Natural circulation with flashing can be utilized in the cooling of the moderator in advanced Heavy Water Reactor. A model natural circulation loop has been designed, fabricated and characterized in order to understand the various transients and the steady state conditions that occur during the flashing driven natural circulation process. Non-dimensional numbers are identified. Numerical simulation, using RELAP 5, has been conducted to model the natural circulation loops. The single channel loop is further modified to study flashing by using two parallel risers. Stable flashing could not be attained with both the channels. However, using parallel channels in the single phase region which combine into a single channel in the zone where flashing occur produced stable flow. It is observed that a stable flashing driven two-phase flow occurs only by reducing the inlet subcooling to a very low value with respect to the riser exit pressure. The presence of inlet restriction does not help in reducing the oscillation period, but the stable flashing was observed at lower power values. The initial pressurization and subsequent depressurization to the desired working pressure of the system reduces the flow instabilities considerably. It appears that non-dimensional Zuber number, Friction number and Froude number can be used to characterize flashing driven natural circulation systems.

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