转运通道中燃料组件自然对流传热特性试验研究

针对燃料组件滞留转运通道期间的自然循环传热过程开展了试验研究。获得了承载器顶角区域加热棒的试验数据,并拟合出传热经验关系式。计算结果与试验结果比较表明,该关系式能较好地计算顶角区域加热棒顶部局部努塞尔数Nu。并通过试验数据证实了在相同的燃料棒热流密度和承载器进口水温条件下,最靠近承载器顶角位置的1号棒的传热能力最差,壁温最高。     

Preface: IAEA specialists' meetings on radiation damage units in graphite and ferritic and austenitic steel

A porous body model, new in its application for predicting temperature distributions in wire-wrapped fuel rod assemblies, has been developed. The model developed for thermal transport in wire-wrapped rod bundles is similar in principle to the one which has long been successfully used for heat transfer in fixed beds of packed solids. Although the model is applicable to bundles in forced and mixed (combined forced and free) convection, attention in this paper is confined to bundles operating in forced (negligible natural) convection only. The results obtained from this analysis were found to predict available data with as good a precision as does the more complex analysis.     

Effective thermal conductivity of a heat generating rod bundle dissipating heat by natural convection and radiation

A numerical study of conjugate natural convection and surface radiation in a horizontal hexagonal sheath housing 19 solid heat generating rods with cladding and argon as the fill gas, is performed. The natural convection in the sheath is driven by the volumetric heat generation in the solid rods. The problem is solved using the FLUENT CFD code. A correlation is obtained to predict the maximum temperature in the rod bundle for different pitch-to-diameter ratios and heat generating rates. The effective thermal conductivity is related to the heat generation rate, maximum temperature and the sheath temperature. Results are presented for the dimensionless maximum temperature, Rayleigh number and the contribution of radiation with changing emissivity, total wattage and the pitch-to-diameter ratio. In the simulation of a larger system that contains a rod bundle, the effective thermal conductivity facilitates simplified modelling of the rod bundle by treating it as a solid of effective thermal conductivity. The parametric studies revealed that the contribution of radiation can be 38–65% of the total heat generation, for the parameter ranges chosen. Data for critical Rayleigh number above which natural convection comes into effect is also presented.     

Natural convection/radiation heat transfer simulations of enclosed array of vertical rods

Abstract

Experiments performed by Arya and Keyhani (1990) measured the temperature of 12 vertical heated rods within a constant temperature, internally finned cylindrical enclosure. Measurements were performed with air and helium in the enclosure for ranges of rod heat generation rate and gas pressure. In the current work, steady three-dimensional computational fluid dynamics simulations of conduction, natural convection and radiation heat transfer within the experiment were conducted to benchmark the simulation techniques. In the computational model, different thermal conductivities were applied to a spacer plate between a plate that held the heaters, and one of the enclosure endplates. This was done to model a range of contact resistance between the plates. This was necessary because the experimental endplate conditions were not completely documented. The calculations accurately reproduced the local and average temperatures when a high contact resistance was modelled. These results emphasise that conditions far from data measurement locations can affect experimental results. Those conditions must be well documented if they are to be used to benchmark computational methods.     

CFD investigation of flow inversion in typical MTR research reactor undergoing thermal–hydraulic transients

Three dimensional CFD full simulations of the fast loss of flow accident (FLOFA) of the IAEA 10 MW generic MTR research reactor are conducted. In this system the flow is initially downward. The transient scenario starts when the pump coasts down exponentially with a time constant of 1 s. As a result the temperatures of the heating element, the clad, and the coolant rise. When the flow reaches 85% of its nominal value the control rod system scrams and the power drops sharply resulting in the temperatures of the different components to drop. As the coolant flow continues to drop, the decay heat causes the temperatures to increase at a slower rate in the beginning. When the flow becomes laminar, the rate of temperature increase becomes larger and when the pumps completely stop a flow inversion occurs because of natural convection. The temperature will continue to rise at even higher rates until natural convection is established, that is when the temperatures settle off. The interesting 3D patterns of the flow during the inversion process are shown and investigated. The temperature history is also reported and is compared with those estimated by one-dimensional codes. Generally, very good agreement is achieved which provides confidence in the modeling approach.     

