Roof reflector for a gas-cooled pebble-bed reactor and process for the disassembly of the roof reflector

The invention concerns a roof reflector suspended from the thermal cover shield of a gas-cooled pebble-bed reactor with a plurality of passages for absorber rods arranged in a triangular lattice, and with channels for the passage of the cooling gas. The roof reflector comprises a plurality of essentially hexagonal graphite blocks arranged in several layers to form vertical, closely-adjacent columns. A process for the disassembly of the roof reflector is further provided.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the construction and disassembly of roof 
reflectors for gas-cooled pebble-bed reactors. 
2. The Prior Art 
Reactor roof reflectors are known which consist of a plurality of graphite 
blocks arranged in close proximity to each other, suspended by means of 
tie rods from the upper part of the pressure vessel enclosing the nuclear 
reactor. A roof reflector of this type may consist, for example, of two 
layers of graphite blocks, wherein each of the blocks is equipped with a 
bore to allow for the passage of the cooling gas, the bores running in a 
broken line to provide shielding against neutrons. (See German Published 
Application DE-OS No. 15 64 186). 
Another known roof reflector consists of a single layer of hexagonal 
graphite columns, the cooling gas passages provided centrally through the 
individual columns being shielded either by means of plates placed over 
the graphite columns or by helically wound stoppers inserted in the bores 
for the cooling gas (See French Pat. No. 14 26 264). 
A roof reflector consisting of hanging columns is also known, the roof 
reflector being suspended from a thermal cover shield. Six each of the 
suspended columns define an opening for an absorber rod. The columns, 
arranged in three layers, are provided with slits for the passage of the 
cooling gas, the slits of adjacent layers being offset with respect to one 
another to provide shielding. (See German Published Application No. DE-OS 
No. 23 54 540). 
All of the roof constructions described above are intended for use with 
gas-cooled nuclear reactors employing spherical fuel elements, the 
so-called pebble-bed reactors. Another roof reflector for a nuclear 
reactor of this type consists of a supporting plate fastened by means of 
threaded connections to the cover of the pressure vessel, and a plurality 
of graphite blocks arranged in at least two layers, a free space being 
provided between the supporting plate and the graphite blocks, the free 
space serving as a collector space for the cooling gas. (See German 
Published Application No. DE-OS No. 26 31 408). The graphite blocks are 
fastened to a retaining plate which closes off the lower boundary of the 
cooling-gas collector space and which is suspended by means of anchors 
from the supporting plate. The supporting plate and the retaining plate 
together constitute a static plate. Both consist of sections having 
overlapping junctions. Passages for the absorber rods are provided in the 
roof reflector. The flow of cooling gas passes through annular channels 
arranged coaxially in relation to absorber rods and possessing two 
different non-coincident, passage cross-sections. The cross-section of the 
upper passage is shielded in each case by means of an annular plate. 
SUMMARY OF THE INVENTION 
The present invention is based on the state of the art described above and 
has as its object the design of a roof reflector generally of the type 
described, in which the graphite blocks may be readily removed in a simple 
manner from their positions relative to the reactor core for the purpose 
of replacement with similar new parts or for repairs on other reactor 
installations. Further, in the design of the roof reflector, the absorber 
rods are arranged in a triangular lattice according to a predetermined 
pattern. 
The object is attained according to the invention in that each group of 
three adjacent columns of blocks is combined to form a unit, the columns 
being held by a support block centrally located beneath at least a portion 
of the three graphite blocks of the lowest layer of each unit and fastened 
by means of a releasable hanger bolt to the thermal cover shield. The 
hanger bolt is passed in the upward direction through a central bore 
provided in each of the units. The graphite blocks of the lowest layer of 
each unit interlock with the associated support block and graphite blocks 
forming upper layers of each unit are locked in position in the axial 
direction by means of positive fitting elements provided on the faces of 
the graphite blocks. One sidewall of each unit is shaped such that each 
group of three abutting units defines a passage for receiving an absorber 
rod. Only one absorber rod passage is associated with each unit. Each of 
the graphite blocks is preferably provided at its upper face with a 
gripper bore. 
