Patent Number: 051456397
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A nuclear reactor plant 100 comprises a concrete containment 101, a nuclear reactor 102 and a turbine 104 to be driven by reactor 102, as shown in FIG. 1. Turbine 104 is used to drive a generator to generate electricity. Reactor 102 includes a reactor vessel 106 and its internals, including a fissionable core 108. The activity of core 108 is regulated by inserting and withdrawing control rods 110. Reactor 102 contains water, up to a nominal level 112, and steam. The water circulates up through core 108 to transfer heat therefrom. Steam resulting from this heating exits vessel 106 via a turbine steam conduit 114. This steam condenses as it drives turbine 104, and the resulting condensate returns to vessel 106 via a feedwater conduit system 116. A dry well 118 houses reactor 102 and is otherwise filled with nitrogen. A gravity driven coolant system (GDCS) 120 is used to at least temporarily replenish coolant lost from vessel 106 during emergency operations. Upon turbine isolation, an isolation condenser system 122 is used to dissipate pressure, decay heat, and sensible heat from reactor 102. Condenser system 122 has a condenser well 124 and an condenser 200 submerged in a condenser pool 125 of water. Condenser pool 125 is vented via a vent conduit 128. During normal operation, a turbine valve 130 and a condenser valve 132 are open. During isolation condenser (IC) mode, condenser valve 132 stays open while turbine valve 130 is closed, diverting steam that would have driven turbine 104 through condenser conduit 134 to condenser 200. Condensate from condenser 200 flows back to vessel 106 via isolation return conduit 136 and valve 138, which is open during IC mode. Note that while conduits 134 and 136 are shown coupling independently with vessel 106, in practice they share connections with turbine conduits 114 and 116, respectively, to minimize the number of penetrations of the vessel wall. Normally, a vapor valve 140 remains closed during IC mode. However, vapor valve 140 can be opened to permit vapor, especially noncondensibles, to transfer from collector plenum 218 via conduit 142 to wet well 126. Valve 140 is typically opened during a passive coolant containment system (PCCS) mode, during which mode a GDCS valve 144 is also opened allowing water from GDCS 120 to flow through a conduit 146 to replace coolant lost from vessel 106. Condenser 200 includes a chamber 202 and a annular manifold 204, as shown in FIG. 2. Chamber 202 comprises a diskshaped base 206, a vertically-extending cylindrical sidewall 208, and a semispherical cover 210. Chamber 202 isolates the enclosed condenser volume 212 from pool 125. An annular partition 214 divides condenser volume 212 into an upper distributor plenum 216 and an annular lower collector plenum 218. Radially inward of collector plenum 218 is an inlet conduit 220, which serves as an extension of steam conduit 134. Inlet conduit 220 extends from the center of base 206 vertically well into distributor plenum 216. Inlet conduit 220 is a thick tube of stainless steel. The thickness helps insulate outgoing condensate in collector plenum 218 from heat of incoming steam rising through inlet conduit 220. Further insulation is provided by a vapor space 222 between partition 214 and inlet conduit 220. This vapor space 222 also allows differential thermal expansion of inlet conduit 220 and partition 214. Collector plenum 218 is coupled through an aperture 224 in base 206 to conduit 136, which serves to drain condensate back to vessel 106. In addition, condenser 200 includes a vapor trap tube 226 which extends vertically more than half-way up collector plenum 218. Vapor trap tube 226 is coupled to conduit 142. When valve 140 is open, noncondensible gases accumulating in collector plenum 218 can escape into wet well 126 through conduit 142. Manifold 204 comprises an outer array of 48 tubes 230 and an inner array of 48 tubes 232, for a total of 96 tubes. Each tube 230, 232 extends radially outward from distributor plenum 216 through sidewall 208 into pool 125, curves through pool 125, and extends radially inward through sidewall 208 to distributor plenum 218. Thus, manifold 204 provides the only fluid path within well 124 between distributor plenum 216 and collector plenum 218. Of course, there is another fluid path between plenums 216 and 218 through reactor vessel 106. A tube support 234 helps maintain the structural integrity of condenser 200. During turbine isolation, steam from vessel 106 rises through conduit 136 and through inlet conduit 220, where it accumulates in distributor plenum 216. Steam accumulating in distributor plenum 216 is ushered out tubes 230 and 232, where it gives up thermal energy to pool 125 and at least partially condenses to water. The condensate flows into collector plenum 218, whence it