Patent Number: 047537736
Section: summary

FIELD OF THE INVENTION This invention relates to a steam generator heated by liquid metal, such as may be used in nuclear energy power plants. More particularly, the invention relates to a steam generator for using the heat from a nuclear reactor coolant system to generate high pressure steam and provide improved fail-safe conditions for a reactor coolant system. BACKGROUND OF THE INVENTION Nuclear reactors cooled by a liquid metal such as sodium are well known, and the circulating hot liquid metal coolant has been utilized for generating power by heat transfer from the liquid metal to water, which in turn is converted to high pressure steam. The steam is then cycled to a turbine-generator power conversion system for generating electricity. A major drawback and a safety problem in such steam generators is the need to protect the system against the violent metal-water reactions that may result from a leak in the liquid metal and/or water circulation systems. Should the liquid metal reactor coolant come into direct contact with steam or water leaking out from the steam generator tube, a violent chemical reaction occurs with a corrosive byproduct (e.g., NaOH) and free hydrogen. Conventional reactor-power plant systems employ an intermediate liquid metal heat exchange circuit to protect the reactor core in the event of a leak. Typically, such an intermediate system includes an expansion vessel, complex piping circuits, a heat exchanger, a pump, liquid metal purification equipment, fill and drain systems, electrical preheat systems, and the attendant instruments, controls and structures for housing and support of these components. From the standpoint of efficiency, design simplicity and conservation of physical space and other resources it would be highly advantageous to eliminate such intermediate systems, however a steam generator design of exceptional reliability or with special protective features such as a double tube wall design would be required. A drawback of known double tube steam generator systems is their inefficiency in transferring heat from the liquid metal coolant to water. Prior art steam generators of double wall construction have relied on inert gas as a heat transfer medium, however an inert gas barrier is extremely inefficient for this purpose. U.S. Pat. Nos. 3,545,412, 3,613,780 and 3,907,026, for example, show apparatuses wherein closely placed tubes containing liquid metal or water are surrounded by inert gas, or wherein water tubes are run through a sleeve containing inert gas separating the water and liquid metal coolant. Other prior art duplex tube steam generators have used bonded tubes or duplex tubes with mercury as the intermediate heat transfer agent. Bonded tubes can experience difficulties associated with loss of contact stress due to thermal aging. Duplex tubes with mercury pose a safety problem for the reactor core, because typical liquid metal coolants, i.e., sodium, react with the mercury to form an amalgam. Furthermore, conventional steam generators are large and bulky due to use, typically, of straight tube design. As a result, integration of a steam generating system with the reactor is often complex and costly. Furthermore, such steam generator designs present difficulties in locating a failed tube and in accomodating tube-to-tube and tube-to-shell temperature gradients. Conventional steam generator systems are also characterized by fabrication and repair drawbacks. Many of the structures are large and custom-manufactured for the particular plant they are used in; and in the event of a structural failure, such as a ruptured water pipe, the entire plant must be shut down in order to isolate the source of the trouble, which can lead to the development of significant temperature transients. Special structures (e.g , gantries or large cranes) may also have to be assembled to repair or replace the damaged components. Finally, conventional steam generator systems often require additional auxiliary systems for decay heat removal. SUMMARY OF THE INVENTION Accordingly, it is a primary object of this invention to provide a novel and highly reliable liquid metal steam generator particularly well suited for application in a nuclear power plant. It is a further object to provide a liquid metal steam generator having improved reliability and safety over prior art designs. It is a further object of this invention to provide a modular steam generator which has an integral barrier between the hot liquid metal and water systems which does not require a pump, separate piping or an intermediate heat exchanger. It is a further object of this invention to provide a steam generator with an efficient heat transfer path between the liquid metal coolant and water. All of the aforementioned disadvantages of the prior art are addressed, and the aforementioned objects attained, by the present invention. The steam generator disclosed herein utilizes stagnant (non-circulating) liquid metal as a heat transfer medium, which is confined to the annulus area of a compact co-axial double tube assembly. Water is conducted through the inner tube, and the double tube assembly is immersed in hot liquid metal coolant. The liquid metal in the annulus area acts as an efficient heat transfer agent between the reactor coolant and the water. A multiplicity of double tube assemblies are grouped together to form tube bundles, and the tube bundles are fabricated to assume a configuration permitting optimal heat exchange from the hot liquid metal coolant (in which the tube bundles are immersed) and the water carried in the inner tube of each double tube assembly. The particular configuration of the tube bundles is such that a compact unit is formed, which additionally provides great surface area for heat transfer between the liquid metal coolant and the water, across the stagnant liquid metal barrier in the annular gap. Many such configurations, affording compactness and efficient heat exchange, are possible. For example, single or multiple