The roof reflector according to the invention may be assembled by the 
simple stacking upon one another of the graphite blocks, and preferably 
comprises three superposed layers of the blocks. The blocks of the lowest 
layer of each unit interlock with an associated support block up to about 
one-third of their length and are advanced to the lower edge of the roof 
reflector (corresponding to the lower edge of the support blocks). The 
lower portions of blocks of the intermediate and the uppermost layers, 
respectively, have fitting elements machined onto them for interlocking 
with mating fitting elements machined onto the upper portions of blocks of 
the lower and intermediate layer, respectively. Except for the anchoring 
of the lower layer of each unit on the corresponding support block and the 
axial locking in position of the other blocks of each column of blocks, 
there is no other inter-connection between the graphite blocks within a 
unit. Likewise, there is no frictional interconnection between adjacent 
units. During disassembly of the roof reflector, the graphite blocks may 
be separated from one another by means of simple lifting; for this, a 
suitably-fitting dismantling tool is inserted into gripper bores in the 
graphite blocks. Following the removal of the graphite blocks, the support 
blocks may be dismantled, since they are equipped with releasable hanger 
bolts. 
Each of the absorber rods is located in a central area between three 
abutting units, each of the three abutting units enclosing an angle of 
120.degree. about the associated absorber rod. As required, the 
predetermined positions of the absorber rods are accurately maintained in 
a regular triangular lattice, except for those absorber rods located near 
the periphery of the roof reflector. Each unit preferably has a sidewall 
portion shaped such that three abutting units serve to define a circular 
passage for receiving an absorber rod. 
A portion of the circular absorber-rod passage located in the uppermost 
layer may have a larger diameter than the portion of the circular passage 
in the lower layer or layers. The diameter of the latter portion is 
adapted to the diameter of the absorber rods, with the provision of an 
adequate tolerance for curvatures and movements, respectively, of the 
absorber rods. 
Advantageously, the shielding of each of the passages for the absorber rods 
is effected by means of a pair of stacked graphite sleeves arranged in the 
respective circular passage portions of the uppermost layer, the internal 
diameter of said sleeves being adapted to the diameter of the absorber 
rods. The graphite sleeves rest upon the graphite blocks of the layer of 
blocks immediately beneath the uppermost layer. Their external diameter is 
dimensioned so as to provide adequate tolerance with respect to the 
surrounding graphite blocks in order to provide compensation for radial 
movements or curvature of the absorber rods. Satisfactory shielding of the 
passages is obtained when the absorber rods are inserted, due to the 
different cross-sections of the passage portion of the uppermost layer 
relative to the other layers and due to the arrangement of the graphite 
sleeves, yet there is no danger of jamming the absorber rods in the 
passages. 
Each graphite block is expediently provided on one surface with at least 
one groove extending in the axial direction such that the grooves of 
adjacently-located blocks form channels for the passage of the cooling 
gas. For reasons of manufacture, the cross-sectional configuration of the 
channels for the cooling gas may be other than circular, but the hydraulic 
flow section of the channels must be taken into consideration in their 
dimensioning. 
To prevent, for reasons of shielding, the presence of openings passing in a 
straight line through the entire thickness of the roof reflector, only the 
cooling gas channel portions of the upper layers of the blocks are aligned 
with one another, while the cooling gas channel portions of the lowest 
layer of blocks are laterally offset with respect to the firstmentioned 
channels. The cooling gas channel portions of the upper layers have, for 
example, a hexagonal cross-section and, for example, two grooves may be 
provided for the formation of cooling gas channels in each block, with the 
exception of those blocks having a curved sidewall portion for defining a 
part of an absorber rod passage. The cooling gas channel portions of the 
bottom layer of blocks may be of a rectangular cross-section, and a larger 
number of cooling gas channels may be present in said bottom layer than in 
the layers of blocks thereabove. 
