Improvement of PWR thermal efficiency has a strong effect on the plant economy. The analysis of heat and mass flux in PWR Steam Generator (SG) is important for improving the thermal efficiency. In this study, the evaluation model of SG with economizer considering the effects of U-Tubes was developed to calculate the vapor flow rate and the pressure drop. The comparison with the measured pressure drop of KWU type SG with economizer of Grafenrheinfeld PWR plant was done. The prediction of pressure drop accuracy was relative good and the improvement of void fraction was needed.

Usually, Trichloroethane has been used for the de-oiling and cleaning of machine parts. But its production and import have been prohibited since 1995 because of its possibility to destroy the ozone layer. Generally for biological and environmental safety, the de-oiling should be done with the physical method instead of the chemical method using detergent or solvent. As one of the physical method, a cleaning by a supersonic wet steam jet has been proposed. The steam jet with water droplets impinges on the oily surface of machine parts and removes the oil or smudge. In the present study, the low-cost and taper-shaped nozzles were fabricated with an electric discharge machining. The jet behaviors from the taper-shaped nozzles were carefully observed by using air and wet steam. The non-equilibrium model of wet steam was proposed and compared with the experimental results.

Some underwater vehicles need propulsion system without propellers to install sensors at the tail and to avoid the entangled seaweeds. Pump jet propulsions are, however, not efficient at the low speed of vehicle. So the new pump jet system was considered to improve the efficiency. Thenew system for the underwater vehicle of lowspeed was proposed. The water jet experiment was conducted to verify the prediction method to design the new system. The high speed water jet was injected into the duct with Coanda nozzle and the duct flow was established. The experimental results agreed well with the prediction and the prediction method was expected to use for the design of new propulsion system.

Since the Tohoku earthquake (March 11, 2011) causes quite a lot of damage to a lot of people , we have been getting much aware of tsunami disaster. While the disaster training is effective for preventing damage by tsunami, we cannot get training in a real tsunami unlike earthquake. The earthquake can be virtually experienced in an earthquake simulation vehicle. For tsunami training as many people as possible, especially children and old persons, we have been developing a portable tsunami virtual reality (VR) system using three key technologies of the head-mounted display (HMD), the laptop computer and the particle-based simulation. We correctly reproduce the tsunami behavior using a particle-based CFD method (SPH) for the purpose of giving tsunami's correct knowledge to trainees. By using the simulated results, we compactly-realized the 3D virtual reality of tsunami using Oculus Rift, one of the recent HMDs, with a small laptop computer.

Steam turbine blade is damaged due to long time use and its performance gets worse. There are many studies about the relationship between performance and blades deterioration. However, most papers consider only a rotor or a stator deterioration. This paper presents a first stage model of an intermediate pressure turbine. The results show that the influence of the stator deterioration is more significant than the rotor, because the change of flow direction from stator affects the power. This paper also conducts a cascade experiment and compares experimental results and calculation result.

A current highly efficient gas turbine is operated at a high temperature and a high pressure condition. Moreover, because of the long-time operations, the turbine blades are degraded due to abrasion, erosion and fracture. Since these cause power decline and efficiency deterioration, understanding of the characteristics is important. In the present study, 3-D shapes of rotor and stator blades in an in-service gas turbine first stage were scanned. A simulation model and a cascade test equipment were created by using these and degradation effects on the gas turbine performance were examined by experiments and numerical simulations. As a result, the shape degradation of the stator blade was found to significantly affect the gas turbine performance.

Water spray is used for cooling suction air of gas turbine. The aim of this study is to investigate the spraying properties and the influence of secular change in the spray nozzle. PDA measurements are conducted in wind tunnel. Four types of nozzle are tested. Same type nozzles with different using period are also tested. The nozzles are new one and old one which is used 1750 hours. The spray properties in high spatial-temporal resolution are provided by the PDA. Also, The droplet size by a pin type nozzle doesn’t change after 1750 hours. On the other hand, the velocity and flow rate decrease after 1750 hours.

