Mechanical Engineering Journal Vol. 4(2017) No. 3

One important safety design consideration for high temperature gas-cooled reactor (HTGR) is air ingress following a rupture of the reactor pressure boundary such as primary piping. The air intrusion to the reactor core held at high temperature through the break will results in significant oxidation of graphite components and fuels. Such oxidation may leads to the weakening of core support structures as well as fuel element damage and subsequent fission product release. This paper intends to propose a practical solution to protect the reactor from severe oxidation against air ingress accidents without reliance on subsystems. Firstly, a change is made to the center reflector structure to minimize temperature difference during the accident condition in order to reduce buoyancy-driven natural circulation in the reactor. Secondly, a modified structure of the upper reflector is suggested to prevent massive air ingress against a rupture in standpipes. As a preliminary study, a numerical analysis is performed for a typical prismatic-type HTGR to study the effectiveness of the proposed design concept using simplified lumped element models. The analysis considers internal decay heat generation and transient heat conduction from inner to outer regions at the reactor core, cooling of vessel outer surface by radiation and natural convection, and natural circulation flow in reactor. The results showed that amount of air ingress into the reactor can be significantly reduced with practical changes to local structure in the reactor.

Under the cooperative effort of the Civil Nuclear Energy R&D Working Group within the framework of the U.S.-Japan bilateral, Argonne National Laboratory (ANL) and Japan Atomic Energy Agency (JAEA) have been performing benchmark study using the Japan Sodium-cooled Fast Reactor (JSFR) design with metal fuel. In this benchmark study, core characteristic parameters at the beginning of cycle were evaluated by the best estimate deterministic and stochastic methodologies of ANL and JAEA. The results obtained by both institutions show a good agreement with less than 200 pcm of discrepancy in the neutron multiplication factor, and less than 3% of discrepancy in the sodium void reactivity, Doppler reactivity, and control rod worth. The results by the stochastic and deterministic approaches were compared in each party to investigate impacts of the deterministic approximation and to understand potential variations in the results due to different calculation methodologies employed. From the detailed analysis of methodologies, it was found that the good agreement in the multiplication factor from the deterministic calculations comes from the cancellation of the differences in the methodology (0.4%) and nuclear data (0.6%). The different treatment in reflector cross section generation was estimated as the major cause of the discrepancy between the multiplication factors by the JAEA and ANL deterministic methodologies. Impacts of the nuclear data libraries were also investigated using a sensitivity analysis methodology. The differences in the inelastic scattering cross sections of U-238, ν values and fission cross sections of Pu-239 and μ-average of Na-23 are the major contributors to the difference in the multiplication factors.

For the prototype sodium-cooled fast reactor, MONJU, the mechanical energy and structural response under energetics caused by neutronic power excursion during Unprotected Loss of Flow (ULOF) accident were preliminarily evaluated. In the first licensing of MONJU, pressure-volume relation (P-V relation) was evaluated based on the maximum theoretical work energy possible for an expanding core. It was adopted in the structural response analysis of the reactor vessel as the input. The maximum theoretical work energy is called Fuel Vapor Work Potential (FVWP) in this paper. In the successive studies of the energetics, mechanical energy was evaluated with the code in which mechanistic modelling of core expansion was implemented and this might reduce the Actual Work Potential (AWP) by an order of magnitude below FVWP. In order to evaluate the realistic structural response of the reactor vessel using AWP, method to convert the AWP to the P-V relation is necessary. Therefore, we developed the method to obtain realistic P-V relation based on the AWP by tracing the surface of the expanding core, and then we evaluated the mechanical energy and structural response under energetics during ULOF accident in MONJU using the developed method. The AWP is evaluated to 3 MJ based on the result of the latest ULOF analysis in which FVWP was evaluated to 30MJ, and sodium slug does not impact on the lower surface of the shield plug and no residual strain of the reactor vessel is evaluated. When FVWP is assumed to be 500 MJ as a hypothetical condition covering the conservative energy production, corresponding AWP is evaluated to 33 MJ. In this case, sodium slug impacts on the lower surface of the shield plug and residual strain of the reactor vessel of 0.008% at the maximum is evaluated, however the integrity of the primary boundary is still maintained.

