15.25. Natural circulation, or more accurately, natural recirculation of the coolant, is the most interesting thermal-hydraulic feature of the SBWR.

It is dependent on the difference in density between the cooler returning and feed fluid mix in the downcomer leg and the steam-water mixture in the core and chimney. As pointed out in §9.131, the flow rate depends on the relative influence of the density difference “driving force” and the frictional flow resistance. Hence, a short core of relatively low flow resist­ance is needed to assure an adequate recirculation rate. To assure reactor stability, the core flow must be maintained above certain levels which depend on the reactor power.

15.26. Reactor stability during normal operation and anticipated tran­sients is an important design consideration for all BWRs. For example, reactivity feedback instability of the reactor core could result in power oscillations. Hydrodynamic channel instability could inpede heat transfer to the moderator and also drive the reactor into power oscillations. Finally, the total system stability depends on the basic process dynamics. Stability criteria are stated in terms of a decay ratio, which is defined as the ratio of the magnitude of the second oscillatory overshoot to that of the first overshoot resulting from a step perturbation. The lower the ratio, the more stable the system. SBWR stability decay ratios have been modeled and found to be at least a factor of 2 lower than those for currently operating BWRs [3].