Natural convection analyses and verification for LMFBR cores

Analytical model requirements for core natural convection analyses are reviewed. Then results from current modeling on intra-assembly flow and heat redistribution are compared with several sources of experimental data. Also, data are described on low flow rod bundle hydraulic characteristics. Numerous sensitivity studies are also presented which show the effect and importance of various parameters on core temperatures during natural circulation, including inter-assembly flow redistribution, pump flow coastdown, rod size and fuel type, control system scram worth and shutdown power level. A system of codes for making the natural circulation predictions is also described, i.e., a plant-wide dynamic code, a whole-core system dynamic code and a hot channel dynamic analysis code. The overall approach of verifying the core related codes is presented, along with the interaction and linkage between all the codes. Confirmation of this system of three codes will bee through prototypic data obtained from EBR-II and FFTF natural circulation experiments.     

Computation of heat transfer through a horizontally arranged HTR-fiber insulation

The heat transfer through a horizontally arranged ceramic fiber insulation operating at high pressure and temperature is studied theoretically. The contributions of heat conduction, natural convection and thermal radiation are considered. With an increasing value of the product of the Rayleigh and Darcy numbers, (Ra, Da), the natural convection is amplified, thus diminishing the efficiency of the thermal insulation. For high temperatures the thermal radiation is no longer negligible. The results of the computer code agree well with experiments available up to a temperature of 670 K and up to a pressure of 40 bar in helium or air. Finally, the heat transfer of a fiber insulation operating under high temperature reactor conditions is predicted, demonstrating that natural convection is nearly suppressed when an insulant bulk density of _b = 165 kg/m³ is provided.     

Indirect estimation method of the turbulence induced fluid force spectrum acting on a fuel rod

This paper proposes a methodology to identify a turbulent flow induced force acting on a nuclear fuel rod based on the indirect input force estimation theory in structural dynamics, which is useful to predict the forcing function when the input force cannot be measured directly. Since the nuclear fuel rod in a PWR (pressurized water reactor) is exposed to coolant flow, the turbulence induced force generates a fuel rod vibration which may cause a fretting wear on the surface of the rod. This study develops a method to estimate turbulence induced force spectrum indirectly for a real scale fuel rod loaded in a nuclear fuel test facility. The proposed method requires a reliable finite element (FE) model which simulates the fuel rod dynamics well; therefore, the FE model is discussed, especially regarding the procedure to determine the effective rod density. Since the pellets rattle inside the tube due to small gaps between the tube and pellets, especially at the beginning of the fuel's life, the contribution of the pellet mass to the density for the FE model cannot be determined clearly. It is shown that the appropriate density can be estimated by comparing the natural frequencies from the modal test results of the rod (with pellet) and the tube (without pellet). Then, the indirect turbulence induced force estimation theory is applied to the fuel rod, and some numerical and test results are discussed to verify the applicability of the suggested method.     

Estimation of fuel and clad temperature of a research reactor during dry period of de-fuelling operation

Heat transfer and fluid flow studies related to spent fuel bundle of a research reactor in fuelling machine has been carried out. When the fuel is in reactor core, the heat generated in the fuel bundle is removed by heavy water under normal reactor operation. However, during the de-fuelling operation, the fuel bundle is exposed to air for some period called dry period. During this period, the decay heat from fuel bundle has to be removed by air flow. This flow of air is induced by natural convection only. In this period, the temperatures of fuel and clad rise. If clad temperature rises beyond a certain limit, structural failure may occur. This failure can result into release of fission products from fuel rod. Hence the temperature of clad has to be within specified limit under all conditions. The objective of this study is to estimate the clad temperature rise during the dry period.In the CFD simulation, the turbulent natural convection flow over fuel and radiation heat transfer are accounted. Standard k-? model for turbulence, Boussinesq approximation for computing the natural convection flow and IMMERSOL model for radiation are used.The steady state and transient CFD simulation of flow and heat is performed, using the CFD code PHOENICS. The steady state analysis provides the maximum temperature the clad will attain if fuel bundle is left exposed to air for sufficiently long time. For safe operation, the clad temperature should be limited to a specified value. From steady state CFD analysis, it is found that steady state clad temperature for various decay powers is higher than the limiting value. Hence transient analysis is also performed. In the transient analysis, the variation of clad temperature with time is predicted for various decay powers. Safe dry time, i.e. the time required for clad to reach the limiting value, is predicted for various decay powers. Determination of safe dry time helps in deciding the time available to the operator to drop the bundle in light water pool for storage. The analysis is found useful in optimizing the de-fuelling process.     

