Patent Number: 055442102
Section: description

MODES FOR CARRYING OUT THE INVENTION Referring now to FIGS. 1 through 6B, pressure vessel apparatus constructed in accordance with the teachings of the present invention is designated by reference numeral 10. Apparatus 10 is a nuclear reactor pressure vessel for use in large size light water integrated reactor systems and it includes vessel main body 12, vessel top body 14, and vessel head 16. Apparatus 10 is positioned within the interior of a containment vessel 200 with base 18 extending below ground level and defining a sump 20 having cooling water 22 disposed therein. Vessel main body 12 rests on seismic base isolators 24 located between the vessel base and the vessel main body. Pressure vessel apparatus 10 defines a pressure vessel interior 26 within which is positioned a nuclear core 28. Suitable support structure 30 such as a core barrel supports nuclear core 28 above the joint 44 of the vessel main body. In the arrangement illustrated, the vessel bottom 32 is convex and contains lead, lead alloy or other material 34. As is conventional, a steam outlet line 36 is in communication with the pressure vessel interior. A water inlet line (not shown) is also employed to furnish feed water to the pressure vessel interior. The vessel main body is constructed of any suitable material such as cast iron or reinforced concrete and includes an outer peripheral wall 38 with internal lining 201 of stainless steel plate extending upwardly from the bottom 32. At the top thereof the outer peripheral wall 38 defines a vessel main body top opening 40. Vessel top body 14 has an outer peripheral wall 42 positioned on the outer peripheral wall 38 of the vessel main body, forming a joint 44 therebetween. Outer peripheral wall 42 defines a vessel top body bottom opening corresponding in size to and communicating with the vessel main body top opening 40. In the arrangement illustrated, vessel top body 14 is of multi-part construction, including a segment 46 having an outer periphery corresponding to that of the vessel main body and a segment 48 which is of reduced circumference. The lower end of segment 48 is of frusto-conical configuration and seats into a recess of like size and configuration at the top of segment 46. Any suitable material such as cast iron or reinforced concrete may be utilized in the construction of vessel top body 14. As illustrated, segment 48 may be formed of reinforced concrete and segment 46 may be formed from cast iron, with internal lining 201 of stainless steel. The outer peripheral walls of vessel main body 12 and vessel top body 14 define a plurality of spaced throughbores 50 extending from the top of vessel top body 14 through the bottom of vessel main body 12. Throughbores 50 extend vertically alongside and spaced from the pressure vessel interior. A plurality of double-ended tendons under tension extend through the throughbores 50 and are secured to the vessel main body and the vessel top body. The tendons, which are designated by reference numeral 52, are directly connected at the bottoms thereof to vessel main body 12 by fixed tendon anchors 54. At the upper ends thereof tendons 52 are connected to vessel top body 14 in three different ways. Some of the tendons 52 are directly connected to the vessel top body 14 by tendon anchors 56 (see FIG. 6B). Suitable means such as an internal thread connection (not shown) is preferably employed to adjust the degree of tension imparted to the tendons 52 operatively associated with tendon anchors 56. These tendons may be provided with couplings (not shown) located beneath the joint 44. With reference to FIG. 6A, some of the tendons 52 may be prestressed close to yield and have their upper ends connected to a dashpot or damper 58 while other of the tendons are slightly prestressed and attached to disc spring mounts 60. The amount of prestress, the number, spacing and proportion of tendons mounted directly, on disc springs, or on dampers is based on a precalculation readily performed by a person skilled in the art to result in the optimum desired response of the vessel top body to given or postulated loads applied thereto. The purpose of the tendons 52 is to carry the vessel pressure during normal service and also to absorb pressure surges which may occur within the pressure vessel interior, for example caused by explosions or the like. Most of the energy will be absorbed by tendons 52 which may stretch into the plastic range up to a certain per cent of their length, e.g. about 3 per cent but less than ultimate. The different mounts of the tendons 52 will limit the response to less than ultimate. Since the tendons 52 essentially provide the sole force (other than gravity) maintaining the vessel top body in place on the vessel main body, stretching or elongation of the tendons 52 will enable the vessel top body 14 to be momentarily displaced in an upward direction relative to the vessel main body 12 to absorb the energy of sufficient increased pressure. In most instances such pressure increases can be expected to be of very brief duration, resulting in the vessel top body 14 again moving downwardly into place on the vessel main body 12. FIG. 4A shows the vessel top body 14 in its normal position relative to vessel main body 12 at joint 44. Upon increase of vessel interior pressure to a sufficient value, the vessel top body 14 will move upwardly as shown in FIG. 4B. Also illustrated in FIG. 4B is a sacrificial weld 62 which has less capacity than the liner 201 and fractures to allow separation of the vessel top body from the vessel main body upon application of sufficient force. As may