WO2004102585A2

WO2004102585A2 - A nuclear reactor

Info

Publication number: WO2004102585A2
Application number: PCT/IB2004/001538
Authority: WO
Inventors: Johann Vorster; Edgar Hornischer
Original assignee: Pebble Bed Modular Reactor (Proprietary) Limited
Priority date: 2003-05-16
Filing date: 2004-05-14
Publication date: 2004-11-25
Also published as: WO2004102585A3

Abstract

This invention relates to a nuclear reactor 10 which includes a hollow core vessel 12 and a modular filler member 20 positioned within the core vessel 12 such that a core cavity 22 is defined around the filler member 20. The filler member 20 is of layered structure comprising a plurality of interconnected layers 42 of blocks 40, each layer 42 comprising a plurality of laterally inter-engaged blocks 40. The invention extends to a method of constructing a central column 20 of a nuclear reactor 10.

Description

A NUCLEAR REACTOR

THIS INVENTION relates to a nuclear power plant. More particularly, it relates to a nuclear reactor, and to a method of constructing a core of a nuclear reactor.

According to one aspect of the invention, there is provided a nuclear reactor which includes a hollow core vessel; and a modular filler member positioned within the core vessel, such that a core cavity is defined around the filler member, the filler member being of layered structure, comprising a plurality of interconnected layers of blocks, each layer comprising a plurality of laterally inter-engaged blocks.

By "modular" is therefore to be understood constructed of a plurality of independent units/building blocks.

At least some of the blocks of any one layer may be vertically off-set/staggered relative to one another, i.e. neighbouring blocks in a layer may be located at different elevations. Neighbouring blocks in a layer may have complementary keying formations. The keying formations and complementary keying formations may be slidably engageable such that assembly or disassembly of the filler member is achieved by relative vertical displacement of the blocks. Further, each block may be connected to at least one adjacent block of an adjacent layer of blocks.

Typically, the filler member will be of a high temperature- and irradiation-resistant material. The filler member may be of graphite. Instead, the filler member may be of a carbon composite material.

The filler member may be fixed in position in the core vessel.

The core vessel may include a top, a bottom and a circular cylindrical side wall extending therebetween. The core vessel may be lined with a reflector material. The core vessel may be circular in cross-section and the filler member may be co-axial with the core vessel.

The filler member may have a body which is circular cylindrical for at least part of its length. A lower end of the filler member may be of reduced diameter.

The core cavity may be annular for at least part of its length.

The nuclear reactor may be a pebble bed reactor and include a plurality of spherical fuel and/or moderator elements in the core cavity. According to another aspect of the invention, there is provided a method of constructing a central column of a nuclear reactor, which method includes arranging a plurality of filler blocks in a plurality of courses in a core vessel, each course comprising a plurality of blocks, such that each block in a course engages with at least one other block in said course and adjacent blocks in adjacent courses are interconnected to form a layered structure, and so that a core cavity is defined in the core vessel around a filler structure so formed.

The method may include inter-engaging neighbouring blocks so that the blocks in a course/layer are vertically off-set/staggered.

To this end neighbouring blocks in a course/layer may have complementary keying formations ie., engaging each block in a course with at least one other block in said course may include engaging complementary keying formations defined on neighbouring blocks.

Typically, the blocks will be of a high temperature- and irradiation-resistant material. The blocks may be of graphite. Instead, the blocks may be of a carbon composite material.

According to yet a further aspect of the invention, there is provided a nuclear power plant incorporating a nuclear reactor as hereinbefore described.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings. In the drawings, Figure 1 shows a longitudinal sectional view through a nuclear reactor in accordance with the invention;

Figure 2 shows a transverse section through a filler member of the nuclear reactor of Figure 1 ; and

Figures 3A - J show a series of steps in a method of constructing a central column of a nuclear reactor in accordance with the invention.

In Figure 1 , reference numeral 10 refers generally to a nuclear reactor in accordance with the invention. The reactor 10 includes a circular cylindrical core vessel or barrel 1 2, for containing nuclear fuel.

