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The critical heat flux (CHF) experiment for the reactor fuel assemblies (FAs) is the most difficult and important one in the reactor thermo-hydraulic study. The FA CHF experiments are performed on the large scale thermo-hydraulic test facilities with large DC power suppliers, complicated test sections and rigid test process. The results are analyzed with subchannel codes to obtain the parameters at the CHF point. Based on the experimental data, the CHF correlation is derived as the tool for the reactor safety analysis and design. After many years’ work, the common understanding of the FA CHF was established and the method of the FA CHF experiment becomes similar. This paper reviewed the progress of the FA CHF experiments, analyzed the modeling of the test sections, introduced requirement of the test facilities, and discussed the hypothesis in the modelling. The benchmark qualification test adopted to verify the test method and test section used in the experiment are discussed in the paper, too.

For the design verification and licensing of newly pressurized water reactors (PWRs), many integral test facilities were conducted. From the above integral test facilities, the obtained experimental results were adopted in the assessment and development of safety analysis codes. In this study, scaling analysis and design parameters of integral test facilities were summarized from the perspective of scaling method, basic scaling choice, equipment design, test matrix and application. The scaling method is the guideline for establishing scaling ratios of integral test facilities and different scaling methods were proposed. With the basic scaling choices of system pressure, geometrical parameters, working medium and equipment material, the design parameters of equipment were discussed. The test matrix and application of different integral test facilities were compared. This work is expected to sum up experience and provide guidance for the design and application of newly integral test facilities.

The thermal diffusion coefficient (TDC) is a dimensionless characteristic parameter to reflect the overall mixing characteristics in bundle channels. The effects of bundle geometries and spacer grids axial arrangement on TDC have not been systematically studied. Experimental study on the mixing characteristics of fuel assembly with 5×5 full-length bundles were carried out in two structural forms. The geometric parameters, mainly are the bundles radial power distribution and the form of spacer grids axial arrangement, which influence on the TDC of fuel assembly has been emphatically analysed. The experimental results show that the current FLLICA subchannel software cannot accurately simulate the transverse macroscopic flow in the subchannel matrix space. Without further optimization of subchannel software, the hot-cold bundles radial power distribution has a great influence on the TDC value of fuel assembly. The TDC value of fuel assembly is closer to true value when the hot-cold rods are centrally symmetrical. The mid-span mixing grid (MSMG) has a small enhancement effect on the thermal diffusion coefficient, and its contribution to the pressure drop of bundles is very low.

Safety systems play a key role in the nuclear industry. In the past decades, various safety systems have been designed and built to enhance safety level in nuclear power plants. The advanced secondary passive residual heat removal system (ASP) is one of several passive safety systems designed to ensure the safety of the reactor core. Experimental study on transient and steady-state characteristics as well as relevant influence factors of ASP system were conducted by using the high temperature high pressure thermal-hydraulic test facility. The results showed that the C-type heat exchanger could effectively remove the residual heat, and get stronger with the increase of loop pressure. The results also showed natural circulation could be set up with good stability, and get little influence from movement of isolating valves, relief valve as well as water volume in steam generator. Besides, to certificate the reasonability and reliability of the ASP system directly, an integral test was conducted.

The experimental investigation on transient heat transfer of supercritical water during depressurization in 2 × 2 bundle with wire wraps has been performed on the SWAMUP-II co-constructed by CGNPC and SJTU. The test section consists of two channels separated by a square steel assembly box with round corners. Water flows downward in the first channel and then turns upward in the second channel to cool the 2 × 2 bundle with wire wraps installed inside the assembly box. The bundle consists of four heater rods of 10 mm in O.D. arranged with pitch-to-diameter ratio of 1.18. Experimental parameter ranges cover pressure from 16 to 26 MPa, mass flux from 850 to 1450 kg/m 2 s, heat flux from 250 to 650 kW/m 2 , inlet fluid temperature from 345 to 365 °C, and depressurization rate of 1 and 2 MPa/min. The experimental data are obtained and heat transfer characteristics during depressurization from supercritical to sub-critical conditions are discussed. The fraction of heat transfer from inner channel to outer channel is lower than 15% under the test conditions. Four different transient heat transfer phenomena during depressurization are observed. The boiling crisis is likely to occur in SCWR during depressurization as the operating condition of SCWR is compared with the test condition. The boiling crisis is less likely to occur with the increase of mass flux, and more likely to occur with the increase of heat flux or inlet fluid temperature. The wall temperature increment during boiling crisis is smaller with higher mass flux, and larger with higher heat flux or inlet fluid temperature. The depressurization rate affects the transient heat transfer itself trivially.

