Flow instabilities under low-pressure and low flow conditions with application to the simplified boiling water reactor

Ibrahim M. M Babelli, Purdue University

Abstract

Thermalhydraulic systems operating under low pressure and low flow conditions are susceptible to flow instabilities such as flow excursion and density wave oscillation. Instabilities may lead to a premature critical heat flux even though the system is well below the enthalpy burnout limit. Flow excursion occurs when the internal pressure drop curve of the facility has a negative slope. Flow excursion is the dominant instability mode for downflow systems under low flow and low pressure conditions but it is not always so for upflow systems. Flow reversal is imminent following flow excursion in a downflow parallel system. A simple criterion is developed for predicting the onset of flow excursion and is verified against experimental data for upflow and downflow systems. A unique feature of the downflow heated systems is that heat addition renders the system unstable even if boiling does not occur in the heated section. As the pressure drop components decrease with reduction in the coolant velocity, the pressure drop due to thermal expansion increases. The increase in the pressure drop due to thermal expansion results in flow excursion if the inlet subcooling is large. If the subcooling is low, however, boiling occurs in the test section which leads to flow excursion. Density wave oscillation occurs in systems with parallel channels when the pressure drop across the channel and the coolant velocity are out of phase. For a system that meets this criterion, an increase in the external pressure drop will decrease the velocity across the system which feeds back a signal to increase the pressure drop. The density wave instability is analyzed using a model that was originally derived by Ishii (1971). In order to apply the predictive tools of the two instability types discussed above to the PUMA RPV, some modifications are necessary to account for the effect of flashing in the chimney on the onset of instability. Vapor generation due to flashing is estimated from a simple model and used in predicting the natural circulation rate during stable operation of the RPV. The thermalhydraulic parameters obtained from the steady-state model are used in the stability analysis.

Degree

Ph.D.

Advisors

Ishii, Purdue University.

Subject Area

Nuclear physics|Mechanical engineering

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