In this study, we propose a sensor probe utilizing electromagnetic metamaterial perfect absorbers to enable the evaluation of soil properties. This device consists of periodically arrayed copper split-ring resonators (SRRs), a moisture-absorbing material, and a mesh-like aluminum structure, forming a perfect absorbing metamaterial. The mesh-like aluminum placed at the bottom surface prevents interference with soil moisture absorption while blocking reflections from the soil, thereby allowing robust soil property evaluation independent of soil reflection waves. The finite element method (FEM) simulations showed that the device with a mesh-like aluminum structure absorbed approximately 83% of the incident electromagnetic waves, compared to a configuration with no material on the bottom surface. Reflection measurements demonstrated that changes in soil moisture levels could be detected in both air and on soil surfaces without being affected by reflection waves. This study enables measurements of soil surface environments by eliminating the artifacts caused by soil reflection waves, contributing to the realization of wide-area soil property monitoring, independent of soil surface conditions.

The purpose of this paper is to discuss a path planning method for navigating narrow L-junctions using iterative reversing maneuvers, as one of the technologies necessary for achieving full self-driving system. First, an analysis of the vehicle's behavior during repeated reversing maneuvers is conducted. Next, based on the analyzed characteristics, a strategy for navigating narrow L-junctions using reversing maneuvers is discussed. Following the proposed strategy, the necessary turning trajectories—constituting segments of the overall path—are formulated sequentially, and the complete path is derived geometrically. Furthermore, an algorithm for generating the entire path from the entrance to the exit is proposed by utilizing the formulated trajectories. The proposed algorithm is validated through numerical simulations. Path generation is performed under varying road width conditions, and it is confirmed that the paths are generated as intended. Additionally, path generation is conducted under a comprehensive range of road width conditions, and it is verified whether the path generation is successful or deemed infeasible. As a result, it is confirmed that the path generation method utilizing reversing maneuvers allows for a wider range of road width conditions to be successfully navigated compared to when reversing maneuvers are not used, indicating the effectiveness of the proposed method. However, after checking for interference with the road boundaries for all path generation results, it is found that in some conditions, the generated paths interfered with the road boundaries, indicating the need for further improvement.

High-speed and high-precision positioning technology is widely used in industrial machinery such as manufacturing equipment and processing machines. In semiconductor manufacturing equipment, a magnetic levitation (maglev) stage is used to prevent the effects of non-linear friction, which deteriorates positioning precision. In the maglev stage, multiple motors and sensors are arranged to enable control of six degrees of freedom, i.e., a multi-input multi-output (MIMO) system. A decoupling method is effective for MIMO control because it reduces the interaction between the motors and sensors and converts the control target into multiple single-input single-output (SISO) systems. The purpose of this study is to develop decoupling that includes vibration cancellation in the high frequency range. The proposed decoupling cancels vibration modes by combining motors for orthogonal translation axes (horizontal and vertical directions) and using orthogonal motors to excite the vibration modes in opposite phases. In the proposed method, the conditions for cancelling vibration modes are derived and shown in the frequency domain. The proposed decoupling method, which includes dynamics, is designed to satisfy the derived conditions. The effectiveness of the proposed approach is verified by analysis and experiments using a prototype coarse-fine maglev stage.

Recently, in vehicle development, the efficiency of the design process and the reduction of development costs are emphasized. Therefore, the development of a human body model that can perform analysis combining various humans and vehicles is required. In this research, we aimed to investigate the effects on posture control motion of vehicle occupants in response to forward deceleration of a vehicle. First, we used multibody dynamics to make a simplified human model consisting of three rigid body segments: head, chest, and lumbar region. Then we performed motion capture experiments to reproduce the motion of the human body when the vehicle decelerates. After this, we used a two-dimensional body model to perform numerical simulations based on the experimental results obtained by motion capture. Then, we identified the parameters for the numerical simulation and simulated the human motion. Next, we used the parameters to reproduce the experimental results. We verified the validity of the human body model by comparing postural changes in the numerical and experimental results. Finally, we conducted a numerical simulation using the identified parameters and clarified how vehicle occupants move their bodies based on the drive torque response of each body segment under different decelerations.

