METHOD OF MOUNTING A CORE BARREL IN A PRESSURE VESSEL OF A NUCLEAR REACTOR

Abstract: In a nuclear reactor (1) having a pressure vessel (3) with a core barrel (9) having a bottom core support plate (11) dis­ posed within the pressure vessel, with the core support plate (11) being engaged about its periphery by a plurality of en­ gagement means (17), of which each has a recess receiving a key (39) associated with said core barrel, the core support plate (11) has an aperture (31) extending therethrough above each recess of the engagement means (17) and the keys (39) are insertable into and mounted in the apertures (31), and have lower sections which extend into the recesses of said engagement means (17).

Description: The present invention relates to a method for installation of a core barrel in a pressure vessel of a reactor where alignment and securement of the core barrel are readily achieved. A nuclear reactor has an upright cylindrical pressure vessel with a hemispherical lower head section and a removable hemispherical head bolted to the upper end of the vessel. A cylindrical core barrel is suspended inside the pressure vessel from a flange extending around the inside of the upper end of the pressure vessel. The barrel includes a bottom core support plate and is positioned within the pressure vessel spaced from the lower head section. The upper region of the core barrel contains reactor control rod guide tubes, while the core internals, such as the fuel element assemblies, the fuel element assemblies support grid structure, and the like are contained in the lower region of the core barrel. As described in EP-A-140.662, the core barrel is primarily supported at its upper end by engagement with the upper end of the pressure vessel, the lower end of the core barrel being restrained by engagement means so as to provide for lateral stability thereof. A preferred means for providing such lateral stability and auxiliary support resides in the provision of a plurality of engagement means which are secured to the lower end of the pressure vessel and extend radially inwardly from the upper portion of the walls of the hemispherical lower head section. Each of the engagement means has a recess in the upper surface thereof and keys, such as T-shaped keys are secured to the bottom of the core support plate, which keys are designed to fit into the recesses in the engagement means for lateral stability and to align the core barrel within the reactor vessel. In the conventional assembly of the core barrel within the reactor vessel, the engagement means with recesses are generally welded to the bottom portion of the pressure vessel and keys are bolted and/or welded to the bottom surface of the core support plate. A very precise alignment is required, between the keys and walls of the recesses or keyways, upon assembly of the core barrel into the pressure vessel. Because of the degree of precise alignment required, the core support plate is provided with a manway or access port, usually through the center of the core support plate, with a cover plate therefor, so that an assembler can be lowered through the manway into the area between the core support plate and the bottom wall of the pressure vessel. The assembler, while in that area, can then make measurements to determine the type and number of shims that are needed to be manufactured and then inserted into the keyways to provide the required alignment. At times, the needed shims can be inserted into the recesses only after removal of tile core barrel from the pressure vessel, with the shims then placed into the recesses and the core barrel re-inserted into the pressure vessel. This system of alignment is expensive and time consuming and is possible only when the size of the reactor is such that an assembler can be physically lowered through the manway into the area between the core support plate and the bottom wall of the pressure vessel. In small reactors, for example, the pressure vessel may be of a size so small that an assembler cannot be physically located in the area between the core support plate and the bottom wall of the pressure vessel to perform the alignment operations above-described. It is therefore the principal object of the present invention to provide a method of installing a core barrel which greatly facilitates aligning the core barrel relative to engagement means for the core support plate. With this object in view, the present invention resides in a method of mounting a core barrel in a pressure vessel of a nuclear reactor with the core barrel having a flange at its upper end supported on a ledge of said vessel and a bottom core support plate disposed within the pressure vessel, the core support plate being engaged about the periphery thereof by a plurality of engagement means, each having a recess formed therein and receiving a key associated with said core barrel, characterized in that said core support plate