棒束通道内混合对流传热实验研究

以自然循环下堆芯内可能会发生的低流量传热为研究背景,对5×5棒束通道内的混合对流传热现象进行了实验研究。实验压力为6 MPa, 质量流量为25~150 kg/(m2·s),热流密度为25~300 kW/m2,实验雷诺数Re为1000~30000,浮升力参数Bo*为2×10-7~3×10-3。实验发现,随着Bo*的增大,棒束通道内传热产生先弱化后强化的趋势。浮升力对棒束通道内传热造成影响的起始点为Bo*=3.5×10-6,当Re >15000时,浮升力依然可对传热造成弱化现象。基于实验数据,提出了适用于棒束通道的混合对流经验关系式。      

Simple Model for Analysis of Circumferential Ridging Deformation of Fuel Rods

A simple model for analysis of fuel rod ridging is proposed. In this model, a piece of radially cracked pellet is considered as a beam, and cladding as a tube shell, allowing fuel rod ridging to be analyzed by applying the beam and shell theories. Ridging height and contact force between the pellet piece and the cladding tube are expressed in a relatively simple form as a function of elastic constants of the pellet and cladding, temperatures, dimensional parameters, etc. Effects of fuel design parameters on the fuel rod ridging are evaluated using this model.     

After heat removal by a closed secondary cooler circuit on the example of the advanced high temperature reactor AHTR 500 with central graphite column

The application of natural convection in connection with an after heat removal concept in general supports the claim for an inherent safety concept for advanced high temperature reactors (HTR). The effectivity of such an after heat removal (AHR) concept will be explored exemplarily for the process-heat reactor AHTR 500 with central graphite column by a thermohydraulic simulation of a secondary cooler circuit which is thermally connected with the primary circuit. This coupling is undertaken by an AHR-cooler located in the upper part of the graphite column. The heat removal from the secondary circuit is taking place outside of the reactor by a secondary heat exchanger under the assumption that the latter is cooled by a water capacity flow on an ambient temperature level. The developed calculation model determines iteratively the dynamic and thermal positions of equilibrium in the primary and secondary circuit which in the after heat removal mode of operation are exclusively run by natural convection. Different types of design for the central column heat exchanger (coaxial tube, U-tube and helically coiled tube heat exchanger) have been compared. For the secondary heat exchanger a parallel tube design has been supposed. The choice of the secondary flow medium as well as the most important limiting quantities influencing the transmission of heat via the secondary circuit during the after heat removal mode of operation are subject of a parameter study.     

Experimental investigation of turbulent transport of momentum and energy in a heated rod bundle

Turbulent air flow in a central channel of heated 37-rod bundles with triangular array at two different pitch-to-diameter ratios (P/D=1.12 and P/D=1.06) was investigated. Measurements were performed with a hot-wire probe with x-wires and an additional temperature wire. Time mean velocities, time mean fluid temperatures, wall shear stresses and wall temperatures, turbulent quantities such as the turbulent kinetic energy, all Reynolds stresses and all turbulent heat fluxes were measured at two different pitch-to-diameter ratios in a central channel of the bundle. It is shown that with decreasing gap width the turbulence field in rod bundles deviates significantly from that in a circular tube. Also, data on the power spectral density functions of the velocity and temperature fluctuations are presented. These data show the existence of large-scale periodic fluctuations of velocity and temperature in the gap region of two adjacent rods. These fluctuations are responsible for the high intersubchannel heat and momentum exchange. Spectral measurements with two hot wire probes imply a distinct similarity of motion of vortices in adjacent subchannels of the bundle.     

