The bed part of a fused-filament-fabrication-based composite 3D printer was simulated, and the use of a hot plate to control the trans-crystallization (TC) thickness in the bed part was evaluated based on the molding conditions under which TC is known to occur. An experiment using plastic, which is commonly used in 3D printers, showed that TC was generated vertically from the surface of carbon fiber. Some resins exhibited an increasing degree of isothermal crystallization as the temperature of the bed was increased, but the effect of the cooling rate on crystallization was significant; TCs with an average thickness of 38.5, 19.3, and 5.2 μm were generated in carbon-filled (CF)/PP, CF/PPS, and CF/PET, respectively. A simulated experiment that investigated the effect of the cooling rate from the melted state showed a clear reduction in TC during rapid cooling, indicating that controlling the cooling rate from the melted state can alter the thickness of the TC at the interface.

Effect of pretreatment by shotblasting to workpieces before lubricating of a non-chemical conversion lubricant in cold forging was examined by making several surfaces with different topographies, using wet shotblasting. Backward-cup extrusion test estimated the performance of the pretreatment. This test provides extremely large surface expansion to bring severe tribological conditions. After the tests, the insides of the cup-like workpieces were observed with scanning electron microscope and surface-analyzed energy dispersive X-ray spectroscopy to measure the distributive amount of the lubricant on the workpiece. The pretreated surface resulting large aspect ratio (Ra / RSm) raised the potential of lubrication performance. Especially, the surface treated with angular shaped media indicated higher potential to improve anti-galling performance.

The free-cutting steel is one of the essential structural steels in electrical appliance industry and automotive industry. Most of the steel contains Pb, as solid lubricant, chip breaker, tool edge stabilizer and tool life extender. MnS grains also play the same role of Pb. However, Pb is toxic so that it will be prohibited to add to the steel in near future. Steel industry has tried to replace Pb with other element(s) and/or compound(s). The trials are still going on. This study proposes a candidate of the workability parameters for free-cutting steels with/without Pb. Investigated steels were loaded quasi-statically (strain rate: 1×10^-3 s^-1) and dynamically (1×10³ s^-1) up to fracture. Half of the specimens were pre-fatigued to form easier slip situation in steel matrix. After fracture, thin steel matrix penetrated into plenty of MnS grains. For the steel with Pb, the ratios of the number of such penetrations to that of total grains maintained almost the same value for all loading conditions. The ratios were varied with the loading conditions for the steel without Pb. These facts meant that Pb was a key element for easier slip of steel matrix. Therefore, this ratio had a feasibility to be one of the workability parameters for the free-cutting steel.

Flow drilling screw (FDS) is a novel joining technique that enables high-speed joining of dissimilar metals with one-shot process of hole machining, female thread forming and fastening from one side. The damage after FDS process and its effect on joint strength and fracture behaviour near the screw by load-bearing tests for FDS joints between aluminum alloy and carbon fibre reinforced thermoplastics (CFRTP) are evaluated. Cross-sectional observation after joining process reveals that delamination in CFRTP is clearly suppressed and matrix resin is filled in the screw thread compared to the case with CFRP using thermosetting resin. In the joint strength test with shear loading and the cross-shaped tensile tests where the screw is pulled out in its axial direction, only the fibre micro-buckling is observed until fracture. These experimental results recommend that CFRTP laminates should be applied as composite side since this technique utilizes frictional heat by the screw that can contribute to plastic flow or deformation of the thermoplastic resin with high fracture toughness. It is also clarified that damage growth is further suppressed by using CFRTP/Al hybrid laminates where aluminum alloy sheets are inserted between CFRTP plies, which leads to improve the joint strength.

An understanding of damage mechanisms induced by drop-off plies in tapered composite laminates is crucial. This study investigated the mechanical properties and damage behaviors of asymmetric tapered unidirectional carbon fiber-reinforced plastic laminates. Damage observations of laminates with simultaneous tapered and staggered tapered structures under monotonic and cyclic tensile loads were done by an optical microscope and X-ray radiography. Based on the results, matrix cracks in the resin pocket and delamination occurred earliest in the simultaneous tapered specimen due to a greater stress concentration in a single large resin pocket in the structure. Intralaminar damages in the staggered tapered specimen with a shorter step spacing occurred mainly in the lower dropped ply as there was a more significant interaction between the neighboring steps in the specimen. The drastic decrease in the stress concentration after the occurrence of damages in the neighboring step(s) then suppressed the occurrence of intralaminar damages in the upper dropped ply. This is in contrast to the specimen with a longer step spacing where the intralaminar damages occurred in both dropped plies due to the distributed stress distribution. Delamination propagated between the dropped plies and continuous (belt and core) plies for both staggered laminates regardless of the length of the step spacing at higher applied stress levels. The results showed that internal ply-drop configuration and step spacing in staggered tapered structures contribute to significant differences in the mechanical properties and damage behavior in the laminates.

