Patent Number: 052689441
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, there is shown a nuclear reactor 2 in which this invention is used. Reactor 2 includes reactor vessel 4, inside of which reactor core 5 is located. Reactor vessel 4 has a generally cylindrical portion 6 and a generally hemispherical lower end 8. Reactor vessel 4 is disposed within a primary shield 10. Primary shield 10 is preferably a concrete structure which surrounds at least a lower portion of reactor vessel 4. Primary shield 10 has an opening 12 therein in which reactor vessel 4 is positioned. Reactor vessel supports 14 hold vessel 4 in place within opening 12. Opening 12 is defined by a plurality of generally planar interior side walls 16 which generally form a polyhedron. In a preferred embodiment, eight side walls 16 are provided, whereby opening 12 is octagonal in cross-section. It will be appreciated, however, that any suitable polygonal cross-sectional shape may be utilized. In a preferred embodiment, a plurality of steel plates 18 may cover the exterior surface of side walls 16. Steel plates may be provided as the forms for pouring the concrete portion of primary shield 10. Plates 18 do not have to be removed from the exposed walls after the concrete dries. Steel plates 18 may be secured to side walls 16 using any suitable means in a manner known to those skilled in the art. It will be appreciated that the use of generally planar side walls 16 will simplify construction of primary shield 10 over existing cylindrical designs since no curved portions need be formed in either the concrete structure or steel plates 18. Variable sized gap 20 between cylindrical reactor vessel 4 and polyhedral shaped opening 12 results in an increase in the amount of space between vessel 4 and side walls 16. The increased space permits improved access to the area between vessel 4 and side walls 16 for maintenance, inspection and other purposes. With existing designs, increasing the amount of space between the cylindrical vessel and the cylindrical side walls of the opening in the primary core required an increase in the size of gap between the vessel and the side walls, which in turn, resulted in an increase in the distance spanned by vessel supports 14 to support the reactor vessel in the primary shield. As discussed below, the use of polyhedral-shaped opening 12 permits the space between reactor vessel 4 and side walls 16 of primary shield 10 to be increased substantially over that which can be achieved using existing cylindrical designs, without requiring an increase in the distance spanned by vessel supports 14. It should be noted that because of the increased amount of space between reactor vessel 4 and primary shield 10, it may be desirable to install additional shielding to at least partially cover gap 20 at the top portion of primary shield 10 to further shield the surrounding area from radiation. Preferably, four or more vessel supports 14 are provided substantially equally spaced about the circumference of vessel 4. Each vessel support 14 is preferably in communication with and at least partially supported by an upper portion of a side wall 16. The ends of each vessel support 14 preferably extend into the concrete of primary shield 10 and are thereby secured in place. Reactor vessel 4 engages and is supported by at least a central portion 21 of each vessel support 14. In a preferred embodiment, reactor vessel supports 14 engage vessel 4 at locations at or adjacent to the locations where cold leg 23 enters vessel 4. In a preferred embodiment, each vessel support 14 is located such that a central portion 21 of each vessel support 14 is aligned with a central portion 22 of one of the planar side walls 16 of primary shield 10. Central portion 22 is the portion of each side wall 16 which is the closest to reactor vessel 4. In a preferred embodiment, the distance between reactor vessel 4 and each side wall 16 at central portion 22 is substantially equal to the typical distance between the reactor vessel and the cylindrical side walls of the primary shield of existing designs. At least part of central portion 21 of each vessel support 14 may be positioned on and supported by the top of the central portion 22 of the side wall 16 with which it is associated. Accordingly, the distance spanned by reactor vessel supports 14 in the present invention would be substantially equal to or less than the distance spanned by vessel supports in existing cylindrical designs. Yet it will be appreciated that because of the polyhedral shape of the opening 12 in primary shield 10, the total amount of space between vessel 4 and side walls 16 is greater in the present invention than would be available in existing cylindrical designs having the same vessel support span distance. Referring to FIGS. 1 through 4, as discussed hereinabove, it is desirable to provide thermal insulation 30 between reactor vessel 4 and primary shield 10. In a preferred embodiment, the type of insulation used is a stainless steel reflective insulation. This type of insulation consists of a plurality of stainless sheets stacked upon one another to form a desired thickness. The thickness of the insulation is preferably about 2 to 6 inches. Each sheet has a reflective surface thereon. The sheets are separated by air gaps. The insulation is positioned such that the reflective surfaces of the stainless steel sheets are oriented to reflect the heat radiated from reactor vessel 4 back toward vessel 4. While this type of insulation is used in the preferred embodiment, it will be appreciated that any suitable type of insulation may be used with this invention. In a preferred embodiment, insulation 30 includes a plurality of generally planar panels 32, 34, 36 of insulation. Side panels 32 are preferably generally rectangular in shape and are positioned around the circumference of a portion of reactor vessel 4 within gap 20. The long sides 38 of side panels 32 are in communication with adjacent long sides 38 of adjacent panels 32 to form an insulation polyhedron 40 preferably having a cross-sectional shape substantially