Experimental Investigations on the ESBWR Stability Performance

The stability performance of the Economic Simplified Boiling Water Reactor (ESBWR) is studied with the downscaled GENESIS facility. The GENESIS design is based on fluid-to-fluid modeling and includes an artificial void reactivity feedback system for simulating the neutronic-thermal-hydraulic coupling. The experiments show that the ESBWR thermal-hydraulic oscillatory mode is very stable at nominal conditions, exhibiting a decay ratio DR = 0.12 and a remarkably low resonance frequency f_res = 0.11 Hz. This result indicates a static pressure head-driven phenomenon since this frequency corresponds well to typical frequencies found for density wave oscillations traveling through the core plus chimney sections. For the reactor-kinetic oscillatory mode, we found a decay ratio DR = 0.30 and a resonance frequency f_res = 0.75 Hz. This corresponds well to density wave oscillations traveling through the core indicating the instability mechanism is driven by the interplay between the core friction and the neutronic response due to void changes in the core. By comparing these results with those obtained with the TRACG computational code, it was found that they agree very well. In addition, the stability performance of the thermal-hydraulic and the reactor-kinetic mode is investigated for a wide range of conditions, confirming the existence of large margins to instabilities of the ESBWR design.