtained with quantitative analysis and chemical speciation of Fe(II/III) and U(IV/VI) ions were identified in the KAERI Underground Research Tunnel groundwater. Chemical speciation of U(IV) in natural groundwater without considering relevant complexation reaction might cause relatively large uncertainties in redox potential calculations.

Decontamination of radionuclides by functionalized mesoporous silica under gamma irradiation

Schiff base functionalized mesoporous silica (SA-SBA-15) was synthesized by the co-condensation method to remove the radioactive corrosion products from contaminated water coming from nuclear installations. SA-SBA-15 nanoparticles were first irradiated in a solid powder form and then applied to remove Cu(II), Ni(II), and Co(II) from their aqueous mixture in the range from 0 to 1000 Gy of gamma irradiation. The FTIR, TGA, zeta potential, XRD, BET, TEM, and CHN analysis results revealed the stability of a ligand support material, mesopore ordering and the functional groups. The structural and functional group endurance under our studied gamma irradiation makes SA-SBA-15 a potential adsorbent for routine decontamination and decommissioning activities as well as in radioactively contaminated emergency scenarios. Furthermore, the adsorbent-metal complexation followed the Irving–Williams order (Cu(II) > Ni(II) > Co(II)) and showed a higher selectivity for Cu(II) with removal capacity up to 99.76 ± 0.01%, thus suggesting its applicability to separate Cu(II) from other metal ions.