The objective of this study was to experimentally evaluate the fatigue limit of 90° unidirectional carbon fiber reinforced plastic (CFRP) laminates. The very high-cycle fatigue properties of the specimens were evaluated using ultrasonic and electromagnetic fatigue testing machines. Ultrasonic fatigue tests were conducted to obtain the fatigue properties under the giga-cycle regime. The specimen geometry was designed to resonate at 20 kHz, the specimen consisted of CFRP laminate and a metal tab to connect to the horn end of the testing machine. Additionally, the free volume of the matrix material, namely, epoxy resin, of the CFRP laminates was evaluated using the positron annihilation method. A slit was introduced in the specimen surface to identify the location of damage development, which facilitated the free volume measurement by positron microscopy. The obtained S-N curves reveal that failure did not occur at strain levels lower than ε_max = 0.75% at the slit tip for all specimens up to N = 1.0 × 10⁹ cycles. The free volume measurement for a specimen set above the threshold strain level revealed that the free volume increased in size and decreased in amount as the number of cycles increased. The test results revealed that the opposite trend existed below the threshold, which suggests that a fatigue limit may exist.

As the application areas of MEMS (microelectromechanical systems) expand, more advanced functions and performance are being demanded from MEMS. To meet these demands, materials with various functionalities such as mechanical strength and shape memory, which are difficult to achieve with Si-based materials alone, are being used in MEMS. When applying new materials to MEMS, it is essential to establish micromachining techniques and control internal stresses in the same way as for Si-based thin films. In this research, a method for measuring the internal stress of thin film structures formed by microfabrication technology was developed. The thin film structure sample to be measured is in the form of a beam with fixed ends. The uniaxial strain due to internal stress is measured using a micro spring, and the internal stress is calculated. A process for fabricating a device that realizes the novel measurement method was devised and the device was fabricated. The microfabrication technique used was a reverse lift-off process, which can form thin film structures with rectangular cross-sections and uniform film thickness. Thin film metallic glass, an amorphous alloy with higher strength and lower Young's modulus than Si-based materials, was used as the new material to be measured. Using the developed measurement method, the internal stress change of the thin film was measured as a function of annealing temperature. As a result, it was confirmed that the internal stress of the thin film changed from the compressive direction to the tensile direction with increasing annealing temperature. The internal stress values measured by the novel method were compared with those measured by Stoney's equation, and the two measurement methods were close, demonstrating the usefulness of the new measurement method.

In this study, we will report the efficiency of hybrid filler for the mechanical properties and the thermal conductivity of Polyamide-6 (PA6) composite. One of the ways to improve the thermal conductivity of the composite is to add fillers to the matrix, but the excessive filler causes the composite to form aggregation and void. They make the mechanical properties and thermal conductivity decline. As the solution to this problem, to use of hybrid filler is expected to realize the lower filler content of composite with keeping the properties. However, the most efficient ratio of hybrid filler is unclear. In this study, PA6 and filler were composited using a twin-shaft melt-mixing machine. A manual injection molding machine was then used to produce test specimens for thermal conductivity measurement and tensile test specimens. Thermal conductivity was measured based on the laser flash method. The structure of the composite was also investigated by fracture surface observation after tensile testing. It is revealed that two types of fillers were composited with PA6 to obtain higher thermal conductivity than that of a single filler with the same filler content. This is because the different shapes of the two fillers made the thermally conductive path.

Dielectric elastomer generator (DEG) could harvest electrical energy from cyclic deformation by changing the capacitance of dielectric elastomer (DE). Due to its advantages of light weight and high energy density, DEG has great development potential in the utilization of renewable mechanical energy. A key bottleneck restricting the development of DEG is the charge leakage phenomenon, which can reduce the DEG energy density. In this work, the effect of the charge leakage phenomenon on DEG energy density was investigated based on experiments and theoretical simulations. The results showed that with the increase of bias voltage and transverse pre-stretching ratio, the DEG energy density of quadrangular and triangular harvesting cycle first increased and then decreased, and the maximum experimental energy densities of the DEG with transversely restrained configuration were 120 mJ/g and 187 mJ/g, respectively. In addition, a simulation method for calculating the DEG energy density based on charge leakage model was proposed to quantitatively analyze the influence of charge leakage on DEG energy density. The simulation results showed that the energy density of the quadrangular harvesting cycle could be significantly improved by decreasing the charge leakage of the DE film.

The purpose of the present study is to clarify the effect of surface texturing on lubrication characteristics of a sliding surface under reciprocating motion and to develop design guidelines for surface texturing in order to obtain lower friction. To achieve this purpose, the lubrication characteristics of a sliding surface with dimple-shaped texturing during reciprocating motion are numerically analyzed by solving the Reynolds equation. The effects of texturing are compared with those of no texturing under various operating conditions as determined from the sliding speed, viscosity coefficient of the lubricant, and surface pressure, and the placement and dimensions of the texturing that result in lower friction are evaluated. The results show that in the oil film pressure distribution generated by the slider without texturing, friction loss is reduced by applying texturing at the location where the maximum value of this oil film pressure occurs. In this case, under the conditions analyzed in the present paper, the texturing reduces friction loss by 5 to 7%.

