Stability of flashing-driven natural circulation in a passive moderator cooling system for Canadian SCWR

•The stability in a passive moderator cooling system of a unique system in the Canadian SCWR.•Identify and analyze unstable oscillations using flashing-driven natural circulation test results.•The flashing-driven oscillations categorized as a flashing-driven Type-I density wave instability including a geysering-like feature.•A stability map on the dimensionless plane with the Subcooling number and Phase Change number.

This paper presents an examination of the instability mechanisms in a Passive Moderator Cooling System for the Canadian SCWR (Supercritical Water-cooled Reactor). The passive system is being developed at AECL using a flashing-driven natural circulation loop. Unstable intermittent and sinusoidal oscillations were identified from experimental data of the flashing-driven natural circulation passive moderator cooling system. The oscillation periods were correlated with the boiling delay time. A stability map for a flashing-driven two-phase natural circulation loop was established on the dimensionless plane with Subcooling number and Phase Change number. It was observed that there is thermal non-equilibrium in the single-phase and two-phase oscillation stages of the flashing-driven natural circulation.