An Alpha imaging detector for on-site measurement of Plutonium and Neptunium

Morishita, Yuki; DiPrete, D. P.*; Deason, T.*; Nagaishi, Ryuji

Radiation Measurements, 181, p.107366_1 - 107366_8, 2025/02

https://doi.org/10.1016/j.radmeas.2024.107366

Nuclear fuel materials such as Plutonium (Pu) and Neptunium (Np) are produced as by-products of reprocessing operations, necessitating precise understanding of their contamination distribution within controlled areas for radiation protection. This study presents the development and application of an alpha particle imaging detector for on-site detection of Pu and Np contamination. The detector's performance was evaluated using various alpha sources, demonstrating promising energy resolution and spatial resolution. Subsequently, Pu and Np oxide samples were measured at the Savannah River National Laboratory, showing the detector's effectiveness in on-site applications. The detector enabled simultaneous measurement of radioactivity and energy spectrum of individual particles, facilitating rapid discrimination between Pu and Np. The imaging detector has potential for enhancing on-site detection of alpha nuclides in nuclear facilities, aiding in decontamination efforts and environmental monitoring.

Numerical simulation on dispersion of hydrogen leaked in particle layers of glass beads and soil

Terada, Atsuhiko; Nagaishi, Ryuji

Nuclear Technology, 210(10), p.1871 - 1887, 2024/10

https://doi.org/10.1080/00295450.2024.2302747

In order to understand dispersion of H2 leaked in packed beds of non-porous/porous particles in a partially open space practically, the dispersion of H2 in the particle layers of glass beads and soil was analytically studied using a CFD code to be compared with the experiments and to elucidate the effects of particle layer. H2 flowed out from a single leak point in the particle layer of non-porous glass beads was affected by buoyancy around the leak point, and diffused directly above the leak point in an elliptical shape faster than in the horizontal direction. After that, when it reached the air layer in the head space above the particle layer, H2 spread horizontally, formed a large concentration gradient near the boundary between the particle layer and the air layer, and further diffused in the air layer until the H2 concentration became about 1/3 or less of the concentration near the surface of particle layer. The calculations largely reproduced the experimental concentration distributions. When the particle layer was porous decomposed granite soil, the diffusion behavior of H2 in the particle layer proceeded in the same manner as in the case of glass beads. However, a large concentration gradient was formed near the boundary between the particle layer and the air layer, and then H2 diffused in the air layer until the H2 concentration became below the lower combustion limit. It was suggested through sensitivity analysis that the air permeability coefficient had a large effect on the time course of H2 concentration distribution. Based on the above, we further simulated H2 behavior in the vessel containing the H2 leaked particle layer. By inserting multiple vent pipes without considering H2 generation distribution and particle properties in the particle layer, H2 accumulated from one pipe was discharged by buoyancy without depending on the H2 generation distribution and particle properties in the particle layer, and air flowed in from the other pipe.

Retention of hydrogen bubbles generated from water radiolysis in carbonate slurry

Ito, Tatsuya; Nagaishi, Ryuji; Kuwano, Ryo*

Nuclear Technology, 210(8), p.1427 - 1443, 2024/08

https://doi.org/10.1080/00295450.2023.2299893

The retention of hydrogen (H) bubbles generated by water radiolysis was quantitatively studied in a high-viscous suspension of carbonate slurry consisting of a mixture of suspended solid (SS) of magnesium and calcium precipitates under strongly alkaline conditions, like the radioactive wastes discharged from the coagulation sedimentation (co-precipitation) process at the multinuclide removal equipment in the Fukushima Daiichi Nuclear Power Station. The H retention properties were evaluated in two types of carbonate slurry with different hydrophilicity: the hydrophilic "current type" and the hydrophobic "return type". Then, their properties were compared with those in another suspension of clay suspension of bentonite. From the comparison between the amounts of chemical adsorption and HO in the slurry, it was confirmed that HO molecules must be shared among the SS particles, and this sharing formed the structural viscosity in the slurry, different from that in the clay suspension where electrostatic bonding between the fine clay minerals forms the viscosity. The retention of H bubbles in (by) the slurry was evaluated from the difference in the amount of H observed with and without stirring the slurry after Co -irradiation. From the comparison of the retention properties of the hydrophilic slurry, the hydrophobic slurry, the clay suspension, and treated water, it was suggested that H2 bubbles were retained not only by the structural viscosity but also by the steric hindrance in the hydrophilic slurry.

The Effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts generated by intrinsic instability in hydrogen-air lean premixed flames under adiabatic and non-adiabatic conditions; Numerical simulation based on the detailed chemical reaction model

Thwe Thwe, A.; Kadowaki, Satoshi; Nagaishi, Ryuji

Journal of Nuclear Science and Technology, 60(6), p.731 - 742, 2023/06

https://doi.org/10.1080/00223131.2022.2143448

In this study, we performed numerical calculations of unsteady reaction flow considering detailed chemical reactions, investigated the unstable behavior of hydrogen-air dilute premixed flame due to intrinsic instability, and clarified the effects of unburned gas temperature and pressure. I made it. The unstable behavior of the flame in a wide space was simulated, and the burning rate of the cellular flame was obtained. Then, the effects of heat loss and flame scale on flame unstable behavior were investigated. The burning velocity of a planar flame increases as the unburned-gas temperature increases and it decreases as the unburned-gas pressure and heat loss increase. The normalized burning velocity increases when the pressure increases and heat loss becomes large, and it decreases when the temperature increases. This is because the high unburned-gas pressure and heat loss promote the unstable behavior and instability of flame.

Simulation of the self-propagating hydrogen-air premixed flame in a closed-vessel by an open-source CFD code

Thwe Thwe, A.; Terada, Atsuhiko; Hino, Ryutaro; Nagaishi, Ryuji; Kadowaki, Satoshi

Journal of Nuclear Science and Technology, 59(5), p.573 - 579, 2022/05

https://doi.org/10.1080/00223131.2021.1990153

The simulations of the combustion of self-propagating hydrogen-air premixed flame are performed by an open-source CFD code. The flame propagation behavior, flame radius, temperature and pressure are analyzed by varying the initial laminar flame speed and grid size. When the initial laminar speed increases, the thermal expansion effects become strong which leads the increase of flame radius along with the increase of flame surface area, flame temperature and pressure. A new laminar flame speed model derived previously from the results of experiment is also introduced to the code and the obtained flame radii are compared with those from the experiments. The formation of cellular flame fronts is captured by simulation and the cell separation on the flame surface vividly appears when the gird resolution becomes sufficiently higher. The propagation behavior of cellular flame front and the flame radius obtained from the simulations have the reasonable agreement with the previous experiments.

Measurement and evaluation of hydrogen production from mixtures of seawater and zeolite in decontamination of radioactive water

Kumagai, Yuta; Nagaishi, Ryuji; Kimura, Atsushi*; Taguchi, Mitsumasa*; Nishihara, Kenji; Yamagishi, Isao; Ogawa, Toru

Insights Concerning the Fukushima Daiichi Nuclear Accident, Vol.4; Endeavors by Scientists, p.37 - 45, 2021/10

Zeolite adsorbents are to be used for decontamination of radioactive water in Fukushima Dai-ichi Nuclear Power Station. Evaluation of hydrogen production by water radiolysis during decontamination is important for safe operation. Thus hydrogen production from the mixture of zeolite adsorbents and seawater was studied because seawater was urgently used as a coolant for the fuels. The hydrogen yield from the mixture decreased at a high weight fraction of zeolites. However, the measured yield was higher than the yield expected from the direct radiolysis of seawater in the mixture, which would decrease proportional to the weight fraction of seawater. The result suggests that the radiation energy deposited to zeolites was involved in the hydrogen formation. From the results, the hydrogen production rate was evaluated to be 3.6 mL/h per ton of radioactive water before decontamination. After the process, it was evaluated to be 1.5 L/h per ton of waste adsorbents due to the high dose rate.

