Role of Feedback Reactivities in the Passive Safety of Nuclear Reactors: The Case of the Fast Flux Test Facility

The design of the next generation of power reactors will emphasize passive safety and enhanced engineered systems. True passivity can be achieved by capitalizing on natural laws to restore reactor stability during an off-normal event. The most effective stabilizing mechanisms relying solely on natural laws— without human interference—are the feedback reactivities produced by a change in the reactor thermal state. During 1986 and 1987, an important research program was undertaken at the Fast Flux Test Facility (FFTF) to advance the understanding of feedback mechanisms and to investigate passive safety in liquid-metal reactors. The experimental program began with a series of static feedback reactivity measurements aimed at separating feedback components and ended with a demonstration of passive safety in a series of loss-of-flow-without-scram (LOFWOS) to natural circulation tests. Described here are (a) the fundamental experimental concepts used to unfold various feedback components, (b) the analysis of integral data used to construct feedback reactivity models, (c) the comparison of FFTF reactivities with mechanistic feedback models in the SASSYS/SAS4A code system, and (d) the pretest calculations for the LOFWOS test series, using the new FFTF feedback models.