Evaluation of accidental sodium leak, combustion, and its thermal consequence is one of the important issues to be assessed in the field of sodium-cooled fast reactor (SFR). The present paper deals with the sodium pool fire and subsequent heat transfer behavior in air atmosphere two-cell geometry both experimentally and analytically because such two-cell configuration is considered as a typical one to possess important characteristic of multi-compartment system seen in an actual plant. The analyses of the experimental data clarify the basic characteristics of sodium pool combustion and consequential heat and mass transfer in the cells, for instance, suggesting several features of multidimensional thermal-hydraulic behaviors such as the strong gas mixture at the combustion cell and the thermal stratification near the opening between the two cells. As a result of the numerical analysis using a lumped-parameter based zonal model safety analysis code ‘SPHINCS’, the applicability of the ventilation model implemented in SPHINCS has been demonstrated. It is also investigated that the buoyancy- driven ventilation is dominant in the experiment.

In order to precisely investigate molten core relocation behavior in severe accidents, we have been developing the detailed and phenomenological numerical simulation code named JUPITER for predicting the molten core behavior with melting and solidification based on computational fluid dynamics (CFD) including the three-dimensional multiphase thermal-hydraulic simulation models. In order to treat complicated core structures, e.g., boron carbide (absorber), stainless steel (control rod, fuel support structure, etc.), Zircaloy (channel box and fuel cladding) and to deal with complicated melt relocation behaviors, high accuracy, efficient, stable and robust numerical schemes are implemented. In this paper, in order to evaluate the validity and applicability of the JUPITER for actual core structures, we perform the preliminary melt relocation analysis in the control rod and fuel support piece and also verify the validity of the JUPITER regarding the melt relocation and solidification processes by the fundamental numerical problem and the experimental analysis. As a result, the preliminary analysis showed that multicomponent melt flow and its melt and solidification were stably worked in the melt relocation simulation. In the validation analysis, the numerical results were in the reasonably agreement with experimental results. Therefore, it was confirmed that the JUPITER has a potential to calculate the core melt relocation behavior in RPVs.

As a part of external hazard probabilistic risk assessment methodology development, a sensitivity study on forest fire hazard curves was performed on condition parameters where frequency/probability variables were varied within respective fluctuation ranges. Important variables related to forest fire breakout time, forest fire breakout points, and forest firefighting operation were identified. The probability fluctuation on forest fire breakout time only affects the intensities of the hazard curves, but not the frequency, because the intensity increases in daytime due to sunshine and the breakout probability in daytime is statistically higher than the daily average. The hazard curves of the reaction intensity and the fireline intensity increased around 4% and 14% respectively in comparison with those without the forest fire breakout time fluctuation. The probability fluctuation on forest fire breakout points affects only the frequency of the hazard curves, but not the intensities. The hazard curves vary around +70% to -40% in frequency, because each forest fire breakout point has different distance to the plant and the forest fire arrival probability varies with propagation duration. The probability fluctuation due to forest firefighting operation only affects the frequency of the hazard curves, but not the intensity. The effect of the forest firefighting operation was conservatively assumed where there is no forest firefighting operation outside a plant, hence all potential forest fires arrive at the plant. The hazard curves remarkably increase around 40 to 80 times in frequency in comparison with those with considering the forest firefighting operation effect outside the plant. This study indicated that the most significant factor in the forest fire hazard risk is whether the forest firefighting operation outside the plant is expected before the forest fire arrival at the plant.

Seismic buckling of vessels is one of the main concerns for the design of nuclear power plants in Japan. Rational design is important, especially for fast reactor plants. Although thicker walls are preferable in terms of prevention of seismic buckling, excessively thick walls cause unacceptable creep-fatigue interaction damage. In a previous study, we proposed an evaluation method for the seismic buckling probability of a reactor vessel considering seismic hazards and showed that among the random variables considered in the evaluation, seismic load had the most significant impact on buckling probability. This suggests that more rational vessel designs can be realized by taking appropriate account of seismic load variations. The load and resistance factor design (LRFD) method enables us to determine design factors corresponding to target reliability by considering the variations of random variables. Therefore, in this study, we used the LRFD method to develop a new design rule for the prevention of seismic buckling of vessels. The equation in the proposed rule is almost the same as that in the Japan Society of Mechanical Engineers fast reactor codes, but every random variable, seismic load and yield stress, has its own design factor. In addition, mean or median values are used in the evaluation instead of design values including conservativeness. The effectiveness of the new design rule was illustrated in comparison with the current provision.

