Application of optimal control theory to aboiling water nuclear reactor is the theme of this paper. The nonlinear model of the direct cycle 159 MW (th) Big Rock Point boiling water reactor derived on the basis of physical laws and empirical relations is linearized around an operating point and the model is verified against experimental results. The optimal control problem of the linearized model is treated as a regulator problem wherein the given quadratic index of performance includling a sensitivity term is minimized in an average sense over a set of feedback gainmatrices which ensure prescribed closed-loop eigenvalues. An efficient computational procedure based on direct cost optimization using a gradient type algorithm is also reported. With the new optimal control strategy proposed in this paper, the stability and sensitivitycharacteristics of the system response can easily bemanipulated. © 1985 IEEE