[en] Flow behaviour for ESS horizontal target is studied experimentally using two dimensional water model. A velocity field of stationary flow in reaction zone has been obtained. Three dimensional effect was also studied as a spanwise flow structure. (author) 3 figs., 3 refs

[en] In the target of the European spallation neutron source (ESS), the beam line is horizontal and the mercury is fully forced by a pump in a parallel channel geometry. The top front of the assembly is a hemicylindrical wall, where the beam enters and is thus called a window. The main body is partitioned by a horizontal separation plate into the upper and the lower flow channels. The target liquid flows in the lower channel to the window region, turns there, and flows out in the upper channel. A thermal-hydraulic study is under way to investigate the general behavior of the flow in this configuration. The present experimental work is the first step of an effort to validate computer codes. The model used in this investigation is two-dimensional and uses water

[en] The SINQ target uses a lead-bismuth eutectic (LBE) mixture in liquid phase as a target material. After a detailed investigation of the natural circulation flow of LBE in a concentric double tube containment, a forced flow using a pump is expected to improve its flow and thermal characteristics and performances. With this concept, a flow problem is considered to be decoupled from the thermal problem for its large-scale motion and performance. As far as flow behavior is concerned, the kind of liquid metal is less relevant, and flow can be studied using other liquid metals such as mercury

[en] The purpose of this study is to investigate thermohydraulic instability characteristics in natural circulation parallel boiling channels upflow system with chimney assembly under low-pressure condition. Limiting heat fluxes of the instabilities were determined; their dependences on system pressure, inlet subcooling, and chimney length were presented graphically. The experiments were conducted based on the selected inlet subcoolings of 5, 10 and 15K; system pressures of 0.1, 0.2, 0.4, 0.5 and 0.7 MPa, consecutively, and maximum heat flux of 577 kW/m2. The series of experiments were carried adopting stepwise increase in input heat flux under constant system pressure and inlet subcooling. Instability boundaries, and the sequence of thermohydraulic pattern toward stability, will be particularly presented. The objective of the study is to propose thermohydraulic stability maps required for determining rational startup procedure of the loop, in which the instability could be predicted and mitigated. The study clarified that the flow modes during startup occur in the following sequence: (1) single-phase flow, (2) geysering, (3) oscillation due to hydrostatic head fluctuation, (4) density wave oscillation, (5) transition oscillation, and (6) stable two-phase flow. The other findings of the experiments are as follows: Low amplitude geysering were not suppressed at 0.7 MPa. Stable natural circulation was achieved with system pressure as low as 0.2 MPa. In-phase oscillation due to hydrostatic head fluctuation only occurred under atmospheric pressure. The adiabatic chimneys installed above the parallel channels significantly enhance the buoyancy force, but it leads to specific instability problems. Finally, the thermohydraulic stability maps are proposed

[en] Natural circulation BWR actively equipped with passive safety features has been promoted to solve the recent challenges in BWR nuclear power and safety technology. With regard to startup stage, various thermo-hydraulic instabilities might be induced due to an elimination of re-circulation pumps. A lot of studies have been made on the instabilities in evaporated system as well as in a reactor. In the instabilities, geysering accompanied with flow reversal phenomena has been investigated in a vertical closed loop, U-shaped closed loop, twin parallel channels, and so on. However, in twin parallel study the effect of non-heated length on geysering has not been sufficiently clarified. The objective of this research is to experimentally investigate the thermo-hydraulic instabilities, particular in geysering, with a natural circulation loop consisting of parallel boiling channels and the single connection channel, which simulates the basic flow around the reactor core in the system pressure range from atmospheric to 0.7 MPa. The parallel boiling channels are consisted of heated and non-heated section. The heated section forms annulus and heated from the inner wall. The input heat flux range of 0 up to 580 kW/m², and inlet subcooling temperatures of 5, 10, and 15 K respectively, are imposed in the experiments. In the parallel channels with non-heated risers, three types of thermo-hydraulic instabilities were detected in the following sequence, geysering, natural circulation oscillation, and density wave oscillation. Especially in geysering, it is induced due to rapid condensation in the non-heated risers and it is not be suppressed even at 0.7 MPa though it has a tendency to be suppressed with an increase in the system pressure. On the other hand, in the parallel channels without non-heated risers, sinusoidal oscillation similar to natural circulation oscillation has been detected, and geysering had never observed. The new findings are that the sinusoidal oscillation is induced because of the hydrostatic head fluctuation in the connection channel, where the flow regime is constantly slug flow. The oscillating period is well correlated with the sum of delay time for boiling and passing time of slug bubbles in the connection channel. From the facts described above, it is found that non-heated region in a channel box should be as shorter as possible to prevent geysering from occurring, and sinusoidal oscillation similar to natural circulation oscillation is induced in any configuration of parallel channels. (authors)