In this study, a second-order three-node triangular element with drilling and strain degrees of freedom (DOF) (GNTri3-2nd) was developed. GNTri3-2nd was formulated by applying an approximation of the generalized finite element method and a second-order Taylor expansion. The boundary conditions of the extra nodal DOF (drilling, normal strain, and curvature) without the zero-energy mode are discussed through eigenvalue analysis. The analysis accuracy of GNTri3-2nd was higher than that of the conventional six-node triangular element (FEM-Tri6) for the cantilever problem. Furthermore, in the case of plane-strain compression with a nearly incompressible material, the analysis performance of GNTri3-2nd was almost identical to that of FEM-Tri6 when a selective reduced integration was applied to the element stiffness matrix. Finally, we applied the GNTri3-2nd to the fundamental rigid-plastic finite element method. The analysis result of GNTri3-2nd is in good agreement with the slip line method and the conventional four-node quadrilateral element.

This paper presents a model predictive control algorithm under input and state constraints. The algorithm is designed to track time-varying reference signals and allows for offset-free tracking when the reference signal is a constant or ramp signal. A double integrator is inserted into the controller as a servo compensator, when the cost function is large, the state of the integrator can be reset at each sampling moment, thereby allowing the cost function to decrease rapidly to improve the system’s performance. While the cost function is sufficiently small, an integral action is employed to achieve offset-free tracking. The control algorithm is reduced to a convex optimization problem under linear matrix inequality constraints.

We report on the effect of elastomer materials on the time response characteristics of tactile sensors using microcantilevers embedded in the elastomer. Our previous work has shown that different electrical time response characteristics of the sensor output can be obtained using piezoelectric or resistive elements. In this paper, focusing on mechanical time response control, we evaluated the response differences based on the viscoelasticity of the elastomer used to embed the microcantilever as a sensing element for tactile sensors. The microcantilever embedded in an acrylic elastomer, which has higher viscoelasticity than the previously used PDMS, exhibits nonlinear responses that are significantly dependent on the rate of deformation. Furthermore, we found that even when the elastomer used to embed the microcantilever is the same material, the time response characteristics vary depending on the material of the contact surface with the object.

A vibration energy harvester generates power by utilizing resonance. Stationary sine-wave excitation is a common method of evaluating vibration energy harvesters. However, environmental vibrations are often non-stationary. For example, human vibrations are often impulse vibrations with large instantaneous accelerations. In this study, we applied impulse vibrations to a piezoelectric vibration energy harvester (PVEH) and evaluated the input acceleration of the impulse vibrations, deflection, and output power waveforms. The PVEH was fabricated into a cantilever shape, and the resonance frequencies were varied by attaching different proof masses. Based on the results of the experiment and FEM analysis, it was observed that the pulse width of the input impulse vibration and the resonance frequency of the PVEH at which the maximum output power was obtained were inversely proportional. The conventional design method of matching the resonance frequency of a PVEH to the number of impulse vibrations per second results in challenges regarding the size and complexity of the device. However, the proposed method focuses on the acceleration waveform of impulse vibrations, making it effective even when the input vibrations are non-periodic. In addition, the proposed design method can generate high power with a small and simple structure. Therefore, it is promising for application in wearable devices.

