GB2185346A - Pressure vessels - Google Patents

Abstract

A pressure vessel has a cylindrical wall formed from a plurality of similar annular structures, each annular structure being formed from a plurality of substantially identical cast iron segments 10 with radially spaced circumferentially extending projections 22, with opposed projections of adjacent segments held in contact by circumferentially extending prestressing tendons 28 and the recesses between the projections providing ducts 26 for axially extending prestressing tendons 27. Segmented top and bottom structures are also provided for the pressure vessel. <IMAGE>

Description

SPECIFICATION Pressure vessels This invention relates to pressure vessels, and more especially, though not exclusively, to pressure vessels for nuclear reactors.

In Patent No. 2093259 there is described a pressure vessel having a cylindrical wall structure formed of a plurality of truncated wedge-shaped cast iron slabs disposed so as to form an annular assembly, with the slabs having cast-in through passages, disposed substantially helically, both right- and left-handed with respect to the cylindrical wall structure, respective passages of adjacent slabs being in alignmentto provide, in the assembled wall structure, a system of right- and left-handed generally helical ducts in which prestressing tendons are accommodated.

The use of cast iron elements has the advantage over prestressed concrete, which had previously been proposed for forming the pressure vessels of nuclear reactors, in that cast iron can be used at a peak stress many times the maximum allowablefor concrete.

An object of the present invention is to provide an alternative method offorming the cylindrical wall structure of a pressure vessel, utilising cast iron el ements,which has advantages overthe structure described above.

According, therefore, to the invention a pressure vessel has a cylindrical wall structure comprising a plurality ofsimilarannularstructures disposed coaxially end to end about a vertical axis, each annularstructure beingformedfrom a pluralityofsubstantially identical cast iron segments, each comprising a central radially extending body portion having a seriesofradially-spaced circumferentiallyextending projections on each side with opposed projections of adjacent segments in contact, therecesses formed between the projections providing vertical ducts through which extend prestressing tendons to provide axial loading, and each annular structure being surrounded by a plurality of axially spaced circumferentially extending prestressingten- dons serving to apply radially inward loading on the segments.

Preferably the outermost projections of adjoining segments are secured together as by bolts.

Preferably, also, fabricated steel orcastiron members extend circumferentially across the outer ends of a pluralityofadjoiningsegmentsandtransmitthe loading of the circumferentially extending tendons to the segments.

The top wall of the pressure vessel conveniently comprises a central circular region formed of an assembly of substantially sector shaped elements surrounded by a plurality of segments prestressed inwards by means of circumferentially extending tendons and downwardsagainstthesegmentsfor- ming the uppermost annular sidewall structure by vertical tendons.

The sector elements of the top wall assembly are preferably keyed to the surrounding segments, which in this case do not make direct contactwith each other, so as to ensure that the segments are urged firmly against the sector elements.

Flexible spacers, for example of rubber, fibre or aluminium could be disposed between the segments if desired. Moreover the segments and sectors ofthe top wall are conveniently formed so that they extend circumferentially the equivalent of a plurality, say four, sidewall segments, the topwall segments being suitably apertured, to receive the vertical prestressing tendons.

An opening is preferably provided at the centre of the top wall assembly to accommodate an apertured plug for refuelling purposes, the sector elements being keyed vertically, forming, in effect, a prestressed iron ring as a boundary to the central plug.

The bottom wall of the pressure vessel is conveniently constructed in a generally similar mannerto the top wall with the central plug in this case penetrated with holes for the control rod actuator tubes, and designed to supportthe radial prestress load.

Voids between sectors will normally be filled with concrete in order to provide radiological shielding, to stabilise the sector panels and to reinforce the penetrations through the top and bottom walls, and the formed pressure vessel will be provided inter wally with a steel liner.

One nuclear reactor pressure vessel incorporating a cylindrical wall structure in accordance with the invention will now be described byway of example with referenceto Figures 1 to7oftheaccompanying schematic drawings, in which Figure 1 represents a vertical axial section in diagrammaticform of the pressure vessel, Figure2 represents a plan section of part ofthe cylindrical wall of the pressure vessel, Figure 3 represents a side elevation of one of the elements employed in the wall, Figure 4 represents a plan view partly broken away ofatopwall sector and topwall segments, Figures 5and 6illustrate a side view and an end view one of the sectors employed in the upper wall, and Figure 7 represents a plan view of part ofthe bottom wall of the vessel.

Referring first to Figure 1,the pressure vessel 1 comprises a cylindrical wall structure 2, closed at the top and bottom by end walls 3,4 respectively. These end walls are formed with various required penetrations, such as coolant circulator penetrations 5, control rod penetrations 6, feedwater and steam penetrations 7 leading to and from steam generators (not shown) disposed within the pressure vessel, and one or more refuelling penetrations 8 for permitting refuelling and other operations on a nuclear reactor core (likewise not shown but similarly accommodated within the pressure vessel). The pressurevessel also contains an impermeable liner9 of steel slightly spaced from the inner surfaces of the cylindrical wall structure and the top and bottom end walls 3,4.