also be seen with reference to FIGS. 4A and 4B, protective sleeves 64 with thermal insulation 202, for example asbestos, are disposed about tendons 52 at the location of joint 44 to afford protection to the tendons during relative movement between the vessel top body and the vessel main body, for example, by isolating the tendons from contact by hot fluids escaping the pressure vessel interior. The sleeves may also serve as guide for the vessel top body during the separation from the main body. In the arrangement illustrated, the sleeves 64 incorporate two components 66 and 68 which telescope relative to one another, sleeve component 66 being attached to vessel top body 14 while the sleeve components 68 are connected to vessel main body 12 and allow relative movement with respect to the tendons. At joint 44, bellows 70, cut and welded from steel tubing or formed of bent steel sheet or the like, extend between the vessel main body and the vessel top body about the joint to form a seal about the joint. Thus, bellows 70 serves to prevent the escape of heated fluids from the apparatus even when pressure surges results in dislocation of the vessel top body upwardly from the vessel main body. Elongated reinforcement members in the form of wires, bands or cables 72 limit the degree of outward movement of the bellows as shown in FIG. 4B. A cushion 203 of metallic lead, or other suitable material, is provided inside the bellows for support during the prestressing of the bellow reinforcement members 72. A passageway 74 leading from the space between the bellows 70 and the vessel top body 14 is in communication with instrumentation 76 to monitor liner leak tightness pressure or other physical conditions. Channels 80 extend circumferentially about vessel main body 12 and vessel top body 14. Channels 80 accommodate elongated reinforcement elements in the form of continuously wound prestressing wire strands or bands 82 under tension in engagement with and extending about the outer peripheral walls 38, 42 and 48. These prestressed reinforcement elements 82 substantially contribute to the structural strength of the vessel main body and the vessel top body and contribute to the ability thereof to resist high pressure generated within the pressure vessel interior. Any "breathing" of the structure may occur if the main body is segmented only after the top body separates from the main body, and will be short lived or not occur at all because of the formidable reserve strength of the prestressed reinforcement elements 82. Vessel head 16 includes two dome-like steel shells 86, 88 having outwardly extending peripheral flanges. In the arrangement shown, a lead, aluminum, or other metallic filler 90 is disposed between shells 86, 88 to distribute forces. Double-ended, elongated tendons 92 extend through spaced openings formed in the head flange comprised of the flanges of shells 86, 88 and extend downwardly to tendon coupling 93 located in throughbores 94 in the vessel top body 14 which are in registry with the openings in the head flanges. The lower ends of tendon couplings 93 are fixedly secured to the vessel top body 14 in any desired manner. The top ends of the tendons 92 are connected to the head shell 86 by head tendon stud anchors 96 which are preferably adjustable to control the degree of tension imparted to tendons 92. An important feature of the present invention resides in the fact that the tendons 52 secured to the vessel main body and the vessel top body permit movement of the vessel top body away from the vessel main body due to a pressure surge within the pressure vessel interior before the tendons secured to the vessel top body and the vessel top head permit movement of the vessel top head away from the vessel top body due to a pressure surge within the pressure vessel interior. As is conventional, the vessel head 16 supports control rod stand pipes 98, drive mechanism, shafts and housings, and various instrumentation which pass through openings in the vessel head (not shown) and extend to the nuclear core 28. FIG. 3 depicts a cross-section of the symmetric core with fuel-bundles within the pressure vessel interior 26. In the interest of simplicity, FIG. 1 only shows the upper portions of two control rod stand pipes 98. Momentary lifting of the vessel top body 14 (and thus vessel head 16) from vessel main body 12 as a result of high pressures within the pressure vessel interior will not cause relative movement between the control rods and the nuclear core because the support structure 30 supporting the nuclear core is itself supported by the vessel top body and will move upwardly therewith. The use of wire strand tendons to hold the vessel head to the vessel top body is a considerable improvement over the conventional threaded studs which have a lower strength capacity and ductility than the tendon studs which are normally at least partially comprised of wire strands. This permits a larger diameter and pressure load on the vessel head, and reduces stress and allows more room for control rods or other penetrations. With the present arrangement, all tension anywhere in the apparatus is carried or assumed by high-tensile wires or bands. The vessel main body, the vessel top body and most of the vessel head are under compression, with the apparatus itself in a state of three dimensional compression. FIG. 7 discloses an alternative form of the apparatus, identified by reference numeral 10A which is essentially of the same construction as apparatus 10 except that it employs a core catcher 100 containing lead or lead alloy for receiving core material and dissipating the decay heat in case of an accident.