Advantageously, horizontal channels are provided to interconnect the 
cooling gas channel portions of one layer of blocks with the channel 
portions of a superposed layer of blocks, the channels being arranged in a 
lower face of the graphite blocks of the second layer from the bottom. The 
flow of cooling gas coming from above is therefore deflected in the 
horizontal direction prior to entering the channel portions of the bottom 
layer of blocks. Gaps existing between adjacent graphite blocks may not be 
relied upon to permit flow of the cooling gas, because the gaps are 
largely closed off when the reactor is in operation. 
According to the invention, the nominal width over the flats of the 
essentially hexagonal graphite blocks is determined by a distance a (FIG. 
1a) representing the spacing between the absorber rods arranged in the 
triangular lattice. Thus, for example, for a triangular lattice of the 
absorber rods with a spacing between rods of a=764 mm, the theoretical 
width over the flats of the graphite blocks would by approximately 254 mm. 
In view of the fact that gaps between the blocks are closed during 
operation of the reactor due to thermal expansion, the actual width over 
the flats of the blocks is less than the theoretical width by some tenths 
of a mm. 
In the case of a nuclear reactor equipped with the roof reflector according 
to the invention and housed in a pressure vessel having a pressure vessel 
cover provided with armored tube passages for the insertion of absorber 
rods, a process is provided according to the invention for the removal of 
the roof reflector wherein the armored tube passages for the absorber rods 
are utilized for the removal of the graphite blocks from the core of the 
reactor. It is not necessary to withdraw the spherical fuel elements for 
the removal and installation of the roof reflector blocks. The armored 
tube passages must have an internal diameter of at least 310 mm (in the 
case where the blocks have a width over the flats of 254 mm, as cited as 
an example given above). Free space for the raising of the graphite blocks 
must be provided above the roof reflector. All metallic installations 
present in the area between the roof reflector and the thermal shield 
which would interfere with the dismantling of the graphite blocks must be 
removed or set aside during disassembly of the roof reflector. 
To dismantle the roof reflector, following removal of each absorber rod, 
the graphite blocks resting on a support block are lifted by means of a 
manipulator from their positions, layer by layer, and removed from the 
pressure vessel through the now-unoccupied armored tube passage. Following 
the removal of all of the graphite blocks associated with each support 
block, the hanger bolt fastening the support block to the thermal cover 
shield is released and the support block is removed through the unoccupied 
armored tube passage. 
The manipulator for disassembling the roof reflector, equipped with a 
shielding device, is placed onto the flange of the armored tube passage 
involved and secured in place. The roof reflector is disassembled 
according to a predetermined dismantling sequence. In the process, a 
gripper forming part of the manipulator is inserted in the bore located on 
the upper face of each graphite block and spread to grip the block for 
removal. The maximum action radius of the manipulator is, typically, 
approximately 400 mm. Three roof reflector units may be removed or 
replaced through each armored tube passage. 
The graphite blocks of the uppermost layer must be lifted a distance equal 
to their entire length above the upper edge of the roof reflector, in 
order to displace them horizontally and bring them to the dismantling 
opening (i.e., the armored tube passage). Subsequent layers are removed in 
a similar manner; the horizontal shifting of the graphite blocks may, 
however, take place in the free space already secured. Finally, the 
support block is dismantled, for which purpose the hanger bolt is released 
from the thermal cover shield. 
A preferred embodiment of the invention is illustrated in the drawings and 
described in detail below.

THE PREFERRED EMBODIMENT 
As shown in the drawings, the roof reflector according to the invention 
comprises a plurality of essentially hexagonal graphite blocks 1, placed 
tightly against one another and arranged in three superposed layers: an 
uppermost layer 2, an intermediate layer 3, and a bottom layer 4. The 
graphite blocks 1 are grouped in the individual layers so that vertical 
columns of the blocks are formed. Each group of three adjacent columns of 
blocks 5 combines to form a unit 6, as shown, for example, in FIGS. 3 and 
4. 
