Fully natural circulation decay heat removal systems (DHRSs) are adopted for sodium fast reactors, which is a passive safety feature without any electrical pumps. It is needed to grasp the thermal-hydraulic phenomena in the reactor vessel and evaluate the coolability of the core under the natural circulation not only for the normal operating condition but also for severe accident conditions. In this paper, the numerical results of the preliminary analysis for the sodium experimental condition with the PLANDTL-2, in which the core and the upper plenum with a dipped-type direct heat exchanger (DHX) were modeled, are discussed to establish an appropriate numerical models for the reactor core including the gap region among the subassemblies and the DHX. The transient analysis simulating the reactor scram reveals that the 3-dimensional large scale flow structure is developed through the gaps in the whole of the core area during the reactor scram. The steady-state analysis coinciding Richardson number between the PLANDTL-2 and the reactor operation condition reveals that the hot spot and cold spot appear depending on the location of the DHX, which is caused by the complex thermal-hydraulic phenomena driven by the natural circulation. From these preliminary analyses, the characteristics of the thermal-hydraulics behavior in the PLANDTL-2 to be focused are extracted.

On-road turbulence caused by atmospheric winds and other automobiles influences the flow field of automobiles and causes variations in aerodynamic forces. Furthermore, the disturbance caused by passing vehicles has a significant impact on drag increase and vehicle stability. Therefore, designing flow control techniques considering this impact will effectively improve the robustness of automobiles under on-road conditions. In this study, we investigated flow phenomena responsible for drag changes under passing vehicle conditions to reduce drag. Two simplified vehicle models were adopted for the test vehicle. These models are one-fifth scale models, which reproduce the flow structure of production vehicles. The tests were conducted in a wind tunnel that simulated the passing environment where the truck model was placed in the adjacent lane. The drag change and flow characteristics of the truck model were measured by changing the relative positions of the vehicle model and the truck model. The Reynolds number was 1.1×106 at the transition to the turbulent boundary layer in front of the vehicle model. An analysis procedure was proposed to identify the components affecting the drag change using the flow characteristics of a passing vehicle. As a result, three key factors that change drag were identified; variations in the pressure field, crosswind generated by passing vehicles, and the remaining part. Crosswinds are especially important for reducing drag because the contribution of the pressure field to the drag is uniquely determined by the flow characteristics of the passing vehicles and almost zero when the overtaking/being overtaken process is considered. These results provide guidance for designing flow control techniques that are robust against disturbances under on-road conditions.

Many sensitivity calculations are necessary to evaluate the effect of the uncertainties of Core Disruptive Accident (CDA) scenarios on debris bed coolability for Sodium-cooled Fast Reactors (SFR). This paper describes a calculation model and a technique for high-speed calculations of debris bed coolability. Firstly, the detailed coolability calculation model taking into account phase change and vapor-liquid advection for a boiling zone in the debris bed is derived as a single equation. The vapor-liquid advection is based on the momentum equation including inertial, viscous, capillary and gravitational effects. In addition, the streamlined calculation which ignores inertial and gravitational effects for the boiling zone is proposed. Secondly, both the detailed and streamlined calculation models are validated through the analysis of the Sandia ACRR-D10 in-pile experiments. Then, the effects of the ignoring on debris bed coolability are evaluated through sensitivity calculations. Thirdly, the speed-up technique for the streamlined calculation model is proposed which solves the boiling zone by the combination of transient and steady state calculations and allows using coarse mesh size and time step. Owing to the technique, the calculation time becomes about 1/50. The application of the speed-up technique to the streamlined calculation model is confirmed under typical SFR conditions. Finally, it is demonstrated that the streamlined calculation model and the speed-up technique enable us to perform many sensitivity calculations in a realistic time. The model and technique developed in this study is practical to evaluate the effect of the uncertainties of CDA scenarios on debris bed coolability for SFR.

Because double-mass dynamic vibration absorbers (DVAs) are superior to single-mass DVAs in terms of their vibration suppression performance and robustness, they have been increasingly studied recently. The optimization of double-mass DVAs is much more difficult than that of single-mass DVAs. However, recently, the ability of formula manipulation solvers typified by Mathematica has greatly improved, and exact algebraic solutions have been obtained for double-mass DVAs. The optimal solution for a double-mass DVA attached to a damped primary system has been reported in the form of an exact algebraic solution in a previous report. That paper reported the algebraic optimal solutions for a series-type double-mass DVA for the compliance and mobility transfer functions of the primary system successfully obtained by applying three different optimization criteria: H∞ optimization, H2 optimization, and stability maximization. In the present article, the numerical solutions to optimization problems for double-mass DVAs that cannot be algebraically solved are presented. There are two types of double-mass DVAs: series- and parallel-type DVAs. When applying the three optimization criteria mentioned above to each of them, there exist a total of 22 different optimal solutions because there are three transfer functions— the compliance, mobility, and accelerance transfer functions—that are typically used to describe the absolute response of the primary system. Of these 22 solutions, 10 solutions for the compliance transfer function are introduced in this article.

In this paper, the fracture problem of a functionally graded piezoelectric material strip (FGPM strip) containing a crack perpendicular to the interface between the FGPM strip and a homogeneous layer under an electric load is considered. Material properties are assumed to be exponentially dependent on the distance from the interface. The superposition technique is used to solve the governing equations. The stresses induced by the electric load in the un-cracked laminate are calculated, and the obtained normal stress is used as the crack surface tractions with opposite sign to formulate the mixed boundary value problem. By using the Fourier transforms, the electro-mechanical fracture problem is reduced to a singular integral equation, which is solved numerically. The stress intensity factors of the internal crack and the edge crack are computed and presented for the various values of the nonhomogeneous and geometric parameters.

