A probabilistic risk assessment (PRA) should be performed not only for earthquake and tsunami which are major natural events in Japan but also for other natural external hazards. However, PRA methodologies for other external hazards and their combination have not been sufficiently developed. This study is aimed at developing a PRA methodology for the combination of low temperature and snow for a sodium-cooled fast reactor which uses the ambient air as its ultimate heat sink to remove decay heat under accident conditions. The annual exceedance probabilities of low temperature and of snow can be statistically estimated based on the meteorological records of temperature, snow depth and daily snowfall depth. To identify core damage sequence, an event tree was developed by considering the impact of low temperature and snow on decay heat removal systems (DHRSs), e.g., a clogged intake and/or outtake for a DHRS and for an emergency diesel generator, an unopenable door on necessary access routes due to accumulated snow, failure of intake filters due to accumulated snow, and possibility of water freezing in cooling circuits. Recovery actions (i.e., snow removal and filter replacement) to prevent loss of DHRS function were also considered in developing the event tree. Furthermore, considering that a dominant contributor to snow risk can be failure of snow removal around intakes and outtakes caused by loss of the access routes, this study has investigated effects of electric heaters installed around the intakes and outtakes as an additional countermeasure. By using the annual exceedance probabilities and failure probabilities, the event tree was quantified. The result showed that a dominant core damage sequence caused by a snow and low temperature combination hazard is the failure of the electric heaters and the loss of the access routes for snow removal due to low temperature and snowfall which last for a day, and daily snowfall depth of 2 m/day.
This paper describes the development of a probabilistic risk assessment (PRA) methodology against a combination hazard of strong wind and rainfall. In this combination hazard PRA, a hazard curve is evaluated in terms of maximum instantaneous wind speed, hourly rainfall, and rainfall duration. A scenario analysis has provided event sequences resulting from the combination hazard of strong wind and rainfall. The typical event sequence was characterized by the function loss of auxiliary cooling system, of which heat transfer tubes could crack due to cycle fatigue caused by cyclic contacts with rain droplets. This cycle fatigue crack could occur if rain droplets enter into the air cooler of the system following the cooler's roof failure due to strong-wind-generated missile impact. This event sequence has been incorporated into an event tree which addresses component failure caused by the combination hazard. As a result, a core damage frequency has been estimated to be about 10-6/year in total by multiplying discrete hazard frequencies by conditional decay heat removal failure probabilities. The dominant sequence is the manual operation failure of an air cooler damper following the failure of external fuel tank due to the missile impact. The dominant hazard is the maximum instantaneous wind speed of 20−40 m/s, the hourly rainfall of 20−40 mm/h, and the rainfall duration of 0−10 h.
The loss-of-reactor-level (LORL) where the coolant circulation path is lost is one of the important accident types of loss-of-heat-removal-system (LOHRS) in loop-type sodium-cooled fast reactors (SFRs). Due to the non-negligible possibility obtained by probabilistic risk assessment (PRA), pipe failures and succeeding sodium leakage in two points both occurred in primary heat transport system (PHTS) was assumed in this study, unlike the conventional studies assuming single pipe failures. The sodium level in reactor vessel (RV) is affected by many elements such as leakage position, guard vessel volume, operational state of pumps, and countermeasures to maintain sodium level which are sodium pump-up into RV and siphon-breaking of the pipe between RV and pump. Thus, a calculation program was developed in this study to evaluate and discuss the effectiveness of the countermeasures and safety margins for the loss of coolant circulation path. In addition, the crack size was discussed and evaluated realistically, and t 2 (t : pipe thickness) was obtained for a sufficiently conservative value, instead of Dt/4 (D : pipe diameter) that was assumed in the conventional studies. Time interval between two leakages was also given by PRA, considering failure rates of the pipes and components. Representative sequences and leakage positions where the sodium level can decline below emergency sodium level (EsL) were chosen, and the sodium level transient in RV was calculated where the crack size of the second leakage was set t 2. The calculations were also conducted where the larger crack size, Dt/4, was set for both the first and second leakage, in order to investigate additional requirements to maintain the RV sodium level. The evaluation results clarified that the coolant circulation loop can be maintained even after the second leakage in PHTS, taking into account the effects by the countermeasures.