The vessel is lined by reflector blocks 21 which constitute a top reflector

14, a bottom reflector 1 6 and a circular cylindrical side wall reflector 1 8 extending therebetween, respectively. The reactor 1 0 further includes a filler member 20 positioned in the core vessel 1 2, the filler member 20 extending upwardly centrally from the bottom reflector 1 6 to the top reflector 14. The filler member 20 has a circular cylindrical body 24 which is co-axial with the core vessel 1 2, having a diameter of about

2000 mm for most of its length. It will be appreciated that the filler member 20 provides a central column of the reactor 1 0 within the core vessel 1 2.

A core cavity, generally indicated by reference numeral 22, is defined around the filler member 20 between an operatively outer surface of the filler member 20 and the side wall reflector 1 8. The core cavity 22 is hence annular in form for most of the length of the filler member 20. In the embodiment shown in the drawings, a lower end 25 of the filler member 20, proximate the bottom reflector 1 6, is of reduced diameter.

The nuclear reactor 1 0 is of the pebble bed type, a plurality of spherical fuel and/or moderator elements, or pebbles, 30 being contained within the core cavity 22.

The body 24 of the filler member 20 is of modular construction, being comprised of a plurality of interconnected discrete blocks 40, typically of graphite. The blocks 40 may instead he of a carbon composite material. Naturally, however, any suitable high temperature- and irradiation-resistant material may be used.

The filler member 20 is further of layered structure having a plurality of layers, or courses, 42 of interconnected blocks 40.

In the embodiment shown, and as can best be seen from Figure 2, each layer 42 (one of which is shown in Figure 2) comprises a central block 44 and eight peripheral blocks 63, 65, 70 having rounded operatively outer faces/sides 64, 72. Each block 40 is connected to a neighbouring block 40 by means of complementary keying formations defined between neighbouring blocks 40 of a layer 42, as described in more detail below.

Each layer further includes two intermediate, or filler, blocks 50 located between the central block 44 and a pair of opposed peripheral blocks 65. The central block 44 and each peripheral block 63, 65, 70 are connected to two adjacent intermediate blocks 50 or to a peripheral block 63, 65, 70, as the case may be, of an adjacent layer/course 42 as described in more detail below.

Reference is now made to Figures 3A - 3J of the drawings, in which, unless otherwise indicated, the same reference numerals used above in relation to Figure 2 indicate similar parts. As will be appreciated from Figures 3A - 3D, a layer 42 of the filler member 20 is built up around a central block 44 which is generally rectangular in shape, having a top 51 and a bottom 53, and ends 54 and sides 56 extending therebetween. A stepped, roughly square, recess 58 is defined centrally in each of the top and bottom 51 , 53. Opposed female keying formations 55 are defined in the sides 56 spaced inwardly of each end 54 and parallel therewith. A circular bore 88 extends centrally through each central block 44 from the top 51 to the bottom 53. When located in register with bores 88 of adjacent central blocks 44, a vertical channel 1 00 (Figure 2) is defined centrally through the filler member 20 which provides, in use, means for channelling coolant through the centre column thereby to remove heat generated during moderation and neutron reflection effected by the graphite blocks. It will be appreciated that the surface area for. heat removal at a centre of the core vessel 1 2 is effectively increased by the centre column 20. The channel 100 may further serve as a location for a neutron source at start up, and may provide means for locating the centre column 20 relative to the top reflector 14, eg. by receiving an engagement formation defined on the top reflector 1 4 within the channel 1 00 thereby to locate the centre column 20 relative to the top reflector 14. Instead, or in addition, instrumentation may be located in the channel 1 00. ^"

Two generally rectangular intermediate or filler blocks 50 are each located on opposite sides of the central block 44 with one side 84 of each filler block 50 being flush against a side 56 of the central block 44 (Figure 3B). Each intermediate block 50 has a female keying formation 80 defined centrally in each end 83 thereof, the keying formations extending parallel with the sides 80 and opening out of a top 92 and a bottom 93 of the block 50.

Four peripheral blocks 70 which are roughly wedge-shaped when viewed in plan (Figure 3C), each having a rounded operatively outer face/side 72 and sides 74 which taper inwardly from the side 72 to a nose 75, are each located between an intermediate block 50 and an end 54 of the central block 44. To this end, each wedge-shaped block 70 has male keying formations 78, of generally rectangular section, projecting from each side 74 thereof closely spaced from the nose 75, which key with a complementary female keying formation 55 of the central block 44, and a female keying formation 80 of a neighbouring intermediate block 50, respectively. A block 70 is thus engaged with the central block 44 and neighbouring intermediate block 50 by aligning a lower surface of a block 70 with an upper surface of the central block 44/intermediate block 50, with the complementary keying formations 78, 55, 80 lined up, and slidably receiving the male formations 78 in the female formations 55, 80 whilst displacing the block 70 downwardly relative to the central block 44 and intermediate block 50. A male, roughly triangular, mating formation 76, is defined on each tapering side 74 of each peripheral block 70 closely spaced from the face/side 72 thereof.

