Selection of natural circulation as the means for providing coolant flow through the reactor, coupled with a 42-kW/liter core power density, results in a number of benefits to help safety SBWR objectives. Compared to existing forced circulat ion plant, the natural circulation SBWR offer low fuel cycle costs, fewer operat ional transients, and increased thermal margin for transients expected to occur. In addition, elimination of recirculation loops , pumps, and controls needed for forced circulation substantially simplifies the design. An isolation condenser (used in many earlier BWR designs) is placed in the isola tion condenser pool. When the reactor vessel is isolated from the turbine conden ser, the isolation condenser control the reactor pressure automatically without t he need to remove the fluid from the reacotr vessel. Thus, conventional BWR safe ty/relief valves- which open and close to discharge reactor vessel steam to the suppression pool-are not needed in the SBWR.
The gravity-driven core cooling system provides a simple approach to emergency c ore cooling, eliminating the need for puumps or diesels. It requires more water in the reactor vessel above the core and the additional depressurization capacit y so that the reactor can be depressurized to very low pressure and so that gra vity flow from the elevated gravity driven cool pool can keep the core covered.
However, because there are no large pipes atteched to the vessel near or below t he core elevation, the design ensures full core coverage for all design-basis ev ents. The additional water provided also has other benefits, such as reduced pre ssure rate for transients and substantially more time before core uncovering in multiple failure scenarios.