1. A sensor monitoring system for displaying a profile of enthalpy rise over a defined area comprising at least a part of a core of a nucelear reactor, which system comprises: a plurality of core exit temperature sensors positioned to monitor at least a portion of the defined area; an inlet temperature sensor outside the core and monitoring the temperature of core coolant at an inlet to the reactor; means, responsive to the outputs from both said temperature sensors and said inlet temperature sensor, for generating corresponding representative fractinal deviation values of the enthalpy rise from reference values; means for interpolating the generated values of the enthalpy rise to provide an output representaitive of interpolated values of the fractional deviations in enthalpy rise over the entire defined area; means for classifying the generated values and the interpolated values into a number of different classes; and means for multidimensionally displaying the classes of the generated values and the classes of the interpolated values to present the enthalpy rise profile over the defined area.