Flame motion variation of a stagnation flow premixed flame with gas component transition between mixture A (methane:84%, ethane:13.3%, propane:2.7%) and B (methane:92%, ethane:6.7%, propane:1.3%) is discussed. Estimation of burning velocity and flame temperature of mixture A and B by CHEMKIN-PRO show little difference. In experiments, transition from mixture A to B and B to A were performed for a stagnation flow premixed flame. The transition time changes from 1 sec to 10 sec, keeping flow velocity and equivalence ratio constant at 0.8 m/s and 0.7. The flame position was measured by a picture taken by high speed video camera with 125 fps. Experimental results show overshoot of flame position during the transition. The overshoot was much significant for shooter transition time.

Presently, hybrid hydrogen-gasoline fueling stations are being constructed at every region of Japan. The hybrid station is more dangerous than a simple hydrogen station, especially in the case of a fire in the gasoline tank. The fire increases the temperaute of the hydrogen storage tank via thermal radiation. In order to simulate the thermal radiation arising from such a heat source, we first estimate its shape factor. we used Monte Carlo method to estimate the shape factor for our analasis model. We clarified the thermal radiation from the fire and the temperature rise in each tank using Monte Carlo method.

A desiccant air conditioning system which can handle the sensible and latent heats in the air separately is generally composed of a dehumidifying rotor with desiccant material, a heat source for regeneration of the material and a heat exchanger controlling the dehumidified air temperature. We proposed a desiccant rotor heated by solar irradiation, concentrated by Fresnel lens, impinging the rotor directly. The irradiation has a distribution along the channel on the wall in a rotor. This study presents local heat transfer coefficient on the wall by performing a CFD simulation assuming laminar forced convection heat transfer in a circular channel. The results showed that the coefficients decreased along the channel. Comparing with the result considering uniform heat flux condition, the decreasing rate was large, and it didn't reach a constant value obtained at the fully developed flow region.

This study describes a heat transfer characteristic of pulsating airflow around heating obstruction mounted in a rectangular duct. Several researchers have reported the possibility of the heat transfer enhancement by pulsating flow. Our study is trying to apply pulsating flow to the heat transfer enhancement method in thermal equipment such as heat exchangers and electronic equipment. In this report, an effect of the clearance between the enclosure wall and the heating obstruction on the heat transfer of the pulsating airflow around the obstruction was investigated in the case of Re = 1044. It is found that pulsating flow can enhance heat transfer around the obstruction except in the case of extremely narrow flow passage.

This paper suggests that there is equality in physics between coaxial cylinder and pipe flow on the friction force per unit area. Among rotational machinery, electric motors and turbines for example are common industry which may have concentric shell structure. As the speed of rotating cylinder ascend, particular flow will occur known as Taylor-vortex flow regime beyond certain rotating speed. The major circulation of which has distance at least between coaxial cylinders. This means mass-flow is stirred enough to transport between inner and outer cylinder to emerge universal turbulence representation. It is investigated that stress force within concentric structure is almost the same as of the pipe flow even if the boundaries have various roughness. Consequently, complex behavior of frictional force including bumped surface can be estimated algebraically using Moody diagram.

There is problems that an electrical rotating machine loses it’s the output and driving efficiency due to overheating during rotating. Therefore, the improvement of heat removal capacity needs to know the internal flow and heat transfer characteristics. The internal structure of the rotating machine has coaxial cylinders consisting of the inner cylinder and outer cylinder. This study produced three shapes of the rotating coaxial cylinders. By performing a visualization using Kalliroscope flakes, the effect of the shape variation on flow characteristics was investigated.

Toward the improvement of performance of an electrical rotating machine such as a motor and a generator, an appropriate design for heat removal is important. A typical motor is composed of the pair of coaxial cylinders with the fixed outer cylinder (i.e. stator) and rotating inner cylinder (i.e. rotor). The present study numerically clarifies the influence of inner shape on heat transfer characteristics in the electrical rotating machine. The heat transfer coefficient of the gap region between the rotor and stator was obtained. As a result, a vortex structure was observed in the slit on the stator wall, and the heat transfer coefficient of with-slit case in the high rotational speed was increased compared with that of without-slit case. Hence, it was implied that the heat flux was increased by the presence of the vortex of the internal slits.

This study investigated pressure drop for gas and non-Newtonian liquid two-phase flows in a horizontal circular microchannel. The microchannel was a fused silica tube with 0.25 mm I.D. Polyacrylamide aqueous solutions with different mass concentration, which exhibit pseudo plastic behavior, were used as non-Newtonian liquid, while the nitrogen gas was used as the test gas. Pressure drop was measured with a calibrated differential pressure transducer. The pressure drop data have been compared with calculations by various correlations, usually used for prediction of the pressure drop in mini/micro channels as well as conventional sized channel. As the comparison results, the data have been correlated with separated flow model by using a newly developed equation of two-phase friction factor as well as homogenous flow model with an effective viscosity accounting for non-Newtonian effects. The Lockhart-Martinelli model also predicted well the present data if an appropriate C-value in two-phase friction multiplier was adopted.

In this study, we performed to observe horizontal vapor-liquid two phase flow of refrigerant R 134a in the rectangular glass channel which hydraulic diameter is 0.84 mm. The test glass tube was added the oscillation to the gravity direction for slug and stratified flow under low mass flux and heat flux. When the channel was oscillated of certain frequency and amplitude, the liquid of the corner part of rectangular channel waved up and down. In slug flow, dry surface of central was supplied the liquid by the oscillation and was formed the thin liquid. In stratified flow, dry surface of top was supplied the liquid and flow pattern changed to annular from stratified flow. The oscillation influences were investigated in a quantitative way.

Experiments were performed to explore the conditions for the onset of secondary droplet formation during single drop impact onto a liquid film(splashing). It was found that there were two types of splashing: the prompt splash and the late splash. In the prompt splash, small droplets were scattered with high velocity immediately after the impact. Whilst, in the late splash, larger droplets of lower velocity were produced after the development of liquid crown. The condition for the onset of splashing was expressed using the two dimensionless numbers: the Weber number, the Ohnesorge number. The correlations were given separately for the prompt and late splashes. Moreover new correlations were developed about prediction of the secondary droplet diameter and velocity.

The coalescence of horizontally approaching twin bubbles is experimentally studied in isothermal system at room temperature. The bubbles are generated at the outlets of two tubes facing each other horizontally. Liquid film thickness formed between approaching bubbles were measured by simultaneous measurement of laser extinction method and laser interference method. This study investigates the rupture location in liquid film and the distribution of the liquid film thickness for water. The variable of the experiment are the air flow rate and the transmitted laser position. The effects of bubble approach velocity and the contact duration were investigated.

Experimental databases and knowledge of turbulent flows in rod bundles are required for validation and improvement of numerical prediction methods for two-phase flow in rod bundles. In this study, we therefore measure liquid velocity in single-phase and bubbly flows by installing a small LDV probe into a rod in a 2x2 rod bundle. The diameter and the pitch of the rods are 10 and 13 mm, respectively. The liquid volume flux is 0.5 m/s and the gas volume flux in the bubbly flow is 0.08 m/s. The experimental results show that (1) the Reynolds shear stress is enhanced in the vicinity of the rod surface due to the increase in velocity gradient at the wall, (2) the ratio of Reynolds shear stress to the axial turbulence energy decreases due to the presence of bubbles, (3) the Prandtl’s mixing length hypothesis holds in the vicinity of the rod even in the bubbly flow and the mixing length in the core region is close to the bubble radius.

There are several methods for heat transfer enhancement. For example, one can attach various fins on the heat transfer surface, process the surface roughly, insert twisted tape, and so on. These methods increase the heat transfer coefficient or area by manufacturing changes to the heat transfer surface. However it is necessary to consider the problem on account of processing heat transfer area. In this study, heat transfer and flow characteristics performance of air-water heat exchanger by under the condition inserting metallic wire was evaluated, and compared the case of the fin. Pressure drop as compared with the case of the fins is significantly reduced.

