Patent Number: 053032756
Section: claims

1. A forced-circulation dual-phase reactor comprising: a reactor vessel;  a reactor core disposed within said vessel and defining a core inlet plenum below said core;  a chimney means disposed above said core for guiding fluid exiting upward from said core along a substantially vertical path so as to support a steam/water column therein;  a fluid return path for recirculating fluid exiting said chimney means to said core inlet plenum, said fluid return path defining an upstream direction and a downstream direction;  pump means for controllably enhancing a pressure differential across a boundary in said fluid return path so as to urge fluid in said downstream direction along said fluid return path; and  fluidic diode means for asymmetrically constraining fluid flow across said boundary, said fluidic diode means providing a path for at least some fluid to bypass said pump means and to pass relatively freely across said boundary in said downstream direction, said fluidic diode means relatively restricting fluid flow across said boundary in said upstream direction;  whereby, when said pump means is not operating, said fluid diode means provides relatively unrestricted natural circulation flow across said boundary, and when said pump means is operating, said fluid diode means relatively restricts backflow across said boundary.  a reactor pressure vessel having a cylindrical vessel wall;  a radioactive core for generating heat, said core being disposed within said reactor pressure vessel and defining a core inlet plenum within said vessel and below said core;  chimney means for supporting a steam/water column;  a cylindrical shroud extending vertically at least partially along the vertical extent of said core to a level below said core, said shroud defining a radially inward boundary of a downcomer, said vessel wall defining a radially outward boundary of said downcomer;  recirculation pump means, including internal components disposed within said vessel, said pump means providing for a pressure differential from said downcomer to said core inlet plenum, said pressure differential being positive when said pump means is operating;  a pump deck located at the base of said downcomer, said internal pump components being mounted on said deck, said deck and said shroud defining a downcomer/plenum boundary between said core inlet plenum and said downcomer; and  fluidic diode means for asymmetrically constraining fluid flow, said fluidic diode means being mounted in said pump deck, said fluidic diode means allowing fluid to pass relatively freely through said pump deck in said downstream direction, said fluidic diode means restricting fluid flow through said pump deck in said upstream direction;  whereby when said pump means is not operating, said fluidic diode means provides relatively unrestricted natural circulation flow through said pump deck, and when said pump means is operating, said fluidic diode means relatively restricts backflow through said pump deck.  when fluid is flowing downstream, said diverter means induce relatively little turbulence so that downstream flow is relatively unrestricted, and  when fluid is flowing upstream, said diverter means induce relatively more turbulence so that backflow is relatively restricted.  when the pump means of said forced-circulation reactor are not operating, allowing at least some natural circulation of flow to bypass said pump means through apertures, and  when said pump means are operating, restricting backflow through said apertures.  when pumps of said forced-circulation reactor are not operating, allowing natural circulation flow to flow in said downstream direction through apertures in said recirculation path boundary so as to impose a relatively small resistance on said natural circulation flow; and  when said pump means are operating to force coolant across said boundary in an downstream direction so as to induce a backflow toward said pressure differential boundary, 2. A reactor as recited in claim 1 wherein the flow resistance through said fluidic diode means in said upstream direction is at least twice the flow resistance through said fluidic diode means in said downstream direction. 3. A reactor as recited in claim 1 wherein said fluidic diode means has no moving parts. 4. A reactor as recited in claim 1 wherein, when said pump means is operating, said fluid diode means relatively restricts said backflow by creating a flow-restricting turbulence. 5. A forced-circulation boiling-water reactor comprising: 6. A reactor as recited in claim 5 wherein said fluidic diode means includes plural radially nested cylinders, said cylinders having respective diverter means for diverting fluid downstream and radially inward so that 7. A method of improving core stability in a forced-circulation dual-phase nuclear reactor comprising the steps of: 8. A method as recited in claim 7 wherein said step of restricting backflow involves generating turbulence across said apertures. 9. A method of improving core stability in a forced-circulation dual-phase nuclear reactor, said reactor including pumps for forcing coolant in a downstream direction across a pressure-differential boundary along a recirculation path, said method comprising the steps of: