High Frame-rate Neutron Radiography Research Articles

Image Enhancement for High frame-rate Neutron Radiography

High frame rate neutron radiography has been utilized to investigate two-phase flow in a metallic duct. However, images obtained by high frame-rate neutron radiography suffered from severe statistical noise due to its short exposure time. In this study, a spatio-temporal filter was applied to reduce the noise in the sequence images obtained by high frame-rate neutron radiography. Experiments were performed at the B4-port of the Research Reactor Institute, Kyoto University, which has a thermal neutron flux of 5.0×107 n/cm2s. Results were also compared with those obtained at the JRR-3, the Japan Atomic Energy Institute,with athermal neutron flux of 1.6×108n/cm2s.

Visualization of Bubble Behavior in a Packed Bed of Spheres Using Neutron Radiography

The present paper describes gas-liquid two-phase flow measurements in a packed bed of spheres using neutron radiography. Porous debris formed during a severe accident of a nuclear reactor should be cooled by a coolant and the cooling characteristics are dominated by two-phase flow behavior in the debris bed at the initial stage of the accident. Therefore, experimental database of the two-phase flow in the porous media has been required for safety analysis of the reactor. However, it is difficult to observe the flow structure, for example, void fraction distribution in such complex flow channel. In this study, the local void fraction in a packed bed which simulates the debris bed was measured by high frame-rate neutron radiography. Experiments were performed in air-water two-phase flow in a vertical pipe. Alumina spheres with 5mm in diameter were packed randomly in the pipe. The bubble behavior between the spheres was investigated by using the void fraction distributions estimated from the neutron radiographs. Although it was difficult to track the small bubbles in the packed bed, the move of the large bubble could be found roughly from the distribution. In addition, the fluctuation of the void fraction was compared with that of the pressure drop in the test section. From these results, the possibility of the gas velocity estimation was shown.

Visualization of Hydrazine Decomposition in a Catalyst Bed by Using Neutron Radiography

Most mono-propellant thruster technologies were developed in the 1960s. Those basic principles and fundamental mechanisms, such as the catalyst and propellant, are still in use without major technical innovation. However, much remains to be clarified in terms of the mechanisms and quantitative limitations of the hydrazine decomposition phenomena inside the mono-propellant thruster. Therefore, in order to enhance the reliability of the propulsion systems, it should be promising to perform the direct observation of the physical and chemical phenomena occurring in the catalyst bed of the mono-propellant thruster. For that purpose, a visualization of the mono-propellant thruster was performed by using high frame-rate neutron radiography technique. The experiments were conducted at the Kyoto University Research Reactor and the hydrazine injection behavior to the catalyst bed was visualized clearly.

중성자 래디오그래피를 이용한 액체금속 유동장 측정

The flow-field of a liquid-metal system is very important for the safety analysis and the design of the steam generator of liquid-metal fast breeder reactor. Dynamic neutron radiography (DNR) is suitable for a visualization and measurement of a liquid metal flow and a two-phase flow in a metallic duct. However, the three dimensional DNR techniques is not enough to obtain the velocity information in the wide channel up to now. In this research, a high speed DNR technique was applied to visualize the heavy liquid-metal flow field in the narrow channel with the HANARO-beam facility. The images were taken with a high frame-rate neutron radiography at 250 fps and analyzed with a Particle Image Velocimetry(PIV) method. The images were compared with the results of the commercial CFX code to study the feasibility of DNR technique for the measuring the heavy liquid-metal flow field. The PIV images could discern the turbulent vortex flow in the two-dimensional narrow channel.

Development of high-resolution high frame-rate neutron radiography

High-resolution high frame-rate neutron radiography system was developed. To achieve the high resolution, various combinations of imaging devices such as a fluorescent converter, high-speed video camera, and image intensifier were tested. Gen III image intensifier (GaAsP) combined with high-resolution high-speed video camera produced good-quality images. By using the new imaging system, measurement error of instantaneous void fraction and decay characteristics of fluorescent converter were estimated. In addition, the imaging system was applied to 3-D measurement of spherical particle position, which enables to measure 3-D velocity field combined with Particle Tracking Velocimetry.