Each unit 6 is held by a support block 7 arranged centrally between and 
beneath at least a portion of the three graphite blocks 1 of layer 4, 
i.e., the bottom layer. Each of the support blocks 7 is suspended by means 
of a respective hanger bolt 8, which is releasable by machine, from the 
thermal roof shield 30. A bore 9 passes centrally through each unit 6, 
with the hanger bolt 8 passing upwardly through central bore 9. The hanger 
bolt 8 is shown in FIGS. 2, 3 and 4 for only one of support blocks 7, but 
it will be understood that such a hanger bolt 8 is to be provided for each 
support block 7. 
The three graphite blocks 1 of the bottom layer 4 of each unit 6 interlock 
with a corresponding support block 7, each block enclosing an angle of 
120.degree. about central bore 9. Each of the remaining graphite blocks 1 
is secured in position in a corresponding column of blocks 5 by means of 
fitting elements 10 provided on mating faces of graphite blocks 1, as 
shown. The fitting elements 10 provided on the mating faces of graphite 
blocks 1 accurately fit together. 
The roof reflector has a plurality of passages 11, arranged in a triangular 
lattice, for receiving the absorber rods. In the example illustrated, 198 
absorber rod passages 11 are provided. Accordingly, the roof reflector is 
subdivided into 198 regions 12. Each region 12 comprises three units 6, 
and has a total of 30 reflector blocks (including the support blocks 7). 
In FIGS. 1a and 1b, a region 12 is shown in each of the different section 
planes, each region 12 being particularly emphasized by means of shading. 
Each of the three units comprising each region 12 is shaded distinctively. 
In the center of each region 12, i.e., at the point where the three units 6 
abut, a passage 11 for the absorber rods is provided. Each passage 11 is 
defined by respective grooves 13 in the immediately adjacent three columns 
of blocks 5 of the units 6 involved. The grooves 13 comprise sidewall 
portions of the respective units shaped so that together they yield a 
circular passage 14 for receiving an absorber rod. As seen in FIG. 2, the 
circular passage portions 14a in the upper layer 2 have a larger diameter 
than the circular passage portions 14b in the layers 3 and 4. The diameter 
of passage portions 14b is determined by the diameter of the absorber 
rods, allowing adequate tolerance for curvatures and/or movements of the 
rods. 
In the absorber rod passage portions 14a of layer 2, two graphite sleeves 
15 are stacked, their inside diameter being adapted to that of the 
absorber rods. The selection of the diameter of the absorber rod passage 
portions 14a must take into account that, on the one hand, the diameter 
must be large enough to provide satisfactory shielding of the open 
cross-sections in the layers 3 and 4 and, on the other, the diameter must 
be such that the graphite sleeves 15 are allowed sufficient tolerance in 
relation to the surrounding graphite blocks 1, in order to prevent the 
jamming of the absorber rods. 
The center of the roof reflector is located at the center point of three 
columns 5 of blocks adjacently located and displaced at an angle of 
120.degree. with respect to each other. This results in a three-way 
rotational symmetry of the roof reflector. Because in each case three 
columns 5 of blocks 1 are combined by means of a support block 7 to form a 
unit 6, the resulting pattern yields a highly irregular separation between 
the roof and the side reflector; i.e., units of the roof reflector form 
part of the side reflector and vice versa. At the junction of the roof 
reflector with the side reflector, there are therefore deviations from the 
disassembly pattern. For the other regions of the roof reflector, the 
disassembly of the graphite blocks 1 and the support blocks 7 follows a 
pattern in which the locations of blocks surrounding each region 12 are 
identified to a coordinate system having x, y, z coordinates. The block 
location information is fed into a dismantling manipulator. Because all of 
regions 12 have identical structures, the same coordinate system may be 
used for the entire roof reflector (with the exception of the peripheral 
areas). 
