AbstractSurveillance of a nuclear reactor core involves determination of the power or temperature distribution of the assemblies. Derived from other assemblies’measured temperatures, the temperature of a nonmeasured assembly is calculated for every assembly with the help of the principal components method (PCM). The basis of this method is presented, and the measured values are interpolated for various geometrical coverings of the WWER-440 core. A number of procedures have been elaborated and investigated, the most successful of which are introduced. Each method offers self-consistent means for determining the numerical errors of the interpolated values. The procedure based on the PCM can be utilized for any reactor type and is fairly accurate.