Patent Number: 039390381
Section: summary

BACKGROUND OF THE INVENTION A nuclear reactor includes a pressure vessel having a substantially cylindrical side wall and a substantially hemispherical bottom wall above which a core vessel is suspended with its lower end spaced above this bottom wall. It is possible for the core vessel to accidentally fall, and since it is spaced for a substantial distance above the pressure vessel's bottom wall, it is possible for the latter to be ruptured and release the reactor core coolant. To prevent this, the prior art has proposed the use of an intercept construction extending upwardly from the pressure vessel's bottom wall towards the core vessel's lower end for intercepting the core vessel if it accidentally falls. The construction is intended to limit, or substantially eliminate, the distance the core vessel can fall. Such a construction must transmit the falling force and ultimately the static weight of the core vessel to the pressure vessel's bottom wall. Incidentally, in addition to preventing the core vessel from falling forcibly against the pressure vessel's wall in the event of an accident, it is important that the core vessel be intercepted so that it is held at least approximately at its normal position where it is within the coolant flow to assure that the core in the vessel continues to receive adequate cooling. An example of a prior art core vessel intercept construction is described in the book VGB-Nuclear Power Plant Seminar 1970, p. 33. This construction is in effect a stool, and the force of holding the core vessel in position, in the event of an accident, is concentrated exclusively over a relatively small, localized area of the spherical bottom wall of the pressure vessel, giving rise to high local stressing of this bottom wall. SUMMARY OF THE INVENTION One object of the present invention is to provide a core vessel intercept construction which, should its use be required, avoids transmittal of the force of the accidentally released core vessel, exclusively to a localized area of the pressure vessel's bottom wall. This object is attained by the present invention through the provision of a structural metal framework supported by the pressure vessel's bottom wall at a plurality of interspaced positions, at least some of these positions being adjacent to the peripheral portion of the bottom wall which is where the latter integrally connects with the pressure vessel's side wall with the latter capable of carrying very substantial force and tension. The framework extends upwardly at least adjacent to the core vessel's lower end. The framework includes an annular series of substantially vertical interspaced columns extending upwardly from the pressure vessel's bottom wall's peripheral portion, and a central substantially vertical column extending upwardly from substantially the center of that bottom wall, struts extending laterally from this central column to the annular series of columns and positioning them on the pressure vessel's bottom wall which, being hemispherical, sharply declines arcuately at its peripheral portion. Substantially flat ductile metal blades form the peripheral series of columns and are positioned in radial planes with respect to the central column, by the mentioned struts, and these flat plate columns are all connected by a metal ring which is substantially concentric, as are the columns of the peripheral series, with the central column. Means are provided for tensioning the central column downwardly so that via the mentioned struts the annular series of columns have their bottoms pulled downwardly against the hemispherical bottom wall of the pressure vessel. A downwardly extending peripheral flange is a part of the core vessel and the framework includes a ring connected with the annular series of columns and which is at least partially inside of this flange and peripherally adjacent thereto. This ring is supported by the upper ends of the annular series of columns and the upper portions of the latter are reduced in cross-sectional area in such a manner that if the core vessel falls, these reduced portions are stressed beyond their elastic limits and deform in a ductile manner to provide for a gradual absorption of the falling force, and, of course, ultimately the static weight of the core vessel, when required. The central column is also made of flat plates radially arranged, each with its upper end reduced in cross-sectional area so as to correspondingly deform in a shock-absorbing manner. The plates of the central column are in the radial planes of the plates forming the annular series of columns. There is at least a slight space between the upper end of this intercept structural framework and the core vessel, to provide room for thermal expansion and contraction of the parts. The core vessel can fall this short distance and thereafter its weight must be supported by the framework. The falling force is absorbed in a shock-absorbing manner by the ductile deformation of the parts, as described above. All of the parts are symmetrically distributed about the central column, and the upper ring of the framework laterally positions the falling core vessel, and therefore, the ductile deformation is uniformly distributed without excessive angular displacement. Thereafter the core vessel is held upwardly approximately reasonably close to its normal position so that its fuel elements continue to receive the cooling of the normal coolant flow.