This study deals with condensing heat transfer in a quadrilobed tube. HFC-134a was used as the working fluid. The effect of the tube configuration on the heat transfer coefficient was evaluated in horizontal flow. Superheated vapor or wet vapor was supplied to the condenser, and heated in water. A straight and twisted quadrilobed tubes were used. The local heat transfer coefficient at the center in the length was evaluated from the wall temperature measured by mounted thermocouples. As the result, for low refrigerant mass flux in the straight tube, deterioration in local heat transfer coefficient under low quality condition was suppressed, because condensate might be accumulated along the center by surface tension. For the twisted tube, on the other hand, heat transfer coefficient was lower than the straight one. In this experiment, since the twist tube improved the water heat transfer, the twist tube produced higher heat transfer rate than the straight one.

Experiments were carried out to investigate the characteristics of high-heat-flux heat removal using a porous-micro-channel (a small rectangular channel filled with a porous-metal plate). The working fluid was water and the main experimental parameters were the inlet liquid subcooling, mass flux, and channel size. It was shown that nucleate boiling heat transfer is enhanced in the porous-micro-channel in comparison with the normal rectangular channel. Another important finding was that as in the normal channel, the micro-bubble emission boiling (MEB) occurs in the porous-micro-channel if the mass flux and liquid subcooling are high enough. In addition, the flow oscillation and noise production that often accompany with the MEB in the normal channel were mitigated significantly in the porous-micro-channel. This indicates that the porous-micro-channel enables the stable and high-heat-flux heat removal by the MEB.

Dropwise condensation (DWC) has high heat transfer coefficient. However, It is difficult to maintain DWC for long time. In order to maintain DWC long enough, it is important to understand the effect of the surface condition. In the present paper, the wettability of the metal-sputtered surfaces was controlled by MEMS technology. The effect of several metal-sputtered surfaces, i.e., Cupper(Cu), Titanium(Ti), Silver(Ag), Chromium(Cr), and Lead(Pb) on the pattern of the condensation, i.e., DWC or FWC, was examined qualitatively. The surface condensation with sputtered thin Silver(Ag) and Lead(Pb) films were DWC. On the other hand, the surface condensation with sputtered thin Cupper(Cu), Titanium(Ti) and Chromium(Cr) films were FWC. For DWC, the surface adsorption force was lower than that of FWC.

Surface tension of liquid metal is sensitive to surface oxidation and it varies temporary in oxic condition. The temporally varing surface tension, i.e. dynamic surface tension, has great impact on many industrial processes relating to liquid metal. In this study, a novel and simple method for the dynamic surface tension of the liquid metals tension is proposed. This method determined the dynamic surface tension on the basis of the profile of a falling capillary jet, which is ejected horizontally. The dynamic surface tension for Wood's metal reaches equilibrium state from a initial value after gradual decreases or increase depending on oxygen concentration. The decrease of surface tension is caused by oxygen adsorption at relatively low oxygen concentration, and high oxygen concentration leads to the increase of surface tension by segregation. The relaxation time of the dynamic surface tension is the order of 10ms, and the dependency of the relaxation time on oxygen concentration can be also iterpreted as the result of oxygen adsorption and segregation.

There is a strong need to enhance the critical heat flux of IVR due to the increase of the plant power. Many methods involving nanofluids, surface modifications, have been investigated to enhance the CHF values, but most of the previous works focus on pool boiling. An experimental research on enhancing CHF by a metal honeycomb porous plate was conducted in saturated flow boiling. The experimental result shows that CHF values are increased in flow boiling condition. The honeycomb structure and liquid supply through the porous media are verified to play an important role in the enhancement of the CHF.

One strategy for severe accidents is in-vessel retention (IVR) of corium debris. IVR consists of external cooling of the reactor vessel in order to remove decay heat from the molten core through the lower head of the vessel. However, heat removal is limited by the occurrence of the critical heat flux condition (CHF) at the outer surface of the reactor vessel. Therefore we proposed the CHF enhancement technique in a saturated pool boiling by attachment of a honeycomb porous plate (HPP) on the heated surface. However, the reactor vessel to install HPP has curvature. The CHF enhancement depend on how HPP are attached firmly to the heated surface, so it is important to establish the method to place the HPP on the surface of reactor vessel with curvature. Therefore, we propose the way using porous cellulose beads and nanofluid. As a result, the CHF is shown to be enhanced double compared with that of a plain surface of pure water.

