The long-term safety of geological disposal of radioactive waste is studied through several simulations. Before underground disposal, radioactive waste is stored for 30 to 50 years at facilities near nuclear power plants to cool it down to around 100 degrees Celsius. It is then placed in steel canisters surrounded by artificial materials such as bentonite and concrete. To determine the long-term safety and stability of this disposal method, we’ve studied the corrosion rate of the steel canisters under different conditions using electro-chemical impedance spectroscopy (EIS). This paper describes the corrosion of the carbon steel and elucidates the corroded condition using EIS measurement. EIS was adopted to estimate the corrosion condition from the impedance frequency characteristic. In our experiment, samples of bentonite and carbon steel were Kunigel V1 compacted to 1.37 Mg/m3 dry density with several different water contents, and SM400 as a low carbon steel. An electric heater was set inside the steel canister to maintain the temperature at 100 degrees Celsius. This model was made to a scale of around 1/120 as a current concept of a vertical disposal plan and reproduced the enclosed situation after underground emplacement of the radioactive waste. During heating, we conducted EIS measurements and set this data result as an equivalent circuit. We noted some different trends of impedance frequency characteristic depending on the bentonite’s water content and the heating time. From this result, we estimated the corrosion condition to analyze the corrosion products.
Nanofluid, a liquid containing choroidal dispersion of nanometer-sized solid particles, enables high-temperature bodies to be cooled more rapidly during quenching than in pure liquid. Drastic rise of the minimum heat flux temperature (TMHF) caused by the layer of nanoparticles formed on the heat transfer surface is the key phenomenon of heat transfer enhancement. In the present work, using alumina, silica, and titanium dioxide as the nanoparticle materials, quenching experiments were carried out to explore the mechanisms of the rise of TMHF in nanofluids; stainless steel 304 and Inconel 718 were used as the materials of the specimen and distilled water was used as the base liquid. In the experiments, TMHF increased in all the nanofluids but the increasing rate was dependent significantly on the nanoparticle material and the nanoparticle layer thickness. To elucidate the mechanisms of the heat transfer enhancement, the relations of TMHF with the three basic surface parameters of roughness, wettability, and wickability were examined but no clear relationship was found. When the metal specimen of higher thermal conductivity is covered with the nanoparticle layer of lower thermal conductivity, the contact temperature during quenching should decrease and the contact duration would be dependent on the thermal properties and thickness of the nanoparticle layer. Assuming that TMHF rises with an increase in the contact duration, a new model describing the rise of TMHF in the nanofluid was proposed.
In a large-scale plant such as a nuclear power plant, thousands of process values are measured for the purpose of monitoring the plant performance and the health of various systems. It is difficult for plant operators to constantly monitor all of the process values. We present a data-driven method to comprehensively monitor a large number of process values and detect early signs of anomalies, including unknown events, with few false positives. In order to learn the complex changing internal state of a nuclear power plant and accurately predict the normal process values, we created a two-stage autoencoder composed of a time window autoencoder and a deviation autoencoder, which is a deep learning network structure corresponding to the characteristics of the process values. We assessed performances of the two-stage autoencoder with simulated process values of a nuclear power plant, a 1,100 MW boiling water reactor having 3,100 analog process values. In situations where it is difficult to predict the normal state (rapid operation mode change, transient state, and small fluctuations in the process values), the two-stage autoencoder properly predicted the normal process values and showed excellent performances with early detection and zero false positives, except for one case. The two-stage autoencoder would be an effective solution for comprehensive plant monitoring and early detection of anomaly signs.