Evaluation of energy spectrum around structural materials in radiation environments

Matsumura, Taichi; Nagaishi, Ryuji; Katakura, Junichi*; Suzuki, Masahide*

Radiation Physics and Chemistry, 166, p.108493_1 - 108493_9, 2020/01

https://doi.org/10.1016/j.radphyschem.2019.108493

In this work, when radiation sources of Cs, Sr and Y were assumed to be put in the front of a plain SUS304 plate as a typical material submerged in water, energy spectra of secondary photons and electrons at the front and back sides of plate were simulated with changing the thickness of plate, and spacing between the source and plate by using a Monte Carlo calculation code of PHITS. In the case of Cs gamma-ray (monochromatic 662 keV), the energy spectra at the front side was smaller than those at the back side due to the existence of plate. Then the dependence of spectra on the plate thickness was observed more clearly at the back side than at the front side. It was clearly shown how the energy spectra of photons and electrons varied with the incident radiation type, the spacing, and the thickness.

Modern Radiation Chemistry (Applications), 26; Evolution of water radiolysis studies for measures against post-severe accidents

Nagaishi, Ryuji

Radioisotopes, 66(11), p.601 - 610, 2017/11

https://doi.org/10.3769/radioisotopes.66.601

no abstracts in English

Irradiation experiments of simulated wastes of carbonate slurry

Nagaishi, Ryuji; Motooka, Takafumi; Yamagishi, Isao

Proceedings of 2016 EFCOG Nuclear & Facility Safety Workshop (Internet), 6 Pages, 2016/09

BB2016-0884.pdf:1.24MB

Overflow of water from waste storage tanks of High Integrity Containers (HIC) in the multi-nuclide removal equipment (ALPS) was discovered at Fukushima Daiichi NPS in April of last year. The mechanism of overflow was not understood very much at that time. To elucidate that for chemical safety in the waste storage, irradiation experiments of simulated carbonate slurry by Co-60 -rays have been conducted in CLADS, JAEA in cooperation with TEPCO, TOSHIBA and KURITA. Hydrogen molecule was the main radiolytic gas product in the slurry, and its amount was enhanced by dissolved species of not only halide ions as seawater components but also carbonate ion as an additive for co-precipitation at a basic condition. The bubbles of molecules were further formed and almost held in the slurry without stirred. These sequentially led to the expansion of slurry, and then to its separation into the shrunk one and supernatant water, which was little accumulated without irradiated.

Development of new type passive autocatalytic recombiner, 1; Characterization of monolithic catalyst

Kamiji, Yu; Taniguchi, Masashi*; Nishihata, Yasuo; Nagaishi, Ryuji; Tanaka, Hirohisa*; Hirata, Shingo*; Hara, Mikiya; Hino, Ryutaro

E-Journal of Advanced Maintenance (Internet), 7(1), p.84 - 89, 2015/05

https://www.jsm.or.jp/ejam/Vol.7No.1/AA/SP14/AA98/AA98.html

For hydrogen mitigation, a new type passive autocatalytic recombiner is under developing. This new recombiner has been developed from automotive monolithic catalyst in order to reduce weight and to improve hydrogen treating capacity, environmental resistance and product quality. In this study, activation energy of hydrogen-oxygen recombination reaction was examined to clarify the basic characteristics of the catalyst. In addition, the degradation of the catalyst by -ray irradiation simulating the environmental condition in nuclear power plants was also examined. As a result, the activation energy was experimentally estimated at 5.75 kJ/mol. Besides, no significant differences were observed in compositional distribution from the EPMA results. On the other hand, specific surface area of the catalyst and surface area of the precious metals were increased. Moreover, catalyst performance test showed that -ray irradiation up to 1.0 MGy can increase activity of catalyst.