An optimal reliable control system has been designed to control the steam generator’s water level in a pressurized water reactor. The control system uses a multiple-controller configuration so that even if part of the controller fails, the control system still functions well. The value of the controller parameters is obtained by means of an optimization algorithm, thus guaranteeing the performance of the controller under normal operation or a partially failed condition. The simulation results clearly demonstrate the performance of the controller, and this success encourages application of the method to the other crucial control systems in nuclear power plants.