In a core disruptive accident scenario, boron carbide, which is used as a control rod material, may melt below the melting temperature of stainless steel owing to the eutectic reaction with them. The eutectic mixture produced is assumed to extensively relocate in the degraded core, and this behavior plays an important role in significantly reducing the neutronic reactivity. However, these behaviors have never been simulated in previous severe accident analysis. To contribute to the improvement of the core disruptive accident analysis code to simulate these eutectic melting and relocation behavior, the thermophysical property database of the eutectic mixture implemented into the analysis code should be developed. As part of this database development, this study measured the thermophysical properties of the eutectic mixture in the solid state. In this study, pre-alloyed austenitic stainless steel containing boron carbide, which has the prescribed concentration of boron carbide and is homogeneous, is used as eutectic samples to reduce the uncertainty of samples and achieve accurate measurement. Based on these evaluation results, the effect of adding boron carbide on the thermophysical properties of austenitic stainless steel are discussed. Furthermore, regression equations that show the temperature (and boron carbide concentration) dependence are created for each thermophysical property.
Pellet-Cladding Mechanical Interaction (PCMI) failure is one of the failure mode which must be evaluated in nuclear fuel safety. PCMI is caused by mechanical load to the cladding due to fuel pellet expansion. Under high fuel burnup condition, the fuel cladding may become degraded by embrittlement under neutron-induced irradiation and hydrogen accumulation due to the waterside corrosion. In order to consider the further deterioration of the material with higher burnup, evaluation using mechanical indicators, e.g. strain and stress, might be required. As fuel burnup proceeds, cracks occur in the pellet due to an internal temperature gradient, which induces the fuel pellet relocation, and fission gas may be accumulated in the pellet and the gap between pellet and cladding. Cracks and fission gas may cause more deformation of the pellet under the power excursion than in low burnup condition. In this study, a transient model is developed, which can mechanistically evaluate the PCMI behavior, in particular, for fuel rods under higher burnup condition. The model is incorporated in a fuel behavior analysis code and verified by benchmarks with other similar codes. The PCMI predictability of this code is validated using the experimental test data.
In a postulated severe accidental condition of sodium-cooled fast reactor (SFR), the eutectic reaction between boron carbide (B4C) as control rod element and stainless steel (SS) as control rod cladding or related structure materials may take place. Thus, the kinetic behavior of the B4C-SS eutectic reaction is one of the important phenomena to be considered when evaluating the core disruptive accidents in SFR. In this study, for the first step to obtain the fundamental information on kinetic feature of the B4C-SS eutectic reaction and compare the pervious findings, the thermal analysis using the pellet samples of B4C and Type 316L SS as a different experimental approach was performed up to 1773 K at different heating rates of 2.5-10 K min-1. The differential thermal analysis (DTA) endothermic peaks for the B4C-SS eutectic reaction appeared from 1483K to 1534K and systematically shifted to higher temperatures with increasing heating rate. Based on this kinetic feature, apparent activation energy and pre-exponential factor for the B4C-SS eutectic reaction were determined by Kissinger method. It was found that the kinetic parameters obtained by thermal analysis were comparable to the literature values of the thinning experiment at high temperatures. In addition, the microstructure and element distribution formed in the interdiffusion layer composed of the B4C-SS system were analyzed by the electron probe microanalyzer (EPMA), which can provide key validation data on elemental interdiffusion behavior in the early stage of the eutectic reaction for reflecting the reaction kinetic modeling.
Fission product removal by pool scrubbing is known to be largely affected by the gas-liquid two-phase flow regime. The pool scrubbing performance of various carrier gases was evaluated in two-phase flow experiments by injecting helium, nitrogen, or argon through a pool of stagnant water in a column with an inner diameter of 0.5 m and a height of 8 m. The gases were supplied through a cylindrical nozzle with an inner diameter 69 mm at superficial gas velocities ranging from 0.013 to 0.053 m s-1. Measurements were conducted using a camera and two sets of a 128 × 128 wire-mesh sensors, separated by 40 mm. The visually observed gas hold-up and wire-mesh sensor-measured average void fraction decreased with increasing fraction of lighter gas in the supplied gas. Detailed analysis of the flow regime using the obtained wire-mesh sensor signals revealed that lighter gases led to a greater fraction of relatively large bubbles in the flow compared to heavier gases, causing the gas phase with the lighter gases to have a higher average rise velocity in the flow. This leads to a hypothesis that, compared to heavier gases, lighter gases break up less or coalesce more in the flow, resulting in distinct two-phase flow characteristics depending on the inlet gas composition.
