Patent Number: 039740277
Section: summary

BACKGROUND OF THE INVENTION A nuclear power plant using a pressurized-water reactor comprises a pressure vessel having a vertically substantially cylindrical side wall and containing the reactor core through which the pressurized-water coolant is circulated. The vessel's side wall has inlet and outlet coolant connections radiating from its upper portion, these outlet connections via pipes connecting with the heat exchangers of steam generators and from which pipes extend back via main coolant pumps to the vessel's inlet connections. The pressure vessel is surrounded by a concrete wall formed by concrete defining a pit in which the vessel is positioned. The relative diameters of the two walls are such as to define an annular space between the two so as to accommodate instrumentation lowered into the pit for inspection of the pressure vessel's side wall condition. In prior art installations this concrete wall has been designed to function not only as biological protection but also to contain missile-like fragments of the pressure vessel in the event its side wall ruptures under the pressure of the pressurized-water coolant circulating within the vessel. Therefore, the concrete construction has required expensive reinforcement, but because of the annular space, the concrete wall has been unable to function to reinforce the vessel's side wall against rupturing. The coolant pipes extending to and from the steam generators also contain the pressurized-water coolant and the steam generators must operate under the internal pressure of the generated steam. These components are also surrounded by concrete walls, but in this instance the walls are spaced substantial distances from the components. This has made it possible to provide the steam generators and coolant pipes with rupture protection in the form of pressure-resistant and heat-resistant concrete segments which are interfitted to surround the components and which are themselves surrounded by high-tensile metal annular elements. In this way rupure protecting encasements are provided for these components, the encasement parts being relatively proportioned so that when the steam generators and coolant pipes are thermally expanded by their normal operating temperatures, the high-tensile metal annular elements, which remain cooler, resist the expansion, the result being that the high-compression strength segments are compressed against the walls of the steam generators and pipes to relieve them of a substantial amount of the stress resulting from their confinement of the high-pressure fluids. Such rupture protection for the steam generators and coolant pipes has been disclosed and claimed in the U.S. Michel et al. patent application Ser. No. 417,798, filed Nov. 21, 1973. Now the previously described annular space between the reactor pressure vessel's side wall and its surrounding concrete wall must be small enough in radial extent to ensure containment of possibly flying fragments of the side wall in the event the latter should rupture under its internal pressure. Therefore, there is no room for working personnel and thus it has heretofore been impossible to encase the pressure vessel's side wall in the same manner as done in the case of the steam generators and coolant pipe lines. If the encasement were to be built on the vessel prior to its installation in its concrete pit, the encasement would fill up the annular space required to inspect the condition of the pressure vessel's side wall. SUMMARY OF THE INVENTION The primary object of the present invention is to provide the pressure vessel with a rupture protecting encasement having the advantages of the encasements which can be used on the steam generators and coolant pipes, while providing for clearance of the annular space between this pressure vessel's side wall and its surrounding concrete wall when pressure vessel side wall inspection is desired. The concrete wall must have holes for the passage of the coolant pipes to and from the pressure vessel and if the described prior art encasement construction is applied to the pipe portions which must pass through the concrete wall, the holes in this wall must be undesirably large. Therefore, another object of the present invention is to provide these pipe portions with rupture protection without requiring their passage holes through the concrete wall to be undesirably large in diameter. The usual pressurized-water reactor pressure vessel side wall does not have a uniform diameter throughout its height. At least its upper portion is of enlarged diameter relative to its portions therebelow, because the wall thickness of the upper portion is increased in the area of the coolant pipe connections to prevent them from materially reducing the side wall's radial and axial strength in that area. This characteristic, plus of course, the radially extending coolant pipes, eliminate any concept of an encasement that is vertically slidable on the pressure vessel's side wall. However, in accordance with the present invention, radially superimposed layers of individually separable, segmentally cylindrical segments are interfitted to form a substantially cylindrical shell enclosing the vessel's side walls. These segments are made of the pressure-resistant, heat-insulating material and are arranged to not only provide a plurality of layers which are subdivided in the radial direction of the vessel's side wall, but also layers which are subdivided in the axial direction of the vessel's side wall. The high-tensile strength metal elements, normally steel, are in the form of cylindrical rings which are stacked end-to-end and form, in effect, a metal wall surrounding the layers of segments. Those of these rings which are below the vessel's coolant connections are positioned prior to the vessel being installed in its pit. One of these rings, or possibly several, are positioned above the coolant pipe connections of the vessel's side wall and can be installed and removed at any time the reactor is shut down. The segments are appropriately shaped and laid to contact the vessel's side wall throughout, including both the upper portion of larger diameter and all lower portions, and the segments can be laid and removed at any time when the reactor is shut down. When the encasement is completed, with the reactor of course in a shut-down condition, start-up of the reactor, with consequent thermal expansion of the pressure vessel's side wall, places the layers of segments in compression because their surrounding metal cylinders remain cooler and, therefore, thermally expand to a lesser degree than the pressure vessel's side wall. The new encasement substantially fills the previously referred to annular space and prevents the use of this space for vessel side wall inspection purposes. However, when the reactor is shut-down and its pressure vessel is at or near room temperature, the encasement parts are free from stress, the segments and cylinders being dimensioned to assure this condition. Then the metal cylinder or cylinders above the coolant pipes of the pressure vessel, may be lifted upwardly out of the annular space, and the outermost layer or layers of the segments may then be removed, thus providing clearance in the radial direction for removal and upward lifting of the segments of the layer or layers beneath the upper portion of the vessel having the larger diameter. In this way the entire encasement, with the exception of the metal cylinders below the coolant pipes, may be removed from the annular space. Being made of high-tensile strength metal, such as steel, these cylinders may have a wall thickness small enough to provide room between their inner surfaces and the pressure vessel's side wall for the insertion of any instrumentation normally required for the inspection of the vessel's side wall. Re-installation of the encasement is, of course, only the reverse of the described procedure. The rupture protection for the coolant pipes is provided in the form of closely interspaced high-tensile strength metal rings which encircle these pipes. The rings are preferably made from rectangular metal bar stock, preferably square in cross section, having a thickness at least equal to the wall thickness of the coolant pipes. In addition, these rings are spaced from each other a distance about equal to their thickness. Their compactness avoids the need for undesirably large pipe passages through the concrete construction.