Patent Number: 047864621
Section: summary

FIELD OF THE INVENTION This invention is concerned with support structures for nuclear reactors. BACKGROUND OF THE INVENTION Nuclear reactors are well known and have been utilized for steam generation by circulation of a liquid coolant from the area of the reactor core to a heat exchanger means. The nuclear reactors have been supported by various types of steel and concrete structures. The prior art support structures have been fabricated from concrete and/or steel columns and cross braces which have been designed with various walls separating the reactor components. These prior art support structures have been difficult to fabricate and have been modified to include seismic reinforcing elements. The seismic reinforcing elements include additional reinforcing rods within the concrete structures, auxilliary snubbers, hangers and bumpers. These elements are expensive to fabricate and install, difficult to inspect and limited in their ability to protect the reactor structures from damage due to the stresses induced by high ground level acceleration. Prior reactor support structures result in a structural configuration that significantly amplifies seismic forces to important safety related components such as the reactor vessel. This is due to the fact that the mass to stiffness characteristics of the support structure results in a fundamental support structure frequency which is within the range of the supported component. This results in ground seismic forces being amplified by the structure to impose large seismic loads to the supported items. There are two ways to deal with the problem of accomodating seismic forces. One way is to make the structures, systems, and components sufficiently strong so they can accomodate these loads. The alternate approach is to change the configuration so that the components do not absorb all of the loads. In the prior art both approaches have been used. Changes in configuration have included providing flexibility in the item of interest; changing the natural frequency of the component such that its frequency of vibration does not coincide with the amplified response of the supporting structure; embedment below grade of the reactor support structure; or providing an energy absorption device. Energy absorption devices include design approaches using snubbers, ductile joints, or seismic isolation pads. The design of liquid metal reactor vessels and piping is more sensitive to seismic disturbances than conventional reactor concepts because of thin walled vessels and piping associated with high temperature liquid metal systems. Prior art designs have had difficulty in providing support concepts which would adequately limit seismic forces to major vessels such as the reactor vessel. Current U.S. designs for commercially (1000 Mwe or greater) sized loop LMFBR plants result in high horizontal seismic shear forces being amplified by the reactor vessel structural support system for sites which have rock type (soil shear wave velocity greater than 3500 ft/sec) sites. Studies for the loop plant have shown that for a given plant configuration the major components needing seismic protection are subjected to less severe seismic design requirements for sites with less firm soil characteristics (soil shear wave velocities less than 2000 ft/sec) due to the beneficial effects of soil-structure interaction For harder sites with rock type of foundation (soil sheer wave velocity greater than 3500 ft/sec) alternative design methods for limiting seismic forces have been investigated. For the U.S. consideration has been given to limiting the plant location to sites with suitable soil and seismic conditions; for the French Superphenix II LMRBR pool plant, consideration is being given to use of seismic isolators for limiting seismic forces to the reactor vessel; and for the United Kingdom's commercial design pool plant (CDFR) the designers approach is to embed the nuclear reactor vessel support structure below grade up to the reactor vessel support ledge in order to reduce forces to the reactor vessel (embedment does not have a large effect on modifying vertical response). Use of embodment or seismic isolators results in a plant design which is costly and more difficult to construct. Limitations of sites to those with the proper seismicity levels and soil conditions is also undesirable since this approach could result in significantly reducing the available number of sites. SUMMARY OF THE INVENTION The invention comprises a monolithic reinforced concrete support structure for a nuclear reactor. The support structure comprises a reinforced concrete base, a unitary reinforced concrete core, said reinforced concrete core containing voids for installation and support of a reactor vessel. Additional voids may be provided to accomodate other major components which need to be seismically protected. An alternate approach for changing the configuration to accomodate seismic forces is provided by applying the principles of structural and soil dynamics (reference 1) utilized for machine foundation design. By having a support structure sufficiently rigid, its resonant frequencies will be far enough above the resonant frequency of the machine that a large fraction of the vibratory motions are not transmitted from the supported machine. Principles for calculation of the natural frequency of structures is provided in reference (2). This invention applies this structural dynamics principle by providing a support design that has a natural frequency much higher than the supported component, and in this manner effectively limiting amplification of a large fraction of the vibratory forces of a seismic disturbance to the supported component. The preferred embodiment of this invention describes a monolithic reinforced concrete structure with a natural horizontal frequency greater than 10 Hertz. The major vessels supported by this structure have a natural horizontal frequency in the range of 4 to 6 Hz. In this manner, the internal structures, systems and components which are supported or are a part of the structure can be isolated from amplification of ground level seismic forces. Only loads near the natural frequency of the monolith are amplified, but since the frequency of the supported component is significantly less than the support, the transmission of the amplified seismic loads to the component will be reduced. The extent to which this tranmission is reduced is a function of the natural frequencies and can be determined from analysis (reference 1 or 2). This analysis may be carried using known techniques as described in (1) Vibrations of Soils and Foundations by F. E. Richart, Jr., J. R. Hall Jr., and RD Woods, Prentice-Hall, 1970 and (2) Dynamics of Structures, by Clough, R. W. Penzien, J. McGraw-H.71, 1975, both of which are incorporated by reference. The undamped natural frequency may generally be expressed by the formula ##EQU1## wherein fn is cycles/second or Hertz; k is the stiffness in units of force required to displace the structure a given distance and m equals the weight of the structure displaced by k. The monolithic reinforced concrete structure of the invention provides a means of supporting a nuclear power plant reactor as well as other major vessels which need seismic protection. The unitary design inherently limits seismic forces to the supported vessels and it is also easier to construct the monolithic support structure than the prior art support structures. Accordingly, it is a primary object of this invention to provide a novel support structure for a nuclear reactor. It is also a further object to provide a novel nuclear reactor support structure that has inherent seismic resistance and can provide a seismically stable support for other major vessels which need protection from seismic disturbances. It is also a further object of this invention to provide a novel nuclear reactor support structure that is relatively easy and quick to fabricate. It is also a further object of this invention to provide a novel nuclear reactor support structure that may be situated on or below grade level. It is also a further object of this invention to provide a novel nuclear reactor structure that simplifies the shielding of the nuclear reactor. These objects are attained by a support structure comprising: A support structure for a nuclear reactor: (a) a unitary reinforced concrete core containing a void for support of a nuclear reactor vessel, said concrete core providing shielding for radiation from the reactor vessel; (b) said reinforced concrete core having means for transferring heat from the reactor vessel; (c) said reinforced concrete core having a distribution of mass and stiffness which provides a fundamental horizontal natural frequency of vibration greater than 10 Hertz and greater than the natural frequency of the supported components. For reactor concepts which have need of seismic protection of other large vessels the unitary core can be increased in size to provide additional voids for these vessels.