Patent Number: 052767205
Section: summary

The present invention relates to nuclear boiling water reactors (BWRs) and more particularly to their containment in the event of a malfunction. During normal operation of a nuclear BWR, steam is generated from either a forced- or a natural-circulation of coolant water in heat transfer relationship with the nuclear fuel housed within a reactor pressure vessel (RPV). The nuclear fuel consists of fuel rods which develop substantial internal heat from the fission of a radioactive material, such as uranium or the like. Even after shutdown of the reactor in the normal course of operation or in the event of an emergency situation, decay reactions occurring in the fuel rods continue to generate heat for an extended period of time. Removal of this decay heat is necessary to maintain the structural integrity of the RPV, but must be effected without releasing radioactive steam or water to the environment. The most serious emergency situation requiring reactor shutdown is generally perceived to be a rupture of the RPV or of a major coolant line connected to the vessel, either resulting in what is known as a loss of coolant accident (LOCA). Another emergency situation which may require only a reduction in heat generation and not a total shutdown of the reactor occurs in connection with the driving of an electric generator with a turbine powered by the steam generated in a BWR. An electric generator can experience a sudden loss of load. Attendant therewith is a concomitant reduction in demand for steam at a rate that exceeds the ability of the reactor control system and the normal cooling system to accommodate. In either of the emergency situations described, the decay or excess heat must be dissipated from the reactor without the release of radioactive materials to the environment. To prevent the release of radioactive products in emergency situations, the RPV typically is placed within a series of containment structures known as primary and secondary containment vessels. The primary containment vessel consists of a drywell and a wetwell. The drywell contains the reactor and the coolant recirculation pumps and in more recent BWRs is a concrete cylinder with a domed top. The wetwell commonly is an annular chamber in which a pool of water is retained by an interior rear wall and by the primary containment vessel. During a LOCA, the steam released by the flashing of the coolant water is forced into the water of the wetwell and condensed, thereby lowering the temperature and pressure of the drywell atmosphere. For this reason, the wetwell is commonly referred to as the pressure suppression pool. Connection between the drywell and the wetwell generally is provided by a number of horizontal cylindrical vents in the lower part of the drywell wall. A reinforced concrete shield building usually constitutes the secondary containment vessel. To remove the decay or excess heat from the reactor after a LOCA, there is normally provided within the secondary containment vessel a containment condenser disposed in a water pool heat sink for receiving and condensing excess steam from the reactor and containment until the decay heat of the fuel rods is dissipated. The water pool heat sink is commonly opened to the atmosphere so that the specific and latent heat of the steam condensed can be removed from the reactor. However, the radioactive steam and condensate themselves must remain inside the reactor containment vessels for environmental reasons. Although long-term heat removed is assured by the containment condenser, the condenser requires some bleeding to the wetwell to remove non-condensable gases that can otherwise accumulate in the containment condenser and deleteriously effect heat transfer. Thus, a bleedline from the containment condenser to the wetwell is provided. The outlet of this bleedline must be submerged in the pressure suppression pool above the uppermost horizontal vent on the drywell side of the drywell-wetwell boundary. By so locating the bleedline outlet, the pressure difference between the higher pressure in the drywell and the lower pressure in the wetwell is used to drive any residual, noncondensed steam and any noncondensibles from the containment condenser and into the wetwell. The condensate is normally recycled back to the RPV. In traditional BWRs, the movement of cooling water to remove decay or excess heat in emergency situations was effected as a result of forced circulation by electric- or diesel-powered water pumps. However, inasmuch as such pumps may fail at a critical time, newer BWRs feature passive- or natural-circulation emergency cooling without the use of active devices such as pumps or the like. With respect to flow from the containment to the containment condenser, natural circulation flow may be achieved by proper location of the condenser to make use as a motive force the pressure differentials that develop between the drywell and the wetwell. Especially for passive- or natural-circulation emergency cooling systems, special consideration must be given to the presence of noncondensibles inside the containment condenser. In particular, noncondensibles may accumulate in the containment condenser and