Four roughly rectangular peripheral blocks 63, 65 (Figure

3D), having convexly curved operatively outer faces/sides 64, are next located between two angularly spaced wedge-shaped peripheral blocks 70. Two of the peripheral blocks 63 are positioned so that sides 67, opposed to their sides 64, lie flush against the ends 54 of the central block 44. The remaining two peripheral blocks 65 are positioned so that their sides 67 lie flush against a side 84 of the neighbouring intermediate block 50. A roughly triangular female mating formation 66 is defined in each end 68 of each peripheral block 63, 65 and mates with a complementary formation 76 of a neighbouring wedge-shaped peripheral block 70 to interconnect the blocks 63, 70 in a dove-tail fashion. The central block 44 is of greater height than the peripheral blocks 63, 65, 70 such that it protrudes above operatively upper surfaces of the peripheral blocks 63, 65, 70. Further, the peripheral blocks 63, 65, 70 are engaged such that the peripheral blocks 63 are at a lower elevation than their neighbouring wedge-shaped blocks 70 and the peripheral blocks 65 are at a higher elevation than blocks 70 which neighbour them, i.e. the blocks 63, 65, 70 are relatively vertically off-set.

As can best be seen from Figures 3E - 3H, a second layer/course 42 of blocks 40 is arranged adjacent to the blocks 40 of the layer 42 shown in Figure 3D of the drawings. As can best be seen from Figure 3G, the central block 44 of the second layer 42 is turned through 90 degrees relative to the central block 44 of the first layer 42. It will be appreciated that adjacent central blocks 44 are turned relatively through 90 degrees throughout the filler member structure. Central blocks 44 in adjacent layers 42 are respectively supported by the intermediate blocks 50 of a layer 42 immediately below such that a clearance space (not shown) is defined between the recess 58 in a bottom 53 of an upper central block 44 and the recess 58 in a top 51 of adjacent central block 44 below the upper central block 44. In this way, the effects of thermal expansion in a centre of a core are reduced. Accordingly, the two intermediate blocks 50 of any one layer 42 are preferably manufactured from the same graphite batch in the same machining step and in the same direction of extrusion of raw graphite material such that they are of the same height and have similar expansion properties.

The adjacent peripheral blocks 63, 65, 70 of adjacent layers 42 are interconnected. To this end, two longitudinally spaced circular apertures 61 are defined in each of a top 51 and bottom 53 of the central block 44, each aperture 61 being spaced inwardly of an end 54 of the central block 44. The apertures 61 -provide, in use, mounting openings in which a dowel fastener (not shown) is receivable, each aperture 61 being mountable in register with another like aperture 91 defined in a neighbouring top or bottom 92, 93 of an adjacent intermediate block 50 in an adjacent layer 42 (Figure 3B). The central block 44 and adjacent intermediate block 50 can thus be interconnected by means of the dowel (not shown). Similarly, two apertures 82 (see Figure 3D) are defined at longitudinally spaced positions in each peripheral block 63, 65 and an aperture 86 (Figure 3D) is defined proximate a nose 75 of each wedge- shaped peripheral block 70. A dowel fastener (not shown) is received in registering apertures 82, 86 of adjacent blocks 63, 65, 70 in adjacent layers 42 to interconnect the blocks 63, 65, 70.

As can best be seen from Figures 31 and 3J, a third layer/course 42 of blocks 40 is arranged adjacent to the blocks 40 of the second layer 42 shown in Figure 3H of the drawings. It will be appreciated from Figure 3J that the peripheral blocks 63, 65, 70 and the filler blocks 50 are of lesser height than a central block 44. More particularly, in the embodiment shown in the drawings, each block 63, 65, 70, is roughly 230 mm in height, each block 50 is roughly 245 mm in height, and each central block 44 is roughly 445 mm in height. The blocks 40 are arranged so that the central column cannot be split along a central plane.