similar to, but smaller than, the cross-sectional shape of opening 12 in primary shield 10. In a preferred embodiment, eight side panels 32 are provided and the cross-sectional shape of polyhedron 40 formed thereby is octagonal. A central panel 34 is positioned below a central portion of generally hemispherical portion 8 of vessel 4 and below the lower edges 44 of side panels 32. The shape of central panel 34 is preferably substantially similar to the cross-sectional shape of polyhedron 40 at side panels 32, but is significantly smaller than insulation polyhedron 40. In a preferred embodiment, the area of central panel 34 is about 0 to 100 percent of the cross-sectional area of insulation polyhedron 40 at side panels 32. Central portion 34 is oriented such that each generally linear edge 42 thereof is disposed generally parallel to the transverse axis of one of side panels 32. It will be appreciated that if the cross-sectional area of central panel 34 is equal to 100 percent of the cross-sectional area of insulation polyhedron 40 then edges 42 of central panel 34 will abut the lower edge 44 of a side panel 32 and no connecting panels 36 are provided. A plurality of generally planar connecting panels 36 connect each edge 42 of central panel 34 with a lower edge 44 of a side panel 32. Connecting panels 36 are preferably generally trapezoidal in shape, with the short side 46 of each connecting panel 36 abutting and connected to an edge 42 of central panel 34 and the long side 48 of each connecting panel 36 abutting and connected to the lower edge 44 of a side panel 32. Each nonparallel side 50 of each connecting panel abuts and is connected to an adjacent non-parallel side 50 of an adjacent connecting panel 36. Insulation panels 32, 34, 36 are preferably connected together to form a substantially continuous enclosure surrounding a lower portion of reactor vessel 4. It will be appreciated that central panel 34 is not always necessary to enclose vessel 4 within insulation polyhedron 40. If no central panel 34 is provided, connecting panels 36 will be generally triangular in shape with the apex of the triangles meeting below a central portion of hemispherical portion 8 of reactor vessel 4 and the base of each triangle abutting the lower edge 44 of a side panel 32. It will be appreciated that insulation 30, and in particular side panels 32, will occupy a portion of gap 20. However, a space, or gap, 52 will remain between each side wall 16 and each side panel 32. In a preferred embodiment, gap 52 is about 1 to 3 inches wide. The remainder of gap 20 which is not occupied by insulation 30 will be located between side panels 32 and reactor vessel 4. Gaps 20, 52 provides areas for cooling air to circulate around insulation 30. In a preferred embodiment, the corners 54 of opening 12 in primary shield 10 and the corners 55 of polyhedron 40 form obtuse angles. In each corner 54 of opening 12, gap 20 is of sufficient size to accommodate reactor monitoring instrumentation, such as an excore detector 56, therein. Excore detectors 56 are used to monitor neutron emissions from reactor vessel 4. It will be appreciated that with this invention, excore detectors 56 may be positioned such that no insulation is disposed between detector 56 and reactor vessel 4. This results in improved performance of the excore detectors over that which can be achieved in existing designs. However, excore detectors 56 may be positioned in any desired location, with or without insulation 30 disposed between the detectors and reactor vessel 4. Each excore detector 56 preferably includes a cylinder mounted on side wall 16 in a corner 54. Excore detectors 56 may be mounted in any or all corners 54 of opening 16. In a preferred embodiment, eight detectors 56 are provided, one in each corner 54. Referring to FIGS. 4 and 5, insulation panels 32, 34, 36 may be secured to vessel 4, side wall 16, the cylinders in which excore detectors 54 are mounted or to any combination thereof. For example, elongated, generally U-shaped tracks 58 may be mounted on the exterior of the cylinders in which the excore detectors 54 are mounted. Tracks 58 receive the edges of side panels 32 therein, whereby each panel may vertically slide into and out of position. Tracks 58 allow easy removal of the panel for maintenance and also simplify installation of the panels. Alternatively, it will be appreciated that insulation panels 32, 34, 36 may be permanently or removably mounted to at least one of vessel 4, side walls 16, and excore detectors 54 using any suitable means. Bottom panel 34 and connecting panels 36 may be removably or permanently secured to any of side panels 32, reactor vessel 4, side walls 16, excore detectors 54 and tracks 58, in any suitable manner known to those skilled in the art. To further aid in installation and removal of insulation 30, each side panel 32 may be composed of a plurality of vertically stacked sections 60, as best shown in FIG. 4. Each section 60 may be of a size such that it may be easily handled. It will be appreciated that this invention may be utilized with a reactor vessel and primary shield of any desired dimensions by varying the sizes of the various elements. For example, to accommodate a larger diameter reactor vessel, the width of side walls 16 may be increased, along with the corresponding width of side panels 32, and the size of panel 34, and connecting panels 36. Further, it will be appreciated that this invention provides a nuclear reactor wherein the space between the reactor vessel and the primary shield is increased in order to provide improved access for installation, removal and maintenance of thermal insulation and reactor monitoring instrumentation disposed therein, yet which does not require the reactor vessel supports to span additional distance in order to accommodate the increased space. Whereas particular embodiments of this invention have been described for purposes of illustration, it will be evident to those skilled in the art that numerous variations in the detail may be made without departing from the invention as defined in the appended claims.