Abstract
In natural circulation loops, the driving force is usually low as it depends on the riser height which is generally of the order of a few meters. The heat transport capability of natural circulation loops (NCLs) is directly proportional to the flow rate it can generate. With low driving force, the straightforward way to enhance the flow is to reduce the frictional losses. A simple way to do this is to increase the loop diameter which can be easily adopted in pressure tube designs such as the AHWR and the natural circulation boilers employed in fossil-fuelled power plants. Further, the loop diameter also plays an important role on the stability behavior. An extensive experimental and theoretical investigation of the effect of loop diameter on the steady state and stability behavior of single- and two-phase natural circulation loops have been carried out and the results of this study are presented in this paper.
Highlights
Natural circulation systems find application in many nuclear reactor systems
Certain reactor systems like the heating reactors and the CAREM PWR [2] use single-phase natural circulation as the normal mode of coolant circulation in the primary system
Two-phase natural circulation systems are extensively used in fossil-fuelled power plants, thermosyphon reboilers, and steam generators including those in nuclear power reactors like PWRs, VVERs, and PHWRs
Summary
Single-phase natural circulation is the mode of coolant circulation in the current pressurized water and heavy water reactors (PWRs and PHWRs) following a pumping power failure. Two-phase natural circulation systems are extensively used in fossil-fuelled power plants, thermosyphon reboilers, and steam generators including those in nuclear power reactors like PWRs, VVERs, and PHWRs. Two-phase natural circulation is the mode of coolant circulation in the current generation of nuclear power plants following a small break LOCA associated with pumping power failure. Natural circulation based reactor designs are being studied even with supercritical water as coolant [5, 6] In view of their wide application, both single-phase and two-phase natural circulation systems are being extensively studied. Apart from the steady-state performance, operational transients like start-up, power raising and step back are required to be stable for natural circulation reactors. Premature occurrence of CHF and burnout is a serious concern during low-frequency and high-amplitude oscillatory flows in small diameter loops
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