Multi Parameters Effect on Thermohydraulic Instability in Natural Circulation Boiling Water Reactor during Startup

Abstract

The purpose of the study is to experimentally investigate driving mechanism of major instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings. Pressure range of 0.1 up to 0.7 MPa, input heat flux range of 0 up to 577 kW/m 2 , and inlet subcoolings of 5, 10 and 15 K respectively, are applied in the experiments. The objective of the study is to formulate a rational startup procedure, in which major thermohydraulic instabilities can be detected and prevented. The study clarifies that four (4) kinds of thermohydraulic instability might occur even up to a higher pressure of 0.7 MPa. The instabilities' sequence is as follows: (1) geysering induced by condensation accompanied by flashing, (2) oscillation induced by hydrostatic head fluctuation, (3) density wave oscillations, and (4) flashing accompanying those instabilities. The experiments confirmed that the geysering region gets narrower and suppressed with the increased system pressure. With chimneys, natural circulation can be achieved reliably and more easily. However, the flashing in the chimney cannot be avoided at low system pressure. Stable two-phase natural circulation can be established if the system pressure is increased beyond 0.7 MPa, after the high frequency density eave oscillation thoroughly suppressed. The experiments were analyzed based on frequency domain of each instability phenomenon.

An Overview of Non-LWR Vessel Cooling Systems for Passive Decay Heat Removal (Technical Letter Final Report)

“…Boure et al [73] performed an extensive review of two-phase flow instabilities and classified the phenomena into different types based on their specific mechanisms and characteristics. For low-pressure boiling systems, typical types of instabilities that should be considered include natural circulation oscillations (also called thermalhydraulic oscillations [74], or hydrostatic head fluctuations [75]), density wave oscillations (DWO), flow pattern transition instability, and geysering…”

The development of a model for predicting the stability boundaries of natural circulation process

“…Such geyser instability was first studied in vertical pipes closed from below and filled with water [3,4]. When a pipe is heated with some heat flow in the lower part the water boiling begins.…”

Report on Year-2 of Water NSTF Matrix Testing

“…Boure et al [11] performed an extensive review of two-phase flow instabilities, and classified the phenomena into different types based on their specific mechanisms and characteristics. For low-pressure boiling systems, typical types of instabilities that should be considered include natural circulation oscillations (also called thermal hydraulic oscillations [12], or hydrostatic head fluctuations [13]), density wave oscillations (DWO), flow pattern transition instability, and geysering.…”