Modeling the Kinetic Behavior of Reflected-reactor Systems near and above Prompt Critical

The safe operation of a fast burst reactor (FBR) depends directly on the knowledge, experience, and judgment of the operating staff to understand the mechanisms by which experiments influence the reactor’s behavior. Fast burst reactors are a unique class of research reactors that generate high yield, self-terminated power pulses from reactivity insertions exceeding prompt critical. These reactors are typically very compact, fueled with fully enriched uranium-235, and unmoderated. Therefore, experiments positioned near or within the core become neutronically linked to the reactor, and influence its kinetic behavior. Since experiments can cover a broad range of conditions, the potential exists for an inadvertent over-insertion of reactivity during a pulse operation, producing a yield capable of damaging the reactor. To reduce this heavy reliance on human component of reactor operation, we evaluated the kinetic behavior of the Sandia Pulsed Reactor III (SPR III), a FBR, over a wide range of experimental conditions, looking for relationships among measured parameters. Given this data, we determined the physical bases for the observed relationships. Further we modified the theoretical models to account for these observed relationships. The result of our analysis is a modified reflected point-reactor kinetics model of SPR III that adequately simulates the experiment-reactor system under all observed conditions. We used experimental data and computer simulations to validate the resulting models. The results show that experiments neutronically behave as loosely coupled reflectors. Further, the kinetic behavior of the reflected reactor during a pulse can be adequately modeled by treating the reflected neutrons in a manner analogous to additional delayed neutron groups. As found in this study, there are multiple components to the time-response of the reflectors. This variation in response is dependent on the material composition and distance of the experiments from the core.