It will be appreciated that the blocks 40 of the second and third (and further) layer(s), positioned as they are adjacent to the blocks of a first layer 42, will be (as is the first layer) relatively vertically staggered. Further, three adjacent layers 42 of blocks yield a repeating section of blocks, which repeating sections lend to easier block replacement.

Each central block 44 is manufactured of extruded graphite such as that available from the SGL Group as Grade C or NBG-1 2 Grade graphite. The peripheral blocks are manufactured of extruded graphite such as that available from the SGL Group as Grade A or NBG-1 0 Grade graphite. Towards the lower end 25 of the filler member 20, the peripheral blocks 63, 65 are eliminated and the central blocks 44 span the full diameter of the column 20. Further, the intermediate blocks 50 are larger in order to fill the space occupied in the layers 42 above by a neighbouring intermediate block 50 and peripheral block 65. Wedge- shaped peripheral blocks 70 key with a neighbouring intermediate block 50 and the central block 44, respectively, by means of their keying formations 78, the male mating formations 76 present in those peripheral blocks 70 of the column above being omitted from the peripheral blocks 70 proximate the lower end 25. The blocks 44, 50, 70 in this region of the column are manufactured from extruded graphite such as that available as Grade B or NBG-1 2 Grade graphite from the SGL Group.

At the lower end 25 of the centre column 20 the diameter of the column is reduced to about 1 600 mm. To this end, the central block 44, intermediate blocks 50 and peripheral blocks 70 are reduced in size.

A basal block 40 of the column, supporting the centre column 20 on the bottom 1 6 of the core vessel 1 2, is of circular cross- section and spans the width of the lower end 25. The basal block 40 is manufactured from a single Grade K or NBG-32 Grade graphite block, suitable for use in the lower flux region at the lower end 25 of the centre column 20. The basal block 40 is doweled to an expansion compensator (not shown) at the bottom 1 6 of the core vessel 1 2 by means of either a central rotation-preventing dowel or multiple angularly spaced sliding dowels. The filler member 20 provides a centre column for a nuclear reactor 1 0 which has neutron moderating and reflecting properties and which affords vertical bending stiffness by having interlocking blocks 40 at different base heights i.e. with misalignment of horizontal faces of neighbouring blocks 40. Any lateral force exerted on the blocks 40 causes neighbouring blocks 40 to interlock by way of the keying formations 55, 78, 80. This locking of the keying formations 55, 78, 80 results in the centre column behaving as would a beam, although it is constructed of discrete graphite blocks 40. Further, the blocks 40 in adjacent vertical relationship, being doweled together, facilitate transfer of the shear load through the horizontal surfaces between discrete blocks 40.

It is to be appreciated that instead of having integral keying formations formed thereon, blocks 40 may be inter-engaged by use of independent locking keys received between neighbouring blocks 40.

In one embodiment of the invention (see Figure 2), the filler member 20 may define at least one opening 200 providing a receptacle for receiving materials for testing, i.e. the reactor 1 0 may provide means for testing the effects of radioactivity on various materials. Instead, the openings 200, defined in the blocks 40 of which the filler member 20 is comprised, may provide means for visual inspection of a block 40 from within by camera, e.g. to ascertain the presence of cracks. Further, as can best be seen from Figure 2, a plurality of superficial coolant flow channels 100, for conducting coolant gas through the core cavity, may be defined in an outer surface of the filler member 20. These may further serve, in use, as radiation-induced stress reducing means by facilitating a reduction of stresses arising as a result of dimensional changes which graphite undergoes on irradiation. In still another embodiment (see Figure 2), each peripheral block 63, 65, 70 has a bore 90 extending therethrough which, when located in register with bores 90 of adjacent blocks 63, 65, 70, defines a vertical channel/passage through the filler member structure for accommodating control rods of the reactor 1 0 or absorber spheres of a reserve shutdown system of the reactor 10. Adjacent to and at the lower end 25 of the centre column 20 the bores 90 are displaced radially inwards and are inclined from the vertical in order to compensate for the reduced diameter of the column 20 in this portion. Preferably, the bores 90 are inclined at about 69 ° . The bores 90 defined in the basal block 40 are similarly inclined but the angle of inclination is increased, such that ends of the bores 90 thereof register with interfaces in the bottom 1 6 of the core vessel 1 2.