Diamond-like carbon (DLC) films had been formed as a surface treatment for intracorporal device for decreasing the coefficient of friction. DLC film showed significant stability without being damaged during acid immersion and high-pressure steam sterilization. However, bacteria that adhere to medical devices lead to the induction of infectious diseases associated with therapeutic actions. Therefore, maintenance of a hygienic surface condition has been strictly required. This study demonstrated the fabrication of DLC films (gas source: CH4), which incorporated Cu (Cu-DLC). The Cu-DLC films were synthesized on a Si (100) substrate via plasma-enhanced chemical vapor deposition and magnetron sputtering. The surface morphology, microstructure, element contents, wear resistance, hardness, and antibacterial properties of the films were experimentally analyzed. Cu particles were considered not uniformly distributed in the DLC film, they embedded in DLC films formed a three-dimensional structure led to higher roughness. The Cu-DLC exhibited wear resistance and higher hardness compared to Cu. After inoculation, Cu-DLC films showed higher antibacterial activity against E. coli than pure DLC. It is expected that the hygienic films with excellent mechanical properties demonstrated in the present study will be utilized in various medical and industrial sectors.

For safe and sound utilization of the applications made of polylactic acid that output an electric signal to an intended mechanical input within an undesirable thermal environment, the transient thermoelectroelastic field is investigated for an infinite cylinder with D_∞ symmetry subjected to shear stress as an intended mechanical input and temperature as an unfavorable thermal environment. By use of the analytical technique constructed previously, the field quantities are represented in terms of the elastic, piezoelastic, and thermoelastic displacement potential functions and the electric potential function, and the governing equations for these functions are presented. Then, the analytical solutions of the transient and non-axisymmetric field quantities are obtained using the Fourier and Laplace transformations with respect to the axial coordinate and time variable, respectively, and the Fourier expansion with respect to the circumferential coordinate. Subsequently, numerical calculations are performed to investigate the field due to the shear stress or temperature. As a result, the structures of respective fields are elucidated and the effect of thermal disturbance on the output signal to mechanical input is investigated quantitatively, both of which illustrate the significance of transient and three-dimensional analysis.

Cold forming has required considerably superior hard coating on a die to manufacture net-shaped products. Especially, under poor lubrication conditions or a dry condition, the hard coating is indispensable for the success of manufacturing. Accordingly, the die has required the superior hard coating to prevent adhesion, reduce friction, and control wear. Recently, it has been also necessary to improve die life and product qualities under severe tribological conditions. However, the comprehension of damage such as cracking, flaking, and galling is insufficient for metal forming. This study focuses on initiation and growth of cracking in the hard coating. Assuming the hard coating is subjected to tensile stress, nominal stress, or frictional shear stress, the performances of typical four hard coatings are investigated by two tests. One is a uniaxial tensile test, and another is a rotating cross-cylinders wear and friction test. These tests are applied to estimate the mechanical performance of hard coatings: CrN, TiN, VC, and CrAlN. After the two test and analyses with detail observation, the CrAlN coating collectively exhibits the highest anti-cracking property of the tested four hard coatings.

Welding is used as a joining method in the construction of many structures. Residual stress is generated near the bead because of locally given heat. It is well known that it degrades fatigue strength. Especially, it is a cause of stress corrosion cracks in stainless steel. Since reduction methods, stress-relief annealing and shot peening are widely used, reduction methods of residual stress have been studied. However, these methods require special equipment and are time-consuming. The authors have proposed a method using vibrations during welding and shown the effectiveness of the method in rolled steel for general structures. On the other hand, the authors have also investigated the effectiveness of the method on stainless steel which is used for important structures. However, the experimental conditions were not the same. In this paper, the reduction of residual stress on build-up welded SUS304 and SS400 specimens were compared in the same experimental conditions and the effects of material properties on the method were investigated. The experiment is conducted for different amplitudes of the ultrasonic vibration load and without vibration load. It is concluded that the reduction rate for SUS304 is greater than that for SS400 and that the greater the amplitude of the ultrasonic vibration load, the greater the reduction rate. Statistical values of residual stress were obtained and there were significant differences in the mean values between each amplitude of ultrasonic vibration load. Additionally, the effectiveness of the method has also been verified. The experimental results were examined by the simulation method using a model considering plastic deformation caused by ultrasonic vibration load. It is found that residual stress is reduced using ultrasonic vibration load during welding.

