Patent Number: 040627268
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to pressure vessels and more particularly to a nuclear reactor pressure vessel nozzle seal. 2. Description of the Prior Art The conventional nuclear reactor pressure vessel comprises a longitudinally disposed cylindrical structure, closed at both ends by a convex base and a domed roof, having reactor coolant inlet and outlet nozzles protruding therethrough. Generally, these nozzles are disposed in a plane transverse to the longitudinal axis of the vessel and angularly separated from each other. Housed within the pressure vessel structure are, among others, the nuclear core, subassemblies and a fluid coolant. Moreover, within the pressure vessel, an annular flange is formed on the inner surface thereof. The flange serves as a means for supporting the reactor core which is suspended from a distribution hoop or shell. The distribution hoop is extended by means of a thermal shield-skirt assembly, which supports the fuel elements in the reactor core and which also serves as a hydraulic guide. In operation, the fluid coolant, in forced circulation, enters the pressure vessel through the inlet nozzles, and flows through the annular hydraulic guide that is formed between the inner surface of the pressure vessel and the skirt. The coolant then rises through the core of the reactor whereupon it is discharged from the vessel through an outlet nozzle which is in fluid communication with the hoop opening through conduit means interposed therebetween. To insure proper circulation, it is imperative that direct communication be prevented between the incoming coolant and the discharging coolant. Toward this end, a leak proof contact between the hoop opening and the pressure vessel outlet nozzle is required. However, although a leak proof contact is necessary to prevent direct fluid communication, structural and differential thermal expansion conditions which can occur between the internal reactor structures and the pressure vessel must be considered. In general, the attendant thermal expansion precludes fixedly joining the conduit means to both the hoop and the pressure vessel wall. Therefore, a leak proof sealing means, either as part of the conduit or in substitution thereof, is required to prevent the commingling of the inlet fluid coolant and the outlet fluid coolant. Further, from a structural consideration it is desirable that the sealing means segregate the fluid coolants without structurally coupling the hoop to the pressure vessel. In the past, a leak proof seal was established by a spring biased contact of a sealing ring or by thermal expansion contact of the conduit. In general, the thermal expansion contact seal consists of carefully and tediously machining the conduit or a ring to be attached thereto to establish a designed clearance or tolerance between the machined conduit or ring face and the pressure vessel nozzle during assembly. The leak-proof condition, however, for this thermal expansion type seal is only achieved at the elevated operating temperatures of the nuclear reactor system when thermal expansion of the hoop and conduit expand to meet the inner wall of the pressure vessel. Moreover, since the pressure vessel also expands during operation, this thermal expansion conduit-seal generally requires a material having a greater thermal expansion coefficient for the hoop and/or the conduit than the expansion coefficient of the pressure vessel, if the leak proof state is to be achieved. The spring contact type seal, moreover, comprises a cylindrically shaped sealing member disposed within and extending from a cylindrical annular cavity concentric therewith. The sealing member is generally machined on one face of its cylindrical shape in order to nestle in close contact with, for example, the pressure vessel wall about the outlet nozzle and thereby prevent leakage therebetween. A spring disposed within the annular cavity interposed between the other face of the cylindrical sealing member and the rear wall of the cavity, or a compression ring, exerts in the axial direction the force necessary to tightly seat the sealing member against the pressure vessel wall. Moreover, to prevent leakage flow from one fluid from traveling through the annulus, between the sealing member and the annular cavity, and across the spring into communication with the other fluid, both the sealing member and the cavity are machined to exact close fitting tolerances such that the sealing member is seated in the cavity with only a very narrow annular gap therebetween. This gap, however, provides a labyrinth-like flow passage for fluid communication and therefore flow leakage is not prevented but merely reduced. Generally, however, this leakage rate is too large and the manufacturing tolerances are too stringent for this type of a seal. Accordingly, there is a need to provide a sealing means which will prevent or at least reduce the leakage flow between the incoming and discharging coolants at all operating conditions without the stringent manufacturing tolerances, or the use of different materials having different thermal coefficients that are characterized by the prior art systems. SUMMARY OF THE INVENTION In accordance with the invention, a reactor pressure vessel-hoop discharge sealing means is provided which eliminates costly machining, removes the thermal expansion determination of the sealing means-pressure vessel clearance, allows a wider selection of materials for the hoop and/or sealing means and establishes a satisfactory leak proof seal in all reactor conditions, operating or during shut down, without structurally coupling the distribution hoop to the pressure vessel. Specifically, a reactor pressure vessel-hoop discharge sealing means that has these features comprises a sealing ring connected to the hoop opening by an impervious expansion bellows. More specifically, the discharge opening seal comprises an annular compression ring member seated in a recess about the hoop opening or a conduit extending therefrom having a sealing ring attached thereto and biased axially therefrom by an expansion bellows weldably connected to both the annular member and the sealing ring. The self actuating bellows seal is designed to insure that the sealing ring will securely contact the pressure vessel in all reactor conditions, operating or not, without structurally coupling the hoop to the pressure vessel. Furthermore, the impervious expansion bellows seal weldably connected to the sealing ring and the annular compression ring provides a boundary across which and around which fluid communication is prevented. Moreover, the machining costs of close tolerance members as found in the prior art thermal expansion and spring type seals are eliminated by this bellows spring system. In addition, this bellows seal design may be remotely assembled by attachment to the hoop prior to the hoop's insertion within the vessel. The various features of novelty which characteristics the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention .