Patent Number: 051456397
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear reactor plants and, more particularly, to isolation condensers for such plants. A major objective of the present invention is to provide an a simpler and more compact isolation condenser characterized by improved flow stability. Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining. Dual-phase reactors store heat generated by the core primarily in the form a phase conversion of a heat transfer medium from a liquid phase to a vapor phase. The vapor phase can used to physically transfer stored heat to a turbine and generator, which are driven to produce electricity. Condensate from the turbine can be returned to the reactor, merging with recirculating liquid for further heat transfer and cooling. Dual-phase reactors are contrasted with single-phase reactors, which store energy primarily in the form of elevated temperatures of a liquid heat-transfer medium. Pressurized water reactors (PWRs) are considered single-phase in that the reactor coolant is maintained in a liquid state, although heat from the pressurized water is used to boil a secondary coolant to drive a turbine. The primary example of a dual-phase reactor is a boiling-water reactor (BWR). The following discussion relating to BWRs is readily generalizable to other dual-phase reactors. Modern BWRs provide for the removal of reactor decay heat from a reactor pressure vessel in the event the turbine becomes isolated from the reactor. During a turbine shutdown, a valve on the main steam line is closed preventing steam from reaching the turbine. Even after the reactor is shut down by fully inserting the control rods, decay heat continues to be generated for a period of days. This heat generates steam, which if left to accumulate in the reactor pressure vessel, could exceed the vessel's pressure-bearing specifications, potentially inducing a breach. An isolation condenser is one type of system designed to handle steam during turbine isolation to avoid excessive pressure accumulation. A typical isolation condenser includes an upper distributor chamber and a lower collector chamber. The chambers are immersed in the water of a condenser pool. The chambers are coupled via an array of vertical tubes which extend therebetween and through intermediate pool water. During isolation, steam is conveyed to the distributor chamber. The steam is forced through the tubes, which through heat exchange with the condenser pool, condense the steam so that water flows into the collector chamber. A drain conduit coupled to the outlet chamber conveys the condensate to the reactor to replenish its coolant supply. The performance of such a condenser can be impaired when the condenser pool has been heated to saturation. At that point, steam generated in the pool can insulate the heat-exchanger tubes, limiting further heat transfer and causing thermal cycling in the manifold. The thermal cycling can stress the condenser, impairing its structural integrity and inducing pool-side flow instability. Other problems with such a conventional isolation condenser concern the amount of material required to ensure the distributor and collector can withstand the large pressure differentials that can develop between their interiors and the condenser pool. Pressure differentials of up to about 1250 pounds per square inch must be accommodated. The relatively flat boundaries, including the tube sheets, of the disk-shaped distributor and collector require considerable thickness to withstand this pressure. The thickness not only adds bulk and mass to the condenser, but subjects it to thermal stresses due to the larger thermal gradients that thicker material can sustain. What is needed is a more compact, lightweight isolation condenser that is less subject to flow instability. In addition, the condenser should be economical to manufacture and maintain. SUMMARY OF THE INVENTION In accordance with the present invention, a dual-phase reactor plant incorporates an isolation condenser that isolates a contiguous volume that is divided by a partition into a distributor plenum and a collector plenum. These plenums are coupled by tubes of a manifold that pass through a condenser pool in which the condenser is disposed. The condenser has a base, a vertically extending sidewall, and a domeshaped top. The cross section of the sidewall is such that the area it encloses measures at least the square of one-fourth its perimeter. The plant includes a containment structure, typically of concrete, that defines a dry well, a wet well, and a condenser well. The reactor is in the dry well, which is otherwise filled with noncondensible gases. The condenser is in the condenser well, immersed in coolant. The wet well holds a suppression pool of coolant. The reactor produces vapor which drives a turbine, which in turn can be used to drive a generator to produce electricity. During normal operation, conduits convey vapor from the reactor to the turbine and condensate from the turbine to the reactor. When the turbine is decoupled from the reactor, a resulting pressure buildup triggers a relief valve that permits vapor to escape to the distributor plenum of the condenser. The vapor is distributed to the manifold tubes where they give up heat energy to the suppression pool. The loss of energy results in condensation of the vapor to liquid. The liquid flows through the sidewall into the outlet chamber. From the collector plenum, the liquid flows into the reactor through a conduit that mates with the reactor below its nominal liquid level. Noncondensible gases accumulating in the collector plenum can be conveyed by a conduit to the suppression pool. The present design provides for enhanced flow stability relative to the conventional condenser. In the latter case, steam forming on the outside of a heat-exchanger tube tends, under the influence of gravity, to flow upward. Since the tubes are vertical, the rising steam forms an insulating sheath about the tube, impairing its heat-transfer capability. Movement of steam reaching the top of a tube is then impeded by the tube plate of the distributor chamber. Thus, steam remains in the manifold vicinity, impeding the performance of the condenser and threatening its integrity. In the present design, the heat-exchange tubes extend radially from the condenser chamber. Rising steam escapes the tube at which it was generated, perhaps passes between the tubes of another array, and then rises unimpeded to the surface of the condenser pool. This relatively free movement of steam induces convection, minimizes hot spots within the pool, and further ushers steam away from the tubes. Thus, optimal heat exchange is maintained and the structural integrity of the condenser is prolonged. The radial arrangement of heat-exchanger tubes has the further advantage that longer tubes do not require a taller condenser. Because vertical constraints do not have to be divided three ways between two chambers and the tubes, the single condenser chamber can be made relatively tall, as can each of its plenums. The relatively tall collector plenum permits a relative high entrance level for a noncondensible gas outlet, providing improved separation of noncondensible gases from condensate. The novel condenser geometry provides favorable distribution of stresses induced by pressure differentials between the suppression pool and the internal volume of the condenser. The improved stress handling allows thinner boundary walls to be used with less reinforcement. This in turn reduces thermal gradients through the walls and hot spots within the walls. Reducing thermal gradients and hot spots reduces thermal stresses and prolongs the useful lifetime of the condenser. The compactness provides proportionate advantages in the size of the containment structure, which strongly impacts plant cost. In addition, the vertical design combined with compactness, along with the employment of a relatively small and light cover, minimize the requirements for overhead access. This makes maintenance more convenient and more economical. These and other features and advantages of the present invention are apparent from the description below with reference to the following drawings.