Cold forging of automotive parts in high strength steel causes high pressure on the die. The high pressure or heavy load reduces a die life and accuracy of the forged parts, so prediction of a more accurate load helps us to design optimum die dimensions in the process. In this report, the more precise frictional coefficient was measured by a ring compression test to predict the more accurate load in cold upsetting of high strength steel. The experimental results indicated a frictional coefficient μ of about 0.19 within height reduction less than 20% under a dry condition. However, when the reduction in height exceeds 20%, the results deviate from the calibration curve assumed to be constant μ = 0.19. This means that the friction changing during compression. Slight adhesion was observed on the die surface, which must directly increase the frictional coefficient. The calibration curve was modified considering the change in friction during compression. Using the modified calibration curves, the experimental results are exactly plotted on the calibration curve which is drawn by changing μ from 0.19 to 0.25 at the reduction in height of 20%. This increasing frictional coefficient was helpful to estimate the accurate load when the rod of high strength steel is upset at high reduction under a dry condition.

Binder jet is one of the metal additive manufacturing methods. In this method, metal powder is used as a raw material, and a metal part is obtained by sintering a green body produced by jetting a binder on a metal powder bed leveled with a roller. Powders produced by the gas atomization method with an average particle size of around 10 μm is mainly used in the binder jetting method for metal additive manufacturing. The water atomization method used in this study has a higher production yield, so the cost of powder is lower than that of gas atomized powder. It is also possible to produce ultra-fine powders. However, there are few studies on the use of water-atomized powder in the binder jetting method. This paper aims to evaluate the basic characterization using water atomized powder for practical use in the binder jetting method. Three different water atomized powders with average particle sizes ranging from 4 μm to 10 μm and the gas atomized powder with an average particle size of 10 μm for comparison were used in the evaluation. The results confirmed that the flowability of water atomized powder decreased as the average particle size became smaller, and the flowability was lower than that of gas atomized powder due to differences in powder shape even at the same average particle size. The apparent density of the green body was found that tapped density of powder had a significant effect. The sintering process confirmed that the green body with smaller average particle size powder had higher sintering performance. This result is in line with the existing theory in the sintering technology using metal powders. It is clear that tap density and powder flowability are important for the practical use of water atomized powder in the binder jetting method.

The purpose of this study was to develop a frictional motion material with excellent corrosion and wear resistance in marine environments by generating surface modification layers obtained by frictional reforming techniques using various fine powders mixed with carrier particles. As one means to achieve this, we constructed a system that simultaneously measures the corrosion and wear resistance characteristics of various frictional-reforming materials in artificial seawater under fretting conditions. Frictional reforming was performed on 18Cr-8Ni austenitic stainless steel using hard fine powders of Ti, TiN, Cr, Cr₂N, and Al₂O₃ mixed with Al₂O₃ carrier particles. The time variations of the frictional and corrosion resistance of these materials in a 3.5 % NaCl solution under fretting conditions were then evaluated simultaneously. The results showed that stainless steel modified by frictional reforming using a mixture of TiN powder and Al₂O₃ carrier particles has excellent corrosion resistance. However, its wear resistance may be inferior to that of stainless steel.

Potential energy of an alloy is an essential indicator for evaluating the stability of the structure in predicting new materials. Therefore, how to calculate the potential energy in material design has become an inevitable problem. While first-principles calculations can provide chemical accuracy for arbitrary atomic arrangements, they are prohibitive in terms of computational effort and time. To enable atomistic-level simulations of both the processing and performance of Aluminum alloys, neural network potential was proposed to predict the binding energy of vacancy-containing aluminum alloys in a highly accurate state. This method combined first-principles calculations and machine learning techniques to explore the intrinsic link between solid solution structure and binding energies. In this study, four binary alloys (aluminum-silicon, aluminum- zirconium, aluminum-magnesium and aluminum-titanium alloys) were investigated. The mean squared errors were used to quantify the quality of the neural network potential models and it was found that the trained model is more stable and exhibits high accuracy for energy prediction. The Monte Carlo simulation results show that using this neural network potential successfully simulated aging process of aluminum alloys, and the neural network potential can be much faster than first-principles calculations, even with high accuracy.

A detailed evaluation for air cooling of fuel debris in actual reactors will be essential in fuel debris retrieval under dry conditions. To understand the heat transfer in and around fuel debris, which is assumed as a porous medium in the primary containment vessel (PCV) mechanistically, we newly applied the porous medium model to the multiphase and multicomponent computational fluid dynamics code named JUPITER (JAEA Utility Program for Interdisciplinary Thermal-hydraulics Engineering and Research). We applied the Darcy–Brinkman model as for the porous medium model. This model has high compatibility with JUPITER because it can treat both a pure fluid and a porous medium phase simultaneously in the same manner as the one-fluid model in multiphase flow simulation. We addressed the case of natural convection with a high-velocity flow standing out nonlinear effects by implementing the Forchheimer model, including the term of the square of the velocity as a nonlinear effect to the momentum transport equation of JUPITER. We performed some simple verification and validation simulations, such as the natural convection simulation in a square cavity and the natural convective heat transfer experiment with the porous medium, to confirm the validity of the implemented model. We confirmed that the result of JUPITER agreed well with these simulations and experiments. In addition, as an application of the updated JUPITER, we performed the preliminary simulation of air cooling of fuel debris in the condition of the Fukushima Daiichi Nuclear Power Station unit 2 including the actual core materials. As a result, JUPITER calculated the temperature and velocity field stably in and around the fuel debris inside the PCV. Therefore, JUPITER has the potential to estimate the detailed and accurate thermal-hydraulics behaviors of fuel debris.

