Countercurrent flow limitation (CCFL) refers to an important class of gravity-induced hydrodynamic processes that impose a serious restriction on the operation of gas–liquid two-phase systems. In a nuclear power plant, CCFL may occur in the liquid level measurement system where an orifice is applied in the pipeline, which may introduce error into the level measurement system. CCFL can occur in horizontal, vertical, inclined, and even much more complicated geometric patterns, and the hot-leg channel flow passage has been widely investigated; however, a pipeline with variable cross-sections, including an orifice, has not yet been investigated. An experimental investigation has been conducted in order to identify the phenomenon, pattern, and mechanism of CCFL onset in this type of geometry. Both visual and quantified experiments were carried out. A high-speed camera was applied to capture the flow pattern. Visual experiments were implemented at atmospheric pressure, while quantified pressurizer experiments were implemented at higher pressures. It was determined that if the condensate drainage is low and the liquid level is also low, with a stable stratified flow upstream of the orifice, there is no oscillation of the differential pressure. However, at higher condensate drainage levels, when the liquid level increases, a stratified wavy flow occurs. One of these waves can suddenly rise upstream of the orifice to choke it, which subsequently gives rise to differential pressure across the orifice, with periodic variation. This pattern alternately features stratified flow, stratified wavy flow, and slug flow, which indicates the occurrence of CCFL. The CCFL occurring under these experimental conditions can be expressed as a Wallis type correlation, where the coefficients m and C are 0.682 and 0.601, respectively.
After the release of new nuclear safety requirements SSR-2 / 1 (the latest version is 2016 Edition) in 2012, IAEA issued a new guideline SSG-30 “safety classification of structures, systems and components of nuclear power plants” in 2014 proposing a new safety classification method. Subsequently, IAEA published TECDOC-1787 “Application of Safety Classification of Structures, Systems and Components of Nuclear Power Plants” in 2016, which further explained the application of SSG-30. The safety classification method of SSG-30 based on the safety functions and design provisions. An SSC implemented as a design provision should, however, be classified directly because the significance of its postulated failure fully defines its safety class without any need for detailed analysis of the category of the associated safety function. The severity of consequences is divided into three levels (high, medium and low). The high, medium and low consequences are defined by the radiation dose criteria for the public and staff in different plant conditions. In order to classify the structures, systems and components of nuclear power plant, the specific meaning of the definition of high, medium and low “consequences” should be clarified firstly. In this paper, the radioactive dose criteria for the public and staff under different working conditions of nuclear power plant are studied and suggestions are put forward to make SSG-30 more operable when used in nuclear power plant SSCs classification.
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