Soft actuators have garnered significant attention for applications in medical and wearable devices owing to their lightweight properties and inherent safety. Nevertheless, standardized performance charts for such actuators remain scarce, complicating the selection of suitable actuators for specific applications. One of the fundamental characteristics of soft actuators is that the force–displacement relationship is influenced by the stiffness of the object with which they interact. To evaluate the sensor characteristics, conventional methods employ rigid load cells, which hinders the simultaneous measurement of force and displacement under varying stiffness conditions. Here, we propose a force sensor consisting of a seesaw-type beam embedded with an internal magnet and a laser displacement meter. External magnets are installed above and below the internal magnet, enabling modulation of the beam’s effective spring constant by varying the distance to the external magnets and their magnetic polarity. This configuration enables both repulsive and attractive magnetic forces, facilitating continuous and wide-range stiffness adjustment. The sensor structure was fabricated using a 3D printer. Experimental validation confirmed that the spring constant was tunable within the range of 5.9 to 20 N/m. Furthermore, the sensitivity of the laser displacement meter remained consistent, with a standard deviation corresponding to an error of approximately 2.2% across varying stiffness conditions. These results suggest that the proposed sensor can measure force while adjusting its stiffness, making it useful for evaluating the performance of soft actuators under different mechanical stiffness conditions.

Tapping inspections of structures are commonly performed manually by technicians. However, in the case of nuclear structures and other hazardous environments, maintaining a safe distance from the inspection target is preferable. Additionally, there is a growing demand for automated inspection methods. This study proposes a novel remote tapping inspection method that eliminates the need for close proximity to the object. Our approach employs a String-Shoot Impactor, a device inspired by a string shooter toy, which propels a looped string into the air using rollers. By attaching a 4 mm diameter stainless steel ball to the string, we enable remote tapping of inspection targets. In this study, remote tapping tests were conducted from a distance of 2 meters, and the impact sounds were analyzed using frequency spectrum mapping. The results successfully distinguished defect areas from intact regions in a concrete specimen with a 300 mm diameter void defect. To further extend the inspection range, a high-speed String-Shoot Impactor was developed. The previous device, driven by a speed control motor with a maximum rotation speed of 1400 rpm, achieved a maximum launch velocity of 9.2 m/s. By replacing this motor with a brushless motor capable of 4000 rpm, the launch velocity was increased to 26 m/s, enabling an inspection distance of up to 3 meters. Additionally, the impactor was redesigned with three rollers, improving string retrieval performance and stabilizing the launching mechanism. Mounting the impactor on a tripod allowed smooth adjustment of the impact point, enhancing usability. The results demonstrate that this method enables remote tapping inspections from distances of several centimeters to several meters and can be applied in various orientations, from horizontal to vertical. Furthermore, by integrating the device into a suspended inspection setup, it becomes possible to inspect structures that are difficult to access from the ground.

This study presents a novel remote impact echo testing method utilizing pulsed water jets for non-destructive inspection of concrete structures. The method provides advantages over traditional impact methods by enabling efficient long-distance inspections while minimizing water consumption. The pulsed water jets also exhibit improved resistance to wind interference and generate a broader range of excitation frequencies, enhancing defect detection accuracy. To validate the method, experiments were conducted using concrete specimens embedded with artificial cavities to simulate structural defects. The system employed a 4 mm diameter nozzle operating at 1 MPa, generating high-speed water pulses with a velocity of approximately 40 m/s. The resulting impact-induced acoustic signals were recorded using a gun microphone, and frequency spectrograms were analyzed to identify defect-related resonant frequencies. The tests confirmed that defects at a depth of 20 mm and diameter of 300 mm could be successfully detected from a distance of 3 m. Additionally, spatial mapping of impact intensity was performed, revealing that regularly spaced impacts improve defect localization accuracy. The results demonstrate that the pulsed water jet method is a viable and effective alternative for remote defect detection in concrete structures, particularly in areas where direct access is restricted.

Recently, detection and separation techniques of micro- and nanoparticles using micro- and nanofluidic channels have been developed. Especially, micro- and nanoplastics have attracted significant attention as pollutants of the water environment. On the other hand, it is known that the separation and recovery of nanoobjects that widely disperse in water are quite difficult. In this study, we investigate the behavior of nanoparticles in nanochannels where nanoparticle transport driven by pressure-driven flow is modified by electrostatic force and electroosmotic flow. Furthermore, nanoparticles are effectively separated by size and captured in the nanochannel by the balance between the Stokes drag and the electrostatic forces acting on particles passing through the nanochannel. The present methods are expected to be one of effective methods for separation, condensation, and recovery of nanoplastics from wastewater as well as transport velocity control for single particle analysis.