Toshiba Corporation, a member of International Research Institute of Nuclear Decommissioning (IRID), has contributed to decontamination works throughout Fukushima Daiichi Nuclear Power Plant from outside ground to inside of the buildings. Speedy decontamination works allow workers to access and stay inside of the contaminated buildings for many hours, and as a result, all decommission works can be accelerated. Some remote decontamination machines to decontaminate the inside of the reactor buildings from a remote safe building have been developed for workers not to be radiated by high-level radiation. Conventionally, operators have remotely controlled the decontamination machine through multiple views of some remote surveillance cameras mounted on it. Because the position data such as GPS data is not available in the buildings, it was hard for operators to detect its absolute position and orientation in the building, and it took much time to recognize targets to be decontaminated. In order to reduce positioning time and make operation works easier, we constructed 3D positioning system to automatically detect the absolute 3D position of the decontamination machine in the reactor buildings from a remote safe building. Moreover, we can also keep records of decontamination works easily by tracking 3D position of the decontamination machine. This paper shows the overview of our approach of 3D positioning and a result of examinations in the mock-up facility that simulates a part of the inside of the reactor building at Fukushima Daiichi NPP.

The new sustainable power generation technique which can convert the salinity gradient energy to the hydroelectric energy is expected. This technique is called Pressure Retarded Osmosis (PRO). Clarification of the relationship between the performance of the PRO module and permeation characteristics is important. It’s already known that increase of salt concentration can increase the permeation flux through the membrane. As the conventional researches, the effects of increase of fresh water concentration and concentration polarization have evaluated. In this research, relationship between the salt concentration and membrane module is focused. The effects of fresh water dissipation and flow state of salt water in hollow fiber membrane module as new factors are researched with experiment and numerical simulation. As result, in the case of low salt concentration, permeation is not caused sufficiently in module. On the other hand, in the case of high salt concentration, very low permeation flux area exists extensively, and the effects of concentration polarization and fresh water dissipation are relatively large. Therefore salt water flow rate and module shape should be changed for each salt concentration.

A new simple analytical method for solving the problem of one-dimensional transient heat conduction in a slab of finite thickness is proposed, in which the initial temperature is assumed zero or constant and the boundary surfaces are assumed to be at constant temperature, constant heat flux, or insulated. In this method, the solution is expressed by an infinite series representation, each term of which is the temperature solution of the corresponding initial-boundary value problem for the semi-infinite solid. Each semi-infinite solid extends to infinity in the positive or negative direction of the x axis and the surface is located at various positions along the x axis. Each term and the partial sum in the infinite series automatically satisfy the heat conduction equation and the initial condition. The solution is easily constructed so that the boundary values of the partial sum converge to those of the heat conduction problem as the number of terms N increases to infinity. The basic concept of the solution method for the problem of one-dimensional transient heat conduction in a slab is described. The solution method is applied to various initial-boundary value problems. The formulas of the typical solutions by this method are found to be the same as those of the solutions obtained by other literature using the method of Laplace transformation, which supports the validity of the new solution method proposed in this paper. The usefulness of this method is also examined.

In the present study a multi-part mirror is developed for focusing only a scattered light effectively. The multipart mirror is composed from several elliptic mirrors. Several elliptic mirrors are installed on the surface of the multi-part mirror. Primary focal points of all elliptic mirrors lie on the surface of the multi-part mirror. Secondary focal points of all elliptic mirrors are located in the same point above the multi-part mirror. The mirror surface of elliptic mirrors is small; so many elliptic mirrors are densely placed on the multi-part mirror. A large amount of light passing from various directions through the primary focal points densely packed on the multi-part mirror surface is reflected from the mirror surface and focused in the secondary focal points. The multi-part mirror was designed as described above. Mirror surfaces of the elliptic mirrors comprising the multi-part mirror and location of primary focal points of the elliptic mirrors were designed. Design parameters necessary to densely place the elliptic mirrors on the multi-part mirror were determined. The actual multi-part mirror was manufactured using these determined design parameters. The manufactured multi-part mirror was exposed to a diffused light. The intensity of scattered light focused in the secondary focal points was measured. The measured light intensity shows the focusing efficiency of the multi-part mirror.

