Patent Number: 043476236
Section: description

DETAILED DESCRIPTION OF THE DRAWINGS Referring now to the drawings, there is illustrated a reactor vessel 10 with inlet connection 12 and outlet connection 14. A single cooling loop is illustrated. However, it is to be understood that in the typical case the reactor vessel would have three or more coolant loops. The system of the invention to be described hereinafter could be applied to one or all of the additional coolant loops. The vessel houses a reactor core 16 incorporating the fuel rods and associated structure. In normal operation, coolant water enters via the inlet connection 12, passes down along the reactor core 16 and then up through the reactor core into a plenum 18 to exit the vessel 10 through the outlet connection 14. For operation of the system according to the invention, the valve 19 is opened to permit the coolant water that has been heated by being passed through the reactor core 16 to enter the downcomer pipe 20. As noted, the downcomer pipe functions as a subcooler by the hydrostatic pressure developed through the vertical extent of the downcomer pipe which increases the difference between the saturation temperature of the coolant and the then existing temperature of the coolant by raising the coolant saturation temperature above the then existing coolant temperature of the coolant. The subcooling effect is obtained without loss of coolant temperature or heat loss. However, it should be noted that the subcooling could be obtained by the injection of cold water from a separate cold-water source or could be obtained by a heat exchanger. The output of the subcooler is connected to the flash jet pump 22. The flash jet pump 22 is shown in FIG. 2 to comprise a nozzle 23 incorporating a convergent section 24, throat 26 and divergent section 28. The entire nozzle 23 is contained within a housing 30 producing an annulus 32 surrounding the nozzle 23. Coolant to replace the coolant loss by a rupture in the primary cooling circuit is drawn through the conduit 34. One source for the make-up coolant is the building sump 50 which collects the coolant lost from the primary cooling system. The coolant passing through the reactor core takes on the heat being developed by the decay heat of the reactor core. A heat exchanger 44 removes heat from the make-up coolant to maintain a system equilibrium. Secondary coolant water for this purpose is available in the event of an emergency. The secondary coolant water passes into the heat exchanger from connection 46 and is drawn off from the heat exchanger at connection 48. A valve 35 in the make-up coolant conduit 34 prevents water from the sump 50 from entering the jet pump 22 and subcooler 20 before the jet pump commences operation. The make-up coolant and the supersonic two-phase flow from the flash jet nozzle 23 pass from the housing into a high-velocity flow section 36 and then into a diffuser section 38. Even after partial mixing of the two streams in section 36, the combined flow is still at supersonic speed until a compression shock completes the mixing and condensation process, generating a substantial pressure rise for pumping the combined flow into the vessel. Thus, the high-velocity section and diffuser section 38 convert the kinetic energy from the high-velocity flow into static pressure in conduit 40 thereby pumping the flow against the hydrostatic pressure head through an open valve 52 and through the inlet connection 12 into the vessel 10. Flash jet pump 22 has a nozzle area ratio (the ratio of the nozzle outlet area to the nozzle throat area) in the range of 10:1 to 50:1. Flashing of hot water in the divergent nozzle section produces supersonic flow at the nozzle outlet. Thus, the thermal energy of the hot water is converted into the kinetic energy of a supersonic two-phase jet. The combination of the subcooler 20 and the convergent nozzle section result in incompressible flow in the convergent section 24 and throat 26, converting into compressible flow in the divergent nozzle section 28. The subcooler 20 interaction with the nozzle 23 results in a steady outlet pressure that is essentially independent of the inlet pressure. This is an important stabilizing effect in conjunction with the flow rate stabilization that is described more fully hereinafter. A startup tank 51 is utilized to maintain a pressure substantially lower than the pressure in the leg of the system incorporating the jet pump 22. A reduced pressure is a prerequisite to the operation of the jet pump 22. Reduced pressure is required to cause flashing and acceleration to supersonic speed of the hot water in the divergent section of the nozzle. After startup, the high-velocity flow and condensation produces a self-sustaining low pressure region for both the pump operation and for the suction of the make-up coolant through conduit 34. The reduced pressure is produced in the startup tank 51 by collecting by gravity in the tank 51 all the water in the subcooler pipe 20, jet pump 22 and conduit 40 and then cooling the water by cold water flow through the coil 57. Cooling of the water causes a partial condensation of water vapor in tank 51 thereby reducing the total pressure in the leg. After the system is operational, the valve 53 may be closed. The initial charge of coolant into the reactor vessel after a loss-of-coolant accident is provided by a transfer system from a storage tank 86 containing a quantity of borated water, as in FIG. 3. Motive power for the pumping action is provided by hot water in a flash jet pump 78 which is essentially similar in its structural particulars to the flash jet pump 22 with exceptions set forth hereinafter. Hot water to power flash jet pump 78 is provided from a hot-water storage tank or the secondary side of the heat exchanger 84 through a subcooler in the form of downcomer pipe 82. The subcooled water passes through a valve 81 into the convergent-divergent nozzle 80 producing supersonic flow that draws the borated water through the conduit 85 and valve 88. The resulting combined flow enters a high-velocity section 89 and passes into the diffuser section 90. An increase in pressure of the resultant flow is produced which is sufficient to force the borated water through conduit 54 and up the conduit 55 through valve 42 and into the inlet connection 12 of the reactor vessel 10. The flash jet pump 78 operates from heated water on the secondary side of the heat exchanger or a storage tank which is always available subsequent to normal operation of the nuclear reactor. The immediate startup of the flash jet pump 78 is obtained by opening the valves 81, 88 and 42. No other moving parts or additional valves are essential to the operation. The area ratio of the nozzle 80 in the flash jet pump 78 in this application is in the range of 15:1 to 70:1. Since the pressure in the secondary side of the heat exchanger will not vary as much as the potential pressure variations in the reactor vessel, it may be possible to reduce or eliminate the subcooler in some applications. Referring particularly to FIGS. 4 and 5, the contrast between choked flow (sonic velocity) and flow in the flash nozzle are illustrated. FIG. 4 illustrates the effect of the inlet pressure on a supersonic nozzle with no subcooling. The maximum mass flow rate for an inlet pressure of 200 p.s.i.a. is substantially three times that for an inlet pressure of 50 p.s.i.a. It will be apparent that with such a system, the design criterion would be strongly dependent upon the minimum pressure at which the system must operate and the system would have excess capacity at all other pressures. As previously noted, the expansion pressure at the outlet of the supersonic nozzle is proportional to the inlet pressure. Referring now to FIG. 5, there is illustrated the mass flow rate versus expanion pressure relationship for the subcooled case. It will be seen that the maximum flow rate does not increase with the inlet pressure. For a given nozzle design, the expansion pressure at the nozzle outlet is independent of the inlet pressure. Thus, a highly stable relationship exists and a constant mass flow rate will be produced substantially independent of the system pressure over a wide range.