Fenglei Niu

Thermal–Hydraulic Performance Analysis for AP1000 Passive Containment Cooling System

In order to improve the safety of new generation nuclear power plant, passive containment cooling system is innovatively used in AP1000 reactor design. However, since the system operation is based on natural circulation, physical process failure — natural circulation cannot establish or be maintained — becomes one of the important failure modes. Uncertainties in the physical parameters such as heat and cold source temperature and in the structure parameters have important effect on the system reliability. In this paper, thermal–hydraulic model is established for passive containment cooling system in AP1000 and the thermal–hydraulic performance is studied, the effect of factors such as air temperature and reactor power on the system reliability are analyzed.Copyright

Analysis of the Thermal Hydraulic Phenomena Caused by Steam Jets in AP1000 Containment

In order to get a better analysis of the thermal hydraulic phenomena related to steam jets in AP1000 containment, a model based on the actual AP1000 containment size has been used to simulate the steam jet injection at different positions with different velocities of the containment. ANSYS CFX is used to get the temperature profiles of the ambient fluid. From the results of the simulation, stratification can be expected in the gas spaces of the containment, because the temperature only changes along with the vertical height, due to the heat or mass transfer processes between the air and the steam jets. It can be predicted that the hot air rises and eventually stratification forms, with lighter air overlying the heavier air. In this paper, strong and weak steam jet injections have been both simulated by ANSYS CFX, however, it turned out that stable stratification phenomena can only be found in weak steam jet injections, strong injections can’t form obvious stratification. This is very important for the design and operation of AP1000 containment, and can help us to study the long-term containment behavior during LOCAs.Copyright

A New Model Towards Thermal Mixing and Stratification in Passive Containment

Thermal mixing and stratification often appears in passive containment cooling system (PCCS), which is an important part of passive safety system. So, it is important to accurately predict the temperature and density distributions both for design optimization and accident analysis. However, current major reactor system analysis codes only provide lumped parameter models which can only get very approximate results. The traditional 2-D or 3-D CFD methods require very long simulation time, and it’s not easy to get result. This paper adopts a new simulation code, which can be used to calculate heat transfer problems in large enclosures. The new code simulates the ambient fluid and jets with different models. For the ambient fluid, it uses a one-dimensional model, which is based on the thermal stratification and derived from three conservation equations. While for different jets, the new code contains several jet models to fully simulate the different break types in containment. Now, the new code can only simulate rectangular enclosures, not the cylinder enclosure. So it is meaningful for us to modify the code to simulate the actual containment, then it can be applied to solve the heat transfer problem in PCCS accurately.Copyright

Effect of air temperature on passive containment cooling system reliability in AP1000

Passive containment cooling system is innovatively used in AP1000 reactor design to enhance the safety. Since the system operation is based on natural circulation, physical process failure induced by uncertainties of physical parameters becomes one of the important failure modes (e.g. natural circulation cannot establish or keep and system design function cannot be accomplished because some parameters such as air temperature deviate from their design values), which should be considered in system reliability evaluation. As the heat sink, air temperature with high uncertainty has important effect on system reliability. In this article, we analyze the pressure variation in the containment along with the air temperature based on system thermal–hydraulic model, and the effect of air temperature on system operation is closely related to the thermal–hydraulic performance of the system. Moreover, the system thermal–hydraulic capacity is influenced by the system component configuration, so we evaluate the system physical process failure probability by Monte Carlo simulation and analyze the effect of air temperature distribution under different system component configurations. Finally, we evaluate the whole system reliability considering the logical relationship between physical process failure and equipment fault by fault tree method. The results illustrate that air temperature distribution has important influence on the system reliability, the system failure probability may be difference by several orders and the main contributors may be different at different plant locations, so climate should be considered in system design and reliability analysis.

Helium Permeability Measurement of SiC Ceramic Composites for Advanced Reactor Application

Permeability of helium gas of Silicon carbide ceramic composites material, which is one of the most important properties in application of SiC composite for advanced reactors, is studied by using a simple, low-cost test system. The test system can not only qualitatively determine whether the sample is permeable or not, but also quantitatively measure the permeability for the permeable ones by water displacement. The tests are conducted with low pressure in room temperature. The permeability of the SiSiC composite depends on the preparation method. In four flat-plate materials prepared by different processes for the test, the splint based SiSiC and cordierite coated fiber reinforced SiSiC are hermetic, the permeability of uncoated fiber reinforced SiSiC and CVD carbon coated fiber reinforced SiSiC are 0.216cm2/s and 0.109cm2/s, which imply that the permeability is cut in half with the coating. The samples are scanned under SEM to analyze their microscopic structures and verified that the difference of permeability is related to their coatings as well as the pores and cracks.Copyright

Young’s Modulus and Gas Tightness Measurement of Ceramic Matrix Composite-SiC for Advanced Reactor Application

Considering the core outlet temperature of high temperature gas-cooled reactor (HTGR) is above 1000 °C and in the accident conditions, a new type of heat-resistant silicon carbide (SiC) ceramic matrix composites needs to be developed. Analysis of the Young’s modulus and gas tightness of ceramic matrix composite-SiC should be paid attention to due to the inherent brittleness and large porosity of SiC ceramics and also the high temperature and high pressure environment where the fuel cladding works. In this paper, a set of measuring device is designed, which is capable of measuring both Young’s modulus and gas tightness of sic ceramic materials. In order to avoid the shatter of ceramic matrix composite-SiC due to excessive deformation in the measurement process of Young’s modulus, Wheatstone bridge principle is used in the experiment to convert the changes of resistance which generated by the micro-deformation of resistance strain gage adhesive on the surface of the specimen into voltage signal, then the relationship between the amplified voltage signal and the strain of specimen can be established, thereby the Young’s modulus of the specimen can be calculated. The measuring method used in this experiment is more rapid, accurate and economic than previous methods. Besides, there is no need for extra measuring device to measure gas tightness of specimen for it has been taken into account in designing process.Copyright

Scaling Analysis of Mixing and Thermal Stratification in Passive Containment

The passive containment cooling system (PCCS) is one of typical passive systems of AP1000, which is a passive condenser system, designed to remove energy from the containment for long term cooling period after a postulated reactor accident, like LOCA or MSLB. One of the key phenomena of PCCS is mixing and thermal stratification inside the containment. In order to establish empirical correlations and develop model of this phenomenon, the experimental study is essential. Because it is difficult to use prototype system for research, so a scaling analysis is needed to design an experimental facility with smaller scale and accelerated time to simulate the prototype system. In this paper, a scaling method for mixing and thermal stratification is given and gets the governing equations and scaling criterions. In final, a group of primary parameters of the experiment, such as mixing time and volume rate of flow, is given in the form of geometric scaling ratio which is chosen by the designer.Copyright