A water-flooded-core accident is a serious potential accident for high-temperature gas-cooled reactors (HTGRs). In this technical note, based on two different water-flooded-core scenarios, preliminary neutronics analysis was performed on a typical HTGR. Preliminary temperature-effect analysis is carried out as well. It is found that the neutron-slowing ability is the key for the effective multiplication factor of the HTGR core. More importantly, when the water-flooded-core accident occurs, the HTGR might return back to supercritical with the core temperature decreasing even if it is safely shut down at high operation temperature.