We conducted an experiment to clarify nitrogen gas behavior at low pressures during reflux condensation in a pressurized water reactor (PWR) with the rig of safety assessment/large scale test facility (ROSA/LSTF) at Japan Atomic Energy Agency. The primary pressure was lower than 1 MPa under the constant core power of 0.7% of the volumetric-scaled (1/48) PWR nominal power, unlike a previous related test with the LSTF. Steam generator (SG) secondary-side collapsed liquid level was maintained at a certain liquid level above the SG U-tube height. Nitrogen gas was injected stepwise into each SG inlet plenum at a certain constant amount. The primary pressure and the degree of subcooling of the SG U-tubes were largely dependent on the amount of nitrogen gas accumulated in the SG U-tubes. Nitrogen gas accumulated from the outlet towards the inlet of the SG U-tubes. Non-uniform flow behavior was observed among the SG U-tubes with nitrogen gas ingress. The RELAP5/MOD3.3 code indicated remaining problems in the predictions of the primary pressure and the degree of subcooling of the SG U-tubes depending on the number of nitrogen gas injection. We investigated further the applicability of the RELAP5 code with different models for the SG U-tubes to the prediction of the non-uniform flow behavior through sensitivity analyses.
A dry method is one of fuel debris retrieval methods for decommissioning of TEPCO’s Fukushima Daiichi nuclear power station. However, the cooling of fuel debris must be fully maintained without water. Japan Atomic Energy Agency (JAEA) has evaluated the air-cooling performance of the fuel debris in the dry method by using JUPITER. Because JUPITER can represent the relocation of the corium, the unknown parameters, such as the composition and the position of the fuel debris at the RPV pedestal, can be reduced. By calculating the heat transfer of the fuel debris based on the corium relocation obtained with JUPITER, more accurate analysis of the air-cooling performance of the fuel debris in the dry method is expected. In order to evaluate the air-cooling performance of fuel debris in the dry method by using JUPITER, the validation of the free-convective heat transfer analysis of JUPITER were performed in this paper. In order to qualitatively evaluate results of JUPITER for configurations closer to experimental conditions and to decide physical values and positions to be measured in the validation, JUPITER was compared with OpenFOAM for the simple cuboid configuration which has the heating and cooling surfaces at the floor and the ceiling, respectively. The comparison proved that JUPITER can calculate the vertical temperature distribution as well as OpenFOAM on the condition of the lower heating amount. In the validation, JUPITER was compared with the heat transfer experiments of free convection in air adjacent to an upward-facing horizontal heating surface. The comparison proved that JUPITER was in good agreement with the experiment on the condition of the lower heating-surface temperature. The result indicated that JUPITER is a helpful numerical method to evaluate the free-convective heat transfer of the fuel debris in the dry method.
Titanium and its alloys have excellent strength properties and corrosion resistance, but they show poor wear resistance, which cannot be improved by heat treatment. The addition of hard ceramic particles to produce a titanium matrix composite is an effective method for enhancing the wear resistance of titanium and its alloys. In this study, TiC and TiB2 were used as reinforcement materials because a composite containing these reinforcements shows high tensile strength and different microstructure. TiB- and TiC-reinforced pure Ti matrix composites were fabricated by using the spark plasma sintering (SPS) method. Dry sliding wear tests were conducted on the composites using a ball-on-disk type testing machine. The effects of the reinforcement material, microstructural features, and reinforcement volume fraction on the wear behavior of the composites were investigated. The specific wear rates of both composites decreased with increasing reinforcement volume fraction. The specific wear rate of the TiC/Ti composite deceased drastically at a reinforcement volume fraction of 25 vol.%. TiC/Ti composites with reinforcement volume fractions of over 5 vol.% showed excellent wear resistance compared with the TiB/Ti composites. The wear behavior of the composite depended mainly on the distribution of reinforcement material and the nature of the reaction products between the matrix and reinforcement particles.
Injection molded Ti-6Al-4V compacts have normally a coarse lamellar microstructure, because it is sintered above the β transus temperature and it is cooled slowly in furnace. However, static mechanical properties such as tensile strength and ductility are equivalent to those of wrought materials, but they have the disadvantage that the fatigue strength is significantly lower than wrought materials. Therefore, improvement of fatigue strength is needed for injection molded Ti alloy compacts. In this study, injection molded Ti-6Al-4V compacts having equiaxed microstructure were prepared by sintering at α + β region temperature which is very lower than normal sintering temperature. When sintering is carried out in α + β region, sinterability is remarkably poor. Therefore, long-time sintering is required. Since the grain growth of the prior β grains was suppressed by the pinning effect of α grains and pores, it was possible to obtain the significantly fine grains as compared with the β region sintered compacts. The α + β region sintered compacts showed high strength, high elongation and also high fatigue strength. Furthermore, by applying HIP treatment, the mechanical properties were improved to satisfy the standard value of wrought Ti-6Al-4V material.