A sodium-cooled fast reactor has been designed to attain a high burn-up core in commercialized fast reactor cycle systems. The sodium-cooled fast reactor adopts a wire spacer between fuel pins. The wire spacer performs functions of securing the coolant channel and mixing between subchannels. In high burn-up fuel subassemblies, the fuel pin deformation due to swelling and thermal bowing may decrease the local flow velocity in the subassembly and influence the heat removal capability. Therefore, understanding the flow field in a wire-wrapped pin bundle is important. This study performed particle image velocimetry (PIV) measurements using a wire-wrapped three-pin bundle water model to grasp the flow field in the subchannel under conditions, including the laminar to turbulent regions. The PIV results confirmed that the normalized flow velocity near the wrapping wire in the low Re number condition was relatively decreased compared to that in the high Re number condition. Meanwhile, in the region away from the wrapping wire, the maximum flow velocity was increased by decreasing the Re number. Accordingly, the PIV measurements using the three-pin bundle geometry without the wrapping wire were also conducted to understand the effect of the wrapping wires on the flow field in the subchannel. The results confirmed that the mixing due to the wrapping wire occurred, even in the laminar condition. These experimental results are useful not only for understanding the pin bundle thermal hydraulics, but also for the code validation.
The void fraction distribution of a fuel rod bundle in a boiling water reactor is a critical parameter for accurately predicting the optimal thermal margin in the design of a reactor core. The rod bundle configuration, such as a part-length rod (PLR) and water rod, can affect void distribution. To clarify the influence of PLR on void fraction distribution, a boiling flow experiment was conducted using a 5 × 5 heated rod bundle that partially simulated a boiling water reactor (BWR) rod bundle, and three PLRs were arranged in the corner. The cross-sectional void fraction distribution was acquired using high-energy X-ray computed tomography at six height levels for wide flow conditions, system pressures of 0.1 − 7.2 MPa, inlet subcoolings of 20 - 90 kJ/kg, mass fluxes of 500 - 1250 kg/m2/s, and linear heat generation rates (LHGR) of 3.2 - 8.6 kW/m. In the PLR region, the local void fraction temporarily decreases because the PLRs disappear, and the flow channel rapidly expands. Together with the downstream PLRs, the voids propagate to the PLR region and concentrate in the center. The void fraction in the corner of the PLR region remains lower. A maximum 26% decrease in the subchannel void fraction was observed in the corner of the PLR region at the system pressure of 7.2 MPa, mass flux of 1.25 × 103 kg/m2/s, inlet subcooling of 50 kJ/kg, and LHGR of 8.6 kW/m.
In the accident at the Fukushima Daiichi Nuclear Power Station, reaction of water vapor with hot zirconium led to the generation of hydrogen and a subsequent explosion in the reactor building. From the perspective of defense-in-depth, multiple hydrogen explosion prevention measures are desirable to improve the safety of nuclear power generation. In this research, we focus on a hydrogen treatment system that re-oxidizes hydrogen into water vapor using a fixed, packed bed of copper oxide pellets. The advantages of this method are that the hydrogen oxidation rate is rapid and no external source of oxygen is necessary. In this study, we conducted experiments and complementary numerical calculations for the hydrogen oxidation reaction using copper oxide pellets. The oxidation reaction of hydrogen by copper oxide is decomposed into five elementary reactions, the rate of each was determined experimentally. The resultant numerical calculation accurately modeled experimentally observed hydrogen oxidation rates and provides insights into the phenomena controlling the reaction progression. The results suggest that the commonly observed induction period is due to the presence of poorly adsorbing sites on the copper oxide surface. Moreover, when water vapor is present, competition between water vapor and hydrogen for adsorption sites further suppresses the hydrogen oxidation reactions.
