Stability of two-phase natural circulation reactor during start-up procedures
Abstract
The new small-scaled light water reactor, known as medium modular water reactor (MMR), is susceptible to flow instabilities due to two-phase natural circulation inside the reactor pressure vessel (RPV). Flow instabilities may be amplified due to strong interaction between the flow and core power through the void-reactivity feedback mechanism. During the start-up of the MMR, system pressure is low. At low pressure, the density ratio can be quite high, which leads to large variation in void fraction due to change in flow quality. In the MMR design, the long riser and large volume of water can lead to thermal non-equilibrium between the phases due to the significant variation of the saturation temperature during start-up. This can result in condensation-induced or flashing-induced flow oscillations at certain reactor conditions. In order to understand and identify the instability phenomena during the start-up of the reactor, a scaled experimental facility is designed based on the sound scaling approach. In order to investigate the instability during the start-up procedure, start-up transient tests, which simulate the whole start-up procedure of the MMR, are performed for different core heat-up rate. These experimental results show that MMR is susceptible to instability during the start-up procedure at very low pressure due to condensation at chimney inlet and increased pressure leads to stable flow. In order to investigate the effect of strong interaction between the flow and power through the void-reactivity feedback mechanism, start-up transient experiments are performed with reactivity feedback. Experimental results show that after the boiling starts, power starts oscillating with certain frequency. Since amplitude of power oscillation is low, void-reactivity feedback effect does not affect the stability of the system for these transient tests. Steady state tests are performed with and without void-reactivity feedback at different system pressures. The flow is stable below a certain core power regardless of the channel inlet subcooling. The unstable region reduces significantly at high pressure compared to low pressure case. High subcooling boundary is not affected by the void-reactivity feedback. As the inlet subcooling is decreased, power starts oscillating with certain frequency and it slightly increases the flow velocity oscillation amplitude. In order to obtain theoretical stability maps, linear stability analysis is performed which includes flashing effect. Predicted stability boundaries show good agreement with experimental data.
Degree
Ph.D.
Advisors
Ishii, Purdue University.
Subject Area
Nuclear engineering
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