Coupled Neutronic and Thermal-Hydraulic Out-of-Phase Stability of Supercritical Water-Cooled Reactors

As the last topic of a series of U.S. reference supercritical water-cooled reactor (SCWR) design stability studies, coupled neutronic-thermal-hydraulic out-of-phase stability is analyzed and compared with that of a typical boiling water reactor (BWR). A modal expansion method based on modes (reactivity modes) of the neutron kinetic equation is applied, and the first subcritical mode of the neutron dynamics model is coupled with the coolant thermal-hydraulic model. The out-of-phase oscillation of the SCWR is found to be dominated by the reactor thermal hydraulics, whereas the BWR is more sensitive to the coolant density reactivity coefficient because of much stronger neutronic coupling. It is also found that the SCWR stability is sensitive to the details of the core simulation model and the hottest channel dominates the stability. The BWR is less sensitive to the core simulation model since it has much stronger neutronic coupling that is controlled by the whole-core average properties. Power and flow rate sensitivity analysis of the out-of-phase stability was also conducted for both the SCWR and the BWR. The SCWR stability is found to be more sensitive to the operating parameters than the typical BWR. Although the water rod heating can improve the SCWR out-of-phase stability, it cannot significantly improve the sensitivity feature.