In accordance with the invention the cylindrical wall structure 2 is formed fromthreeannularstruc- ture 11,12,13 stacked one upon the other, and each formed from a plurality of similar cast iron segments 10 (Figures 2 and 3). Each segment (three of which are shown in Figure 2), comprises a central web 20 disposed in a respective radial plane, and four radially-spaced transversely-projecting lobes 21,22,23, 24 on each side of the central web 20,the circum- ferential lengths of the lobes decreasing from the outerto the inner lobe so thatthe end faces ofthe lobes on each side of the web lie in respective radial planes. Adjacent segments are disposed with the outer edges of the lobes in contact, the outer lobes 21 being secured together by bolts 25.

When the segments 10 are assembled to form a complete annular structure the recesses between the lobes of adjacent segments providethreevertically extending channels 26 between four concentric bearing rings provided by the contacting lobes 21 to 24.

The segments ofthe three annular structures 11, 12,13 are disposed immediately above one another, so that the ducts 26 extend through the complete cylindrical wall structure to provide passagesforver- tically extending prestressing tendons 27 as will subsequently be described.

Around the outside of each annular structure extend a plurality of circumferential prestressing tendons 28, fabricated steel or cast iron pans 29 transmitting the wound tendon load to the sidewall segments 10. Each pan 29 conveniently traverses a plurality, in this case three, of the segments the aim being to load all the segments equally.

The top wall 3 ofthe pressure vessel comprises a central circular region formed from a plurality of sec torshaped elements 16 surrounded bya plurality of segments 17. As shown in Figure 4 each of the ei- ements 16 and segments 17 extend circumferentially for a distance corresponding to a plurality, in this case four, of the sidewall segments 10.

The topwall segments 17 are formed with two lobes 18which provided between them vertically ex- tending ducts 19. The ducts 19 align with respective ducts 26 in the side wall 2 for the passage of thevertical prestressing tendons 27. Steel tubes 30 surround the prestressing tendons 27 and extend vertically from the tendon anchorage zone at the top of segments 17 down to the top of the sidewall segments 13.

Thetopwall segments 17 aresimilarlysurrounded by circumferential prestressing tendons 28, cast iron or steel pans, as at29 in Figure 4, fortransmittingthe wound tendon loadto the segments 17.

However the circumferentially extending lobes 18 ofthetopwall segments 17 do not contact each other, the outermost lobes being separated by spacers 31 of rubber, fibre or aluminium composite or like resilient material, so thatthe radial prestressing load is transmitted totally to the sectors 16.

Each ofthe latter comprises, as shown more clearly in Figures Sand 6, of a central radially extending web 32 joining upper and lowerwedge-shaped flanges 33,34, with transverse strengthening ribs 35, theflanges of adjoining sectors being bolted together in the completed assembly. The inner ends of the sectors 16terminate short of the centre to accommodate an apertured plug 36 through which the refuelling pipe 8 extends, the sectors being keyed vertically at their inner ends to form, in effect, a prestressed iron ring as a boundarytothe central plug zone.

Owing to shear loading, the sectors 16 are firmly connected to the surrounding segments 17, for example by dowels as at37 in Figure 4. However in a modification (notillustrated) each of the sectors 16 and the respective segment 17 could be cast in one piece. Certain of the sectors 16 are provided with openings (not shown) for the feedwater and steam penetrations 7.

The bottom wall 4 is constructed in a somewhat similarmannerto the top wall 3, although appropriate ones ofthe sectors 38 (Figure 7) are con- structed so as to provide openings for the coolant circuiators. Figure 7 indicates howthis is achieved.

The sector width is selected so that, for example, threesectorwidths envelope the circulator penetration shown at 39. The penetration 39 is formed by two similar units 41 bolted together and to a third unit 42 at vertical interfaces three truncated sectors 43 making up the assemblage. Keys and dowels (not shown) provide radial, and bolted flanges circumferential, interconnections.

In the completed pressure vessel the voids between sectors forming top and bottom walls are filled with concrete (as at 44 in Figure 5) in orderto provide radiological shielding, to form an added structural reinforcement, to stabilise the sector webs, and to back up steel penetrations throughthe sectors as in a concrete vessel. The concrete could be introduced after the vessel has been partly prestressed, the remaining fraction of the prestress being selected to prestress the concrete to its maximum, so as to give it structural value. Alternatively the vessel could be fully prestressed before the introduction of the concrete.