To enhance resilience of next-generation nuclear structures, it is necessary to develop design methodology that mitigates impacts of failure caused by extremely high temperature conditions which might lead to a severe accident. The purpose of this study is to understand its deformation behavior under extremely high temperature conditions and to identify the areas that should be focused on to mitigate impacts of failure. For this purpose, this study has conducted a detailed structural analysis of a reactor vessel (RV) in a loop-type sodium-cooled fast reactor using a commercial finite element analysis code, FINAS/STAR. In a postulated scenario of loss of heat removal system, the RV was heated from the normal operation condition to the sodium boiling temperature in the upper sodium plenum for 20 hours, assuming depressurization. The analysis has revealed less significant stress and strain which were sufficiently lower than failure criteria. The upper body of the RV was identified as the important area in terms of mitigation of structural failure. The RV was eventually deformed downward about 160 mm, but it resulted in no failure. This analysis implies maintaining the RV sodium level in a long term, thereby enhancing the RV resilience.

The Japan Atomic Energy Agency has been developing “Advanced Reactor Knowledge- and AI-aided Design Integration Approach through the whole plant lifecycle (ARKADIA)” to offer the best solutions for challenges in the design and operation of nuclear plants. A part of ARKADIA for design study, which included design optimization of components, is named as ARKADIA-Design. In the development of ARKADIA-Design, we have been developing a process to automatically optimize design parameters of structural components subjected to various kinds of loads, including thermal transients. In this paper, we propose a simplified procedure to estimate the failure probability of components subjected to thermal transients for design optimization. An objective function of this optimization is defined on the basis of failure probability of the components, because failure probability can be commonly used as an indicator of component integrity for various mechanisms, and it helps future introductions of a risk-informed performance-based approach to component design. To enable the necessary number of estimations for design optimization with practical calculation time, we aimed to reduce the number of analyses required for one estimation. For this purpose, we adopted the first-order second-moment (FOSM) method as the estimation method for failure probability in the process of optimization. An orthogonal table in the experiment design method is utilized to define the conditions of the analyses for evaluation of the mean and variance of thermal transient stress, which are used as inputs in the FOSM method. The superposition of ramp responses is also utilized to evaluate the time history of thermal transient stress instead of finite element analysis. The proposed procedure was applied in a demonstration study to optimize the thickness of a cylindrical vessel subjected to thermal transients derived from shutdown. We confirmed that the procedure can evaluate the failure probability depending on the cylinder thickness with practical calculation time.

To develop rationalized maintenance plans for nuclear power plants, the characteristics of each plant must be considered. For sodium-cooled fast reactor (SFR) plants, constraints on inspections exist due to the specialty that equipment retaining sodium must be handled, which is one of the important points that must be considered in maintenance rationalization. In this study, we propose a maintenance optimization scheme, which is a design support tool, using risk information to develop a maintenance strategy based on the system based code (SBC) concept. The SBC concept intends to provide a theoretical procedure to optimize the reliability of structure, system and components (SSCs) by administrating every related engineering requirements throughout the life of the SSCs from design to decommissioning. “The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Code Case, N-875, Alternative In-service Inspection Requirements for Liquid-Metal Reactor Passive Components” was developed based on the SBC concept. The basic procedure of the Code Case has also been implemented in “The ASME Boiler and Pressure Vessel Code, Section XI, Division 2,” which provides requirements for Reliability and Integrity Management programs for nuclear power plants, including advanced reactors. The purpose of this study is to establish detailed procedures for the maintenance optimization scheme based on the procedure in Code Case N-875. Furthermore, a quantitative trial evaluation of the core support structure of the next SFR under development in Japan is also performed using the maintenance optimization scheme.

This paper describes the results of seismic analyses conducted to clarify the effects of the plasticity of the pipe and its supports on the seismic response of the entire piping system. First, a study (Study 1) was performed to evaluate the effects of the plasticity of the pipe and its support independently, and of both the pipe and the support on the response of the entire piping system. Second, another study (Study 2) was performed to evaluate the effects of the magnitude of the yield load and post-yield stiffness on the restoring force characteristics of the supports. Study 1 produced remarkable insight on the seismic design of piping systems. The plasticity of the pipe and the support significantly reduced the support load and the elbow strain. In particular, the effect of the plasticity of the support was larger. Study 2 showed that the yield load of the support significantly affects the response of the piping system, whereas the post-yield stiffness does not affect much compared with the yield load.