Application of high frame-rate neutron radiography to liquid-metal two-phase flow research

Liquid metal two-phase flows in metallic vessels were studied by using high frame-rate neutron radiography. Both a bubble column and a gas-lift loop arrangement have been considered. Liquid velocity field of two-phase flow in a flat bubble column with rectangular cross-section was measured precisely by the particle tracking velocimetry. In a gas-lift loop, simultaneous measurements of void fraction by using high frame-rate neutron radiography and four-sensor probe were also performed to observe the bubble-probe interaction. Asymmetric Abel inversion was applied to compare the radial void fraction profiles. Measured radial void fraction profiles obtained by neutron radiography and electrical conductivity probe agreed well with each other. From these measurements, the measurement error and basic characteristics of gas–liquid metal two-phase flow were clarified.

Void fraction and velocity measurement of simulated bubble in a rotating disc using high frame rate neutron radiography

To evaluate measurement error of local void fraction and velocity field in a gas-molten metal two-phase flow by high-frame-rate neutron radiography, experiments using a rotating stainless-steel disc, which has several holes of various diameters and depths simulating gas bubbles, were performed. Measured instantaneous void fraction and velocity field of the simulated bubbles were compared with the calculated values based on the rotating speed, the diameter and the depth of the holes as parameters and the measurement error was evaluated. The rotating speed was varied from 0 to 350 rpm (tangential velocity of the simulated bubbles from 0 to 1.5 m/s). The effect of shutter speed of the imaging system on the measurement error was also investigated. It was revealed from the Lagrangian time-averaged void fraction profile that the measurement error of the instantaneous void fraction depends mainly on the light-decay characteristics of the fluorescent converter. The measurement error of the instantaneous local void fraction of simulated bubbles is estimated to be 20%. In the present imaging system, the light-decay characteristics of the fluorescent converter affect the measurement remarkably, and so should be taken into account in estimating the measurement error of the local void fraction profile.

Void fraction and velocity measurement of simulated bubble in a rotating disc using high frame rate neutron radiography

To evaluate measurement error of local void fraction and velocity field in a gas-molten metal two-phase flow by high-frame-rate neutron radiography, experiments using a rotating stainless-steel disc, which has several holes of various diameters and depths simulating gas bubbles, were performed. Measured instantaneous void fraction and velocity field of the simulated bubbles were compared with the calculated values based on the rotating speed, the diameter and the depth of the holes as parameters and the measurement error was evaluated. The rotating speed was varied from 0 to 350 rpm (tangential velocity of the simulated bubbles from 0 to 1.5 m/s). The effect of shutter speed of the imaging system on the measurement error was also investigated. It was revealed from the Lagrangian time-averaged void fraction profile that the measurement error of the instantaneous void fraction depends mainly on the light-decay characteristics of the fluorescent converter. The measurement error of the instantaneous local void fraction of simulated bubbles is estimated to be 20%. In the present imaging system, the light-decay characteristics of the fluorescent converter affect the measurement remarkably, and so should be taken into account in estimating the measurement error of the local void fraction profile.

Design and Characteristics of the JRR-3M Thermal Neutron Radiography Facility and Its Imaging Systems

The JRR-3M thermal neutron radiography facility was constructed in the JRR-3M of the Japan Atomic Energy Research Institute in 1991 and has been used as a research facility for various kinds of research fields. The characteristics of the facility have been briefly reported and declared to be excellent in neutron flux and in collimator ratio. Additionally, the authors have measured the beam flatness and the scattered components and have compared these measured characteristics with the design values. The neutron source, the beam tube, and the radiography rooms are described in detail and their data are applied to the analyses of the characteristics. The description of the radiography rooms includes beam size definition tubes and beam shutters, which are the most important components in the room. Also described are the restrictions on the size and shape of the sample, the background dose rates, and equipment information. The high-performance of the facility enables advanced imaging techniques such as high spatial resolution imaging and high temporal resolution imaging. The high-resolution static neutron radiography system using a cooled charge-coupled device camera has the capability to take neutron radiography images with 72 μm of spatial resolution. The high frame rate neutron radiography system has the capability to image high-speed phenomena with 4500 frames/s of temporal resolution. Both neutron radiography systems are described in detail especially for key components such as fluorescent converters and cameras.

