Review of the State of the Art on Startup Transients in Natural Circulation Boiling Water Systems

A proper understanding of the system behavior and other important parameters during startup of a Natural Circulation Boiling Water Reactor (NCBWR) is of primary importance in thermal hydraulic safety analysis. The use of natural circulation has introduced operational uncertainties, especially related to the startup phase of the reactor, when the system has to pass from single-phase to two-phase operation. Natural circulation systems may exhibit thermal-hydraulic instabilities under low-pressure low-power startup conditions. At low-pressures, a natural circulation loop typically has three operation ranges: single-phase stable region, two-phase unstable region and two-phase stable region. The instabilities in the region between single-phase and stable two-phase NC mode are mainly induced by the phenomenon of geysering in the heated channel and flashing in the riser section. The findings and understanding obtained through various experimental and analytical research is summarized in this paper. Numerical modeling of such low-pressure natural circulation phenomenon was also revisited and their start of the art is reported. The paper also discusses the various startup procedure proposed and employed for startup of NCBWRs.