Gas entrainment (GE) from cover gas, which is an inert gas to cover sodium coolant in the reactor vessel, is one of the key issues for Sodium-cooled fast reactors (SFRs) design to prevent unexpected effects to core reactivity. In this research series, evaluation method has been investigated for surface dimple depth growth of unstable drifting vortex dimples on the liquid surface in the reactor vessel. By using a computational fluid dynamics (CFD) code, analyses have been conducted to estimate the drifting vortex on water experiments in a circulating water tunnel. The unstable drifting flow vortexes on the water surface were generated as wake vortexes behind a plate obstacle. Downward flow velocity was induced by the bottom slit’s flow passing along the flow channel. In the previous study, the initial conditions of the gas entrainment were evaluated based on existing non-dimensional numbers method by using the STREAM-VIEWER code. However, the CFD predication accuracy of the detailed flow field itself was not clear especially for vortex frequency in the wake flow and detailed velocity profiles in the flow channel. In this study, to clarify the accuracy of CFD analysis, Strouhal numbers of vortex frequency and detailed flow velocity profiles were compared with experimental data which were measured by Particle Image Velocimetry (PIV) method. As the results, the Strouhal numbers of the vortex frequency behind the plate obstacle reasonably agreed with the experimental data.
In order to improve the safety of nuclear power plants, it is necessary to make sure measures against their severe accidents. Especially, in the case of a sodium-cooled fast reactor, there is a possibility that molten core material would be discharged through control rod guide tubes into the inlet coolant plenums beneath the rector cores in the event of a core disruptive accident (CDA). It is important to ensure in-vessel retention that keeps and confines damaged core material in the reactor vessel even if the CDA occurs. In this study, effective cooling of the melt in coolant was confirmed by comparing the experiment and analysis. CDA scenario initiated by a unprotected loss of flow condition , which is a typical cause of core damage, is generally categorized into four phases according to the progression of core-disruptive status, which are the initiating, early-discharge, material-relocation and heat-removal phases for the latest design in Japan. During the material-relocation phase, the molten core material flows down mainly through the control rod guide tube and is discharged into the inlet coolant plenum below the bottom of the core. The discharged molten core material collides with the bottom plate of the inlet plenum. Clarification of the accumulation behavior of molten core material with such a collision on the bottom plate is important to reduce uncertainties in the safety assessment of CDA. In present study, in order to make clear behavior of core melt materials during the CDAs of sodium-cooled fast reactors, analysis was conducted using the SIMMER-III code for melt discharge simulation experiments by Imaizumi et al. in which low-melting-point alloy was discharged into a shallow water pool. As the result, temperature and pressure behaviors during the discharge almost coincided between the analysis and the experiment. Therefore, it can be concluded that the validity of the analysis cell system model was confirmed.
The numerical scheme of the O-integral is formulated for precisely evaluating the stress intensity factor of an embedded crack with arbitrary shape in an infinite elastic body. In this study, we evaluate the O-integral using an efficient numerical integration method by introducing an iso-parametric element and the Gauss-Legendre formula, which is typically used in the finite element method. To verify the numerical procedure introduced herein, the mode-I stress intensity factors for cracks are evaluated based on a circle, an ellipse, and a perturbated circle cracks. Result shows that the K results obtained using the proposed method are consistent with the exact solution. Therefore, fatigue crack propagation is successfully simulated using the O-integral for the elliptical crack.