degrade the heat transfer to such an extent that decay or excess heat removal is not possible. Venting the noncondensibles and any residual, noncondensed steam from the containment condenser to the wetwell using the pressure differential between the drywell and wetwell provides a partial solution. However, the steam vented to the wetwell carries both specific and latent heat which will be transferred to an upper gaseous layer inside the wetwell. The higher temperature of the upper layer will increase the pressure inside the wetwell and ultimately inside the drywell. Noncondensibles in the condenser will not be transported to the wetwell through the ventline if the wetwell has a higher pressure than the drywell. The heat removal through the condensor will then deteriorate by the accumulation of noncondensibles and pressure in the drywell will increase through the continuing generation of steam caused by the decay heat. Heat transfer will only resume after the pressure in the drywell exceeds that in the wetwell and the noncondensibles have been vented to the wetwell. Inasmuch as there is no passive means to cool the wetwell, the pressure will remain high unless active cooling systems are provided. Thus it may been seen that there remains a need for an improved emergency cooling system for nuclear BWRs. BROAD STATEMENT OF THE INVENTION The present invention is directed to an improved emergency cooling system method for nuclear BWRs. By providing for improved heat transfer between a containment condenser which receives a heat transfer fluid and noncondensibles in the gaseous phase from the RPV and a water pool vented to the environment outside the containment structures of the BWR, the instant invention is able to efficiently remove excess or decay heat from the reactor core contained in the RPV during a LOCA to ensure the structural integrity of the containment structures surrounding the BWR in the event of an emergency situation. The improved heat transfer is effected by the enhanced removal from the containment condenser of noncondensibles which impede heat transfer. Moreover, the enhanced removal of noncondensibles from the containment condenser allows these noncondensibles and any noncondensed steam to be vented into the drywell of the BWR instead of to the wetwell. Consequently, the vacuum breaker check valve between the drywell and the wetwell, as well as active cooling systems for the wetwell, may be eliminated. It is, therefore, an object of the instant invention to provide for an improved emergency cooling system for nuclear BWRs. The improved emergency cooling system may be especially adapted for incorporation into a nuclear BWR wherein a reactor pressure vessel containing a nuclear core and a heat transfer fluid for circulation in heat transfer relationship with the core is housed within an annular sealed drywell and is fluid communicable therewith for passage thereto in an emergency situation the heat transfer fluid in a gaseous phase and any noncondensibles present in the RPV, an annular sealed wetwell houses the drywell, and a pressure suppression pool of liquid is disposed in the wetwell and is connected to the drywell by submerged vents. The improved emergency cooling system has a containment condenser for receiving condensible heat transfer fluid in a gaseous phase and noncondensibles for condensing at least a portion of the heat transfer fluid. The containment condenser has an inlet in fluid communication with the drywell for receiving from the drywell heat transfer fluid in a gaseous phase and noncondensibles, a first outlet in fluid communication with the RPV for the return to the RPV of the condensed portion of the heat transfer fluid and a second outlet in fluid communication with the drywell for passage to the drywell of the noncondensed balance of the heat transfer fluid and the noncondensibles. The noncondensed balance of the heat transfer fluid and the noncondensibles passed to the drywell from the containment condenser are mixed with the heat transfer fluid in the gaseous phase and the noncondensibles from the RPV for passage into the containment condenser. The improved emergency cooling system also has a water pool in heat transfer relationship with the containment condenser and thermally communicable in an emergency situation with an environment outside of the drywell and the wetwell for conducting heat transferred from the containment condenser away from the wetwell and the drywell. In one embodiment of the invention, the improved heat transfer and enhanced noncondensible flow from the containment condenser is effected by providing the containment condenser with a shroud defining a plenum in fluid communication with the first outlet and a plurality of tubes for passage therethrough of the heat transfer fluid in a gaseous phase and the noncondensibles from the drywell. The tubes may extend in fluid communication with a steam dome connected to the inlet and have annular centers and inner surfaces. At least a portion of the heat transfer fluid may condense in the tubes and flow through the tubes along the inner surfaces. The tubes may be oriented at an angle from between 20.degree. and 40.degree. with respect to vertical so that