Abstract: A safety grade protection system for monitoring parameters indicative of the status of a core of a nuclear reactor, and reducing the criticality of the nuclear reaction when the onset of unsafe conditions are detected. The invention directly detects, in real time, the power at a plurality of axial and radial locations within or about the core and provides a three-dimensional real-time core power output. The system processes those outputs together with a measure of core coolant inlet temperature, pressure and flow rate to calculate, in real time, the fuel centerline temperature, the heat transfer condition at the fuel elements' boundary and the rate of power change within the core.