A probabilistic fracture mechanics (PFM) analysis code, PASCAL, has been developed by Japan Atomic Energy Agency for failure probability and failure frequency evaluation of reactor pressure vessels (RPVs) considering neutron irradiation embrittlement and thermal transients. To strengthen the applicability of PASCAL, considerable efforts on verifications of the PASCAL code have been made in the past years. As a part of the verification activities, a working group consisted of different organizations from industry, universities and institutes, was established in Japan. In the early phase, the working group focused on verifying the PFM analysis functions for RPVs in pressurized water reactors (PWRs) subjected to pressurized thermal shock (PTS) events. Recently, the PASCAL code has been improved in order to run PFM analyses for both RPVs in PWRs and boiling water reactors (BWRs) subjected to a broad range of transients. Simultaneously, the working group initiated a verification plan for the improved PASCAL through independent PFM analyses by different organizations. Concretely, verification analyses for a PWR-type RPV subjected to PTS transients and a BWR-type RPV subjected to a low-temperature over pressure (LTOP) transient were performed using PASCAL. This paper summarizes those verification activities, including the verification plan, analysis conditions and results. Through the verification studies, it is confirmed that the mathematical models and probabilistic calculation algorithms incorporated in PASCAL work appropriately and the applicability of PASCAL has been improved.

The basemat uplift is a phenomenon that the bottom of the basemat of a building partially rises from the ground due to overturning moments and vertical motions during earthquake. The degree of the basemat uplift can be indicated using a ground contact ratio which is defined as a ratio of the contact area of the bottom of the basemat to its entire area. The problem is that, at a large earthquake, the basemat uplift becomes large, so-called low ground contact ratio state, and the basemat falls resultant to recontact between the basemat and the ground, and large acceleration response occur on the floor of the building. It is a crucial aspect in the seismic evaluation of a nuclear facility building. It affects not only structural integrity of the building but also the response of the equipment installed in the building. However, the building behavior under the low ground contact ratio state lacks sufficient study. In this study, we conducted seismic response analyses for the building using a three-dimensional finite element model and simulated shaking table experiments focused on the basemat uplift and confirmed the validity of this analysis method. Since the basemat uplift is a strong non-linear phenomenon, we conducted computer simulations under the same analysis conditions with three different analysis codes, namely E-FrontISTR, FINAS/STAR, and TDAPIII, and compared the results. We investigated the influence on the structural response caused by the difference of the adhesive force of the basemat and the bearing ground at the low ground contact ratio state. In addition, we studied the influence of numerical parameters to the structural response through sensitivity analyses. This paper reports the analysis results and the insights obtained from our investigations.

The significance of probabilistic risk assessments (PRAs) of nuclear power plants against external events was re-recognized after the Fukushima Daiichi Nuclear Power Plant accident. Regarding the seismic PRA, handling correlated failures of systems, components, and structures (SSCs) is very important because this type of failure negatively affects the redundancy of accident mitigation systems. The Japan Atomic Energy Research Institute initially developed a fault tree quantification methodology named the direct quantification of fault tree using Monte Carlo simulation (DQFM) to handle SSCs’ correlated failures in detail and realistically. This methodology allows quantifying the top event occurrence probability by considering correlated uncertainties related to seismic responses and capacities with Monte Carlo sampling. The usefulness of DQFM has already been demonstrated. However, improving its computational efficiency would allow risk analysts to perform several analyses such as uncertainty analysis efficiently. Therefore, we applied quasi-Monte Carlo and importance sampling to the DQFM calculation of simplified seismic PRA and examined their effects. Specifically, the conditional core damage probability of a hypothetical pressurized water reactor was analyzed with some assumptions. Applying the quasi-Monte Carlo sampling accelerates the convergence of results at intermediate and high ground motion levels by an order of magnitude over Monte Carlo sampling. The application of importance sampling allows us to obtain a statistically significant result at a low ground motion level, which cannot be obtained through Monte Carlo and quasi-Monte Carlo sampling. These results indicate that these applications provide a notable acceleration of computation and raise the potential for the practical use of DQFM in risk-informed decision-making.

Basically, decommissioning of nuclear facilities is a project that does not generate new profit because it is carried out with the reserve funds from operation, etc. Therefore, its cost should be minimized with optimization by shortening the process and minimizing the waste, etc. Meeting the requirements of exposure risk (safety) also affects the optimization. In this study, we decided to integrate these evaluation methods to develop a comprehensive optimization evaluation method. In this study, we established an average process for the current decommissioning plans of Japanese nuclear power plants and developed a cost evaluation method including sensitivity analysis. As a result of examining the feasibility of the deferred dismantling strategy using the above calculation method, it became clear that although there is a reduction in disposal and dismantling costs due to the natural decay of radioactive materials, the maintenance and management costs during the safe storage period account for a large proportion of the costs, and for this reason, immediate dismantling is unconditionally advantageous, at least in Japan. The components of optimization described above are naturally subject to various uncertainties and risks. For example, there are regulatory risks, and the location of waste disposal site is subject to social acceptance, so there is a great deal of uncertainty. In the future, these factors will be incorporated into the evaluation and studied, and the optimal strategy for decommissioning and what kind of uncertainty should be focused on will be clarified quantitatively.

