Experimental stability maps for a two-phase natural circulation reactor with and without void-reactivity feedback effect

Highlights: • A scaled experimental facility is designed based on the sound scaling approach. • Experiments are performed to identify the instability phenomena. • Four heater rods are used to simulate the chaotic flashing phenomena. • Experiments are performed with and without void-reactivity feedback. • Stability maps are obtained at different system pressures. -- Abstract: The new small-scaled light water reactor design, known as integral modular water reactor (IMR), 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 the core power through the void-reactivity feedback mechanism. During the start-up of the IMR, system pressure is low. At low pressure, the density ratio is quite high, which leads to large variation in void fraction due to change in the flow quality. In the IMR 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 along flow direction. This can result in flow oscillations at certain operating 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. The scaling laws are used to obtain design parameters to maintain the similarities between the prototype and the experimental facility. Four heater rods are used to simulate the chaotic flashing phenomena. The 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 region of stability grows in size as the core power is increased. The unstable region reduces significantly at high pressure compared to low pressure case. A new approach is presented to simulate the void reactivity feedback in a scaled experimental facility. High subcooling boundary is not affected by the void-reactivity feedback. As the inlet subcooling is decreased, power starts oscillating with the certain frequency and it slightly increases the flow velocity oscillation amplitude. It is found out that the coolant power may oscillate in phase or out of phase with the void fraction depending on the fuel rod time constant, which may destabilize or stabilize the system