Patent Number: 053032756
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to energy generation systems and, more particularly, to a forced-circulation dual-phase reactor. A major objective of the present invention is to provide for enhanced core neutron power stability during pump shutdowns in a forced-circulation boiling-water reactor (FCBWR). Nuclear reactors generate heat as a byproduct of fissioning in the reactor core, and generally remove this heat from the core using a liquid transfer medium. In dual-phase reactors, the core heat vaporizes the liquid transfer medium; this energy in the form of vapor pressure is readily transferred from the reactor for use elsewhere. The predominant type of dual-phase reactor is the boiling-water reactor (BWR). Accordingly, much of the following discussion concerning BWRs is readily extrapolated to other dual-phase reactors. In a BWR, heat generated by nuclear fission in a core can be used to boil water to produce steam. Water passing through the core without being vaporized is recirculated within a reactor vessel to provide a continuous flow of water through the core. The steam that is generated can be separated from the water and transferred from the reactor vessel to deliver energy. For example, the steam can be used to drive a turbine, which in turn can be used to drive a generator to produce electricity. In the process, the steam condenses and can be returned to the vessel as feedwater. The condensate is merged with the internally recirculated water and continues to aid heat transfer. BWRs can be distinguished by the means employed to recirculate the water within the reactor vessel. Forced-circulation boiling-water reactors (FCBWRS) rely primarily on pumps to drive the water along a recirculation path. Natural-circulation boiling-water reactors (NCBWRS) rely primarily on the driving force provided by the density difference between a downcomer and a steam column above the core. NCBWRs have the advantage of simplicity. However, their inherently lower pumping capacity limits reactor power output. Accordingly, the largest capacity BWRs are all FCBWRS. The distinction between FCBWRs and NCBWRs notwithstanding, FCBWRs are preferably designed to take advantage of natural circulation to allow decay heat to be removed from the core in the event the pumps are shut down. In a NCBWR, water rising up from the core is guided vertically to promote steam-water separation and to support a relatively low-density steam/water head above the core. Water recirculates down the downcomer annulus between the reactor vessel and the chimney and core. The water in the downcomer is denser than the steam and water mixture in the core and chimney region. The difference in density forces circulation up through the core and chimney and down through the downcomer. Natural circulation provides limited power output in part because its limited circulation rates allow the water flowing through the core more time than is optimal to be converted to steam. The excess boiling results in a larger volume of steam in the core. This larger steam volume adversely affects core stability, as the stability-decay ratio of the nuclear fission rate is dependent on the ratio of two-phase pressure drop to single-phase pressure drop. In NCBWRs, this problem is addressed by limiting the amount of heat generated by the core, and thus the power output of the reactor. FCBWRs, on the other hand, are typically designed so that they exceed the power output that would be available using natural circulation alone. Total pumping power failure in an FCBWR operating at full capacity could result in excess boiling and core instability. To minimize the likelihood of total pumping power failure, several independent pumps are provided. Despite the levels of safety afforded by redundant pumping, it is still worthwhile to enhance the throughput due to natural circulation in a FCBWR. Natural circulation is especially attractive as a safety backup because it does not depend on active components. Thus, improvements in natural circulation are highly desirable in FCBWRs. The above-identified patent discloses the use of bypass valves to augment natural circulation in a FCBWR. The valves are designed to open automatically when the pumps stop pumping, thereby increasing the flow cross section and enhancing natural circulation. The valves are automatically closed while the pumps are operating to block backflow through the valves. The bypass valves have moving parts, which can give rise to reliability issues. An object of the present invention is to provide the advantages that these bypass valves provide for FCBWRs without requiring moving parts. SUMMARY OF THE INVENTION The present invention is employed in the context of a FCBWR that promotes natural circulation when pumps are not operating. To this end, the FCBWR can include a chimney supporting a buoyancy head above a reactor core. The pumps impose a pressure differential across a boundary in a fluid circulation path. The present invention provides apertures through the boundary to enhance natural circulation induced by the buoyancy head supported by the chimney. In accordance with the present invention, these apertures are fluidic diodes that constrain backflow across the boundary when the pumps are operating. In a typical implementation of the invention, the chimney function can be provided by an appropriately designed steam-separator, and the circulation path boundary can be a pump deck. A fluidic diode functions as a check valve except that it has no moving parts and does not completely seal. Typically, a fluidic diode includes an aperture that is relatively unobstructed during "downstream" flow. However, "upstream" flow is diverted across the aperture causing turbulence with further upstream flow. The turbulence constrains the upstream flow through the aperture. One type of fluidic diode comprises a set of nested cylinders. Each cylinder includes a diverter that extends radially inward and downstream. The diverters guide downstream flow into a central aperture without substantial turbulence. Upstream flow is trapped between cylinders and is diverted radially inward across the aperture. The crossflow into the aperture mixes with further upstream flow inducing turbulence and thus restricting the upstream flow. In the context of the present invention, the downstream flow that occurs with relatively low resistance is natural circulation flow; the backflow induced during pumping is the upstream flow that faces greater resistance. The fluidic diode thus approximates the effect of a check valve that admits downstream flow but prevents upstream flow. Since the fluidic diode does not completely seal, its effectiveness is less than 100%. However, a functional advantage can be attained provided the resistance to upstream backflow is at least twice that in the downstream flow during natural circulation. A major advantage of the present invention is that core neutron power stability is enhanced in the event of a pump shutdown, without the disadvantages of moving parts that 1) can fail to move as intended, for example, a value might stick in the closed position when it should open in response to a pump trip; 2) break due to fatigue and possibly adversely impact other reactor components. Stability is improved since enhanced natural circulation decreases the ratio of the two-phase pressure drop to the one-phase pressure drop in the core. This in turn improves the stability-decay ratio of the nuclear fission rate, and thus core neutron power stability. Since fluidic diodes contain no moving parts, they provide increased reliability. The invention is compatible with existing FCBWRs and does not require significant redesign efforts. The invention can respond automatically to pump power failures without relying on active feedback systems. Thus, the invention provides for reliable and effective response to reduction in forced recirculation. These and other features and advantages of the present invention are apparent in the following description with reference to the drawings below.