Polymer optical fibers (POFs) are promising candidates for sensor applications due to their high strain resistance, flexibility, and cost-effectiveness. The performance of POF-based sensors can be enhanced through precise micro-processing of the fibers. However, conventional methods for observing POF cross-sections often fail to capture the detailed profiles of micro-processed POFs, leading to inaccuracies in sensor design and analysis. In this work, we developed a new protection layer-assisted cross-sectional observation method to preserve the surface profiles of POFs during cutting and polishing. As a proof of concept, we applied this method to perfluorinated (PF)-POFs dry-etched by reactive ion etching (RIE). Epoxy resin was used as a protection layer to maintain the structural integrity of the fibers during the cutting process. The cross-sectional profiles of the micro-processed PF-POFs were successfully captured using a laser microscope. These profiles, previously difficult to predict, were found to be crucial for analyzing the strain sensor performance. Finite element method simulations further demonstrated the impact of these geometric features on the strain characteristics of the PF-POFs. Our results confirm that the proposed method is effective for constructing accurate design and analysis models of micro-processed POFs, paving the way for improved sensor technologies.

At the Fukushima Daiichi nuclear power station, fuel debris is cooled under immersion. However, under an unexpected significant drop in water level, the water comes into contact with fuel debris that possesses a porous structure. In this scenario, rapid cooling of the fuel debris is essential. However, the cooling time has not been estimated, because the thermal behavior of high-temperature debris, including capillary phenomena, is not well understood. In the present study, the droplet evaporation characteristics on metallic porous media featuring pores smaller than 1 mm were investigated. To derive lifetime curves for droplets, the authors conducted experiments using 316L stainless steel and bronze porous materials with pore diameters of 1, 40, and 100 μm to establish droplet lifetime curves. The experimental findings indicate that the Leidenfrost phenomenon is mitigated on porous surfaces because the vapor escapes through the pores of the porous material. Further, as the temperature increases, an oxide film having a fine structure activates capillary action in bronze porous media. By contrast, a stainless-steel porous medium prevents capillary phenomena owing to its low wettability, inhibiting droplet absorption and dispersion into the pores. Consequently, if the fuel debris has similar characteristics to steel porous media, rapid cooling via the capillary action is unexpected. Finally, the cooling heat flux and cooling time of fuel debris are estimated according to experimental results. The estimation indicates that even in the case of stainless steel, the fuel debris can be cooled from 200 to 100 °C within 60 min under a low-decay heat condition. However, when the decay heat exceeds a certain value, the fuel debris cannot be cooled in either the stainless steel or bronze cases. Therefore, for risk management, the cooling system should be established for situations where the decay heat is substantial and simple spray cooling is not sufficient.