An experimental study on transient heat transfer of supercritical water during pressure variation across critical pressure in a 2 × 2 rod bundle has been carried out on the Supercritical WAter MUltiPurpose test loop II (SWAMUP-II). The bundle consists of four heated rods with an Outer Diameter (O.D.) of 10 mm and a pitch-to-diameter ratio of 1.18. The 2 × 2 rod bundle with wire wrap is installed into a square assembly box. The experimental conditions are as follows: pressure ranging from 16 to 26 MPa, mass flux ranging from 850 to 1450 kg/m 2 s, heat flux ranging from 250 to 650 kW/m 2 , inlet fluid temperature ranging from 345 to 365 °C, and depressurization/pressurization rate of 1 MPa/min. The experimental data are obtained and the transient heat transfer characteristics during pressure variation across critical pressure are discussed. The heat transfer near the critical pressure is unstable as the physical properties of water changes fiercely, and the boiling crisis is likely to occur during both pressurization and depressurization, so the pressure range from 21 MPa to critical pressure is defined as a transition pressure section. The thermal-hydraulic parameters affect the transient heat transfer alike during both pressurization and depressurization, and obviously within the transition pressure section. The boiling crisis is more remarkable and the wall temperature increment is bigger with lower mass flux, higher heat flux or higher inlet fluid temperature. In order to avoid boiling crisis within the transition pressure section, the mass flux should be increase to or kept at higher level, the heat flux should be kept at or reduced fast to lower level, and the flow boiling should be kept at subcooled boiling.

Both experimental and theoretical works had been done to study the effects of mixture parameters on the performance of corrugated plate (CP) dryer. A water-air experimental branch had been established to simulate water-steam separation in a CP dryer. The effect of ambient relative humidity on the dryer experiment is analyzed. The diameter distribution of water droplets in air-water mixture at (CP) dryer entrance was measured by laser particle size analyzer. The effects of droplet diameter, moisture content and mixture flow velocity on the separation efficiency, pressure drop and the re-entrainment were studied. At the same time, a theoretical model for trajectory tracking of water particle in CP channel was established to illustrate experimental results. The conclusion shows that: 1) this effect of ambient relative humidity cannot be ignored or error could be introduced to experimental results. 2) The separation efficiency increases with the increase of inlet particle size, increases with the increase of inlet moisture content and increases first and then decreases with the increase of inlet velocity. 3) Critical airflow speed of re-entrainment decreases as the inlet moisture content increases. 4) The pressure drop is exponentially increasing with inlet airflow speed.

The newly designed advanced secondary passive heat removal system (ASP) was one of the most important cooling systems to cope with station blackout (SBO) accident in the generation II + 1000 MWe PWR nuclear power plant (NPP). Advanced secondary passive heat removal system test facility (ASPTF) was constructed to obtain the operation performance of ASP under SBO conditions. The geometrical scaling ratio of ASPTF is 1/4 in the height and 1/208 in the volume with respect to the generation II + 1000 MWe PWR. The drag coefficient experiments were conducted to match the prototypical resistance characteristics of ASP in this paper firstly. Then, a prolonged SBO accident sequence was experimentally investigated in ASPTF. The experimental results reveal that the secondary side pressure of steam generator (SG) maintained at the set value in the early stage with the reciprocating opening of vapor discharge to atmosphere valve (VDA). After the startup of ASP, the secondary side pressure of SG decreased and heat generated in core simulator was effectively removed by natural circulation through ASP within 3 h (h) of the design value. Three hours later, the flow rate of ASP became unstable with the baring of heat exchanger (HX) tube. The stable natural circulation of ASP was recovered after the injection of water in the water tank (WT) after 4 h. A sensitivity investigation on operation performance of ASP also was performed based on RELAP5 code. This study not only helps to improve the understanding of important thermal hydraulic phenomena in ASP, but also sheds some lights on operation optimization for the PWRs with ASP.