This paper describes a measurement method for longitudinal displacement of wheel/rail contact point in a railway vehicle using a wheelset to measure the wheel/rail contact forces, which is generally used for the sake of the running safety evaluation in Japan. The clarification of phenomenon related to the wheel/rail contact has very important role to discuss the curving performance and the running safety of railway vehicles. The longitudinal displacement of the contact point is related to the wheelset angle of attack, which is related to the steering performance of the wheelset. The authors propose algorithms to extract informations about the contact point from the cyclic signal of the strain gauges that are put on the wheel to measure the contact forces. Experiments on roller rigs were carried out to evaluate the reliability and the accuracy of the proposed algorithm. The running tests were also carried out to demonstrate the applicability of the proposed algorithm to the real condition used. Those test results show that the measurement result using proposed algorithm is reliable as a longitudinal displacement of the contact point.

Nuclear power plants and chemical plants contain many piping, which degrades with age. As a result of a reduction in structural strength, guillotine breaking and rupturing can occur, leading to severe damage to plants. Piping is subjected to various types of tension and bending, but the influence of the load history on the integrity of piping containing a flaw is poorly understood. The goal of the present study was to develop an improved integrity assessment method for a stainless steel pipe with a part-through notch that takes into account the load history. This involved first determining the feasibility of the leak-before-break (LBB) concept for piping subjected to a combined load and then evaluating the stress at the crack penetration point and estimating the plastic collapse strength. An austenitic stainless steel pipe (SUS 304) with a length of 110 mm, a diameter of 32 mm, and a wall thickness of 3 mm was used as a specimen. A part-through notch with a notch angle of 90° was cut in the center of the specimen by wire electric discharge machining. Loading tests were carried out using statically indeterminate fracture mechanics testing equipment developed by ourselves. This allowed the loading history to be varied because the equipment is capable of applying arbitrary sequences of tension and bending. The LBB concept is considered to be applicable if the crack penetrates before reaching the maximum stress. The plastic collapse strength was then determined using the double elastic slope method. The LBB concept was found to be applicable. In addition, the stress at the crack penetration point was approximately equal to the maximum stress. The results of the present study indicated that the plastic collapse strength of a part-through notched specimen can be safely estimated using the theoretical plastic collapse strength of a through-wall notched pipe.

High-temperature control blade degradation tests simulating a beginning phase of a severe accident in BWRs has been comprehensively performed in Japan Atomic Energy Agency (JAEA). In the latest test, a mock-up of BWR bundle material has been investigated under postulated Fukushima Dai-Ichi (1F) unit 2 accident conditions in a complex heating transient scenario including a phase of lack of available steam. The progress in control blade degradation was monitored with help of an in situ video and the detailed analysis of the solidified metallic melt, so-called “metallic debris”, was carried out by conventional SEM EDS method. These results indicated that the composition of the metallic debris at different elevations has been significantly changed due to the redistribution and relocation of steel elements under the influence of B and C, sometimes accompanied by a formation of high-melting-point layers. The results of this paper significantly contribute to the physical understanding of control blade degradation phenomenology during beginning phase of a core degradation for a special case of steam-starved conditions at 1F unit 2.

An evaluation methodology of a thermal-hydraulics based on a mechanism in light water reactors (LWRs) is needed from a viewpoint of the safety analysis during normal operation and unanticipated transient such as under a severe accident. Currently, the evaluation of safety for the nuclear reactor has been implemented by a best estimate (BE) code and subchannel analysis code. These analysis codes contain models and empirical correlations. Therefore, the full-scale mock-up tests are needed to evaluate the reliability and validation of code. And the model and empirical correlation are allowed to be applied only in the range where the experiments were implemented. The large mock-up tests are once again needed in order to consider the new geometry and boundary conditions when the design of components is changed. Hence, the 3D detailed numerical simulation by the mechanistically based method is expected to be applied for the preliminary analysis to improve the design of fuel assemblies and evaluate the safety. This 3D detailed numerical simulation can reduce the large mock-up tests. The detailed numerical simulation method can provide much information relating to the two-phase flow such as the bubble size, its velocity, and detailed void distribution which, for example, are needed to predict the critical heat flux based on the mechanism. Moreover, JAEA is implementing the development of the nuclear-thermal-coupling code by using a detailed two-phase flow analysis code based on the VOF method like a JUPITER code. In this study, the numerical simulation of two-phase flow in the 4x4 bundle was examined by numerical simulation code JUPITER in order to examine the possibility of the JUPITER code for the large scale two-phase flow analysis. The simulation results are verified by the previous experimental data of two-phase flow.

In this paper, a new model of magnetically levitated (Maglev) vertical axis wind turbines (VAWTs) is presented for power generation purposes. The rotor is suspended by two permanent magnet attractive type passive magnetic bearings and one control coil; it is possible to rotate the rotor without any mechanical contact. The proposed model solves the most common problems which are found on the other Maglev VAWTs such as reducing the power consumption during the levitation to zero amperes by the zero-power control method and reducing rotation loss during rotation. In addition, the model has some main advantages consisting in the ability to avoid the resonance at the critical speed and increase the maximum rotation speed by changing the air gap between the rotor and passive magnetic bearings to adjust the radial stiffness of the rotor. The design of the passive magnetic bearings is investigated by the finite element analysis, and the optimum shape is discussed. The results of levitation tests are presented for the model to show the effectiveness of levitation current reduction and the adjustment of radial stiffness. The results of rotation tests show that the resonance can be avoided by changing the radial stiffness which also makes it possible to rotate the rotor over the critical speed. Moreover, the results of free-run tests show that the rotation loss of the proposed Maglev system is quite low.