[en] A valveless pump consisting of a pumping chamber with an elastic tube was simulated using an immersed boundary method. The interaction between the motion of elastic tube and the pumping chamber generated a net flow toward the outlet through a full cycle of the pump. The net flow rate of the valveless pump was examined by varying the stretching coefficient, bending coefficient, and aspect ratio of the elastic tube. Photographs of the fluid velocity vectors and the wave motions of the elastic tube were examined over one cycle of the pump to gain a better understanding of the mechanism underlying the valveless pump. The relationship between the gap in the elastic tube and the average flow rate of the pump was analyzed

[en] Core make-up tank (CMT) is full of borated water and provides makeup and boration to the reactor coolant system (RCS) for early stage of loss of coolant accident (LOCA) and non-LOCA. The top and bottom of CMT are connected to the RCS through the pressure balance line (PBL) and the safety injection line (SIL), respectively. Each PBL is normally open to maintain pressure of the CMT at RCS, and this arrangement enables the CMT to inject water to the RCS by gravity when the isolation valves of SIL are open. During CMT injection into the Reactor, the condensation and thermal stratification are observed in CMT and the rapid condensation disturbed the injection operation. The optimal design of the flow distributor is very important to ensure structural integrity of the reactor system and their safe operation during some transient or accident conditions. In the present study, we numerically investigated the performance of flow distributor in tank with different shape factor such as the total number of the holes, the pitch-to-hole diameter ratios (p/d), the diameter of the hole and the area ratios. These data will contribute to the design the flow distributor. In the present study, the model of the flow distributor in tank is simulated using the commercial CFD software, Fluent 13.0 with varying the different shape factor of the flow distributor such as the total number of the holes, the diameter of the holes and the area ratio. As the diameter of the hole is smaller, the velocity difference between holes, which is located at upper position and lower position of the flow distributor, also decreases. For larger area ratio, the velocity of the holes is slower. When the diameter of the hole is large enough for the velocity difference between holes to be large, however, the velocity of the holes is not in inverse proportional to the area ratio

[en] In general the integral reactor has such characteristics, the integral reactor requires a high flow rate of coolant safety injection at the initial stage of the accident in which the core level is relatively fast decreased, A medium flow rate of coolant safety injection at the early and middle stages of the accident in which the coolant discharge flow rate is relatively large due to a high internal pressure of the reactor vessel, and a low flow rate of coolant safety injection is required at the middle and late stages of the accident in which the coolant discharge flow rate is greatly reduced due to a decreased pressure of the reactor vessel. It is noted that a high flow rate of the integral reactor is quite smaller compared to a flow rate required in the commercial loop type reactor. However, a nitrogen pressurized safety injection tank has been typically designed to quickly inject a high flow rate of coolant when the internal pressure of the reactor vessel is rapidly decreased, and a core makeup tank has been designed to safely inject at a single mode flow rate due to a gravitational head of water subsequent to making a pressure balance between the reactor vessel and core makeup tank. As a result, in order to compensate such a disadvantage, various type systems are used in a complicated manner in a reactor according to the required characteristic of safety injection during an accident. In the present study, we have investigated numerically the performance of the multi stage safety injection tank. A parameter study has performed to understand the characteristics of the multi stage safety injection tank. The performance of the multi stage safety injection tank has been investigated numerically. When an accident occurs, the coolant in the multi stage safety injection tank is injected into a reactor vessel by a gravitational head of water subsequent to making a pressure balance between the reactor and tank. At the early stages of the accident, the high flow rate of coolant in the multi stage safety injection tank is injected into a reactor. Subsequently, a medium flow rate of coolant is injected at the early and middle stages of the accident

[en] A core makeup tank using a gravitational head of water subsequent to making a pressure balance between the reactor and tank and a nitrogen pressurized safety injection tank are used in the passive loop type reactors such as AP600, AP1000 and so on. A nitrogen pressurized safety injection tank has been typically designed to quickly inject a high flow rate of coolant in a safe manner when the internal pressure of the reactor vessel is rapidly decreased due to a large break loss of coolant accident, and a core makeup tank has been designed to safely inject into the reactor at high pressure for the early stages of the accident. The pressure balance type of safety injection tank is designed to have proper height difference from the injection nozzle on the reactor vessel, and it is connected to the reactor vessel through the injection line and pressure balance line. The pressure balanced type of safety injection tank is isolated from the reactor vessel by the isolation valves on the pressure balance line and the check valves on the injection line during normal operation. By opening the isolation valves on the pressure balance line, the cooling water in the safety injection tank can be injected into the reactor by the gravitational head when the steam from the reactor coolant system is injected into the safety injection tank through the pressure balance line and the internal pressure of the reactor coolant system and safety injection tank reaches the equilibrium state. In the present study, a parametric study has been performed to understand the injection characteristics of the balance type of safety injection tank. A sensitivity analysis has been performed to investigate the effect of the tank shape on injection flow rate and depletion time of the safety injection tank, changing the safety injection tank diameter and total height. The initial injection flow rate of the safety injection tank increased with the tank total height.

[en] Highlights: • Performance analysis of a multi-stage safety injection tank by a numerical method. • MSIT can inject coolant from a single tank in multiple stages during a LOCA. • Effects of break size, heights, and pressure-loss coefficients of injection lines. • MSIT can provide continuous coolant injection without problems of delays. A performance analysis of a multi-stage safety injection tank (MSIT) was carried out using a numerical method. A MSIT is capable of injecting coolant from a single tank in multiple stages in the event of a loss of coolant accident (LOCA) at a nuclear power plant. The performance and consequences of two- and three-stage MSITs were compared. The effects of break size, installation heights of the injection lines, and pressure-loss coefficients of each injection line were scrutinized. By selecting pertinent design parameters, the MSIT can satisfy its design requirements, by providing continuous coolant injection without problems of delays or overlaps for a minimum of 72 h after a LOCA.