condensate will collect on one side of the tubes, making the condensate thinner along the rest of the tube and thereby increasing the heat transfer rate from the containment condenser to the surrounding water pool. Flowtrips may be incorporated into the tubes adjacent the plenum for dropletizing the condensed portion of the heat transfer fluid flowing along the inner surfaces of the tubes. Dropletizing increases the shear between the condensate and the noncondensibles and thereby dragging the noncondensibles out of the tubes. In another embodiment of the invention, the improved heat transfer and enhanced noncondensible flow from the containment condenser is effected by providing the containment condenser with a shroud defining a plenum in fluid communication with the first outlet and a plurality of vertical tubes for passage therethrough of the heat transfer fluid in a gaseous phase and the noncondensibles from the drywell. The tubes may extend in fluid communication with a steam dome connected to the inlet and have annular centers and inner surfaces. At least a portion of the heat transfer fluid may condense in the tubes and flow through the tubes along the inner surfaces. Flowtrips may be incorporated into the tubes for dropletizing the condensed portion of the heat transfer fluid flowing along the inner surfaces of the tubes and for directing the droplets produced by the dropletizing to the annular centers of the tubes. It is also an object of the invention to provide a method for cooling a nuclear BWR in the event of an emergency situation. The method is especially suited for nuclear BWRs wherein a reactor pressure vessel (RPV) containing a nuclear core and a heat transfer fluid for circulation in heat transfer relationship with the core is housed within an annular sealed drywell and is fluid communicable therewith for passage thereto in an emergency situation the heat transfer fluid in a gaseous phase and any noncondensibles present in the RPV, and annular sealed wetwell houses the drywell, and a pressure suppression pool of liquid is disposed in the wetwell and is connected to the drywell by submerged vents. Heat transfer fluids are evaporated by the decay heat of the reactor core placed in the RPV. The vaporized fluids will flow into the drywell via a vent connected to the RPV that is opened upon detection of a LOCA. Therefore, in its preferred embodiment, the method includes passing at least a portion of the heat transfer fluid and the noncondensibles from the RPV into the drywell for mixing with heat transfer fluid and noncondensibles from a later step of the method. The mixed heat transfer fluids and noncondensibles are then passed from the drywell and through a containment condenser for condensing at least a portion of the heat transfer fluid. The condensed heat transfer fluid is returned to the RPV. The noncondensed balance of the heat transfer fluid and the noncondensibles are returned to the drywell wherein they are mixed with the heat transfer fluid and the noncondensibles from the RPV for passage into the containment condenser. In one embodiment of the instant method, the containment condenser is provided with a shroud defining a plenum in fluid communication with the RPV and a plurality of tubes for passage therethrough of the heat transfer fluid the noncondensibles from the drywell. The tubes may extend in fluid communication with a steam dome in fluid communication with the drywell and have annular centers and inner surfaces. At least a portion of the heat transfer fluid may condense in the tubes and flow through the tubes along the inner surfaces. The tubes may be oriented at an angle from between 20.degree. and 40.degree. with respect to vertical so that condensate will collect on one side of the tubes, making the condensate thinner along the rest of the tube and thereby increasing the heat transfer rate from the containment condenser to the surrounding water pool. Flowtrips may be incorporated into the tubes adjacent the plenum for dropletizing the condensed portion of the heat transfer fluid flowing along the inner surfaces of the tubes. Dropletizing increases the shear between the condensate and the noncondensibles and thereby dragging the noncondensibles out of the tubes. In another embodiment of the instant method, the containment condenser is provided with a shroud defining a plenum in fluid communication with the RPV and a plurality of vertical tubes for passage therethrough of the heat transfer fluid and the noncondensibles from the drywell. The tubes may extend in fluid communication with a steam dome in fluid communication with the drywell and have annular centers and inner surfaces. At least a portion of the heat transfer fluid may condense in the tubes and flow through the tubes along the inner surfaces. Flowtrips may be incorporated into the tubes for dropletizing the condensed portion of the heat transfer fluid flowing along the inner surfaces of the tubes and for directing the droplets produced by the dropletizing to the annular centers of the tubes. These and other objects, features and advantages of the instant invention will be readily apparent to those skilled in the art based upon the disclosure contained herein.