Nuclear Reactor Pressure Vessel Support System

This invention relates generally to nuclear reactors and more particularly to a support system for nuclear reactor pressure vessels.

The nuclear reactor pressure vessel is one of the main components in any nuclear reactor system, for it is within this pressure vessel that the fission reaction which is the object of any nuclear reactor system occurs. To provide redundancy in the sealing of the nuclear reactor in the unlikely event of an accident, so as to further protect the external environment, the entire nuclear reactor primary system is enclosed within a containment structure. The containment structure, generally fabricated of reinforced concrete, is comprised of a plurality of individual cells, and each cell encloses one primary system component, thereby isolating each primary system component. In the instance of the pressure vessel, the containment structure forms a well radially surrounding the pressure vessel. This containment well is generally placed apart from the pressure vessel so that inspection and monitoring of the external side of the pressure vessel can occur.

As is customary in the nuclear field, the pressure vessel is not self-supporting, but is mounted on and supported by the wall of the containment well. The means of mounting the pressure vessel on the containment structure wall must take into account the postulated accidents mandated by safety considerations which impose substantial stresses on the pressure vessel.

One means of securing the pressure vessel to the containment structure in the prior art was by means of a double-bolting system. A flange extended around upper periphery of the pressure vessel, and this flange is bolted to a stationary outer ring. The stationary outer ring is in turn, bolted to a support ledge of the containment well. The plug riser, through which the rotatable plugs situated within the pressure vessel is supported, is then secured to the stationary outer ring. For ease of design, the tops of the rotatable plugs, the stationary outer ring, and the support ledge are substantially at the same elevation.

The use of the outer ring bolted to the flange of the pressure vessel introduces a joint which is open to both the external environment and the interior of the pressure vessel. This joint has to be sealed to prevent the escape of fluids from the interior of the pressure vessel. The addition of such a seal further complicated the support and stress problems.

The aforementioned problems of the prior art are eliminated by this invention by providing a pressure vessel flange around the outer periphery of the pressure vessel, and a vertical extension of the pressure vessel flange formed integral therewith. The vertical extension and the containment support ledge are located at the same elevation, and the support ring is located on the upper surface of both. The support ring is then bolted to both the containment structure and the flange vertical extension. Secured to the flange vertical extension is the plug riser, which is additionally secured to a radially outward extension of the rotatable plug. In this location, the means of sealing to prevent the escape of contained fluid through the plug riser also functions to seal the joint between the pressure vessel flange and the stationary outer ring, thereby eliminating the need for a separate seal for the latter joint.

Reference is now made to the description of the preferred embodiment, taken in connection with the accompanying drawings, in which:

FIG. 1 is an elevational view of a nuclear reactor pressure vessel utilizing this invention; and

FIG. 2 is a detailed view of the vessel flange, sealing and support structure of this invention.

Throughout the description which follows, like reference characters indicate like members of the various figures of the drawings.

Referring now more particularly to FIG. 1, a nuclear reactor vessel 10 houses a nuclear core 12 comprised of a plurality of assemblies of fuel elements of fissionable and fissile material. Coolant flow inlet means 14 and coolant flow outlet means 16, for the ingress and egress of reactor coolant, are formed integral with and through the cylindrical vessel walls 10. The reactor core and vessel internals do not form a part of this invention. For a more detailed description of the reactor core and pressure vessel internals, reference may be had to the application of William Pennell and William Rowan, Ser. No. 503,148, filed Sept. 4, 1974 for a nuclear reactor, and assigned to the same assignee as the present invention. The pressure vessel 10 is sealed at the top by a nuclear reactor closure head 18. In the liquid-metal fast breeder reactor shown, the closure head 18 is comprised of a plurality of rotatable plugs 20, 22 and 24. The large plug 24 is of annular configuration and is coaxial with the pressure vessel 10. The intermediate plug 22, also of annular configuration, is positioned within an opening 26 in the large plug 24. The circular small plug 20 is eccentric to the intermediate plug 22 and the large plug 24, and is situated within an eccentric opening 28 in the intermediate plug 22. The intermediate plug 22 rotatably supports the small plug 20 through an annular plug riser 30. The intermediate plug 22 is rotatably supported by the large plug 24 through the annular plug riser 32. The large plug 24 is rotatably supported by the flange 36 of the pressure vessel 10 through the annular plug riser 34. The plug riser arrangements do not form a part of this invention. For a detailed description of desirable constructions of plug risers, references may be to application for U.S. letters patent Ser. No. 537,474 filed Dec. 30, 1974 entitled Rotating Plug Bearing and Seal Arrangement in the name of Elman E. Wade.

