Start-up transient test simulation with and without void-reactivity feedback for a two-phase natural circulation reactor

Abstract

The new small-scaled light water reactor design, known as integral modular water reactor (IMR), has an advantage compared to conventional light water reactor in terms of costs by adopting innovative technologies. However, this reactor may be 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 IMR, 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 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 prototype and experimental facility. Modified power scaling is used for transient tests. Four heater rods are used to simulate the chaotic flashing phenomena. In order to investigate the instability during the start-up procedure, start-up transient tests are performed with and without void-reactivity feedback for different core heat-up rate. These experimental results show that IMR is susceptible to instability during the start-up procedure at very low pressure. These oscillations are condensation induced oscillations and occur due to high subcooling in chimney section. It is investigated that, since amplitude of power oscillation is low, void-reactivity feedback effect does not affect the stability of the system for this type of given power transient.