An example is given to illustrate the probabilistic reactor dynamics theory developed earlier. A reservoir with two output valves is considered in which the valves are subject to random failures. The operator must maintain the tank pressure within an upper and a lower bound whatever the initial system state may be. A complete description is given of the physical system, of the instrumentation, and of the human model that was selected to describe the operator’s behavior. The results are expressed in terms of state occupation probabilities and mean exit times. A Monte Carlo algorithm is briefly described.