A control system is described which will shut down a reactor in close to the minimum time that will allow subsequent restart on demand, despite fission product buildup. Errors in the reactor model, restrictions on operating conditions, and plant perturbations are taken into account. The system uses an on-line computer, requires no extra reactor instrumentation, and provides the operator at all times with an estimate of the minimum time to transfer the reactor from its current state to a suitable shutdown condition. During normal operating conditions, the operator may use the system to evaluate strategies satisfying various restart conditions. The results of simulation tests on the system are included.