can drain through base aperture 224 and conduit 136 back to reactor vessel 106. Vapor and noncondensible gases rise through liquid accumulated in distributor plenum 218. Most of the residual vapor condenses before leaving the liquid. Noncondensible gases of course do not liquefy and thus accumulate at the top of collector plenum 218. Under conditions producing sufficient noncondensible gas to interfere with the operation of condenser 200, valve 140 is opened to allow the noncondensible gases to escape into wet well 126. A major advantage of condenser 200 is that there is only one boundary potentially subjected to severe pressure differentials, e.g., 1250 pounds per square inch. This boundary is constituted by sidewall 208 and cover 210. These components have cylindrical and spherical geometries that enclose maximum volume with for a given peripheral area, thus exhibiting favorable pressure bearing characteristics. In the conventional isolation condensers, two pressure bearing chambers are required, each having one or more flat surfaces exposed to the condenser pool. This less optimal geometry requires the additional thickness, reinforcement, and bulk. Of course, condenser 200 does provide separate distributor collector plenums 216 and 218. However, partition 214 which separates them can be relatively thin since the pressure differential across partition 214 is relatively small due to the fluid coupling through manifold 204. A flat geometry is provided by base 206, which is best suited for interfacing with conduits 134, 136, and 142. Since base 206 opposes concrete containment 101, it does not have to bear a pressure differential. By way of comparison, a typical conventional condenser has two disk-shaped chambers with 7' diameters and wall thicknesses on the order of 3.75', although tubes extend to a diameter of 8'; the weight of such a condenser is about 43 tons. Similar capabilities can be provided in accordance with the present invention where sidewall 208 has a diameter of 3' and a thickness of 2.5". Cover 210 can be even thinner at 2.0". The gross weight of condenser 200 is about 13 tons, a reduction of about 70%. In order to provide thermal insulation, inlet conduit 220 can be about 2.0" thick. However, partition 214 can be relatively thin at about 0.375" thickness. Note that the relatively small and lightweight cover 210 provides relatively convenient access to the internals of condenser chamber 202. Each tube 230, 232 is shown as being in a single radial plane so that it enters collector plenum 218 in the same circumferential coordinate as it exited distributor plenum 216. Alternatively, the tubes of the manifold can be coiled so that greater lengths can be achieved for a given manifold diameter. In addition, the lengths of tubes in inner and outer arrays can be equalized by assigning tighter pitches to the coils of inner array. Whether coiled or not, the tubes should return to the chamber at a level below the top of the exit for the noncondensible gases. While the preferred condenser chamber geometry includes a disk-shaped base, a cylindrical sidewall, and a semispherical cover, other geometries are provided for. For example, the base can be semispherical so that the condenser chamber superficially resembles a reactor pressure vessel. With this capsule geometry, the condenser chamber can be spaced above the bottom of condenser well 124, the semispherical bottom being adapted for resisting potentially severe pressure differentials. While cylindrical and spherical geometries are ideally suited for resisting pressure and isolating an enclosed volume from the condenser pool, other geometries are provided for. However, the condenser chamber should enclose a contiguous volume containing both the distributor plenum and the collector plenum. More specifically, the geometry of the chamber is such that every point on a line segment having as its endpoints points within said volume is in that volume. In other words, it is convex. Furthermore, while the chamber geometry need not be spherical and cylindrical, it should be similar enough to those geometries to provide a substantial share of their advantages. The sidewall need not be as compact as a cylinder, which has a circular cross section, but it should be better than a box with a square cross section. In other words, the horizontal cross-sectional area of the sidewall should be greater than the square of one fourth of its perimeter. The cover should be dome-shaped, in other words, it should enclose a volume when mated with its reflection in a horizontal plane. Alternatively, every point on the cover surface, except at the flange, should have a center of curvature within the chamber volume. These and other modifications to and variations upon the described embodiments are provided for by the present invention, the scope of which is limited only by the following claims.