Application of optimal control theory to a pressurised water reactor is the theme of this paper. The multi-input mathematical model of the H.B. Robinson pressurised water reactor plant, reported by Kerlin et al is simplified to a single-input one. The optimal control problem of the linear model is treated as a regulator problem wherein the given quadratic index of performance including a sensitivity term is minimised in an average sense. An efficient computational procedure based on direct cost optimization using a gradient type algorithm is also reported. Copyright © 1987 by The Institute of Electrical and Electronics Engineers, Inc.