Normal Operation Transients Research Articles

Roadmap for Recommended Guidelines of Leak Detection of Subsea Pipelines

The leak of hydrocarbon-carrying pipelines represents a serious incident, and if it is in a gas line, the economic exposure would be significant due to the high cost of lost or deferred hydrocarbon production. In addition, the leakage of hydrocarbon could pose risks to human life, have an impact on the environment, and could cause an image loss for the operating company. Pipelines are designed to operate at full capacity under steady-state flow conditions. Normal operations may involve day-to-day transients such as the operations of pumps, valves, and changes in production/delivery rates. The basic leak detection problem is to distinguish between the normal operational transients and the occurrence of non-typical process conditions that would indicate a leak. To date, the industry has concentrated on a single-phase flow, primarily of oil, gas, and ethylene. The application of a leak-monitoring system to a particular pipeline system depends on environmental issues, regulatory imperatives, loss prevention of the operating company, and safety policy rather than pipe size and configuration. This paper provides a review of the recommended guidance for leak detection of subsea pipelines in the context of pipeline integrity management. The paper also presents a review of the capability and application of various leak detection techniques that can be used to offer a roadmap to potential users of the leak detection systems.

Recent improvements of probabilistic fracture mechanics analysis code PASCAL for reactor pressure vessels

A probabilistic fracture mechanics (PFM) analysis code PASCAL was developed in Japan for probabilistic integrity assessment of reactor pressure vessels (RPVs) in pressurized water reactors (PWRs) considering neutron irradiation embrittlement and pressurized thermal shock (PTS) transients. To strengthen the practical applications of PFM methodology in Japan, PASCAL has been upgraded to a new version, PASCAL5, which enables PFM analyses of RPVs in both PWRs and boiling water reactors (BWRs) subjected to a broad range of transients, including PTS and normal operational transients. In this paper, the recent improvements in PASCAL5 are described such as the incorporated stress intensity factor solutions and corresponding calculation methods for external surface cracks and embedded cracks near the RPV outer surface. In addition, the analysis conditions and evaluation models recommended for PFM analyses of Japanese RPVs in BWRs are investigated. Finally, PFM analysis examples for core region of a Japanese BWR-type model RPV subjected to two transients (i.e., low-temperature over pressure and heat-up transients) are presented using PASCAL5.

Transient analysis of the main heat-transfer system of the pool-type sodium-cooled fast reactor

<p indent="0mm">Sodium-cooled fast reactors are the first choice of the fourth generation of advanced nuclear energy systems. They have attracted significant attention in the development of advanced nuclear energy technology owing to their advantages of proliferative nuclear fuel, transmutation of radioactive waste, and high thermal efficiency power generation. The design and construction of the secondary sodium-cooled fast reactor are ongoing in China. The physical characteristics of fast neutrons and the special thermal, physical, and chemical properties of liquid sodium metal cause special and complex scientific problems in the design of sodium-cooled fast reactors. In this study, taking the energy transfer process of sodium-cooled fast reactors as the object, we establish mathematical and physical models for analyzing large-scale transients in various energy transfer and conversion processes. The model is verified and validated by the natural circulation test of the Phoenix reactor and the international benchmark of the fast flux test facility loss in flow tests. The trend of the calculated values is consistent with that of the test data, and the results are in good agreement. For a typical pool-type sodium-cooled fast reactor, four types of transient events in the power plant are determined by engineering evaluation and deductive analysis. Among them, there are nine normal operation transients, 23 expected transients, 10 abnormal transients, and 7 emergency transients. Based on the established mathematical and physical models, the analysis results of each type of typical transient are given, which provide an important basis for the design of the system and equipment/components and lay a scientific and technological foundation for completing the design of the transient condition and operation mode of the sodium-cooled fast reactor as well as safety evaluation.

Current Ratio Based Breakage Protection for Flexible DC Distribution Systems

Unlike the short circuit faults, the breakage fault does not offer distinctive characteristics compared with normal operation transients in DC distribution system and the fault information is difficult to be analyzed, which bring great challenges to breakage protection. In this case, this paper introduces a fault characteristic analysis method for breakage faults and proposes a novel breakage protection scheme based on ratio of positive and negative fault currents for DC distribution network. By establishing the fault circuit equations, the breakage fault features are analyzed for protection. Meanwhile, the ratio of current summation over their subtraction is used to further amplify fault characteristics, which overcomes the reliability problem of conventional breakage protection. A six-terminal flexible DC distribution network is built in PSCAD/EMTDC to verify the proposed protection method. Simulation results show that the protection is robust to the measurement noise and can identify an internal breakage fault and realize the broken pole selection rapidly.

Selection of Core Outlet Temperature and Impacts on Fast Reactor Economics

The selection of the operating temperatures for a sodium-cooled fast reactor (SFR) always involves a trade-off between the plant performance and cost. In this work, the general trends with an increase of the core outlet temperature were calculated for the Advanced Burner Reactor (ABR). First, the benefits of higher temperatures in terms of the higher plant efficiency were calculated for several power conversion systems. To characterize the disadvantages of higher temperatures, the safety margins were investigated for the design conditions, normal operational transients, and beyond-design-basis accidents. The limiting criteria were identified for both oxide and metal fuel core designs. In addition, the effect of the higher temperatures on the structural thicknesses was estimated. A preliminary cost analysis incorporating both benefits and cost penalties of higher temperatures showed the economical benefit potential of higher temperatures, provided that the safety requirements are satisfied by the design and/or material selection.

Fault Detection on Multicell Converter Based on Output Voltage Frequency Analysis

Multilevel converters use a large amount of semiconductors, allowing the reconfigurate of the converter to work even on internal fault condition. This paper presents a method to detect faulty cells in a cascaded multicell converter requiring just one voltage measurement per output phase. The method is based on high-frequency harmonic analysis, using a dynamic prediction of their behavior, avoiding erroneous detection on transients while keeping the precision under real fault events. Once the faulty cell is detected, it can be bypassed allowing the converter to keep working according to previously reported techniques. Experimental results confirm accurate and fast fault detection, with a good rejection to normal operation transients.