Recently, probabilistic evaluation has been required in the seismic design. In this study, a design method of piping system supported by elasto-plastic damper based on dynamic reliability of pipings was proposed. First, an analytical model of an L-type cantilevered piping system with weight at the top end subjected to seismic input was derived, considering the nonlinear characteristics of the elasto-plastic dampers. Installation of a lead extrusion damper (LED) was assumed as the elasto-plastic damper. The force-displacement relationship of the LED was given by a bilinear hysteretic curve. Seismic input was modeled as white Gaussian noise. The vibration behaviors were calculated for various seismic inputs, and the stress and the energy absorption were calculated. On the basis of the results, the evaluation indices, such as the dynamic reliability, the energy absorption ratio, and the balance of energy absorption, were calculated. Finally, optimization of the support locations was investigated on the basis of the view point of the dynamic reliability. In addition, the effects of the capacities of the two supports on the evaluation index are also investigated. As a result, J_piping was highest when Node 10 was supported at B. J_energy was higher when Supports A and B were close to the weight but a little apart from each other. J_balance was lower when Supports A and B were close to each other. Furthermore, it was obtained that the effect of Support B capacity on the response behavior of pipings is larger than that of Support A capacity.

This paper presents a disturbance compensation approach using a model-based feedforward control with an adaptive algorithm for the two-dimensional (2D) shaking table systems. In the system, the overturning moment due to a specimen largely deteriorates the motion performance of the table, thereby decreasing the reproducibility of the desired earthquake acceleration. To solve the problem, first, as one of the disturbances, the overturning moment is modeled for the target shaking table system. In the modeling process, a physical model is derived based on the geometrical arrangement and the equation of motion, wherein the moment is modeled as a disturbance acting on each actuator. Based on this model, the feedforward compensators based on the mathematical disturbance model are adopted to cancel the disturbance. In addition, an adaptive algorithm is employed to reduce the effect of the modeling error and/or parameter variation. The designed compensators are validated by conducting experiments using a 2D laboratory prototype of the shaking table system.

This paper presents a practical modeling of the fluid in a suppression pool (SP) for seismic analysis by using three-dimensional finite element method (3D FEM) models of the reactor building of an advanced boiling water reactor (ABWR). We performed a seismic analysis considering the fluid-structure interaction by using the virtual mass method in MSC Nastran, which was verified on the basis of vibration tests in this study, and the effects of the fluid on the structures around the SP were evaluated. For the analysis, we developed the following 3D FEM models: a full model, impulsive model, and no-fluid model. The full model considered both the convective and impulsive effects of the fluid, and the impulsive model lacked the convective effects of the full model (modeling the fluid as masses). The no-fluid model removed the impulsive effects from the impulsive model (removing the fluid from the full model). From the analysis, we obtained the maximum acceleration, von Mises stresses, and pressure fluctuations at the structures around the SP. The obtained results were compared with each other. They indicate that the fluid hardly affects the seismic responses of the structures, though some effects of the fluid were found. On the basis of the effects we found, we propose a practical modeling of the fluid and a modeling flowchart to use it. The modeling flowchart enables us to practically model the fluid in the SP for seismic analysis using ABWR 3D FEM models.

This paper presents a local-in-time (LT) discrete adjoint-based topology optimization method for unsteady incompressible viscous flows incorporating the lattice Boltzmann method (LBM). For the optimization of unsteady flows, straightforward global implementations of the time-dependent optimization are usually adopted. However, such global implementations require that the entire flow solution history be available to calculate the solution of the adjoint equation reversed in time. For 3-D design optimization problems, the storage requirements can become prohibitively large. In this paper, the LT discrete adjoint-based method is applied to a LBM-based topology optimization to reduce the storage requirement. The basic idea of the LT method is to divide the entire time interval into several subintervals and to approximate the global sensitivity derivative as a combination of local sensitivity derivatives computed for each time subinterval. In this approach, flow solutions for only a single subinterval need to be stored. Since each time subinterval includes only a few (possibly one) time steps, the data storage requirements can be tremendously reduced. This method is applied in a pressure drop minimization problem considering unsteady viscous fluid. Two- and three-dimensional numerical examples are provided to confirm the validity and utility of the presented method.

The AE (Acoustic Emission) method is a technique for detecting ultrasonic sound waves generated by plastic deformation, crack initiation and crack growth by using an AE sensor. The AE method is used in checks of petroleum tank drawer bottom corrosion, degradation of steel and concrete structures, bearing degradation checks, etc. The purpose of this study is to apply AE to crack checks of rotary equipment (drive shaft or coupling). First, we conducted a comparative test of AE propagation characteristics, followed by a comparative test of the crack detection signal by a small rotary bending fatigue testing machine. From the AE count rate in the rotary bending fatigue test, the number of cycles of the fatigue test can be divided into three areas. In the initial stage, an AE wave is generated by the dislocation motion in the material. An AE wave is estimated to occur due to the progress of a crack generated on the test piece surface. In this study, we propose a fluid-mediated AE method with an improved S/N ratio and showed that it is possible to detect a fatigue crack in a drive shaft or coupling by the proposed method.