Visualization and measurement of two-phase flow by using neutron radiography

To apply neutron radiography (NR) technique to fluid research, high frame-rate NR with a steady thermal neutron beam has been developed in the present research program by assembling up-to-date technologies for neutron source, scintillator, high-speed video and image intensifier. This imaging system has many advantages such as a long recording time (up to 21 min), high-frame-rate (up to 1000 frames s −1) imaging and no need for triggering signal. Visualization of air-water two-phase flow in a metallic duct was performed at the recording speeds of 250, 500 and 1000 frames s −1. The qualities of those consecutive images were good enough to observe and measure the flow structure and the characteristics. It was demonstrated also that some characteristics of two-phase flow could be measured by using the present imaging system. Image processing technique enabled measurements of various flow characteristics in two ways. By utilizing geometrical information extracted from NR images, data on flow regime, rising velocity of bubbles, and wave height and interfacial area in annular flow were obtained. By utilizing attenuation characteristics of neutrons in materials, measurements of void profile and average void fraction were performed. It was confirmed that this new technique may have significant advantages both in visualizing and measuring high-speed fluid phenomena when the other methods such as an optical method and X-ray radiography cannot be applicable.

Quantitative limits of thermal and fluid phenomena measurements using the neutron attenuation characteristics of materials

Temporal and spatial resolution of the neutron radiographic technique were investigated in order to apply this technique to the visualization and measurement of thermal and fluid phenomena. The temporal resolution of three imaging methods of temporally resolved neutron radiography-static neutron radiography with a pulsed neutron beam and high frame rate neutron radiography with either a pulsed or steady neutron beam-was studied. It was determined that the temporal resolution was determined by the sensitivity and light decay time of the image detector and statistical variation of neutrons, and the resolution limits of static and dynamic imaging methods were estimated to be a few microseconds and a few hundred microseconds, respectively. An image processing method was proposed to measure flow characteristics such as void fraction. By performing an error analysis to calculate the limit value of liquid film thickness that can be measured by neutron radiography, it was determined that the limit value of a rectangular channel gap or round tube diameter should be smaller than 3.25 or 4.00 mm, respectively, for measuring the void fraction of air-water flow within an error of 10%. The void fraction measuring method was experimentally confirmed by comparing the void fraction values in a rectangular duct with a 2.4-mm gap obtained by neutron radiography with those obtained by optical and conductance probe methods. It was shown quantitatively that the measurement error decreased when consecutive frames were temporally integrated.

Feasibility of high frame-rate neutron radiography by using a steady thermal neutron beam with 10 6 n/(cm 2s) flux

The feasibility of high frame-rate neutron radiography by using a steady thermal neutron flux on the order of 10 6 n/(cm 2s) was investigated and the application of this technique to fluid visualization was examined. The Kyoto University Research Reactor, whose neutron flux at the imaging plate was 1.2 × 10 6n/(cm 2 s), was used as a neutron source. The imaging system for the high frame-rate neutron radiography with the steady thermal neutron beam was constructed by combining a high sensitivity scintillator 6LiF/ZnS:Ag, a high-speed video which could take images with the light intensity of 1.5 lux at the recording speed of 1000 frames/s and an image booster whose gain was about 100. Visualization of air—water two-phase flows in a rectangular duct with 2.4 mm gap and 40 mm width were successfully performed with the steady thermal neutron beam and this imaging system at the recording speed of up to 500 frames/s. The limit of high frame-rate neutron radiography in an ideal system without background radiation such as scattered neutron and with neutron detection efficiency of 100% was preliminarily investigated by calculating the measurement error of neutrons due to the natural statistics of neutrons in the low neutron flux field. It was shown from this study that the high frame-rate neutron radiography by the steady neutron flux on the order of 10 6 n/(cm 2 s) has a good possibility.

Visualization of fluid phenomena using a high frame-rate neutron radiography with a steady thermal neutron beam

Experiments on visualization of fluid phenomena in a metallic vessel were conducted using a high-speed video camera with a steady thermal neutron beam. The Japan Research Reactor 3M, whose flux at the imaging plate is 1.5 × 108 n/cm2 s, of Japan Atomic Energy Research Institute was used as a neutron source. The imaging system for high frame-rate neutron radiography with the steady thermal neutron beam consisted of a high sensitivity scintillator, 6LiF/ZnS:Ag, an image intensifier whose gain was a factor of 100 000 at the maximum, and a high-speed video which could record phenomena at 1000 frames/s for 14 min by using an ordinary VHS tape with the recording time of 120 min at the normal speed. Visualization of air-water two-phase flows in a rectangular duct with 2.4 mm gap and 40 mm width were successfully performed with the steady thermal neutron beam and this imaging system at frame rates up to 1000 frames/s. The shapes and the behavior of bubbles and liquid films were clearly observed. The rising velocities of slug bubbles and the flow regimes could be also measured. It was clarified that this new technique may have significant advantages both in visualizing and measuring high-speed phenomena when the visible light is not applicable.