U-shaped tube bundles, a helical coil or concentric or interlocking multiple helical coils, or, most preferably, a serpentine (sinusoidal) coil. The large number of double tube assemblies also provides increased safety in operation, because in the event of an inner tube failure, the metal-water reaction is confined to the annulus area of the duplex tube. The liquid metal in the annular gap is the same as or compatible with the liquid metal coolant, therefore an outer tube failure has no hazardous effects. The steam generator of the present invention may be viewed as the juxtaposition of three closed systems: a circulating water system, a stagnant liquid metal barrier system, and a circulating liquid metal coolant system. The circulating water system begins at a water inlet that may be connected to an outside feedwater source. From the inlet, the water proceeds via a multiplicity of water-carrying tubes into the body of the steam generator, each of the tubes joins a separate outer tube to form a concentric double tube assembly, and bundles of such double tubes are wound in a particular configuration as mentioned above to form a heat exchanger unit or module. By heat transferred from the outside of the double tube across the annular gap, the water is converted to superheated steam which exits the system at a steam outlet, which may in turn be connected to a turbine generator for the production of electricity. The stagnant liquid metal barrier system begins at a disengaging chamber, which is completely closed within the steam generator during normal operation of the system. Water-carrying tubes enter the disengaging chamber, where the tubes join with the enclosing outer tubes of the concentric double tube assemblies. The annular gap formed by the joining of inner (water-carrying) and outer tubes is in open communication with the disengaging chamber. The multiplicity of double tubes, as mentioned above, forms a heat exchange unit or module, having a configuration such as a single or multiple U-turns, a helical coil pattern, or a serpentine (sinusoidal) coil pattern. The double tube continues from the heat exchange unit to a closed disengaging chamber where the outer tubes of the double tube assemblies end, and the inner tubes continue on to a steam outlet. The initial disengaging chamber for the outer tube may be the same as or different from the terminal disengaging chamber for the outer tube. Part of the volume of the annular gap between the inner tube and the outer tube of each double tube assembly is filled with a liquid metal which effectively transfers heat from the outside of the double tube assembly to the inner (water-carrying) tube. The volume of the disengaging chamber(s) and any unfilled volume of the annular gap are preferably filled with an inert gas, such as argon. The circulating liquid metal coolant system begins at a hot liquid metal coolant inlet which may be connected to the cooling system of a nuclear reactor. Hot liquid metal enters through the hot liquid metal coolant inlet and is directed into contact with the double tube bundles. Heat from the liquid metal coolant is transferred across the barrier liquid metal in the annular gaps of the double tube assembles to the water carried in the inner tubes, creating superheated steam. After transferring heat to the double tube heat exchange unit, cold liquid metal coolant flows away from the unit and is directed out of the steam generator via a cold liquid metal coolant outlet, which may be connected to the core inlet area of a nuclear reactor. Preferably the steam generator assembly described herein is interconnected with a nuclear reactor vessel as detailed in commonly assigned, co-pending U.S. application Ser. No. 582,096, filed Feb. 21, 1984, which is incorporated herein by reference. The double tube design of the steam generator allows the closest possible contact between the three closed systems while still providing a barrier between the liquid metal coolant and the water. Using liquid metal as a heat transfer agent is much more efficient than inert gas. Using a multiplicity of double tube assemblies increases the heat transfer surface area in direct contact with the hot liquid metal coolant, while dramatically reducing the volume of liquid metal coming into contact with water, in the event of a leak in an inner tube. In addition, using a coil configuration (e.g., helical coil, serpentine coil, etc.) conserves space and inherently accommodates thermal gradients while permitting unobstructed flow of the liquid metal coolant. Generally, the steam generator comprises a vessel that is subdivided into upper (hot) and lower (cold) liquid metal plenums. In operation, hot liquid metal flows into the steam generator upper plenum, flows through a distributor inlet above the one or more heat exchange units (modules), flows downward over the heat exchange units, transferring heat through the barrier liquid metal (in the double tube annular gaps) to the water flowing within the inner tube of the double tube assemblies. The cooled liquid metal exits into the steam generator lower plenum and is discharged from the steam generator vessel. Optionally, an electromagnetic or centrifugal pump may be connected to the lower plenum, e.g., in the core of the steam generator (see FIG. 1), and a portion of the liquid metal coolant reaching the lower plenum passes into the pump and is discharged at high velocity through a pump eductor back to the reactor. The remaining liquid metal coolant in the lower plenum enters the eductor and passes, mixed with the flow from the electromagnetic pump discharge, through a diffuser to convert the velocity head to a pressure head, and thence to the reactor inlet. As disclosed in more detail below, the double tube assemblies may be used directly for decay heat removal, eliminating the need for a separate decay heat removal system. In addition, the embodiments of the steam generator described herein premit the use of an external air cooling system as an alternative means of decay heat removal.