In an experimental study that simulated a fast breeder reactor (FBR) vessel near the coolant surface, it was reported that the long distance travel of temperature distribution causes a new type of thermal ratcheting, even in the absence of primary stress. When the distance of temperature travel is moderate, the accumulation of the plastic strain due to this mechanism is finally saturated. Through the large number of Finite Element Analysis, we have found the strong relationship between hoop-membrane distributions of accumulated plastic strain and residual stress in this saturated case. Focusing on this relationship, we have aimed to predict the saturated distribution of the plastic strain based on the residual stress distribution that is required for the elastic shakedown behavior. In this paper, based on classical shell theory, we formulated the plastic strain distribution that brings uniform hoop-membrane stress in the given region. And then, we compared the formulated strain distribution with the accumulated plastic strain distribution obtained by finite element analyses using an elastic-perfectly plastic material. As a result, we confirmed that the formulated strain distribution can be used as the prediction of the plastic strain distribution for the cases with moderate distance of temperature travel. For the cases with long-distance travel of temperature, the region with plastic strain expanded with the repetition of temperature travel. By considering the effect of this expansion, the formulated strain distribution can be used as conservative prediction of the accumulated plastic strain also for the cases with long distance temperature travel.
Recent years have witnessed attempts to employ a system with rigid and extremely flexible components (SREF), usually consisting of strings, membranes, and so on, to realize huge structures for spacecraft in orbit. In general, such flexible components have two states, i.e., with and without tensional force. Previously, the authors proposed an effective method for analyzing SREF motion, which is based on an analogy between the state transitions of the SREF and contact problem of rigid bodies. The state transitions of the SREF are detected via a linear complementarity problem that is used for contact problem in the analysis method proposed by Pfeiffer et al. (Pfeiffer and Glocker, 1996). The authors had applied this method to an SREF consisting of two masses and two strings, where the motion of the system was limited to one dimension. In this study, the method is extended to an SREF having planar motion. As an analysis object, an SREF consisting of two masses and two strings is introduced. Finally, the results of numerical analyses are compared with those of an experiment under same parameters, and the validation of the proposed method is demonstrated by the comparison.
This paper describes the optimum tuning of damped Helmholtz silencers. In this study, Helmholtz resonators are used as silencers that suppress acoustic resonance in host acoustic fields. Side branch silencers and Helmholtz silencers are commonly known as a type of vibration absorber; however, damped silencers have not been thoroughly studied thus far. Therefore, prior to this paper, we reported the optimum tuning of damped side branch silencers using modal analysis and two fixed point method. In this paper, we additionally describe the optimum tuning of damped Helmholtz silencers in a similar fashion as in our previous paper. The resonance mechanism of Helmholtz silencers is different from that of side branch silencers. The coupled vibration between the host acoustic field and Helmholtz silencer was theoretically analyzed using modal analysis in this study. An equivalent discrete model was obtained using the equations of motion using modal coordinate systems. Using the equivalent discrete model, the open-end correction of the neck of the Helmholtz silencer was considered, and the number of degrees of freedom of the equivalent discrete model was reduced to two to derive optimum tuning conditions using the two fixed point method. The optimum natural frequency ratio and loss factor of the Helmholtz silencer were derived using the vibration model with two degrees of freedom. The theoretical analysis was validated through simulations and experiments.
The purpose of this paper is to forecast vehicle accelerations by using a Long Short Term Memory (LSTM) approach. Such a predictive capability can be particularly helpful in the case of medical emergency vehicles. During emergency transport, a patient is likely to experience vehicle accelerations in both longitudinal and lateral directions. Although the effect of these accelerations acting on a patient can be reduced by actively controlling the attitude of the ambulance bed, there is inevitably a delay from the time the acceleration is measured until the attitude of the bed reaches its target state. In our approach, we forecast future accelerations from past time series data by using LSTM, a recurrent neural network architecture utilized in deep learning, to predict future accelerations at each time step. Using various driving scenarios, the LSTM is trained with different training data and different numbers of hidden layers, units, and epochs. To evaluate the performance and usefulness of the approach, real-time simulations are conducted using measured longitudinal and lateral vehicle acceleration data. Forecast accuracy is assessed for the trained LSTM with different parameters, and the results show the capability of producing accurate real-time forecasts for certain parameter settings. Comparisons of the LSTM’s forecast results with the results of an autoregressive integrated moving average model shows the advantages of the LSTM approach especially for unsteady time series data that includes such elements as sudden or large acceleration changes.