On LBE natural convection and its water experimental simulation

This paper deals with stability and distribution of coolant flow in a pool type of lead-bismuth eutectic (LBE) cooled accelerator driven system (ADS) under natural convection conditions. Two approaches are proposed for solving the problems, namely, (i) theoretical model for stability analyses; (ii) water experimental simulation for the flow distribution in the natural convection state.The stability analysis is carried out based on a simplified integral flow model, where hot and cold free surface oscillations are taken into account. There is a transition from an absolutely stable state under the forced convection condition to a damped oscillation state under the natural convection condition.Reynolds, Richardson (Grashof) and thermal power similitude laws are revealed for the water experimental simulation. The water model scale, power and other factors can be determined by the similitude laws. By choosing the inlet and outlet temperatures even the local Reynolds number law can be approximately satisfied. By the variation of the inlet temperature in the water experiment several similitude solutions are presented. In particular, they are shown in detail for the model scale one to one. It is concluded that the water experiment can be applied for the simulation of LBE natural convection flow in a pool type of reactor.     

Power–feedwater temperature operating domain for a BWR driven by natural convection

The aim of this work is to analyze feedwater temperature effects on reactor power in a BWR driven by natural convection (NCBWR) applying a methodology based on Monte Carlo's simulation. The analysis was performed with the reactor operating with different control rod patterns. The Monte Carlo's methodology was applied systematically to establish the operating domain, due that NCBWRs are not yet in operation, the analysis of the nuclear and thermal–hydraulic processes must rely on numerical modeling with the purpose of developing or confirming the design basis. The relative standard deviation obtained with a size of 20,000, data showed that for 2% change in the feedwater temperature, the response in thermal power change is 6.5% for 100% of rated power and 100% of rod pattern. Additionally five more rod patterns are presented in this paper. The regression analysis with the feedwater temperature showed statistically valid linear, quadratic and cubic representations, which are described in this paper for six different arrangements of reactor control rods. The results obtained in this study can be applied to operation and design of NCBWR.     

Flow mixing inside a control-rod guide tube – Experimental tests and CFD simulations

This paper covers a combined experimental and computational effort carried out at Vattenfall Research & Development AB in order to study the thermal mixing in the annular region between a top tube and a control-rod stem. The low frequency thermal fluctuations in this region can result in problems with thermal fatigue and have caused cracks in the control-rod stems of several nuclear reactors ( [Kobayashi et al., 2009] and [0070] ).The flow in the vertical annular region formed by the top tube and the control-rod stem is characterized by the mixing of hot bypass flow with cold crud-removal flow. The crud-removal flow is flowing upwards along the control-rod stem, and the warmer bypass flow is entering through eight horizontal holes positioned in the lower part of the guide tube and four holes in the upper part of the top tube, forming jets.Two full-scale models of a control rod, including the control-rod stem and the guide tube, were constructed. The first model, designed to work at atmospheric conditions, was made of Plexiglass, in order to be able to visualize the mixing process, whereas the second one was made of steel to allow for a higher temperature difference between the two flows, and the heating of the top tube.CFD simulations of the case at atmospheric conditions were also carried out.Both the experiments and the simulations showed that the mixing region between the cold crud-removal flow and the warm bypass flow is dominated by large flow structures coming from above. The process is characterized by low frequency, high amplitude temperature fluctuations. The process is basically hydrodynamic, caused by the downward transport of flow structures originated at the upper bypass inlets. The damping thermal effects through buoyancy is of secondary importance, as also the scaling analysis shows, however a slight damping of the temperature fluctuations can be seen due to natural convection due to a pre-heating of the cold crud-removal flow. The comparison between numerical and experimental results shows a rather good agreement, indicating that experiments with plant conditions are not necessary since, through the existing scaling laws and CFD calculations, the obtained results may be extrapolated to plant conditions.The problem of conjugate heat transfer has not yet been addressed experimentally since complex and difficult measurements of the heat transfer have to be carried out. These types of measurements, along with corresponding CFD calculations, constitute one of the main challenges to be dealt with in ongoing work.     