Resistance spot welding has recently been proposed for joining steel and aluminum alloys to reduce the weight and increase the strength of automobile bodies. However, joining Fe and Al results in the formation of intermetallic compounds (IMCs) at the joining interface. A thick IMC layer reduces the joint strength, but it is difficult to control the IMC layer thickness. Therefore, in this study, a joining process that simultaneously achieves resistance heat generation and plastic deformation using die- and punch-shaped electrodes in resistance spot welding equipment is developed. The formation state and extent of IMCs for this process are investigated. The formation state of IMCs is evaluated experimentally for various electrical currents. Numerical simulations are conducted to calculate the temperature variation of the joining interface for various currents. It is found that the IMC is thin at the center of the joint and thick and spike-shaped at the edge of the joint. Furthermore, the cross-tension strength is found to increase with increasing electrical current. The stable formation of IMCs at the center of the joining interface is considered to be due to an increase in temperature in this region with increasing current. Joints with high cross-tension strength form spike-shaped IMCs with differences in thickness. It is considered that the spike-shaped IMCs create a snagging effect, enhancing the strength of the joint interface.

The service life of mechanical structures is influenced by the inelastic strain generated during manufacturing and operation. Therefore, non-destructive estimation of three-dimensional inelastic strain is essential to ensure structure safety even during operation. Accordingly, an inverse problem method was proposed to estimate the inelastic strain over the entire component based on the eigenstrain theory using non-destructively measured surface displacement. However, this method assumes micro-deformation, in which the causal inelastic strain and resulting displacement are linearly related. In practice, the value of inelastic strain is large, which results in large deformation and a nonlinear effect on displacement determined by the magnitude of the inelastic strain. This study considers the inverse problem of estimating the creep strain in a turbine blade for power generation from the displacement caused by the creep strain and proposes a method to derive the exact solution even for large deformation problems. Numerical simulations using a simplified turbine model show that the estimates converge to the correct solution value with relatively high accuracy in the absence of measurement errors. Moreover, the result of the iterative calculations used in the proposed method converge around the correct value, even in the presence of measurement errors in the displacement by the laser displacement meter.

This study evaluates our previously proposed correlations for the wall condensation heat flux q_c and the wall functions of dimensionless profiles for temperature of steam and air mixture T_g and steam mass fraction X_s. These correlations are expected to be used in computational fluid dynamics (CFD) analysis with coarse computation cells to evaluate thermal hydraulic behavior in containment vessels (CVs) of nuclear power plants under accident conditions. We carried out numerical simulations of experiments for wall condensation from mixtures of saturated steam and air in a vertical tube (diameter, 49.5 mm) by using the CFD code FLUENT, compared the simulated T_g and q_c values with measured values, and evaluated the profiles of dimensionless mixture temperature T⁺ and steam mass fraction Y_s⁺ which are used for wall functions in turbulent analyses with coarse computation cells. We determined the simulated T_g and q_c values agreed with the measured values within uncertainties, but the decrease of q_c in the flow direction was not well simulated. The computed T⁺ values were smaller than the measured values but almost within the uncertainty, and the computed Y_s⁺ values agreed well with the measured values obtained from the assumption of the saturated conditions (i.e. T_s = T_g). These results confirmed that our previously proposed correlations for the q_c and wall functions can be used in a CFD analysis with coarse computation cells.

Flow-accelerated corrosion (FAC) is the major pipe wall thinning phenomenon to be managed for its potential of catastrophic pipe rupture. Present management rule for pipe wall thinning in Japanese nuclear power plants is based on pipe wall thickness measurements resulting in large number of measurements in each plant. For future improvement of the plant management in Japan, introduction of prediction method or prediction code for thinning rate and residual lifetime evaluation is expected. To predict the maximum wall thinning rate or minimum wall thickness efficiently for each pipe component, one-dimensionally along the pipelines, relative wall thinning trend of each component type, “geometry factor”, combined with the effect of upstream component, “declining effect”, are essential, which are both derived from hydraulic features. In this study, the geometry factor and the declining effect for pipeline elbows are evaluated considering specific geometric parameters through series of computational fluid dynamics calculations. For the geometry factor, correlation equation is arranged as a function of curvature radius and bending angle of elbows, showing evident effect of each geometric parameter. And for the declining effect, firstly, the effect of plane angle formed by the upstream and downstream elbows is evaluated that the in-plane configuration with the flow streaming forward would be the most conservative condition. Then, based on the in-plane configuration, correlation equation for the declining effect is arranged as a function of the distance between the two elbows. It is found that even when the distance is very short, the remaining portion of the upstream effect on the downstream elbow would be much less than previous knowledge which may improve the overprediction in proximate condition. By implementing these correlations into author’s FAC prediction model, residual wall thickness of plant pipe elbows is shown to be predicted mostly within 20 % from the measured data.