[en] A supercritical CO₂ gas turbine is suited to couple to the Na-cooled fast reactor. In this gas turbine cycle, a compressor is used in the neighborhood of the critical point. Then, the behavior of compressible flow near the critical point is very important. However, this is not clarified so far. In Tokyo Institute of Technology, a new project has been started in July, 2007 under the sponsorship of MEXT. In the framework of this project, CFD analysis research is conducted for the flow of the axial compressor blade. The Initial results of CFD analysis by FLUENT are given. The compressor model is equal to one set of a guide vane, a rotor blade and a stator blade. First, analysis was conducted for the No.10 stages blade, whose conditions are remote from the critical point and the possibility of divergence is very small. Secondly, parametric analyses have been carried out by varying the value of specific heat and also density under the ideal gas approximation so as to examine the flow characteristics of the effects of approach to the critical point. (author)

[en] Pulse Ultrasonic Doppler method for flow metering system has being developed. The principle is an integration of instantaneous velocity profile over a pipe diameter so that it is expected to be able to eliminate installation problems such as entry length as well as to follow transient flow rate precisely. Flow metering principle by pulse ultrasonic Doppler method in a circular pipe depends on the alignment of measuring lines and reflectors are needed for flow rate measurement using Pulse Ultrasonic Doppler method, however, it's not easy to seed the water with some particles at power plants. On the other hand, time of flight ultrasonic flow metering system does not need any reflector; however, it needs profile factor for the accurate flow rate measurement. In the present study, application of time of flight (TOF) ultrasonic flow metering system and Ultrasonic Doppler method (UDM) flow metering system have been investigated for the hybrid ultrasonic flow rate measurement system. (authors)

[en] Natural circulation BWR actively equipped with passive safety features has been promoted to solve the recent challenges in BWR nuclear power and safety technology. With regard to startup stage, various thermo-hydraulic instabilities might be induced due to an elimination of re-circulation pumps. The objective of this research is to experimentally investigate the thermo-hydraulic instabilities in natural circulation loop, consisting of parallel boiling channel simulating the simplest flow behavior around core and chimney in the system pressure range from atmospheric to 0.7MPa. The heated section forms annulus and its channel gap can be selected either 2mm or 5mm. As the investigation of the instabilities, especially effect of channel gap, the following findings can be clarified: Three types of thermo-hydraulic instabilities, which are induced in this order, geysering, natural circulation oscillation induced by hydrostatic head fluctuation, and density wave oscillation, have been observed with an increase in the input heat gradually. An important finding of this study is that geysering, which is induced in relatively lower input heat, changes the oscillation phase in the parallel boiling channel from out-of-phase to in-phase when channel gap is set at 2mm and atmospheric pressure. In contrary, in the other experimental condition, geysering always oscillates in out-of-phase with flow reversal phenomenon. It is considered that the friction loss in the channel gap might significantly affect the oscillation phase. Therefore, it is concluded that in a narrower channel it is difficult to induce out-of-phase geysering with flow reversal phenomenon especially at atmospheric pressure

[en] The objective of this experimental study is to clarify the heat transfer characteristics of the Passive Containment Cooling System (PCCS) with vertical heat transfer tubes for investigating the influence of non-condensable gas on condensation. The research was carried out using a forced circulation experimental loop, which simulates atmosphere inside PCCS with vertical heat transfer tubes if a loss of coolant accident (LOCA) occurs. The experimental facility consists of cooling water supply systems, an orifice flowmeter, and a tank equipped with the heat transfer pipe inside. Cooling water at a constant temperature is injected to the test part of heat transfer pipe vertically installed in the tank by forced circulation. At that time, the temperature of the cooling water between inlet and outlet of the pipe was measured to calculate the overall heat transfer coefficient between the cooling water and atmosphere in the tank. Furthermore, the digital video camera is used to measure the condensate drops via the inspection windows. Thus, the heat transfer coefficient (h_c) between heat transfer surface and the atmosphere in the tank considering the influence of the non-condensable gas was clarified. An important finding of this study is that the amount of condensation in the steamy atmosphere including non-condensable gas depends on the coolant Reynolds number, especially the concentration of non-condensable gas that has great influence on the amount of heat transfer. (author)

[en] This paper describes development of new measurement methods utilizing ultrasonic array transducer toward decommission of a damaged nuclear reactor, particularly in Fukushima Daiichi nuclear power plant. Determination of water leakage point is the first key technology since submersing damaged fuel debris is indispensable for retrieving the debris in terms of a biological protection. Accordingly, flow visualization method was firstly demonstrated. Two dimensional flow map was reconstructed applying ultrasonic Doppler velocimetry, which is also known as Ultrasonic Velocity Profiler (UVP). Effective determination was realized by wide and detailed area mapping approaches. Two sectorial array sensors were employed to obtain coarse flow map and the ultrasonic sensors. After rough determination of leaking point, detailed mapping was conducted using one phased array sensor. Flow configuration was modelled by cylindrical water tank (diameter of 600 mm and height of 1500 mm) and drain hole (diameter of 40 mm). By combining these two approaches, the leaking point was successively determined. As with the determination of leakage point, accurate shape of fuel debris is important information. This paper demonstrates accurate shape estimation method applying ultrasonic aperture synthesis method. A linear array sensor was employed in this purpose. Echo signal collected for each element was collected, and detailed shape was reconstructed by solving inverse scattering problem. As a result, a shape of modelled debris rock was clearly obtained compared to conventional method. (author)