This paper discusses that the unsteady aerodynamic forces caused by vehicle motion in road input have a large influence on the vehicle behaviors. In the present coupling analysis, unsteady aerodynamic forces were obtained by using the response functions for vehicle motion derived in our previous study. By attaching a protrusion on the roof which only measures about 3% of the vehicle height, pitch and heave motions are affected significantly by the change of unsteady aerodynamic effects. Due to the coupling of the unsteady aerodynamic forces and vehicle suspension responses, the vehicle behavior depends on the road input frequency. The pitch motion is enhanced in low-frequency road input (<1.6 Hz for a real vehicle) and is suppressed at high frequencies (>1.6 Hz). Meanwhile, the heave motion is suppressed over 1.2 Hz. Such vehicle behaviors were verified in actual running tests, and were consistent to the subjective evaluation of the drivers feeling. Comparing the unsteady aerodynamic coefficients to the equivalent vehicle specification factors, the aerodynamic inertia caused by pitch motion contributes to 39% of the rear vehicle mass inertia effect in pitch motion. This aerodynamic effect is important in designing suspension specifications of vehicles. These results show that the vehicle stability can be affected even by small detail in vehicle shape due to the unsteady aerodynamic forces.
A pantograph receives aerodynamic force while a train is traveling. As the aerodynamic force increases in proportion to the square of the flow velocity relative to the pantograph, its influence on the pantograph becomes apparent, especially for high-speed trains. When a high-speed train runs in a tunnel, the flow velocity relative to the pantograph is faster than that in an open section. In this study, we measured the flow velocity around a train model running in a tunnel using a rake of total-pressure tubes mounted on the train model. The measured waveforms were distorted owing to the influence of the frequency characteristics of the measurement system. Therefore, we developed a restoration method and applied it to the measured waveforms to obtain the restored waveforms of the flow velocity. With the restored waveforms, we obtained several statistical values, such as the average flow velocity and standard deviation of the fluctuating flow velocity around the train model. Furthermore, we proposed a method for predicting the flow velocity around the train model, including fluctuating components in a frequency range of a pantograph contact performance. The proposed method can predict the flow velocity at the panhead of a pantograph by considering the average flow velocity profile and turbulence component profile in the tunnel cross section, which could not be taken into account so far.
This paper proposes a novel DVA (Dynamic Vibration Absorber) consisting of a ball-like mass embedded in a spherical viscoelastic material to meet practical demands for the multi-modal and multi-axis vibration reduction against elastic vibrations of structures. This DVA is called eMDVA (embedded Mass DVA) here, and the embedded mass can vibrate every direction in the viscoelastic medium. The unique concept of the eMDVA is inspired by the damping effect caused by passengers on railway vehicles. This paper describes a basic configuration of the eMDVA and some numerical studies using finite element (FE) vibration analysis to design the eMDVA. From the numerical investigation, it has been found that the natural frequency of a single-mass eMDVA can be controlled by changing the combination of the sizes of the viscoelastic sphere and the embedded mass. The frequency response function (FRF) of the acceleration of the embedded mass versus excitation force has a dominant single peak corresponding to one of the natural frequencies. These results indicate that the proposed eMDVA is suitable as a DVA, and it can be designed to tune the target vibration frequencies of host structures. As a more realistic analysis, numerical investigation for the thin and long plate-like host structure (a 1:10 scale model of the floor structure of a railway vehicle) was conducted, and multi-modal vibration reduction has been observed by applying the eMDVA consisting of two sets of viscoelastic spheres and embedded masses with different sizes. From these numerical investigations, it has been shown that the proposed eMDVA has promising potential as a multi-modal damper for elastic vibrations.