Investigations on Startup Instabilities and Power Ramping Procedures for Natural Circulation Boiling Systems

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The thesis contains studies on startup instabilities and methods of power ramping in natural circulation boiling water systems. Numerical models of a test facility and its prototype have been developed using RELAP5 code. Startup transients in both single and double channel systems have been analyzed and their structural nature has also been determined. Numerical simulations on power ramping have been performed and suitable startup procedures for the boiling systems have been evolved. Experimental observations in a test facility have been used for model validation and analysis. In the recent past, the use of natural circulation in boiling water reactors (NCBWRs) has been highly recognized for its inherent passive safety feature and the potential for improved economy in nuclear power generation. However, the use of natural circulation has generated concern regarding operating conditions, especially related to the startup phase of the reactor. During startup, an NCBWR is susceptible to flow oscillations when the system has to pass from single-phase to two-phase operation. The startup instabilities are often induced by the phenomena of geysering in the heated channel and flashing in the riser section, which may result in mechanical vibrations, system control problems and critical heat transfer issues. A proper understanding of these instabilities through experimental and numerical studies is of primary importance before attempting to devise an appropriate startup procedure. For numerical simulation of two-phase flow instabilities, a thermal-hydraulic system code such as RELAP5 can be employed. However, the use of such codes to simulate the transients during startup conditions is yet to be fully established. Although earlier reports suggested that RELAP5 cannot deal with flashing instabilities, recent studies towards enhancing the predictive performance of the code under low-pressure subcooled conditions indicate promising results. In the present work, numerical models of a natural circulation test facility and its prototype (a pressure tube type NCBWR) have been developed with RELAP5 code and verified for their grid independence. The model of the test facility has been validated for its steady state as well as transient predictions with the help of experimental observations. The transient predictions and parametric trends obtained by the numerical model of the prototype have been compared with those of the numerical model of the test facility. Thus, the ability of RELAP5 code to predict the transients during startup of a natural circulation boiling water reactor has been verified.
Supervisor: Monmohan Pandey