The shape of axial primary water stress corrosion cracking (PWSCC) flaws in a dissimilar material weld (DMW) joint is relatively rectangular rather than semi-elliptical. The K-solutions for a rectangular flaw as a PWSCC flaw for a Code Case of ASME Section XI were proposed in the ASME Sec. XI Standard Committee. During the discussion in a WG of the Code Committee, sample problems were proposed to confirm necessity of K-solutions for a rectangular flaw. In the sample problems, the K-solutions, that is Gi coefficients, of a rectangular flaw and a semi-elliptical flaw were compared for flaws with a different flaw depth and length. The deeper and the narrower the flaw, the larger the difference of the Gi coefficients between a rectangular flaw and a semielliptical flaw. Also, PWSCC propagation calculations of both types of the flaws assumed in a DMW joint of a pipe with double V groove, with single V groove or with single V groove were performed by using the K-solutions of a rectangular flaw and a semi-elliptical flaw. It was assumed that the initial semi-circular flaw grew with a semi-elliptical shape and the initial rectangular flaw grew with a rectangular shape. These calculation results were compared with the result of Advanced FE (AFEA) analyses which simulated actual axial flaw growth behavior. In the sample problems, the initial flaw was a semi-circular flaw within a weld width or a rectangular flaw with a length of the weld width. As a result, the calculation results of semi-elliptical flaws were quite similar to those of AFEA. The condition of a rectangular flaw with the initial flaw length of the weld width was too conservative, which was in the code description of the first draft Code Case. If the initial semi-circular flaw reached the weld boundary at the surface point and changed to a rectangular flaw, the PWSCC propagation behaviors were reasonably conservative. The results of the sample problems were reflected to the draft Code Case, which was approved by the Standard Committee of ASME Section XI on January 2025.

Three-dimensional (3D) cryogenic bioprinting has emerged as a promising technique for fabricating cell-laden structures by freezing bioinks during printing. This method enables precise spatial arrangement of cells while preventing cell degradation during the structural fabrication. However, conventional cryogenic bioprinting often requires cryoprotective agents (CPAs), which may induce cytotoxicity. To address this issue, CPA-free cryogenic 3D printing was examined and evaluated for constructing cell-embedded ice structures. This approach employs inkjet printing to eject ultra-small 70 pL droplets onto a liquid nitrogen-cooled substrate, achieving ultrafast freezing that suppresses ice crystal formation. The effects of droplet ejection frequency on ice structure formation and cell viability were systematically investigated. Ice pillars were printed at frequencies of 5, 20, 50, 100, and 200 Hz, and their morphologies were analyzed. At 5 Hz, droplets remained distinctly separated, forming a well-defined layered structure, whereas at 20 Hz, they partially merged while maintaining a high aspect ratio. In contrast, at 50 Hz and above, incomplete freezing between droplets resulted in lower aspect ratios and structural instability. Cell viability after freezing and thawing showed no clear difference between 5 Hz and 20 Hz frequency ejections, but was lower than that achieved with conventional monolayer freezing. This decrease in viability is considered to result from a combination of factors, including insufficient cooling in the upper regions of the ice pillars, slower warming during thawing, and localized recrystallization. Additionally, overhanging ice structures were successfully fabricated by adjusting droplet spacing, demonstrating the feasibility of support-free complex ice structure printing. These findings highlight the potential of cryogenic 3D printing for producing intricate ice-based architectures. Further optimization of cooling conditions and droplet control is essential to improve the survival rate of embedded cells and enhance the applicability of cryogenic 3D bioprinting in biofabrication and tissue engineering.

We report a triaxial force plate (FP) fabricated by a metal three-dimensional (3D) printer and sampling moiré (SM) method for displacement measurement. Ground reaction force (GRF) is an important index for foot biomechanics. For a detailed analysis of human locomotion, it is desired to examine the GRF distribution across the sole. FPs are widely used to measure GRF. Development of an FP array capable of multipoint measurement by miniaturizing each FP and arraying them is required. However, conventional FPs with built-in force sensor elements are difficult to array due to the proportional increase in wiring complexity with the number of FP elements. Another approach involves FPs that use mechanical components and external displacement meters, which can be arranged in an array, but this also leads to an increase in the number of displacement meters. Recently, we developed an optical FP capable of triaxial force measurements by capturing a two-dimensional grating attached to the mechanical component through a prism by a single camera and analysis with the SM method at a high resolution. This measurement method is applicable to multipoint simultaneous displacement measurements of an FP array owing to the extension of the camera’s field of view. On the other hand, from a mechanical perspective, it is desirable to manufacture the array structure as one batch. Here, we propose an FP mechanical component integrated with a plate and spring fabricated by a metal 3D printer. Experiments for measurement of spring constants were conducted, which showed a linear response to the applied force in each direction. Using the FP system with a prism and camera with the SM method, the proposed FP demonstrated the feasibility of triaxial force measurement. Therefore, the proposed FP could be applicable to the arraying of FPs.