The formation and control method of the coolant free surface is one of the key technologies for the design of windowless targets in the Accelerator Driven Systems (ADSs). In this work, a new design of windowless target for ADS based on upward swirling flow is proposed to form a stable free surface without recirculation zones. Experimental and numerical investigations on the free surface flow have been performed in a prototypical windowless target using water as working fluid. The experimental results show that a stable free surface is achieved when the mass flow rate is higher than 4.0 m3/h and the shape of free surface profile greatly depends on the inlet flow rate. CFD simulations have been carried out using VOF method with different turbulent models. The numerical results show that the prediction of BSL Reynolds stress model agrees qualitatively well with the experimental results. The free surface profile of Lead–Bismuth-Eutectic (LBE) in the windowless target is very similar to that of water according to the simulation results. The feasibility of the new windowless target for ADS based on upward swirling flow is preliminary validated.

Experimental investigation was performed on the critical heat flux (CHF) in thin rectangular channels with very wide mass velocity range at low pressures. Different test sections were adopted with the different heated length and heated width. Both the vertical upward flow and downward flow were tested during the experiment. The results prove that the effect of the heated length on the CHF exists in the high mass velocity test but it is not very strong. The CHF rises with shorter heated length. At low mass velocities, the CHF at downward flow is much lower than that at upward flow. Empirical correlations were compared with the experimental data both from present and others’ studies. Empirical correlations provided by Katto and Gambill gave out good prediction for the low upflow and the high downflow respectively. The Sudo correlation had wide flow range but it overestimated at low mass velocity and underestimated at high mass velocity. A new CHF correlation for the large flow rate shows good agreement with the present and others’ studies. High pressure drop was found in the experiment at high flow rate conditions. It was analyzed and discussed in the paper.

An experimental investigation was performed on the density wave oscillation (DWO) with two parallel rectangular channels, which have a cross section of 25 mm × 2 mm and a heated length of 1000 mm. Test parameters are 1 MPa to 10 MPa for pressure, 200–800 kg/m 2 s for mass velocity, and 10–50 °C for inlet subcooling. The results show that in general the flow becomes more stable while mass velocity, pressure, and inlet subcooling are increased. The period of oscillation becomes shorter if mass velocity is increased or inlet subcooling is decreased. Pressure has little effect on period of the DWO in this research. The dimensionless subcooling number N sub and phase change number N pch were adopted to compare results from rectangular channels with those from round tubes. The comparison indicates that the data from rectangular channels agree with those from the round tubes. The RELAP5 software was used to simulate the DWO in rectangular channels. The prediction show good consistency with experimental phenomenon. However, different two-phase flow model behaves differently when pressure changes in prediction.

Natural circulation is one of the most important thermal-hydraulic phenomena that makes the fluid flow along a closed loop without any external driving force. With this merit, it is adopted by the passive heat removal system to bring the residual heat out of the core at accidents, and by the primary system of some new conceptual reactors instead of pumps to drive the coolant in the loop at operation. To investigate the reactor natural circulation and verify system thermal-hydraulic codes, it is a way to construct an integrated effect test facility and perform experiments on it with the scaling criteria. With one-dimensional assumption, the natural circulation system was simplified as the heat source, heat sink and pipes, and described by two groups of equations independently for the single-phase and two-phase flow conditions. Based on these equations, a set of non-dimensional equations were derived and the criteria were obtained both applicable for single-phase and two-phase natural circulation. According to these criteria, the practical application was analyzed and discussed. In the paper, the property similarity was strongly suggested in most cases. Though equal height simulation was widely used in the past, the reduced height simulation is a good way to reproduce three-dimensional (3D) phenomena that are of concern in the investigation. The CHF simulation is not suggested. The mass of metal and its distribution is of concern instead of heat transfer at transient simulation.

For some Pressurized Water Reactors (PWR) operated on automobiles, boats or deep sea vessels, natural circulation simulation is important for understanding their safety during severe accidents, or system characteristics when natural circulation is adopted instead of the primary pumps to drive coolant in their loops. Based on the scaling laws of natural circulation for stationary reactors, the scaling laws for moving conditions are derived in this paper by analyzing accelerations and their distribution in a moving reactor with rigid motion theory, and introducing these accelerations into the momentum equations representing a one-dimensional natural circulation model. With modified equations, a set of motion simulation criteria was obtained, and equal height simulation and unequal height simulation were studied. A reduced height simulation is helpful for ensuring that three-dimensional phenomenon are reproduced, but time scaling is needed in a motion simulation.