Dynamic Behavior of Boiling Water Reactors

A study of the basic processes involved in boiling water nuclear reactor dynamics is presented. The main emphasis of this research has been placed on the physical interpretation of these processes. It is shown that this type of reactors have two regimes of operation: linear, during normal operation, and nonlinear, if they become unstable due to the thermohydraulic feedback. Both of these regimes are studied using low-order physical models.

The main result obtained from the linear study is the pole-zero configuration of the reactivity-to-power transfer function. It is determined that three zeros and four poles are needed to properly represent this transfer function. Physical processes are identified with these transfer function features. Based on the understanding of these processes, an automated algorithm to estimate boiling water reactor stability from neutron noise measurements is developed and implemented as a computer code.

The causative mechanism leading to the appearance of the limit cycle in boiling water reactors is identified from the nonlinear study. The relationship between the different process variables during limit cycle oscillations is studied. It is shown that these oscillations could reach large amplitudes.

The stability of the limit cycle is also studied. It is shown that the amplitude of the limit cycle can become unstable and produce period-doubling pitchfork bifurcations which scale according to Feigenbaum's universality theory. As a consequence of the bifurcation process, aperiodic solutions of the deterministic reactor equations are found to be possible.

Finally, nonlinear noise propagation is studied. A nonperturbative technique is developed for detecting the onset of linear instability and the transition to the nonlinear regime.