A variety of water level measurement systems are proposed to resolve the problem of reactor vessel level measurement. Two such systems, the heated thermocouple and the multiple differential pressure cell system, are used commercially. A third system based on ex-core neutron detectors was tested at The Pennsylvania State University Breazeale nuclear reactor facility and at the Idaho National Engineering Laboratory Loss-of-Fluid Test Facility. Results of these tests show that such a system is sensitive to both large- and small-break loss-of-coolant accidents and to voiding in the upper plenum of the vessel.