It is important to predict the critical heat flux(CHF)in flow boiling under vibration condition for the safety technology of nuclear reactor and boiler during earthquake.In the present study, the onset of nucleate boiling (ONB) and CHF under the vivlation condition were investigated experimentally. The experiment was conducted using a copper thin-film and the subcooled water at 0.1 MPa. The liquid velocity was 0.25, 0.67, 1.00, 1.33, and 1.67 m/s, respectively; the liquid subcooling was 3 K. The acceleration was 4.15 m/s² , the frequency was 2 Hz. As a result , CHF was increased with low liquid velocity. On the other hand, CHF was decreased with high liquid velocity.It seems that the flow regime changed by increased liquid velocity.

Liquid inlet behavior to a heat surface in micro-bubble emission boiling (MEB) was investigated by PIV measurement. Subcooled pool boiling experiments were performed using a heat surface with a diameter of 10mm. An upper end of a heater block was used as the heat surface. The subcooling was varied from 40 K to 70 K. The flow visualization around the heat surface was carried out using a high speed video camera and a light sheet. The micro-bubbles generated in the MEB were used as tracer particles and the velocity field was obtained by PIV analysis of the acquired image sequence. The distribution characteristics and fluctuation of the velocity were studied.

This paper deals with subcooled flow boiling experiments using various different dissolved air concentrations in the Perfluorohexane (PFH) in order to clarify its effects on flow boiling heat transfer through a tube with an inner diameter of 4 mm, excess temperature, boiling curve and heat transfer coefficient were calculated and discussed. As a result, dissolved air was shown to affect boiling characteristics. In particular, dissolved air significantly affected the boiling curve in the low heat flux region, and the boiling curve was continuously decreased with increasing the effective heat flux for mass flux of 300 kg/(m2s). Dissolved air also significantly affect the onset of boiling. These effects lead to increasing generation of bubble.

Effects of flow directions of the heating medium on heat transfer performance of upward evaporating refrigerant flows in a plate-fin heat exchanger with single refrigerant channel were examined using R134a as the refrigerant. The refrigerant channel was sandwiched by two water channels. Hot water flew upward or downward to form a parallel or counter flow heat exchange. To understand the heat flux distribution the temperature distributions on the outside wall of the water channel were visualized by an IR camera. As the results, it was shown that the difference in heat transfer rate between the parallel and counter flow was a little because of the large temperature difference in heat transfer. The pressure loss of the refrigerant flow was larger for the parallel flow than the counter flow. It could be estimated from the wall temperature distribution that the increase in pressure loss might be caused by inhomogeneous phase distribution of refrigerant flow.

Enhancement in performance of heat transfer is one of the most significant issues for a heat exchanger in exhaust heat recovery systems. In design of the heat exchanger, increase of heat transfer area and enhancement of heat transfer are generally considered. On the other hand, a practically-designed heat exchanger cannot provide enough performance in terms of heat transfer due to restrictions of installation space of the system and pressure drop of working fluid. In this study, the optimum design of a heat exchanger has been considered for the solution of a trade-off problem between heat transfer rate and pressure drop of exhaust gas based on genetic algorithm coupled with the mathematical model for performance analysis of the heat exchanger. This paper also has presented a validation study of the mathematical model with the experimental benchmark. And then, the optimum design method for the exhaust gas heat exchanger has been mentioned.

This study investigated the effect of latent heat storage wall on axial heat transportation in oscillatory flow by numerical simulation. To clarify an effect of the oscillation frequency, nondimensional frequency (Womersley number) was changed 0.78 ~ 3.13. The stroke of oscillation is 45 mm (Peak-to-Peak). The tube was inner diameter of 6 mm, wall thickness 1 mm, and tube length 250 mm. The difference of temperature of tube both ends 30 K, and the wall materials made composite heat storage material (CompositePCM). As a result, the effective heat transportation coefficient showed up to 3.13 times in comparison with the acrylic tube, and the magnification increased with decreasing frequency.