Any of the known systems of prestress tendons can be employed for the vertical tendons 27; each tendon may, for example, comprise a plurality of individual stranded steel cables, each separately ten- sioned and anchored in any suitabie known manner.

Similarlythe tensioning and anchoring ofthe cir- cumferential tendons, which may also beformed from a plurality of steel cables, can also be carried out in any of the known manners.

The invention has the advantage over the construction described in Patent specification 2093259 referred to above in that the cast-iron segments are more easilyfabricated than those designed to accommodate helical prestressing tendons, leading to a cheaper overall construction, and in addition the necessity to thicken the upper and lower end wall segments, in order to accommodate the number of tendons emerging therefrom, is avoided,sincethe numberofsuchtendons is reduced.

Claims (26)

1. A pressure vessel having a cylindrical wall structure comprising a plurality of similar annular structures disposed coaxially end to end about aver- tical axis, each annular structure being formed from a plurality of substantially identical cast iron segments, each comprising a central radially extending body portion having a series of radially spaced, cir cumferentially extending projections on each side, which projections serve to space the body portions from each other, the recessesformed between the projections providing vertical ducts through which extend prestressing tendons to provide axial loading, and each annular structure being surrounded by a plurality of axially spaced, circumferentially extending prestressing tendons serving to apply radially inward loading on the segment.

2. A pressure vessel as claimed in Claim 1 wherein the outermost projections of adjoining segments are secured together by bolts.

3. A pressure vessel as claimed in any preceding Claim wherein members extend circumferentially across the outer ends of a plurality of adjoining segments such thatthe loading of the circumferentially extending tendons is transmitted to the segments by the members.

4. A pressure vessel as claimed in Claim 3 wherein said members are formed of fabricated steel.

5. A pressure vessel as claimed in Claim 3 wherein said members are formed of cast iron.

6. A pressure vessel as claimed in any preceding Claim wherein the cylindrical wall structure is closed at the top and bottom by walls each comprising an assembly of separate sections.

7. A pressure vessel as claimed in any preceding Claim wherein the opposed projections of adjoining segments are in contact.

8. A pressure vessel as claimed in Claim 6 wherein the top wall comprises a central circular region formed of an assembly of substantially sector shaped elements surrounded by a plurality of segments prestressed inwards by means ofcircum- ferentially extending tendons and downwards against the segments forming the uppermost annular sidewall structure by vertical tendons.

9. A pressure vessel as claimed in Claim 8 wherein the sector shaped elements of the top and bottom walls are formed with major apertures.

10. A pressure vessel as claimed in Claim 8 wherein said sector shaped elements are keyed to the surrounding segments, said segments not being in direct contact such that the said segments are urged firmly against said sector shaped elements.

11. Apressurevessel as claimed in Claim 10 wherein flexible spacers are disposed between said segments.

12. A pressure vessel as claimed in Claim 11 wherein said flexible spacers are formed of rubber.

13. A pressure vessel as claimed in Claim 11 wherein said flexible spacers are formed offibre.

14. A pressure vessel as claimed in Claim 11 wherein said flexible spacers are formed of a deformable metal.

15. A pressure vessel as claimed in Claim 14 wherein said deformable metal is aluminium.

16. A pressure vessel as claimed in Claim 11 wherein said flexible spacers are formed of a fibres metal composite.

17. A pressure vessel as claimed in Claim 8 wherein said segments and sector shaped elements ofthe top wall are formed such thatthey extend circumferentially over the equivalent of a plurality of sidewall segments, the top wall segments being suitably apertured so as to receive said vertical prestressing tendons.

18. A pressure vessel as claimed in Claim 17 wherein said segments and sector shaped elements of the top wall are formed such that they extend cir cumferentially over the equivalent of four side wall segments.

19. Apressurevessel as claimed in Claim 8 wherein an opening is provided at the centre ofthe top wall assembly to accommodate an apertured plug, said sector shaped elements being keyed vertically, forming, in effect, a prestressed iron ring as a boundary to the central plug.

20. Apressurevessel as claimed in Claim 19 wherein said apertured plug is used for refuelling purposes.

21. A pressure vessel as claimed in Claim 6 wherein an opening is provided at the centre of the bottom wall assembly to accommodate a central plug, said plug being penetrated with holes.

22. A pressure vessel as claimed in Claim 21 wherein control rod actuatortubes may be inserted into the holes in the central plug.

23. A pressure vessel as claimed in Claim 6 wherein the bottom wall is designed to support the radial prestress load.

24. A pressure vessel as claimed in Claim 8 wherein voids between the sector shaped elements are filled with concrete.

25. A pressure vessel as claimed in any preceding Claim wherein the vessel is provided internally with a steel liner.

26. A pressure vessel substantially as described herein with reference to the accompanying drawings.