Patent Number: 053032748
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to passive cooling of a nuclear reactor containment. The invention is particularly directed to an isolation condenser type passive cooling system which can be installed externally of the nuclear reactor containment as well as building structure in which the containment is situated, the cooling system being such as minimizes need to penetrate the containment and building enclosure with cooling system components. U.S. Pat. Nos. 5,059,385, 5,082,619 and 5,106,571 disclose use of isolation condensers in connection with passive removal of initial and decay heat loads generated in a nuclear reactor system containment as a result of and upon occurrence of a LOCA, i.e., a loss-of-coolant accident in the system. The cooling systems disclosed in these pending applications also can dissipate initial heat by venting the reactor pressure vessel and/or the containment drywell space to a suppression pool of water confined in a chamber surrounding the reactor pressure vessel. Venting to the suppression pool also can be used with respect to condensate recovery of the isolation condensers, and non-condensable gasses such as nitrogen, which are cooled in an isolation condenser and separated from the condensate. Venting from the containment drywell of heated, pressurized fluid and venting of condensate and non-condensable gasses from the isolation condensers to the suppression pool, is possible because a pressure differential exists between these fluids and gasses on the one hand, and the airspace above the suppression pool water on the other hand. In other nuclear reactor systems, LOCA heat loads are dissipated in different manner. For example, a type of nuclear system that was built in some numbers in the 1960's and 1970's has a containment which includes an upper space in which the nuclear reactor is disposed, and a lower space defining a suppression pool chamber in which cooling water is present with there being an airspace above the water. The upper and lower spaces are separated by a horizontal structural element, e.g., a concrete floor. A concrete pedestal extends upwardly a distance from the concrete floor in the upper space and serves as a mounting on which the reactor pressure vessel is received and supported. A plurality of vertically disposed vent tubes are arranged in circle array in the floor and have entry ends communicating with the upper space, lower outlet ends of these vent tubes locating submerged in the suppression pool water. On happening of a LOCA, initial heat is dissipated by heated, pressurized fluid present in the upper space or drywell venting through the vent pipes into the suppression pool wherein steam condenses and non-condensable gasses such as nitrogen cool and vent from the pool water to the airspace above the water. Initial heat also can be dissipated by recirculating water from the reactor vessel to a cooling operation (unless a reactor vessel rupture is present), which cooling operation may for environmental safety reason, involve an intermediate heat exchange location and a final heat exchange location, the latter being one outside the containment. Recirculation of the suppression pool water in like manner can be practiced to take into account that the suppression pool will heat up quite quickly. Decay heat dissipation will be handled by the same suppression pool and reactor vessel water recirculation functions. It is to be noted though that these systems do not employ passive heat removal capacity. While the last-discussed systems are designed to handle any anticipated LOCA heat load, there is a drawback and potential risk that the cooling function of the suppression pool as it regards non-condensable gasses, can be rendered ineffective. Such happening can come about if a reactor core meltdown attends the LOCA. In that event, the meltdown may cause or contribute to a breaching of the concrete floor structure thereby communicating the drywell of the upper space directly with the airspace above the pool in the lower space rather than such communication being indirect through the suppression pool first. The result is that no lower pressure space exists in the containment to which the higher pressure non-condensable gasses can be vented and cooled by passage through the suppression pool. The systems with the above-recited shortcoming embody massive containment structures. This works against conveniently and simply making system modifications to counter the effects of meltdown as described above and provide for cooling, both as to initial heat removal and the longer term decay heat dissipation. OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide nuclear reactor system satellite heat removal means in the form of a passive containment cooling system specially intended to be retrofittable to an existing nuclear reactor system of a type wherein a drywell space containing the nuclear reactor is located above and over a suppression pool which overcomes the drawbacks of the prior art. It is a further object of the invention to provide a passive containment cooling system which includes a wetwell airspace that serves as a location to which LOCA produced non-condensable gasses present in the containment can be vented in the event a reactor meltdown causes breach in the floor separating the containment drywell from an underlying suppression pool wetwell airspace, such happening thereby destroying presence in the containment of a lower pressure space to which gasses at a higher pressure can be vented. It is a still further object of the invention to readily and conveniently provide passive containment cooling for nuclear reactor systems of types heretofore lacking such cooling capacity. Briefly stated, there is provided a satellite heat removal means which can be embodied in a system as part of an original design but more particularly is intended to be retrofitted to an existing nuclear reactor system to serve optionally to supplement heat removal from the system nuclear reactor containment upon happening of a LOCA, and to assume all system containment drywell venting in the event reactor core meltdown results in breach of the containment floor structure separating the containment drywell and wetwell spaces, which breach would deprive the containment of a space to which non-condensable fraction of LOCA generated heated fluid in the containment could be vented, cooled and stored. The satellite heat removal means includes a structure external of but preferably situated alongside the nuclear reactor containment, a heat exchanger is disposed in a pool of cooling water located in an upper chamber of that structure, while a pool of water also is present in a structure lower chamber. The heat exchanger is communicated with the containment drywell by inlet and outlet conduits so that heated fluid in the containment can enter and be cooled in the heat exchanger with a cooled condensate fraction being returned to the drywell, and a non-condensable gas fraction vented to the lower chamber pool of water. A gas space above the lower chamber pool of water substitutes as the wetwell gas space to which non-condensables vent in place of the containment wetwell gas space that was breached and thus merged with the containment drywell space as an incident of the LOCA. In accordance with these and other objects of the invention, there is provided a nuclear reactor system which includes a containment structure having an upper drywell space and a lower wetwell space, these spaces being separated one from another by an intervening floor member. A nuclear reactor pressure vessel is disposed in the drywell space and a reactor core is present within the pressure vessel. A suppression pool of water is confined in the wetwell space and a gas space is present above a normal level of water in this suppression pool. Means are provided for venting a heated and pressurized fluid present in the structure drywell space incident a pressure vessel loss-of-coolant accident to a submerged location in the suppression pool thereby to remove heat from and reduce pressure in the drywell space by condensing a water fraction of the heated fluid in the suppression pool water, a non-condensable fraction of said heated fluid venting to the wetwell gas space. Satellite heat removal means are operable for effecting additional drywell heat removal during the accident and all drywell venting in the event the floor member structure is breached by a core meltdown during the loss-of-coolant accident with consequent merger of the gas space and drywell so that the containment lacks a space to which the heated fluid non-condensable fraction can vent. The satellite heating removal means includes a satellite structure external of the containment structure and has upper and lower chambers. At least one heat exchanger is located in the upper chamber and a pool of cooling water in the upper chamber surrounds the heat exchanger. Vent means communicate the cooling water with ambient environment. An inlet conduit communicates an inlet end of the heat exchanger with the containment drywell whereby heated fluid present in the containment drywell can flow into the heat exchanger with the containment drywell. An outlet conduit communicates an outlet end of the heat exchanger with the containment drywell. A condensate/non-condensable gas collector is in said outlet conduit. Condensate collected in the collector passes therefrom to the containment drywell and a non-condensable gas fraction collected in the collector passes into a vent pipe which vent pipe outlets submerged below a level of water in a water pool present in the lower chamber, there being a gas space in that chamber above the water level. The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.