Experimental analysis of the start-up of a natural circulation loop in single and two-phase flow

With reference to passive heat removal systems for advanced nuclear reactors, an experimental campaign has been carried out to study the start-up of a natural circulation loop in single and two-phase flow. The effect of the heat source power and initial mass inventory have been investigated. The facility consists of a loop 4.828 m high and 10.275 m long, that includes an electrically heated bayonet heat exchanger and a pool heat sink, and can operate at a maximum pressure of 20 bar. Both single- and two-phase flow (air and water) have been tested and typical oscillations have been observed. Characteristic transient phases have been defined by analyzing the time histories of pressures, temperatures and flow rate in the loop and in the water pool. During the transients, the bayonet heat-up phase is followed by a phase of natural circulation activation, then a natural circulation development phase and, in some tests, a nearly steady-state phase. The duration of the phases has been studied with different mass inventories and heat source power. It has been observed that the natural circulation is established more quickly at higher power; the final pressure increases with the power and decreases with the mass inventory. The natural circulation flow rate at the end of the transients depends on the heat source power, with a small influence of the mass inventory value. At high water filling of the loop, single-phase natural circulation is developed, a fast pressure transient occurs and the pressure in the loop has a diverging behavior. At lower water fillings, two-phase two-component flow (air, water and steam) occurs and the system pressure tends to reach a near-steady-state value.