The final goal of this study is to clarify the mechanism behind enhancements of wall surface wettability and boiling heat transfer due to the radiation induced surface activation (RISA) under high-temperature and high-pressure conditions equivalent to thermal environment in a core of light water reactor. Detailed discussions about dynamic wettability at ambient temperatures up to 250 degrees centigrade were made based on the experimental data. The results showed that the advancing contact angles were decreased due to gamma-ray irradiation at all conditions tested and revealed that the dynamic wettability was enhanced due to the RISA even under high-temperature and high-pressure conditions.

It is expected that IVR (In-Vessel Retention) for a large PWR is feasible if we can make critical heat flux (CHF) high. The target of this study is to develop an IVR technology using honeycomb cooling devise which can make CHF more than double. For this target, the following subjects are to be examined and we will develop a road map for this technology; ・Uncertainty evaluations for melting debris relocation and for heat flux distribution along lower head wall of reactor pressure vessel ・Heat flux performance of honeycomb cooling devise under two-phase natural circulation ・Feasibility study for implementing method of the devise and for a safety system under actual reactor situations

For improvement of Sever Accident (SA) codes, the core melting process has to be clarified because the numerical methods in SA codes for analyzing the core melting process are simplified. To clarify the core melting process quantitatively, it is necessary to understand the melting behavior of fuel elements which consist of fuel assemblies and control rods. Then we have been developing the numerical simulation method for fundamental melting behavior of fuel elements based on the POPCORN code. The POPCORN code was developed in Japan Atomic Energy Agency and is based on the Moving Particle Semi-implicit (MPS) method. In this paper, we introduced the numerical method for melting behavior in multi-component system including chemical reaction to the MPS method. In a melting process of fuel rod, some materials move in fuel rod and melting point changes. And chemical reactions including zirconium oxidation is also affect the melting behavior because of a heat generation. To consider these effects on melting behavior, we focus on oxidation movement in fuel rod and develop the numerical method for melting behavior in multi-component system based on diffusion of oxidation.

Ex-vessel cooling is an important subject in the field of nuclear safety so that many researchers focus on CHF (critical heat flux) enhancement study of pressure vessel reactor. In this study, pool boiling experiment is carried out to test CHF value. Meanwhile, a new structure design, honeycomb plate, is introduced for enhancement study of CHF. During experiment, different kind of plates are utilized. It is found that CHF can be enhanced after using honeycomb structure. Also, porous plate can further increase CHF compared with solid plate.

The current Japan-US nuclear cooperation agreement, which is the result of strenuous diplomatic negotiations in the 1970s-80s, should best be extended beyond July 2018 as it stands now. However, in view of various unfavorable circumstances (such as the delayed operation of the Rokkasho-mura Reprocessing Plant, Monju, MOX programs), further determined efforts are needed on the Japanese side to improve the domestic climate by removing as much as possible the existing negative factors. Above all, Japan should endeavor to dissipate misgivings abroad about clandestine military diversions, somewhat provoked by Mr. Donald Trump's recent statements as well as Chinese accusations.

Performance tests were conducted for Filtered Containment Venting System to verify the decontamination factor for aerosol in a high temperature steam flow. The test results confirms that the filter used in this system achieves a decontamination factor larger than 1000. In addition, a filter to capture organic iodine was developed in this study using silver zeolite. Absorption performance of silver zeolite was tested. And the results demonstrated that the newly developed filter's decontamination factor for methyl iodide is larger than 50 and the filter has enough absorption capacity for organic iodine generated in severe accidents. The filtered containment venting systems with organic iodine filters developed in this study was installed at Kashiwazaki Kariwa Nuclear Power Station.

Fukushima Daiichi NPP accident would be terminated, if sufficient accident countermeasures, such as water proof door, mobile power, etc. In case of Europe, it had already installed the heat removal system and filtered containment venting system (FCVS) from the lessons of TMI and Chernobyl Accidents. The new regulatory standard, the filtered vent system (FCVS) should be installed, and prevent the radioactive material in case of the severe accident and the overpressure breakage prevention of a primary containment vessel (PCV) and also stabilize the FCVS. The authors examined the severe accident process in the 2^nd unit of Fukushima Daiichi NPS, and found the vent by FCVS should be done before water injection into the core. By conducting two-phase flow visualization test, the authors also pointed out the necessity of stabilization in the scrubbing pool. TRAC code analysis was also conducted to clarify the mechanism of the geysering and stabilized by using an orifice plate to pressurize the scrubbing pool.

