Flow excursion time scales in the advanced neutron source reactor

Abstract

Flow excursion transients give rise to a key thermal limit for the proposed advanced neutron source (ANS) reactor because its core involves many parallel flow channels with a common pressure drop. Since one can envision certain accident scenarios in which the thermal limits set by flow excursion correlations might be exceeded for brief intervals, a key objective is to determine how long a flow excursion would take to bring about a system failure that could lead to fuel damage. The anticipated time scale for flow excursions has been examined by subdividing the process into its component phenomena: bubble formation, flow deceleration, and fuel plate heat-up. Models were developed to estimate the time required for each individual stage. Accident scenarios involving sudden reduction in core flow or core exit pressure have been examined, and the models compared with RELAP5 output for the ANS geometry. For a high-performance reactor such as the ANS, flow excursion time scales were predicted to be in the millisecond range, so that even very brief transients might lead to fuel damage. These results have been useful for determining the significance of momentary flow excursion events calculated for accident situations in the ANS reactor. In addition, the methods presented are applicable for evaluating the timing of flow excursion transients in other facilities as well.