The plugs 20, 22, and 24, support a plurality of penetrations, typical of which are the in-vessel transfer machine 38, the ex-vessel transfer machine 40, control rod drives 42 and surveillance ports 44. Radially surrounding the pressure vessel 10 is the containment structure 46. For a more detailed description of such penetrations reference may be had to the aforementioned Pennell-Rowan application which does not form a part of this invention.

Referring now to FIG. 2 which compresses a detailed view of the pressure vessel support structure, the pressure vessel flange 36 extends around the upper periphery of the pressure vessel 10. Formed integral with the flange 36 is an annular vertical extension 48. The containment structure well 46 has a containment support ledge 50 which radially surrounds the pressure vessel 10. As previously mentioned, the large plug 24 is positioned within the nuclear reactor pressure vessel 10. The upper surfaces of the plug 24, the vertical extension 48 of the flange 36 and the support ledge 50 are at substantially the same elevations. Located on the upper surface 52 of the vertical extension 48, and the upper surface 54 of the support ledge 50 is a support ring 56. The support ring 56 is secured, by means of bolts 58 which pass through aligned opening 80 formed in ring 56 and flange 36, to the vertical extension 48 of the flange 36. The support ring 56 is also secured, by a plurality of bolts 60, to the support ledge 50. If so desired, a thermal pad (not shown) can be located between the support ring 56 and the support ledge 50 to insulate the hotter metal support ring 56 from the concrete support ledge 50. An angular reactor cavity seal 82 is secured to both the support ledge 50 and the support ring 56 radially outwardly from securing bolts 60 to prohibit the escape of fluids. This double-bolting method of securing the pressure vessel 10 to the containment structure 46 provides great capability for energy absorption by plastic deformation of the bolts to limit the load imparted to the support system. Forces imparted on the bolts 58, 60 by the pressure vessel 10, in the unlikely event of an accident, will cause the bolts 58, 60 to stretch. The stretching of the bolts 58, 60 will absorb the forces exerted by the pressure vessel 10, and though some bolts 58, 60 will shear, sufficient bolts 58, 60 will remain to support the pressure vessel 10.

The large plug 24 has a radially outward extension 62 located adjacent to the flange vertical extension 48. The annular support riser 34, by which the flange 36 and the vertical extension 48 support the large plug 24, is secured respectively to the plug radially outward extension 62 and the flange vertical extension 48. An annular space 66 is formed between the flange vertical extension 48 and the plug radial extension 62. This annular space 66 could serve as an escape path for contained fluid from inside the pressure vessel 10 to the external environment through the plug riser 34. Means 68 for sealing the annular space 66 are located within the annular space 66. As shown the sealing means 68 comprises a liquid dip seal, in which liquid sodium is present during both operation and refueling. As the contained fluid attempts to escape it exerts a pressure on the liquid sodium in the dip seal 68, and the liquid sodium rises within the annular space 66. The annular space 66, and the annular plug riser space 84 between the two elements of the plug riser 34 with which it is in fluid communication, is sealed. Therefore, the gas in these two spaces 66, 84 is compressed, and forces the liquid sodium down the annular space 66. In this manner, the fluid from the inside of the pressure vessel 10 is prevented from escaping.

In addition to preventing the escape of contained fluids through the plug riser 34, the sealing means 68 also prohibit the escape of contained fluid through the joint 70 between the stationary outer ring 56 and the flange vertical extension 48. By positioning the latter jointure downstream from seal 68, the sealing means 68 seal two possible fluid escape routes with a single seal. If radiation streaming is a concern along a path formed between vessel 10 and the outer periphery of ring 24, an annular shearing ring 74 may be secured to the pressure vessel 10 and the flange 36 in the transition area from the pressure vessel 10 to the flange 36. This ring 74 reduces the radiation streaming which may flow through the annular space 66.

Thus, the disclosed invention provides a system for supporting the nuclear reactor pressure vessel within the containment, and eliminates the need for one of the seals of the prior art by enabling a necessary seal to seal two fluid escape routes.