After the Fukushima Daiichi nuclear disaster, TEPCO launched a project to develop the Filtered Containment Venting System (FCVS), in which our very bitter experiences in the accident are reflected. One of the main purposes of the development of the FCVS is to enhance operability of venting under the severe plant conditions such as the SBO during progressing of severe core damage, and another is to enhance removal performance of radio-nuclides with the newly added filtering equipment, which is installed in the venting line from the PCV to the outer. The Kashiwazaki-Kariwa NPS unit 6 and 7 are the first reactors applied the FCVSs. Performance of the FCVS has been verified by an extensive test program includes scaled filter venting test under various conditions. In this paper, we show some test results on aerosol decontamination factor (DF) under steam condition. The test results show that the DF under steam condition is higher than air condition.

After the Fukushima Daiichi nuclear disaster, Tokyo Electric Power Company (TEPCO) launched a project to develop the Filtered Containment Venting System (FCVS), in which our very bitter experiences in the accident are reflected. One of the main purposes of the development of the FCVS is to enhance operability of venting under the severe plant conditions such as the SBO during progressing of severe core damage, and another is to enhance removal performance of radio-nuclides with the newly added filtering equipment, which is installed in the venting line from the PCV to the outer. The Kasshiwazaki-Kariwa NPS unit 6 and 7 are the first reactors applied the FCVSs. Performance of the FCVS has been verified by an extensive test program which includes scaled filter venting test under various conditions. In this paper we show schematic of our test facility and some test results on Iodine gas decontamination factor (DF). The test results clearly show that the DF of the system is above the FCVS design target value (DF>1000).

During a severe nuclear power plant accident, increasing of containment pressure are assumed to be possible. When the containment is damaged by excess pressure or temperature, radioactive materials are released from the containment. A filtered containment venting system (FCVS) reduces containment pressure and the radioactive release into the environment. The FCVS removes radioactive materials via various process. A pool scrubbing is an important process and flow dynamics (inertial collision, submergence, bubble diameter) and chemical conditions (pH, solubility) affects the radioactive materials removal performance. For an iodine (I2), chemical conditions are important factor in this process. This study has evaluated the relationships between the iodine decontamination performance and the chemical conditions in the pool scrubbing process.

Fukushima Daiichi NPP Accident caused core melts and hydrogen explosions on their reactors and a large amount of radioactive gas release in environment, forcing many people refuge. The core melts were not induced by decay heat only as many nuclear scientists believe but initiated by heat of chemical reaction between hot cladding material Zry and water. Hydrogen gas generated by the reaction flew up into operating floor above containment to explode it. Two type of radioactive gas release were recorded at the accident by radiation monitor provided near the site gate, showing the large amount of the radioactive gas release was caused by failure of opening vent system of No.2 reactor, as well as indicating that decontamination effect of BWR wet vent through suppression chamber is so large enough to extingish the need of refuge from radiation exposure, even at core melt accident.

The dose rate field data recorded at Fukushima Daiichi site shows the effect of radioactivity release by hardened vents at 1F3 was 1/100~ 1/1,000 compared with the effect of direct radioactivity release from 1F2 PCV. That causes were clarified by the evaluation of hardened vent, that is,the steam containing a large amount of radioactivity in 1F3 PCV was decontaminated by scrubbing in the suppression pool (below 1/10) and diluted with a large quantity of steam generated by pressure reduction boiling(~1/35). The scrubbing effect and the dilution effect acted synergistically. The hardened vents performed twise at the early stage of ore damage of 1F3 were estimated to be highly effective for transferring low boiling point radioactive products such as I,Cs and CsI to the suppression pool. The preliminary dynamic analysis with TRAC code suggested the enthalpy loss of steamemmitted through the vent piping line to atomospheric environment was very small. The study will be continued.