As indicated above, disassembly involves positioning of a suitable 
manipulator device over an armored tube passage provided in the pressure 
vessel cover for the insertion and removal of absorber rods. The armored 
tube passages are in line with absorber rod passages 32 in the thermal 
cover shield 30. The manipulator removes, layer by layer, the graphite 
blocks held by a support block 7, then releases the hanger bolt 8 
fastening the support block 7 to the thermal cover shield 30, and removes 
the support block. To facilitate removal of the support block, a suitable 
releasable coupling 34 (FIG. 2) may be provided on hanger bolt 8. As shown 
also in FIG. 2, a pin 36 is inserted into a horizontal bore in support 
block 7, the pin having a threaded vertical bore for receiving a threaded 
lower end portion of hanger bolt 8. 
A plurality of channels 16 is provided in the roof reflector for the 
passage of the cooling gas, the passages being formed by grooves machined 
into adjacent graphite blocks 1. In the layers 2 and 3, the cooling gas 
channels 16 (FIGS. 1a and 2) are formed by grooves 17 (FIGS. 3 and 4) in 
the sidewalls of graphite blocks 1. The grooves are shaped so that at the 
coincidence of three abutting graphite blocks 1, a channel 18 (FIG. 1a) 
having a hexagonal cross-section is defined. The channels 18 of the layers 
2 and 3 are mutually aligned. 
Grooves 17 extending in the axial direction are also provided in the 
graphite blocks 1 of the bottom layer 4; these blocks are, however, 
arranged such that their grooves are offset in relation to the grooves 17 
in the two upper layers, at the periphery of the graphite blocks 1. Thus, 
when the blocks are assembled, the roof reflector has no openings 
extending in a straight line through its entire thickness, and 
satisfactory shielding against escape of neutrons is provided. The grooves 
17 in the lower layer 4 may be shaped as indicated, so that cooling gas 
channels 19 in the lower layer having rectangular cross-sections are 
formed. Three cooling gas channels 19 are associated with each unit 6. 
In the lower front surface of the graphite blocks 1 of the intermediate 
layer of blocks 3, horizontal grooves 20 are provided, grooves 20 having a 
configuration such that at the abutting surfaces of two adjacent graphite 
blocks 1 a horizontal cooling-gas channel 21 is formed. Three of these 
horizontal cooling-gas channels communicate with each channel 18 of 
hexagonal cross-sections and serve to interconnect the latter with three 
channels 19 in the lower layer 4 having rectangular cross-sections. 
FIGS. 3 and 4 shows a gripper bore 22 machined into the upper face of each 
graphite block 1. The gripper of a manipulator may be inserted into a bore 
22 and spread to facilitate removal of the graphite block 1. The process 
of dismantling the roof reflector has been described hereinabove, and need 
not be repeated. 
The disclosures of German Published Applications DE-OS No. 23 54 540, 
published May 15, 1975 DE-OS No. 25 09 025, published Sept. 2, 1976 and 
DE-OS No. 26 31 408, published Jan. 19, 1978, are expressly incorporated 
herein. Application DE-OS No. 23 54 540 is particularly incorporated for 
its teaching of a gascooled pebble-bed reactor having a roof reflector and 
passages for the insertion and removal of absorber rods. 
Application DE-OS No. 26 31 408 is particularly incorporated for its 
teaching (in FIG. 1 thereof) of a roof reflector for such a reactor with 
inserted absorber rods as well as a support plate fastened to the liner of 
a pressure vessel; the support plate serves a shielding function and may 
be regarded as a thermal roof shield. Application DE-OS No. 26 31 408 
further shows an armored tube passage for the absorber rods. 
Application DE-OS No. 25 09 025 is particularly incorporated for its 
description of a manipulator apparatus for placement in a prestressed 
concrete pressure vessel. The process described in DE-OS No. 25 09 025 
does not exploit the armored tube passages in the pressure vessel (as does 
the process of the present invention), but instead uses the loading 
openings for the fuel pellets. Nonetheless, DE-OS No. 25 09 025 discloses 
the manner of operation of a manipulator suitable for carrying out the 
process of the present invention.