The Applicant believes that the filler member 20 of the invention will provide a central column in a nuclear reactor 10 of the pebble bed type which is constructed primarily, if not exclusively, of graphite and which is able to endure temperatures of between 800°C and 1 200°C generated during normal operation of the reactor 1 0 as well as temperatures as high as 1 600°C in excessive conditions.

Further, the filler member 20 is typically constructed of graphite blocks 40 of small size and therefore affording desired structural properties/strength.

It is believed that a centre column yielded by the filler member 20 will, as a consequence of the staggered/off-set arrangement of the blocks 40 of which it is comprised, be stable and of sufficient strength and stiffness to withstand lateral loads exerted on it by the fuel and/or moderator spheres 30.

Further, as a majority of the graphite blocks 40 in the centre column construction (excluding the central blocks 44) are supported vertically by only one other graphite block 40, the effects of stresses generated in the graphite blocks 40 due to differences in expansions of supporting graphite blocks 40 are minimised. The centre column 20 will thus maintain its structural integrity at the high temperatures generated in the reactor core.

Claims

CLAIMS:

1 . A nuclear reactor which includes a hollow core vessel; and a modular filler member positioned within the core vessel, such that a core cavity is defined around the filler member, the filler member being of layered structure, comprising a plurality of interconnected layers of blocks, each layer comprising a plurality of laterally inter-engaged blocks.

2. A nuclear reactor as claimed in Claim 1 , in which at least some of the blocks of any one layer are vertically off-set relative to one another.

3. A nuclear reactor as claimed in Claim 1 or Claim 2, in which neighbouring blocks in a layer have complementary keying formations.

4. A nuclear reactor as claimed in Claim 3, in which the keying formations and complementary keying formations are slidably engageable such that assembly or disassembly of the filler member is achieved by relative vertical displacement of the blocks.

5. A nuclear reactor as claimed in any one of Claims 1 to 4, inclusive, in which each block is connected to at least one adjacent block of an adjacent layer of blocks.

6. A nuclear reactor as claimed in any one of Claims 1 to 5, inclusive, in which the filler member is of graphite.

7. A nuclear reactor as claimed in any one of Claims 1 to 5, inclusive, in which the filler member is of a carbon composite material.

8. A nuclear reactor as claimed in any one of the preceding claims, in which the filler member is fixed in position in the core vessel.

9. A nuclear reactor as claimed in any one of the preceding claims, in which the core vessel includes a top, a bottom and a circular cylindrical side wall extending therebetween, the filler member extending upwardly from the bottom of the core vessel to the top of the core vessel.

1 0. A nuclear reactor as claimed in Claim 9, in which the core vessel is circular in cross-section and the filler member is co-axial with the core vessel.

1 1 . A nuclear reactor as claimed in Claim 1 0, in which the filler member has a body which is circular cylindrical for at least part of its length.

1 2. A nuclear reactor as claimed in Claim 1 1 , in which a lower end of the filler member, proximate the bottom of the core vessel, is of reduced diameter.

1 3. A nuclear reactor as claimed in Claim 1 1 or Claim 1 2, in which the core cavity is annular for at least part of its length.

1 4. A nuclear reactor as claimed in any one of the preceding claims, which is a pebble bed reactor, the reactor including a plurality of spherical fuel and/or moderator elements in the core cavity.

1 5. A method of constructing a central column of a nuclear reactor, which method includes arranging a plurality of filler blocks in a plurality of courses in a core vessel, each course comprising a plurality of blocks, such that each block in a course engages with at least one other block in said course and adjacent blocks in adjacent courses are interconnected to form a layered structure, and so that a core cavity is defined in the core vessel around a filler structure so formed.

1 6. A method as claimed in Claim 1 5, which includes engaging neighbouring blocks in a course so that the blocks in a course are vertically off-set.

1 7. A method as claimed in Claim 1 5 or Claim 1 6, in which engaging each block in a course with at least one other block in said course includes engaging complementary keying formations defined on neighbouring blocks.

1 8. A method as claimed in any one of Claims 1 5 to 1 7, inclusive, in which the blocks are of graphite.

1 9. A method as claimed in any one of Claims 1 5 to 1 7, inclusive, in which the blocks are of a carbon composite material. A nuclear power plant which includes a nuclear reactor as claimed one of Claims 1 to 14, inclusive.