Many lessons can be learned from the Fukushima Daiichi Nuclear Power Plant (NPP) accident. After the loss of Emergency Core Cooling System (ECCS) and IC core cooling, fuels in the core melted down. Leak of fission product and hydrogen began because of high-temperature damage to the PCV packing. A hydrogen explosion occurred in the upper floor in the reactor building in Units 1, 3 and 4. At Unit 2, reactor core isolation cooling (RCIC) continued to function for about 3 days. Soon after the loss of RCIC water injection, the water level in the RPV declined. Drywell pressure increased from 400 kPa to 750 kPa, and PCV top flange might leak began through silicon rubber O-ring. It was an initiation of severe contamination around the NPS. In the afternoon on March 15, wind blew toward Iidate village. Melted core relocation into lower plenum caused the radiation level increased. The radiation level was measured by containment atmospheric monitoring system (CAMS).

It is well known that a radioactivity ratio of cesium-134 to cesium-137 is quite a beneficial index when ones want to specify the source of radioactivity including cesium emitted to the environment. Concerning the Fukushima Daiichi nuclear power plant accident, so much research has been carrioud out on this radioactivity ratio. The present paper reviews results of measurements and analyses of this quantitiy, and also shows calculation results of the in-core cesium radioactivity ratio distributions in the units 1, 2 and 3. Calculation results reveal that there is significant dependence of the radioactivity ratio on the fuel shuffling batch and axial loaded position of the fuel.

It is necessary for nuclear power plants (NPPs) to analyze the hydrogen behavior in the containment vessel in order to prevent such accidents as the hydrogen explosion in the Fukushima Daiichi nuclear power plant. For the purpose, the nuclear engineers make the hydrogen risk assessment through numerically analysis. In this study, we carried out comparative numerical analysis with PANDA benchmark from viewpoint of turbulence (LES) models and mesh system. As the results, it is found that the results with orthogonal mesh is better than those with non-orthogonal and deformed mesh. We also found that the numerical results depend on the turbulence and the LES, especially LISEA, are the better than the RANS models but computationally expensive.

By using the FCVS technology, we had started to develop a high decontamination air cleaning system to remove multi-nuclides for radiation protection to conduct decommissioning the Fukushima NPP (Fig.17). Development of high efficiency multi-nuclide aerosol filters for radiation protection during a process of cutting core debris at Hokkaido University. A plasma cutter, laser cutter, wire cutter, drilling machine, etc., will be used and will generate aerosols. Therefore, the air cleaning system should be needed for removing core debris. In order to develop an air clean up system for radiation protection during a cutting core debris of the Fukushima Daiichi NPP as a process of their decommissioning, a high efficiency filters will be developed, such as a wet-type aerosol filter, a metal fiber filter and a silver zeolite.

Rasa-AgX as an adsorbent of radioactive organic iodine has been commercialized and is being used in filtered venting system at Japanese nuclear power plants (NPPs) as their countermeasures of severe accidents. However, as the venting conditions vary in different NPPs, the adsorbents with matched characteristics are demanded. For this reason, a new radioiodine adsorbent, AgR, has been developed. Although the Ag content is as low as 10%, AgR still shows the adsorption efficiency of 99.5% even if dew point distance (DPD) is 0K (Residence time is 0.24 sec.). Furthermore, since AgR exhibits a weak reactivity with hydrogen, it can work very well in the atmosphere with the concentration of 10% hydrogen. In addition, AgX shows excellent long-term storage stability and water resistance under high humidity.

Recently, various remote cutting and decontamination technologies for decommissioning works in nuclear facilities have been developed. One of the technologies is a laser technology which has good capabilities for such kinds of works. Remote controllability is one of the advantages of a laser system. Because in general the laser head is light and compact compared to the other tools, the head can be easily handled by remote operation equipment such as manipulators. As the other example of the advantages, a laser system does not generate secondary waste like contaminated water generated from waterjet methods. This paper provides the outline of the remote laser cutting and decontamination system recently developed by IHI.