Experimental study on convective heat transfer of water flow in a heated tube under natural circulation

This paper reports on an experimental study on transitional heat transfer of water flow in a heated vertical tube under natural circulation conditions. In the experiments the local and average heat transfer coefficients were obtained. The experimental data were compared with the predictions by a forced flow correlation available in the literature. The comparisons show that the Nusselt number value in the fully developed region is about 30% lower than the predictions by the forced flow correlation due to flow laminarization in the layer induced by co-current bulk natural circulation and free convection. By using the Rayleigh number Ra to represent the influence of free convection on heat transfer, the empirical correlations for the calculation of local and average heat transfer behavior in the tube at natural circulation have been developed. The empirical correlations are in good agreement with the experimental data. Based on the experimental results, the effect of the thermal entry-length behavior on heat transfer design in the tube under natural circulation was evaluated.     

A model for the performance of a vertical tube condenser in the presence of noncondensable gases

Some proposed vertical tube condensers are designed to operate at high noncondensable fractions, which warrants a simple model to predict their performance. Models developed thus far are usually not self-contained as they require the specification of the wall temperature to predict the local condensation rate. The present model attempts to fill this gap by addressing the secondary side heat transfer as well. Starting with a momentum balance which includes the effect of interfacial shear stress, a Nusselt-type algebraic equation is derived for the film thickness as a function of flow and geometry parameters. The heat and mass transfer analogy relations are then invoked to deduce the condensation rate of steam onto the tube wall. Lastly, the heat transfer to the secondary side is modelled to include cooling by forced, free or mixed convection flows. The model is used for parametric simulations to determine the impact on the condenser performance of important factors such as the inlet gas fraction, the mixture inlet flowrate, the total pressure, and the molecular weight of the noncondensable gas. The model performed simulations of some experiments with pure steam and air-steam mixtures flowing down a vertical tube. The model predicts the data quite well. The model described also provides a basis under which the presence of aerosol particles in the gas stream could be analyzed.     

Two-dimensional simulations of natural convection/radiation heat transfer for BWR assembly within isothermal enclosure

Abstract

The current scoping study identifies the significant heat transfer effects for a 7 × 7 boiling water reactor (BWR) assembly within an isothermal basket opening inside a transport cask. A two-dimensional finite volume mesh is constructed that models the assembly components and cover gas. Computational fluid dynamics (CFD) simulations calculate the buoyancy induced gas motion, conduction and radiation within the components. Simulations use different basket surface temperatures, fuel heat generation rates and cladding surface emissivities, for both nitrogen and helium cover gases at atmospheric pressure. An analytical conduction/radiation model is developed for the thermal resistance between the channel and basket. Results using buoyancy induced gas motion compared to stagnant gas simulations show that natural convection is significant only at low basket temperatures, with nitrogen gas. Helium and high basket temperature simulations exhibit no significant temperature reduction from natural convection. Simulations with varying cladding emissivity ? show that a 10% increase in ? causes a 7˙2% decrease in the interior temperature difference for nitrogen and a 5˙3% decrease for helium.     

Natural convection heat transfer of water on a horizontal downward facing stainless steel disk in a gap under atmospheric pressure conditions

An experimental study on natural convection heat transfer on a horizontal downward facing heated surface in a water gap has been carried out under atmospheric pressure conditions. A total of 7204 experimental data points are correlated using Rayleigh versus Nusselt number correlations in various forms, based on different independent variables. The effects of different characteristic lengths and film temperatures are discussed. The buoyancy force acts as a resistance force for natural convection heat transfer on a downward facing horizontal heated surface in a confined space. For the estimation of the natural convection heat transfer under the present conditions, empirical correlations in which Nusselt number is expressed as a function of Rayleigh number, or Rayleigh and Prandtl numbers both, may be used. However, the best accuracy is provided by an empirical correlation which expresses the Nusselt number as a function of the Rayleigh and Prandtl numbers, as well as the gap width-to-heated surface diameter ratio; and uses the temperature difference between the heated surface and the ambient fluid in the definition of Rayleigh number. The characteristic length is the gap size and the film temperature is the average fluid temperature.