[en] Flow distributors are generally observed in several nuclear power plants. During core make-up tank (CMT) injection into the reactor, the condensation and thermal stratification are observed in the CMT, and rapid condensation disturbs the injection operation. To reduce the condensation phenomena in the tank, CMT was equipped with a flow distributor. The optimal design of the flow distributor is very important to ensure the structural integrity the CMT and its safe operation during certain transient or accident conditions. In the present study, we numerically investigated the performance of a flow distributor in tank with different shape factors such as the total number of holes, pitch-to-hole diameter ratios, diameter of the hole, and the area ratios. These data will contribute to a design of the flow distributor

[en] An immersed boundary (IB) method for simulating a three-dimensional valveless pump is described. The valveless pump is treated as an elastic tube connected at its ends to a rigid tube. The governing equation for the motion of the elastic tube is derived by using the variational derivative of the deformation energy. Our method is based on an efficient Navier-Stokes solver that uses the fractional-step method and a staggered Cartesian grid system. The fluid motion defined on an Eulerian grid and the structure motion defined on a moving Lagrangian grid are independently solved, and their interaction is formulated by using momentum forcing. A net flow is generated inside the valveless pump through the periodic pinching of the elastic tube at a position that is asymmetric with respect to its ends. Two valveless pumps are chosen, a single valveless pump and a double valveless pump. The effects on the average flow rate of varying the pinching frequency and the pinching position were investigated. The interaction between the wave dynamics and the inertia of the returning flow was examined for a closed loop system.

[en] Highlights: ► Dynamics of a capsule in moderate Re Poiseuille flow were explored numerically. ► Capsule tends to tumbling motion for larger membrane elasticity and higher Re flow. ► Capsule undergoes swinging motion for larger size and aspect ratio of the capsule. ► Capsule tends to migrate to a specific lateral equilibrium as Re increases. ► Equilibrium position varies differently around the transition of the dynamic motion. - Abstract: The dynamic motions and lateral equilibrium positions of a two-dimensional elastic capsule in a Poiseuille flow were explored at moderate Reynolds number (10 ⩽ Re ⩽ 100) as a function of the initial lateral position (y₀), Re, aspect ratio (ε), size ratio (λ), membrane stretching coefficient (φ) and bending coefficient (γ). The transition between tank-treading (TT) and swinging (SW) to tumbling (TU) motions was observed and the lateral equilibrium positions of the capsules varied according to the conditions. The initial behavior of the elastic capsule was influenced by variation in the initial lateral position (y₀), but the equilibrium position and dynamic motion of the capsule were not affected by such variation. The capsules had a stronger tendency toward TU motion at higher values of Re, φ and γ, whereas the capsules underwent TT or SW motion as the values of ε and λ increased. Under moderate Re Poiseuille flows, capsules tended to migrate across streamlines to a specific equilibrium position. The lateral equilibrium position shifted toward the centerline at larger λ and migrated toward the wall at larger ε,φandγ. As Re increased, the equilibrium position first shifted toward the bottom wall, then toward the channel center. However, different equilibrium position trends were obtained around the SW–TU transition. The capsule undergoing TU motion tended to migrate downward toward the bottom wall more than the capsule undergoing SW motion, all other conditions being similar.

[en] Highlights: ► Dynamics of a capsule in moderate Re Poiseuille flow were explored numerically. ► Capsule tends to tumbling motion for larger membrane elasticity and higher Re flow. ► Capsule undergoes swinging motion for larger size and aspect ratio of the capsule. ► Capsule tends to migrate to a specific lateral equilibrium as Re increases. ► Equilibrium position varies differently around the transition of the dynamic motion. - Abstract: A valveless pump consisting of a pumping chamber with an elastic tube was simulated using an immersed boundary (IB) method. The interaction between the motion of the elastic tube and the pumping chamber generated a net flow toward the outlet throughout a full cycle of the pump. The net flow rate of the valveless pump was examined by varying the stretching coefficient (φ), bending coefficient (γ), the aspect ratio (l/d) of the elastic tube, and the frequency (f) of the pumping chamber. As the stretching and bending coefficients of the elastic tube increased, the net flow through the valveless pump decreased. Elastic tubes with aspect ratios in the range of 2 ⩽ l/d ⩽ 3 generated a higher flow rate than that generated for tubes with aspect rations of l/d = 1 or 4. As the frequency of the pumping chamber increased, the net flow rate of the pump for l/d = 2 increased. However, the net flow rate for l/d = 3 was nonlinearly related to the pumping frequency due to the complexity of the wave motions. Snapshots of the fluid velocity vectors and the wave motions of the elastic tube were examined over one cycle of the pump to gain a better understanding of the mechanism underlying the valveless pump. The relationship between the average gap in the elastic tube and the average flow rate of the pump was analyzed. A smaller gap in the elastic tube during the expansion mode and a wider gap in the elastic tube during the contraction mode played a dominant role in generating a high average flow rate in the pump, regardless of the stretching coefficient (φ), the aspect ratio (l/d) of the elastic tube, or the pumping frequency of the pumping chamber (f).