There are many hemodynamic factors which have been suggested to relate to artery shape regulation but it has not been certain which factors are more effective. We propose a new method to evaluate the relevance of hemodynamic factors to artery shape. First, after selecting some associated factors said to be related with artery shapes, we performed multi-objective optimization that sets two of their factors as objectives to obtain optimized shapes using computational fluid dynamics and genetic algorithms. Then, we checked how similar the original shape was to the optimized shapes using Objective Difference Index (ODI). After this process was applied to seven typical carotid artery bifurcation shapes (seven actual cases), the relevance of each combination of two factors was evaluated. We selected five factors: a) to minimize maximum time-averaged Wall Shear Stress (WSS), b) to maximize minimum time-averaged WSS, c) to minimize WSS gradient, d) to minimize WSS temporary gradient, and e) to minimize inner surface area. At the first stage, shapes were optimized by using only radius as the variable using a fixed center line. We set six kinds of combinations of factors that have trade-off relationships. As a result, ODI in the cases of a) and e) was the smallest, having a value was about one-twentieth that of ODI for the second smallest combination factors of c) and e). Combination factors of minimizing both maximum WSS and artery radius were evaluated to be the most relevant to artery radius in the six kinds of tested combinations. At the next stage, by setting both the radius and center line as variables, it became clear that this combination was also related to the position of the center line. We confirmed our method effectively evaluates the relevance of factors to artery bifurcation shapes.

The objective of this study was to propose a CPG network which can generate the swimming motion of the crawl stroke. First, the CPG network for legs performing a flutter kick was constructed by connecting the neural oscillators for the leg joints. The flutter kick motion was successfully generated by the proposed CPG network. The propulsion by the generated flutter kick motion was confirmed by the simulation of the swimming movement. Next, the CPG network for both the arms and legs were constructed, in which the neural oscillator for the arms initiated the trigger signal to start the prescribed stroke motion. By changing the intrinsic cycle of the neural oscillators for the legs, both six- and two-beat crawls could be realized. It was also found that a stable region with respect to the relationship between the intrinsic cycles of the neural oscillators for the arms and legs certainly existed for the six-beat crawl, although the intrinsic cycles of the arms were three times longer than those of the legs in this case. The propulsion by the generated swimming motion was confirmed by the simulation of the swimming movement both for the six- and two-beat crawls. Finally, the roll angle of the swimmer was fed back into the CPG network in order to restore the balance in the roll direction. Restoring the balance in the roll direction was successfully realized by the proposed feedback algorithm. The resultant motion showed a complicated behavior, such as skipping strokes.

Rolling stock is sensitive to crosswind because of its large side area, so crosswind stability must be assessed when rolling stock is developed. In the European Union, aerodynamic force and moment under crosswind need to be measured in a wind tunnel experiment in accordance with European Norm EN14067-6, which describes the methods of crosswind stability assessment. In this report, we present a measurement system developed on the basis of the European Norm to check the aerodynamic force and moment of the designed rolling stock in a compact wind tunnel. For precise measurement, by effectively using a limited air flow rate, an adequate quality of flow must be acquired from the nozzle and the same flow around the train model must be simulated as the European wind tunnels, in which the measured aerodynamic force and moment were described as the reference value in the European Norm. To achieve this flow, a wide nozzle and a splitter plate were developed. For the wide nozzle design, first, from the relationship between the train model scale and the specifications, the train model scale and the outlet size of the wide nozzle were determined, and an adequate contraction from the inlet area to the outlet area was designed. The splitter plate height was optimized in a simulation to satisfy the boundary layer thickness and acquire sufficient vertical space above the train model. The results revealed that the developed measurement system can satisfy the equipment requirements and flow specifications defined in the European Norm. Additionally, the rolling moment around the leeward rail of a wind tunnel benchmark vehicle model, which is defined in European Norm, was measured within a mean tolerance 0.086 from the reference value described in the European Norm. This shows that the developed measurement system for a compact wind tunnel has the same measurement accuracy as the reference wind tunnel described in the European Norm.