Thermal fatigue cracks have been found at mixing tees in nuclear power plants. The mixing flow of high and low temperature fluids causes temperature and stress fluctuations in the pipe wall and these result in fatigue crack initiation. The authors have conducted a fluid-structure coupled simulation to estimate the fluid and pipe wall temperatures in a mixing tee in their previous study. In the present study, the authors simulated thermal stress using the previous simulation results of the pipe wall temperature. The simulated thermal stress was validated using the stress obtained from the temperature on the pipe inner surface measured by mock-up tests. The test section of the tee pipe was made of stainless steel and consisted of a horizontal main pipe with a diameter of 150 mm and a T-junction connected to a vertical pipe with a diameter of 50 mm. The ranges of the large temperature and stress fluctuation areas on the pipe inner surface calculated by the fluid-structure coupled simulation were narrower in the axial direction of main pipe compared with the results of test. On the other hand, the profiles of the circumferential direction were reproduced by the fluid-structure coupled simulation. The maximum values of the temperature and stress fluctuation ranges were overestimated. The stress fluctuation obtained from the measured temperature showed the equibiaxial behavior where the axial and circumferential stresses had a proportional relationship. Such characteristics of stress fluctuation were reproduced well by the numerical simulation. Not only the stress fluctuation range but also the number of cycles for the stress amplitude were estimated using the time history of the thermal stress and the rain-flow counting method. The distribution of the number of cycles for the stress amplitude estimated by the simulation was similar to that obtained from the measured temperatures.
Lightweight and rigid honeycomb cores have a feature as deployable structures, i.e., their shapes can be changed as desired from the viewpoint of manufacturing processes. This paper presents a geometrical design method for cylindrical honeycomb cores that satisfy high rigidity and deployability, considering core orientation, core distortion, and applicable manufacturing processes. The proposed method is based on a conventional construction process of flat honeycomb cores and applied to a cylinder segmented with longitudinal and latitudinal lines. Cylindrical honeycomb cores with the radial core direction are possibly suitable for the application of radial forces; however, core walls are distorted and the shape cannot be maintained uniformly. To solve this problem, the core direction is revised based on the correction angle. As a result, it is observed that the obtained cores are not distorted and core directions can be designed approximately along radial directions. Geometrical restrictions and feasible design ranges are discussed in this paper. It is possible to fabricate the cores using the conventional expansion process. This implies that they are promising as deployable structures that can be constructed or deconstructed at workplaces.
Torque-unit Manipulator(TUM) is a design concept of (space) manipulator and a Multibody system with nonholonomic constraints. Each joint of TUM is free and a device which is called “torque unit” is attached on each link. The “torque unit” can be made easily of a rotary actuator and a disc. Position-controllability of the links of TUM has been shown with conventional control laws for manipulators. However a problem arises that the discs in torque units usually have residual angular velocities when the links approach desired positions. The reason of the problem is that TUM is a kind of nonholonomic system, hence the angular velocity of each disc at each time depends on the motion history of the links. Controllability of the angular velocities of the discs to desired values has been considered by planning motion trajectories of links. Then, a control scheme for the TUM from zero-velocity initial state to zero-velocity final state has been given. However the trajectories of the links were not smooth enough. Then, a condition is made clear for the trajectories of the links to satisfy for bringing the angular velocity of a disc to zero in this paper. Also trajectories of the links are planned which is smooth enough as an example of application of the condition.
We propose an effective timing of intermittent force application by a gait assistance robot with a wire-driven system to increase the toe trajectory throughout the swing phase. We tested different timings of force application at the shank, employing the short-term assistance of the robot to increase toe clearance throughout the swing phase. The force was applied to the shank to generate knee flexion torque because the shank motion generated by the knee flexion motion makes the largest contribution to toe clearance. Four timings of the force application were considered: when starting knee flexion before toe-off, when lifting the foot, while maintaining knee flexion after toe-off, and when finishing knee flexion after toe-off. Furthermore, we evaluated changes in the toe trajectory and articular angles of the lower limb for each timing condition. We used a timing detection method for the robot conducting tensile force control based on information from the hip, knee, and ankle angles. For all participants, an increase in the knee flexion angle in the early swing phase due to the force application increased the toe clearance throughout the swing phase. We thus conclude that force application when the user begins lifting their toe is effective in increasing toe clearance throughout the swing phase.