3D analysis of BWR stability in low flow condition

The phenomenology of BWR instability under certain low flow condition has been studied as a generic issue by 3D reactor kinetic analysis. A BWR dual pump trip transients was considered as a standard problem of the LaSalle-like event, together with several sensitivity studies by applying intentional conditions to the important operational parameters such that the instability might be easily initiated. Important knowledge obtained from this study were on the spatial and dynamic behaviors of void, flow and reactor power during the instability phenomena. The amplitude of void oscillation has been found to become larger in the lower core region close to the boiling boundary. Also, the effects of inlet subcooling and bottom heavy power distribution have been found to be of similar nature in enhancing the instability phenomena. Concluding remarks: The BWR stability in natural convection condition has been studied as a generic issue, but based on the calculation of the LaSalle-like event considering dual recirculation pump trip for a typical 1,100 MWe BWR unit, applying some intentional conditions for generating the instability phenomena . The study is not yet completed, but many knowledge useful for safety evaluation of the phenomena have been obtained. The instability in the natural convection is more enhanced by increasing the core inlet subcooling, or decreasing the inlet enthalpy. In the meantime, the bottom heavy power distribution has been known to be another cause of instability, as being identified in this work. These two phenomena were found to be of similar nature in that the boiling boundary is made to move toward the higher power region, where the largest void perturbation would possibly be occurring in the assumed power condition. The phenomena would be better understood by looking into the spatial void behaviors. A point to note is the spatial void behavior during the instability phenomena. The amplitude of void oscillation is found always larger in the lower core region close to the boiling boundary, and its oscillatory wave propagates upward with continuous time delay. Local void oscillation could be occurring ahead of other regions for some duration depending on the power distribution conditions, in which a very small power ripple has been generally observed as a precursor event to the possible global core oscillation. The reactivity feedback associated with space-time void behavior during the instability phenomena is an important point to study from the safety aspect, and 3D analysis would have to be emphasized for best estimating the phenomena. The point to note as a future problem is whether the reactivity perturbation during the instability could actually approach to or exceed a dollar level under the conditions to be possibly existing. Further studies are underway on this aspect. (authors)