Patent Number: 052689441
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors, and, more particularly, to insulating and shielding the reactor vessels of such reactors. It is standard practice for the reactor vessels of nuclear reactors to be surrounded by a primary shield which assists in shielding the area surrounding the vessel from radiation emitted from the reactor vessel. Typically, there is a small amount of space, or volume, between the reactor vessel and the inner wall of the primary shield. The size of the space depends upon the size of the gap between the reactor vessel and the primary shield. Primary shields have traditionally been made of concrete. The designs or configurations of primary shields have typically been controlled by the amount of radiation shielding which must be maintained. The high temperatures associated with the reactor vessel during operation, typically on the order of 500.degree. to 600.degree. F. may result in deterioration of the portion of the primary shield adjacent to the reactor vessel. To lessen the likelihood of deterioration of the primary shield, it is desirable to keep the temperature thereof from exceeding about 150.degree. to 200.degree. F. Accordingly, it is necessary to thermally insulate the reactor vessel from the primary shield. Traditionally, the reactor vessel is disposed within a generally cylindrical opening in the primary shield with approximately a 6 to 10 inch annulus, or gap, between the reactor vessel and the primary shield. The reactor vessel insulation has typically been attached directly to the reactor vessel during construction of the reactor facility. Subsequent removal or replacement of the insulation has been very difficult and time consuming because of the limited amount of space between the reactor vessel and the primary shield. It has been known to increase the size of the gap between the cylindrical reactor vessel and the cylindrical primary shield to increase the amount of space therebetween, and thereby enhance accessibility of the insulation. However, this approach has proven to be an unsatisfactory solution because it requires increasing the distance which must be spanned by the reactor vessel supports which support the reactor vessel in the primary shield. It would also be desirable to increase the amount of space between the primary shield and the reactor vessel in order to provide more space for external reactor monitoring instruments, such as excore detectors. In existing reactors, such detectors are positioned within the space between the reactor and the primary shield, or within the concrete of the primary shield. With either of these types of excore detector installation, the reactor vessel insulation is disposed between the reactor vessel and the excore detectors. The presence of such insulation may adversely affect the operational performance of the excore detectors because the strength of the signal being monitored by the excore detectors may be reduced by the signal's passage through the insulation. There is a need for a nuclear reactor wherein additional space is provided between the reactor vessel and primary shield for installation, removal and maintenance of the reactor vessel insulation, and for accommodation of and improved access to reactor monitoring instrumentation, but which does not require the reactor vessel supports to span increased distance, or gap, to accommodate such increased space. SUMMARY OF THE INVENTION The present invention includes a primary shield having a plurality of generally planar vertically disposed internal side walls defining a polyhedral-shaped opening therein. The reactor core is disposed within a generally cylindrical reactor vessel having a generally hemispherical lower end. The reactor vessel is supported within the opening in the primary shield such that the primary shield substantially surrounds the region of the reactor vessel wherein the core is located. Reactor vessel supports hold the reactor vessel in place within the opening in the primary shield. The reactor vessel supports are aligned with and supported by at least a central portion of at least some of the planar side walls of the primary shield. The reactor vessel is secured to the reactor vessel at or near the regions where the cold legs attach to the reactor vessel. Generally planar panels of reactor vessel thermal insulation surround a lower portion of the reactor vessel wherein the core is disposed. The insulation panels form an insulation polyhedron, the shape of which may generally correspond to the shape of the polyhedral opening in the primary shield. The insulation polyhedron includes a plurality of generally rectangular side panels each of which is oriented generally parallel to a side wall of the primary shield. In addition, the longitudinal axis of each side panel is generally parallel to the longitudinal axis of the reactor vessel. The long sides of each panel abut the adjacent long sides of adjacent panels to form a polyhedron. Corners having generally obtuse angles are formed where the panels abut one another. A generally planar central panel may be centrally disposed below the hemispherical portion of the reactor vessel and below the lower ends of the side panels. The central panel is oriented generally perpendicular to the side panels. The central panel preferably has a shape substantially similar, but smaller, than the cross-sectional shape of the polyhedron bounded by the side panels. A plurality of generally planar connecting panels may connect the linear edges of the central panel to the bottom edges of each side panel, such that substantially the entire lower portion of the reactor vessel is surrounded by a continuous layer of insulation. Alternate embodiments include the use of connecting panels without a central panel, and the use of a central panel without connecting panels. The insulation panels may be removably connected to the side walls of the primary shield, to the reactor vessel, to instrumentation mounted within the area between the reactor vessel and the primary shield, or to any combination thereof. The increased space between the reactor vessel and the primary shield resulting from the polyhedral shape of the primary shield improves accessibility to that area for installation, removal and maintenance of the insulation and instrumentation disposed therein. Reactor monitoring instrumentation, such as excore detectors, may be positioned in one or more corners formed where the side walls of the primary shield meet. Positioning the detectors in the corners permits them to be positioned directly adjacent to the reactor vessel without insulation disposed therebetween. Reactor vessel supports are provided for supporting the reactor vessel in the opening in the primary shield. At least four vessel supports are generally equally spaced around the circumference of the reactor vessel. The reactor vessel engages each vessel support at a central portion thereof which is generally aligned with a central portion of a side wall of the primary shield, where the primary shield is closest to the reactor vessel. Positioning the reactor vessel supports such that they are engaged by the reactor vessel in locations near the central portions of the side walls enables the amount of space between the reactor vessel and the primary shield to be increased without increasing the distance, or gap, which must be spanned to support the reactor. It is an object of this invention to provide a nuclear reactor having increased space between the reactor vessel and primary shield which does not require an increase in the distance spanned by the reactor vessel supports to accommodate such an increased space. It is a further object of this invention to provide a nuclear reactor in which reactor vessel insulation and instrumentation disposed within the space between the reactor vessel and the primary shield are more easily accessible for installation, removal and maintenance thereof. It is another object of this invention to provide a nuclear reactor in which excore detectors, and other reactor monitoring instrumentation, may be positioned directly adjacent to the exterior of the reactor vessel without insulation being disposed therebetween. It is yet another object of this invention to provide a nuclear reactor wherein one or more sections of reactor vessel insulation are easily removable for maintenance and/or replacement. These and other objects of the present invention will be more fully understood from the following description of the preferred embodiment of the invention with reference to the drawings appended hereto.