We have aimed to realize high-fidelity neutronics/thermal-hydraulics coupling simulation to provide simulation results that can be used as validation data for reactor analysis codes. We have developed a multi-physics platform, JAMPAN, to conduct neutronics/thermal-hydraulics coupling simulation by connecting independent codes. It is required to reduce empirical correlations as far as possible to perform high-fidelity neutronics/thermal-hydraulics coupling simulation. Hence, a continuous energy Monte Carlo code MVP is adopted as a neutronics analysis code and a detailed and phenomenological numerical analysis code JUPITER is adopted as a thermal-hydraulics analysis code. Our simulation target is a single BWR fuel assembly. Hence, the simulation results need to reproduce the varying heat generation owing to the varying void fraction. Therefore, we carried out MVP/JUPITER coupling simulation in a BWR 8 × 8 single fuel assembly and confirmed that the void fraction and heat generation distribution are reasonable qualitatively. Furthermore, it is necessary to clarify the effect of the parameters of the coupling simulation on the results, and the time interval is one of the coupling parameters to improve the reliability of the simulations. We carried out MVP/JUPITER coupling simulation in a 2 × 2 fuel pin system using the multi-physics platform JAMPAN to investigate the effect of the time interval on the results. The 2 × 2 fuel pin system, which is the smaller unit of the actual single fuel assembly for a BWR, is adopted as a simulation target to investigate the effect with efficiently. It is found that the effect of the time interval on the simulation results is small.

This paper outlines the methodology and analysis of the at-power Fire Probabilistic Risk Assessment (FPRA) of the Shimane Unit 2 Nuclear Power Plant (BWR). For most BWRs in Japan, the Fire PRA model is in the development phase. The Shimane Unit 2 Nuclear Power Plant is one such plant, for which the FPRA project began in 2018 as a joint effort by The Chugoku Electric Power Company (Chugoku electric) and Hitachi-GE Nuclear Energy. The PRA process followed the EPRI/USNRC Fire PRA Methodology (NUREG/CR-6850, 2005) as main guidance, with various other NUREG documents as supplementary reference. The project was divided into two phases; phase 1 focused on collecting the requisite plant information to develop a conservative initial model for initial quantification, and phase 2, which removed the conservatisms in phase 1 by analyzing the fire scenarios in more detail. This paper provides an analysis of plant features from the perspective of fire PRA elements, focusing on the relative risk mitigation between initial and final quantification for specific compartments of interest. Moving forward, refinement measures will be taken to remove over-conservatism for risk-significant fire scenarios, and the model will be updated to reflect the latest design changes. The effort included plant walkdowns to confirm various as-built plant characteristics. The PRA team, with support from Chugoku Electric, visited the Shimane site a total of four times to collect data regarding plant partitioning, ignition sources, distance between source and target, and control room features. As a key milestone, the project scope also included an online, external review at the end of Quantitative Screening to assess conformance with the ASME/ANS PRA Standard.

