Patent Number: 
Section: description

The first BWR plants used a steel containment in the shape of an inverted light-bulb, completely surrounding any reactor vessel pressure relief valves on all main steam lines, and also fully enclosing a coolant re-circulation system. Several containment designs are currently used, as shown in FIGS. 1a through 1c. The containment shown in FIG. 2 is the predominant type of BWR containment. With reference to FIGS. 1 and 2, wherein like numerals denote like structures, a BWR containment 20 generally is divided into two chambers: a dry-well 22 that houses a reactor vessel 24 and other primary system components; and a generally toroidal suppression chamber 26, also known as a xe2x80x9cwet-wellxe2x80x9d, which contains a pool of water 28 used for pressure suppression and as a heat sink. If a pipe rupture occurs inside a containment 20, the dry-well 22 becomes pressurized by steam blowing down from a primary coolant system. The steam enters a plurality of downcomers 30 and is routed to each wet-well 26 where it is discharged beneath the water surface 32 of a pool 28. The pool 28 is designed to condense the steam discharged to the wet-well 26 and thus mitigate a post-accident containment pressure transient. The pool 28 also provides a source of water for an emergency core cooling system (ECCS) (not shown). The ECCS is designed to inject water back into the reactor vessel to make up for lost water in the event of a loss of coolant accident (LOCA). The ECCS also re-circulates water through the core following a LOCA to provide for long term post-accident core cooling. A LOCA results from a high pressure coolant system pipe rupture. It is postulated that such a rupture will cause large quantities of debris, such as pipe and vessel insulating material, and other solids, to be washed into the wet-well 26. Conventional suction strainers 40 (see FIGS. 3 and 4) installed on ECCS suction lines 42 currently installed in BWR plants may not be able to handle the amount of debris expected to result from a LOCA. As a result, conventional strainers 40 are being replaced with replacement strainers 44 that are much larger and heavier. As best seen in FIG. 4, the replacement strainers 44 consist of multiple strainers 46 strainers formed into trains that connect to a suction pipe 48 through a penetration 50 in the wet-well wall 52. The larger replacement strainers 44 include more inertial mass than the conventional strainers 40 that may cause the strainers 44, and the attached suction pipe 48, to move significant amounts during seismic and other events in response to applied loads. In particular, when a large pressurized pipe ruptures with great force, resulting in an LOCA, the strainers 44 in the wet-well 26 are subjected to large hydrodynamic forces that can deflect the strainers 44 and subject the attached suction pipe 48 to large reactive forces. Forces may be applied to the strainers 44 and the suction pipe 48 in directions indicated by arrows A, B, C and D of FIGS. 3 and 4. The force indicated by arrow A in FIG. 3 acts in a transverse direction, causing the entire suction strainer assembly to move either towards or away from the penetration 50. Additionally, suction pipe 48 moves in response to the transverse force A. Forces applied in directions indicated by arrows B and D of FIG. 4 act on the penetration in a radial direction. Radial forces B and D may also combine to exert a rotational force upon penetration 50, indicated by arrow C in FIG. 4. As seen in the figures, the transverse force A, radial forces B and D and rotational forces C affected the conventional strainers 40. But, when the conventional strainers 40 are replaced by larger, heavier trains of strainers 44, the effect of forces A, B, C and D is magnified and has a greater effect on penetration 50 because the trains 44 have a larger inertial mass. Simply by increasing the size of the strainers 44, conventional penetration designs become unable to handle the expected inertial forces. Thus, the present invention is directed to a flexible penetration attachment that moves in response to postulated inertial forces acting upon either or both of the strainers 44 and the suction pipe 48. The flexible penetration attachment of the present invention is shown in FIG. 5. A flexible penetration attachment 60 is shown that allows the suction pipe 48 to communicate with liquid in the wet-well 26 through an aperture 62 in a wet-well wall 52. The penetration attachment 60 includes a boot 64, preferably fabricated, for defining an outer circumference 68 of the aperture 62. Suction pipe 48 extends through the boot 64 to fluidly communicate with the water in the wet-well 26. The outer circumference 68 of the aperture 62 is larger than the outer circumference 70 of the suction pipe 48. As a result, a space 66 is defined between the aperture outer circumference 68 and the outer circumference 70 of suction pipe 48. Because suction pipe 48 usually includes a circular cross-section, space 66 is usually annular, though other geometries may also fall within the objectives of the present invention. A resilient seal 72, also preferably annular, having a generally U-shaped cross section is placed within the annular space 66. More preferably, resilient seal 72 is fabricated from an elastomeric compound having high strength. The resilient seal 72 includes a first end 74 that is fixedly secured to the outer circumference 70 of the suction pipe 48, and a second end 76 that is fixedly secured to the outer circumference 68 of the aperture 62 defined by the fabricated boot 64. As can be seen from FIG. 5, the resilient seal 72 is fixed only at the first end 74 and at the second end 76. A central portion 80 of the seal 72 is able to flex between the first end 74 and the second end 76 of the seal such that the suction pipe 48 is allowed to move a pre-determined distance in response to a force in any direction. In particular, suction pipe 48 is able to move in the transverse, radial and rotational directions because the suction pipe 48 is not directly secured to the outer circumference 68 of the aperture 62. Instead, because the resilient seal 72 is fixedly attached with straps 78 between the outer circumference 68 of the aperture 62 and the outer circumference 70 of the suction pipe 48, the seal 72 allows the suction pipe to move in response to forces placed upon it. At the same time, the seal 72 prevents leakage from the suppression pool 26 through the annular space 66. It may be appreciated that the size of the seal 72 and the size of annular space 66 are dependent upon the size of the replacement strainers 44 in each individual plant. If the replacement strainers 44 and the suction pipe 48 are predicted to move a larger distance in response to predicted loads, then the size of both annular space 66 and central portion 80 of seal 72 may be larger. But, in any event, the seal 72 may be sized so that the suction pipe 48 is allowed sufficient room to move transversely, radially or rotationally in response to the largest predicted force placed upon the combined suction line 42 and the replacement strainers 44. In addition, both the boot 64 and the seal 72 may be sized to easily accommodate any installation differences that may be present due to existing penetrations and complicated piping systems. The seal 72 is also relatively easily disassembled to allow for penetration leakage testing or ECCS suction line repair. The present invention therefore provides a relatively simple replacement flexible penetration attachment 60 for connecting trains of strainers 44 submerged within a wet-well pool 26 to a suction pipe 48 through the existing apertures 62 in the wet-well wall 52. The flexible penetration attachment allows connection through an existing penetration without transferring significant loading for which the original penetration was not designed to absorb, and also minimizes leakage through the penetration. The features and objects of this invention have been disclosed. But it should be realized that the various changes and modifications that are possible will be self-evident to those of skill in the art in which the present invention pertains, and may be made without departing from the scope of the invention, which is limited only by the appended claims.