is formed with apertures which extend therethrough above each recess of said engagement means; and after installation of said core barrel in said pressure vessel, the dimensions of said recesses relative to the respective apertures are measured and keys are machined in accordance with such measurements, and said keys are then inserted into, and mounted in, said apertures, with their lower sections fitted into the recesses of said engagement means for maintaining said core barrel in alignment with said vessel. The keys preferably have an upper flanged portion which fits on a shoulder in the core support plate about the aperture, such that the key is secured in place with its upper surface flush with the upper surface of the core support plate and with the flange secured to the core support plate by welding or by bolted connections. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, wherein: Figure 1 is a vertical cross-sectional view of the lower portion of a nuclear reactor pressure vessel showing two engagement means and the core support plate and alignment system of the present invention; Figure 2 is a plan view of the core support plate showing four engagement means of the present invention taken along lines II-II of Figure 1; Figure 3 is a view taken along the lines III-III of Figure 1 below the core support plate; Figure 4 is a perspective view of a key formed according to the present invention having a cylindrical upper section and rectangular lower section; and Figure 5 is a top plan view of the key illustrated in Figure 4, with bores formed in the flange. Referring now to the drawings, Figure 1 illustrates the lower portion of a nuclear reactor 1 which has a substantially cylindrical, vertically oriented pressure vessel 3, the pressure vessel having, a vertical wall 5 and hemispherical bottom wall 7. A core barrel 9 is disposed within the pressure vessel 3, the core barrel 9 having a bottom core support plate 11, which supports various components of the reactor constituting the core internals such as the fuel element assemblies, the fuel element assemblies support grid structure, and the like, designated at 13, all as conventionally provided. The upper section of the core barrel 9 conventionally is provided with an annular, radially outwardly extending flange, not shown, and an upper region of the pressure vessel 3 has an annular, radially inwardly extending ledge, also not shown, upon which the core barrel flange is seated. Spaced about the upper wall portion 15 of the hemispherical bottom wall 7 are a plurality of engagement means 17, four such engagement means being shown in the drawings, although the number thereof may vary dependent upon the size and type of reactor. The engagement means 17 preferably have a vertical flow channel therethrough and is also preferably formed as a pair of brackets 19 that are attached to the inner wall surface 21 of the upper wall portion 15 of the hemispherical bottom wall 7, and extend radially inwardly therefrom, and a crossbar or connecting portion 23 having a recess 25 therein, with a flow channel 27 formed between the wall surface 21, brackets 19 and connecting portion 23. The core support plate 11 is thus engaged about its periphery by a plurality of engagement means. In accordance with the present invention, the core support plate 11 is aligned and secured on the engagement means 17 within the pressure vessel 3 without the need for the entry of personnel into the area 29 between the core support plate 11 and the hemispherical bottom wall 7 of the pressure vessel. The core support plate 11 is provided with a plurality of apertures 31, preferably having a circular wall, which extend vertically therethrough, about the peripheral region thereof, also preferably with a countersunk portion 33 adjacent the upper surface 35 of the core support plate 11 which forms an annular shoulder 37. An aperture 31 is provided in the core support plate 11 for alignment with each of the engagement means 17, above the recess 25 of the engagement means. To align and affix the core support plate 11 to the engagement means 17, key members 39 are provided, an embodiment of which is illustrated in Figures 4 and 5. With the use of apertures 31 having a circular wall thereabout, the key member has a cylindrical upper section 41, a lower non-cylindrical, and preferably rectangular section 43, and a top outwardly radial flange section 45, the upper surface 47 of which is flat. As illustrated, the keys 39 are secured in the apertures 31, with the flanged section 45 fitting within the countersunk portion 33 of the core support plate 11, and the top surface 47 of the key is flush with the top surface 35 of the core support plate 11. The keys are then secured to the core support plate such as by welding or by the use of bolts 49 which extend through bores 51 in the flange 45 that extend into complementary bores 53 in the shoulder 37 formed in the core support plate 11 about the aperture 31. Upon securement, the lower section 43 of the keys 39 extend into and register in the recesses 25 in the engagement means 17 to provide lateral and radial support and alignment. The lower section 43 of the key 39 is machined so as to provide a mating relationship in the recess 25 of the engagement means, or, if desired, shims may be added to provide such relationship. The provision of collapsible shock absorbers between the core support plate 11 and the engagement means 17 is provided by placement of collapsible cylinders such as stainless steel cylinders 57 in cavities 59 formed in the surface 61 forming recess 25 and if desired, in complementary cavities formed in the bottom 65 of the key 39, the cylinders preferably of a mushroom shape. In the method of installing and aligning a core barrel in a nuclear reactor pressure vessel according to the present invention, a pressure vessel 3 is provided with a plurality of engagement means 17, having recesses 25 therein, about the lower inner wall thereof, i.e., spaced about the inner wall surface 21 of the upper wall portion 15 of the hemispherical bottom wall 7 of the pressure vessel 3. A core support plate 11 of a core barrel 9 is provided with a plurality of apertures 31 therethrough, with an aperture 31 provided for alignment with each of the recesses 25 of the engagement means 17. The core barrel 9 with its core support plate 11 is inserted into the pressure vessel 3 from the top and the apertures 31 aligned with the recesses 25. The assembler then determines, through the apertures 31, the exact dimensions required for the lower section 43 of each key 39 and the key is machined to the desired tolerances. Where desired, shims to provide the desired clearance may also be manufactured and inserted. The keys 39 are then inserted from the top, into the apertures 31 with the lower section 43 fitted in the recesses 25. The keys 39 are then secured to the top of the core support plate 11 by welding or by use of bolts 49 with the upper surface 47 of the key 39 being flush with the upper surface 35 of the core support plate 11. The present invention permits the measurement of the relative position of the keys in the core support plate and the recesses or keyways in the engagement means without physically locating an assembler in the area between the core support plate and the hemispherical bottom wall of the pressure vessel. With the present arrangement, the conventional manway and cover plug of the core support plate can be eliminated. Also, with the present invention, there is no need to remove the core barrel from the reactor vessel for insertion of custom-made shims in the recesses of the engagement means.

Claims: A method of mounting a core barrel in a pressure vessel (3) of a nuclear reactor with the core barrel (9) having a flange at its upper end supported on a ledge of said vessel (3) and a bottom core support plate (11) disposed within the pressure vessel, the core support plate (11) being engaged about the periphery thereof by a plurality of engagement means (17), each having a recess (25) formed therein and receiving a key (39) associated with said core barrel, characterized in that said core support plate (11) is formed with apertures (31) which extend therethrough above each recess (25) of said engagement means (17); and after installation of said core barrel (9) in said pressure vessel (3), the dimensions of said recesses relative to the respective apertures (31) are measured and keys (39) are machined in accordance with such measurements, and said keys (39) are then inserted into, and mounted in, said apertures (31), with their lower sections (43) fitted into the recesses (25) of said engagement means (17) for maintaining said core barrel (9) in alignment with said vessel (3). A method as claimed in claim 1, characterized in that said lower sections (43) of said keys are machined so as to have rectangular cross-sections. A method as claimed in claim 1 or 2, characterized in that each aperture (31) includes an annular shoulder (37) formed in said support plate (11) at a predetermined depth and said keys are machined with a flange (45) of a predetermined height so as to form a substantially horizontal upper surface with said core support plate (11) after placement of said key (39). A method as claimed in claim 3, characterized in that said flange (45) of said key (39) is welded to the core support plate (11). A method as defined in claim 3, characterized in that bores (51) are provided about the periphery of said flange (45) of said key (39) and complementary bores (53) are provided in the shoulder (37) of each aperture (31) of said core support plate (11) and bolts (49) extend through the bores (51) in said flange (45) and into said complementary bores 53.

Inventors: TOWER, STEPHEN NELSON, VERONESI, LUCIANO
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: G21C5
IPC codes: G21C5