Experimental Investigations in a Reactor Cavity Cooling System with Advanced Instrumentation for the Study of Instabilities, Oscillations, and Transient

A research team at University of Wisconsin - Madison designed and constructed a 1/4 height scaled experimental facility to study two-phase natural circulation cooling in a water-based reactor cavity cooling system (WRCCS) for decay heat removal in an advanced high temperature reactor. The facility is capable of natural circulation operation scaled for simulated decay heat removal (up to 28.5 kW m--2 (45 kW) input power, which is equivalent to 14.25 kW m--2 (6.8 MW) at full scale) and pressurized up to 2 bar. The UW-WRCCS facility has been used to study instabilities and oscillations observed during natural circulation flow due to evaporation of the water inventory. During two-phase operation, the system exhibits flow oscillations and excursions, which cause thermal oscillations in the structure. This can cause degradation in the mechanical structure at welds and limit heat transfer to the coolant. The facility is equipped with wire mesh sensors (WMS) that enable high-resolution measurements of the void fraction and steam velocities in order to study the instability's and oscillation's growth and decay during transient operation. Multiple perturbations to the system's operating point in pressure and inlet throttling have shown that the oscillatory behavior present under normal two-phase operating conditions can be damped and removed. Furthermore, with steady-state modeling it was discovered that a flow regime transition instability is the primary cause of oscillations in the UW-WRCCS facility under unperturbed conditions and that proper orifice selection can move the system into a stable operating regime.