[en] The fission gas produced by the Thorium Molten Salt Reactor (TMSR) can be removed with the bubbling method based on the theory of interfacial mass transfer. During this process, bubbles will go across several elbow pipes inevitably, leading to additional difficulties in estimating the mass transfer rate. By now, literature concerning this topic are quite limited so this paper is mainly focus on the phase distribution and evolution of local bubble phase parameters like interfacial area concentration (IAC) inside the elbow. To do this, a series of high-speed camera shooting experiments are performed at a 90° vertical-upwards elbow area. The experiments give qualitative results of bubble distribution with different void fractions and Reynolds numbers. Then a three-dimensional numerical simulation is carried out. By comparing the numerical results and experimental data, we can find that the phase distribution has a coincidence, so the three-dimensional two phase flow structure inside elbow is revealed and the bubble distribution characteristics can be accounted. Bubbles tend to accumulate at the inner side of elbow and display a 'bimodal-type' distribution at the outlet due to a near-wall vortex pair. Thus the local IAC shows a great fluctuation across the elbow, which may weaken the mass transfer efficiency inside the elbow in some way. (author)

[en] In this study, the bubble separation behavior in a gas–liquid separator is numerically investigated on the basis of the Euler–Lagrange approach, in which the forces acting on bubbles in a swirling flow field are modeled to calculate the trajectories of the bubbles. By adopting this approach, the effects of five parameters, namely, back pressure, Reynolds number, bubble diameter, void fraction, and swirl number, on separation performance in terms of pressure loss, separation efficiency, separation length, and split ratio are computed and analyzed. On the basis of the analysis, correlations of separation length with the two main parameters are established, which can serve as a basis for the optimal design of separator.

[en] When the claddings of the PWR fuels breach, the fission gas accumulated in the gap of pellets and claddings releases into the coolant. Naturally, the process of fission gas release is a process of two-phase flow, while the microscopic mechanics is still undiscovered. To reveal the law of the interaction between the coolant and the gas during the process of fission gas release, a CFD method is built to simulate the transient process of fission gas release, in which VOF and k-ε model is used. The result shows that the coolant will flow into pellet-cladding gap, then it will be evaporated, which will lead to the increasing of the pressure in the gap, and the fission gas will release into the subchannel;the processing that the fission gas releases from the pellet-cladding gap can be divided into 2 steps. In step 1, the differential pressure between the pellet-cladding gap and the subchannel is relatively larger, which makes the gas jet into the subchannel, and the duration of this step is short, and the ratio of the release of the fission gas is higher, meanwhile, it changes violently. In step 2, the difference of the pressure between the pellet-cladding gap and the subchannel is relatively smaller and stable, and the fission gas enters the subchannel by the convective mass transfer through the vortex in the breach. The duration of this step is short, and the ratio of the release of the fission gas is lower and stable. (authors)

[en] Venturi tube is based on turbulent flow, whereby the microbubbles can be generated by the turbulent fragmentation. This phenomenon is common in several venturi bubblers used by the nuclear, aerospace and chemical industries. The first objective of this paper is to study the liquid-phase velocity field experimentally and develop correlations for the turbulent quantities. The second objective is to research velocity field characteristics theoretically. Stereoscopic PIV measurements for the velocity field have been analyzed and utilized to develop the turbulent kinetic energy in the venturi tube. The tracking properties of the tracer particles have been verified enough for us to analyze the turbulence field. The turbulence kinetic energy has a bimodal distribution trend. Also, the results of turbulence intensity along the horizontal direction is gradually uniform along the downstream. Both the mean velocity and the fluctuation velocity are proportional to the Reynolds number. Besides, the distribution trend of the mean velocity and the velocity fluctuation can be determined by the geometric parameters of the venturi tube. An analytical function model for the flow field has been developed to obtain the approximate analytical solutions. Good agreement is observed between the model predictions and experimental data

[en] In this paper, the numerical simulation for the gas–liquid flow in a separator applied in the fission gas removal system for thorium molten salt reactor was investigated. The numerical model was established in the frame of Eulerian–Eulerian approach, in which the modeling of the forces acting on the bubbles was introduced. Based on the model, numerical simulations with three flow rates were carried out. Three key parameters (the pressure loss, the separation length, the liquid entrainment ratio) concerned with the separation performance were compared between the numerical results and the experimental data, the results indicate that the calculated results agree well with the experimental data. Hence, the numerical approach shows a promising tool for the performance prediction and the optimization of the gas–liquid separator. (author)