This study investigates the effects of the Fluid-Structure-Soil Interaction (FSSI) on the seismic responses of a deeply embedded nuclear facility with large water pools under severe earthquakes. This paper presents an efficient seismic analysis method for considering FSSI using 3D FEM for deeply embedded typical RC (Reinforced Concrete) shear wall nuclear buildings. In the proposed method, the commercial SSI analysis software, ACS SASSI was used with Option AA-F, which treats ANSYS fluid element FLUID80 dynamic matrices in ACS SASSI environment. Using the proposed method, it was confirmed that large pool water in the building provides significant local deformation of the pool walls, while the effects on other parts of the structure are almost negligible from engineering point of view. To capture the structure behavior during severe earthquakes, the nonlinear behavior of the structure was taken into account. The proposed FSSI method was applied using the ACS SASSI NQA Option NON software. To model the RC walls nonlinear behavior, their nonlinear back-bone curves (BBCs) were computed using both Japanese and US standards. Then, the nonlinear analysis SSI responses with both standards were compared. It was confirmed that the nonlinear structure behavior produced a visible shift of the ISRS (In-Structure Response Spectra) peak responses to the lower frequencies in comparison with the response of linear analysis. It was also confirmed that the amplification of ISRS in nonlinear analyses comparing with elastic linear analyses are strongly influenced by the frequency content of in the in-column input motion at foundation level. Consequently, it was shown that differences between the formulations and requirements in the two standards could affect seismic responses more or less depending on the frequency content of in-column input motion, structure dynamics, and nonlinear modeling of structural behavior.

A large amount of concrete contaminated by ¹⁴CO₂ will be discharged during the decommissioning of aged nuclear power plants. Rubbing, which separates cements from concrete debris, is one method available to reduce the volume of the waste if most of the ¹⁴C is present in the cement inside the concrete. We confirmed that more ¹⁴CO₂ adsorbs onto the cements than onto the aggregates by a factor of approximately 100. A rubbing test was performed to obtain the mass balance and decontamination factor (DF) using simulated concrete debris not contaminated with ¹⁴CO₂. The cements and fine aggregates were removed from the debris as fines by rubbing using a Los Angeles testing machine. Steel balls with larger sizes and in greater quantities were used to increase the rubbing effect. In addition, the production rates of the fines for debris subjected to a heat treatment were compared with those of the fines for debris not subjected to a heat treatment. Residues in the mill of the Los Angeles testing machine were washed to remove deposits remaining on the surface. We concluded that an effective volume reduction could be achieved by rubbing the heat-treated debris using additional steel balls with a larger size. The DF was not improved by washing the surface residues. However, washing can resolve concerns regarding radiation protection because of scattering of fines contaminated with ¹⁴CO₂ in the treatment of the residues after rubbing. If the volume reduction is performed using a system similar to that used in a concrete recycling plant, the production of class H aggregates is necessary to achieve a sufficient DF. In this case, air ventilation and removal of radioactive dust in the exhaust air using, e.g., a bug filter, should be required to reduce the radiation hazard to workers and to the surrounding environment.

After the Fukushima Daiichi nuclear power plants accident, the scope of Probabilistic Risk Assessment (PRA) application was enhanced to meet the new Japanese regulatory standard for nuclear power plants. In addition to that, an importance for risk management has come to be re-recognized in Japan. To use PRA in risk management, Japanese utilities are proceeding the projects for enhancing the quality of PRA model to comply with international standards. The Chugoku Electric Power Company is also upgrading the internal event at power PRA model of the Shimane Unit 2 Nuclear Power Plant (Boiling Water Reactor (BWR)) including both the level 1 and level 2 (except the source term analysis) and enhancing PRA quality to reflect international state-of-the-practice approaches and to meet (ASME/ANS, 2013) requirements (Capability Category II (CC-II) or higher). Our PRA upgrading process is divided into 3 phases: “Phase I”, “Phase II” and “As-is”. In the Phase I and Phase II, the PRA model was upgraded. An external expert review was conducted using (NEI, 2019) and (ASME/ANS, 2013) in the end of Phase II. In the As-is Phase, Shimane Unit 2 PRA is further being updated and upgraded. PRA was updated to reflect the as-built and as-operated plant features resulted from changes made to the plant design and procedure by licensing activities which were performed in parallel with PRA enhancement in the Phase I and Phase II. In addition, we updated the component failure data newly issued for Japanese nuclear power plant and reflected the findings on PRA for other Japanese plant. We addressed Facts and Observations (F&Os) raised in the external expert review conducted in the Phase II. This paper reports the processes and the details of improvements of the internal event at-power PRA for Shimane Unit 2 implemented in the As-is Phase.