To solve the problem of unmanned plugging weld for the steam generator in high temperature gas cooled reactor, a new seven-degrees-of-freedom robot with 3-PSP and SCARA structure is designed based on the scheme of serial-parallel robot. In order to achieve simple and stable motion control, the D-H method and analytical method are used to study its serial-parallel kinematics and establish an accurate kinematics model. Based on MATLAB software and existing kinematic models, the Monte Carlo method is introduced to analyze the position and posture accessibility of the robot which has redundant degrees of freedom, and the rationality of the institutional design is verified. To ensure the high precision performance of the robot, the positional accuracy of the prototype is tested and analyzed under the different operating conditions by means of the standardized robot performance tests. In addition, in order to ensure the quality of plugging weld, through the T22 base material for several groups of TIG welding process experiments and mechanical properties testing to determine the reliable plugging welding process parameters. The results show that the robot can meet the requirements of plugging welding, and has accurate motion control, good reachability, high position and posture accuracy and strong expandability.

In a previous paper, a closed-loop mechanism represented by successive excitation processes was introduced to explain the reason for the nonlinearity-induced transition from unstable response to stable vibration. In one of the processes, the dynamic pressure is excited by the centrifugal force. Based on this, this paper investigates a vibration reduction method that introduces a circumferential flow whose velocity relative to the rotor is negative. This flow is predicted to weaken the excitation effect of the centrifugal force on the dynamic pressure. An improved semi-analytical model is developed for conservative estimate of the vibration reduction effect by considering the boundary condition that the relative flow velocity vanishes at the cylindrical wall. Because there arise circumferential flow velocity terms in various governing equations, the influences of the terms on the response are examined, thereby identifying important mechanism that contributes to the vibration reduction. The present semi-analytical method is helpful for this identification as well as computationally efficient analysis.

Human-Robot Collaboration (HRC) systems are becoming widespread in industrial applications owing to the advantages of enhancing work productivity and reducing production expenses. In this study, we focus on HRC systems that involve physical interactions between humans and robots. The relationship between force and position at the robot’s end-effector is generally modulated using impedance or admittance control techniques to implement these systems. Furthermore, varying the target impedance of robots has been shown to enhance their performance in HRC tasks. This report presents a novel approach to admittance learning strategy aimed at minimizing human effort during physical human-robot collaboration tasks. A damping generation scheme based on Gaussian basis functions is introduced, enabling the generation of a diverse range of smooth damping profiles via the modulation of the weights of these functions. The Gaussian design is based on a frequency analysis of human movement, with weights adjusted via gradient descent to minimize the interaction force. A learning algorithm based on the generalized simplex gradient approximation technique is proposed to accommodate the noisy evaluation function, utilizing data from previous iterations to enhance estimation accuracy. The effectiveness of the proposed method is experimentally demonstrated through comparison to conventional methods, as well as trials involving a complex task.

In this paper, a control system for an active vibration reducer is proposed to damp sloshing in a liquid container while a cart runs on an uneven road. An active vibration reducer with a parallel linkage mechanism with six degrees of freedom was installed on the cart, and used to damp the sloshing. The acceleration added to the container by changing the altitude of the cart was calculated through the kinematics of the cart with an active vibration reducer, and the cart’s acceleration was measured using an accelerometer installed on the cart. The sloshing generated by the acceleration was suppressed by the action of the reducer, controlled through a reference model following control. The residual sloshing was damped by a frequency-dependent optimal servo system. The cart ran along straight and curved paths on an uneven road in an experiment, and the proposed control system demonstrated satisfactory damping performance.

For nuclear power plants, probabilistic risk assessment (PRA) should be performed not only against earthquake and tsunami, which are critical events especially in Japan, but also other external hazards such as strong wind. The aim of the present study is to develop a practical PRA methodology for sodium-cooled fast reactors (SFRs) against strong wind, paying attention to the final heat sink, ambient air, that removes decay heat under accident conditions. First, this study used Gumbel distributions to estimate hazard curves of the strong wind based on weather data recorded in Japan. Second, it identified important structures, systems and components (SSCs) for decay heat removal, and developed an event tree that results in core damage, focusing on the impacts of missiles (e.g., steel pipes) caused by strong wind. It also identified missiles that can reach SSCs at elevated places, and calculated the fragility of the SSCs against the missiles as a product of two probabilities. One is a probability of the missiles that would enter an inlet or outlet of the decay heat removal system, and another is a probability of failure caused by missile impacts. Finally, it quantified conditional decay heat removal failure probabilities by introducing the fragilities into the event tree. The core damage frequency (CDF) was estimated at about 5 × 10^-10/y. The dominant sequence is that strong wind causes offsite power loss and missiles, the missiles penetrate the diesel fuel tank, cause a fire, and the fire increases air temperature around the reactor building where air cooler inlets of decay heat removal systems are installed, leads to loss of power for the diesel generator for forced circulation cooling, resulting in loss of decay heat removal. Through the above, this study has developed the practical PRA methodology for SFRs against strong wind.