[en] Background: Canned nuclear coolant pump is welded to the steam generator and main pipe line. The pressure fluctuation generated in the pump may affect both the steam generator and main pipe line. Purpose: The research was conducted for the purpose of studying the characteristics of pressure fluctuation at inlet and outlet of the nuclear coolant pump. Methods: The frequency spectra of pressure signal located at inlet and outlet of the nuclear coolant pump were obtained with CFD (Computational Fluid Dynamics). Results: The result showed that the pressure fluctuation of the inlet position was dominated by the blade passing frequency, and the pressure fluctuation of the outlet position was much more complex. It was influenced by not only the blade passing frequency but also the axis frequency. The counter-flow in the diffuser and flow separation also influenced the outlet pressure fluctuation. Conclusion: CFD is suitable for canned nuclear coolant pump pressure fluctuation analysis. (authors)

[en] [Background] When fuel rod cladding failure occurs, the defect needs to be determined and information needs to be provided for system response in time. Escape rate of fission gas is adopted to reflect the defect size. However, there is hardly researches on mechanisms of gas release. [Purpose] This paper studies the effect of unsteady process, the pressure and temperature of the coolant on the instantaneous escape rate of the rod cladding breach by experimental simulation. [Methods] First of all, the experimental apparatus was designed on the base of geometric similarity, flow similarity and flashing similarity. Then, influence of coolant pressure and temperature in a subchannel was investigated by using computational fluid dynamics (CFD) methods. Effect of flashing on unsteady process and escape rate in short-term were analysed. [Results] The results indicate that unsteady process has no distinct effect on release of gas at defect size of 0.5 mm. Long-term escape rate remains steady during experiments and release process can be described by first-order kinetics. At the same coolant pressure, long-term escape rate increases when coolant temperature changes from 90℃ to 110℃. At the same coolant temperature, long-term escape rate decreases when coolant pressure changes from 0.3 MPa to 0.5 MPa. [Conclusions] A negative correlation is found between long-term escape rate and subcooled temperature, which indicates that liquid layer formed on the defect has impact on fission gas release. (authors)

[en] Abstract Background: One advantage of the liquid fueled Thorium Molten Salt Reactor (TMSR) is that the fuel can be burned up deeply with the neutron poisonous gas removal system that can be achieved by bubbling degassing approach. In order to quantify the separation efficiency for the separator and provide useful guidelines to determine the dimensions for the separator, the separation trajectories of bubbles with different sizes in the swirl flow inside the separator are needed. Purpose: This study aims to develop an alternative method to predict the bubble's motion which can avoid the troubleshooting induced by the two-phase simulation. Methods: The Lagrangian approach is modified to obtain the continuous phase velocity by a single phase simulation, and curve fitting is applied to further approximation of this velocity. Combing the known velocity distribution with explicit mathematical expression and the interaction model for the gas-liquid interface, a mathematical model to calculate the bubble motion is well posed. Results: A comparison between the numerical results and the experimental data indicates that the result agrees well with each other. As for the separation length concerned, the maximal error is less than 20%. Conclusion: It has theoretical guidance meaning for research on the movement of bubbles in cyclone separator. And it can be used to guide the determination of swirl chamber space. (authors)

[en] When an over-flow condition occurs in a PWR nuclear power plant, the venturi flowmeter downstream of the make-up pump is required to form a cavitation-induced choked flow to protect the flowrate from exceeding the limit. Using the method of FLUENT numerical simulation and high-speed camera experiments, three different cavitation models were used to study the cavitation-induced choked flow phenomenon, cavitation development law and flow characteristics of the venturi tube. The results show that the ZGB cavitation model and the SST k-ω turbulence model can be used to simulate the cavitation flow limitation phenomenon of the venturi tube accurately. During the flow limitation, a periodic cavitation phenomenon will occur inside the Venturi tube. Under the action of the wall re-entrant jet, micro flow behaviors, such as small bubbles falling off, tail bubbles falling off and the whole vapor cavities falling off, will happen. (authors)

[en] The online separation of the neutron poison in MSR is important to increase the burn-up level. A serpentuator used for the online separation is proposed in this paper to make full use of mass transfer between bubbles and liquid. In order to establish a valid numerical simulation method to predict the mass transfer rate of bubbles in a high Reynolds number flow. Two-fluid Model is used in this paper to study the performance of the eddy-cell model and the slip-penetration model; moreover, the variation of the mass transfer efficiency over the flow parameter is also studied and discussed in this paper. The results show that the mass transfer efficiency predicted by the eddy-cell model is close to that of the slip-penetration model; local mass transfer efficiency is enhanced by a larger bubble. (authors)