When a molten jet falls into a shallow pool in the containment vessel during a severe accident in a light water reactor, melt spreading is expected to occur on the floor surface with extremely complex fluid phenomena. In order to analyze such complex behavior, the authors identified important phenomena related to the melt spreading behavior based on experiments conducted under dry and wet conditions. The identified influential factors in under-water melt spreading are the heat transfer between the melt and overlying water, melt-coolant interaction caused by water confinement at the time of molten jet impingement, molten jet breakup into melt slugs, dispersion of melt slugs and cooling by water recirculating inside the partially solidified debris bed. In particular, a numerical model to treat the above-mentioned melt-coolant interaction and the subsequent chain of phenomena was developed. The model was named as "molten jet quench model" by the authors and was implemented in MSPREAD. In the developed model, the diameter of the spherical melt slugs and the radius of the floor surface region where melt continues to be slugs can be given as user inputs. Four sensitivity analysis cases were performed with varying diameter of spherical melt slugs and the radius of the floor surface region together with no quench cases based on the PULiMS-E10 test conducted by the Royal Institute of Technology in Sweden. Comparisons of the time histories of the spreading area, pool water temperature, spreading shapes, and post-test debris cross section indicate that the molten jet quench model can explain the short spreading distances, thicker solidified debris, and even higher pool water temperatures observed in PULiMS-E10.

As an outer protective engineering structure for a nuclear power plant (NPP), a reinforced concrete (RC) containment wall of NPP should be evaluated for safety requirement related to external threats. Therefore, much attention has been paid to investigate the impact resistance ability of the RC plate structure subjected to projectile impact. In these studies, most of them focused on local damage of RC plate structures struck by a rigid projectile along the normal direction of the impact surface, while only few studies have focused on oblique impact. We have therefore conducted a series of impact tests under different impact conditions covering impact angles, stiffnesses of projectiles, etc. The objective is to figure out the different impact behaviors of the RC plate structures. We also intended to put forward a numerical method based on the test results, and to validate the proposed method through comparisons between experimental and numerical results. In the test results, the reaction forces of RC plate were measured by using the load cells installed on the four corners of RC plate’s back surface. In the oblique impact, a special support structure was manufactured to fix the RC plate. It is speculated that the different supporting structure may influence the test results of reaction force. Thus, this work concentrates on the effect of the stiffnesses of the supports for oblique impact on the reaction forces of RC plate. Until now, static loading tests were carried out to confirm the stiffnesses of the components of supporting parts. This paper reports the findings obtained from the comparison between the numerical results using the experimental values as spring constants in the finite element (FE) model and the reaction force measurement results.

In reprocessing plants of Japan, Zr and ultra-low carbon 304 stainless steel (R-SUS304ULC) piping are connected by a joint formed by explosive bonding with Ta sandwiched between R-SUS304ULC and Zr. The joint exhibits excellent corrosion resistance during normal operation. However, Ta causes corrosion and generates hydrogen in NaOH solutions, which are used to decontaminate the equipment in reprocessing plants. This has led to concerns about hydrogen embrittlement of the joints via hydrogen absorption. However, there is a lack of studies on the hydrogen absorption behavior of such joints, and it is difficult to evaluate the amount of hydrogen absorbed by the joints under various decontamination conditions. In this study, we conducted immersion tests of the R-SUS304ULC/Ta/Zr joint in NaOH solution to investigate the effect of immersion environment on hydrogen absorption behavior. Electrochemical measurements were conducted to examine the hydrogen absorption environment. The results showed that the amount of hydrogen absorbed into Ta in the joints decreased compared with that in pure Ta, regardless of the immersion time. The galvanic current measurements of Ta connected R-SUS304ULC or Zr in an NaOH solution indicated that the hydrogen evolution reaction was separated to the R-SUS304ULC and Zr surfaces and that the hydrogen absorption of Ta was suppressed.