Abstract: A real time analysis of nuclear reactor vessel integrity is preformed by monitoring reactor coolant temperature and pressure and fast neutron fluence, and generating therefrom a visual comparison of the reference nil-ductility transition temperature, RT.sub.ndt, required for flaw initiation and the actual RT.sub.ndt through the entire depth of the vessel wall at selected critical locations. When flaw initiation is indicated at any depth, the RT.sub.ndt required for flaw propagation arrest is also generated and displayed. A display can also be generated to provide guidance for optimum performance during heatup and cooldown. Thermocouples located at the reactor core inlet are used in conjunction with the resistance temperature detectors, RTDs, conventionally provided in the cold leg conduits to generate a more accurate measurement of the coolant temperature at the critical locations in the vessel.