US3206373A - Support structure for vertical stack of solid moderator - Google Patents

Description

Se t. 14, 1965 e. DUPUY 3,205,373

SUPPORT STRUCTURE FOR VERTICAL STACK OF SOLID MODERATOR Filed May 16, 1963 2 Sheets-Sheet l INVENTOR GRn/w DUPUY ATTORNEYS Sept. 14, 1965 Filed May 16, 1965 Fly. 5

SUPPORT STRUCTURE FOR VERTICAL STACK OF SOLID MODERATOR G. DUPUY 2 Sheets-Sheet 2 INVENTOR G'EPARD .Dl/PUY ATTORNEYS United States Patent .0

The present invention relates to a support structure for the vertical stack of solid moderator of a nuclear reactor, and especially a heterogeneous reactor cooled by a circulation of gas under pressure.

Among the various solutions which are possible in this type of reactor, it has already been proposed to construct the moderator stack using vertical columns adjacent to each other, each column consisting of prismatic bricks of identical transverse cross-section which are placed one above the other. The coupling between the various columns of the stack is carried out by means of parallelipipedal keys fitted inside radial and longitudinal grooves which are preferably formed in the center of the lateral faces of opposite bricks of adjacent columns. A stack of this type offers a large number of advantages especially as regards the stability and mechanical strength of the moderator structure. In fact by virtue of the narrow clearances provided between the adjacent columns on the one hand and between the coupling keys and the bottom of the keyways on the other hand, each column is capable of either expanding or contracting freely about its own axis. The lattice pitch on which the said columns are arranged thus remains unchanged during the operation of the reactor.

It should be pointed out, however, that this constancy of lattice pitch is not wholly achieved in reactors of types which are at present known. In fact, the stack rests on a sole or support platform which is usually constituted by a single-piece metallic plate supported on the bottom of the reactor vessel by transverse posts or vertical posts. The said posts are accordingly liable to buckle to a slight extent during the operation of the reactor, especially by reason of the temperature dilferences between the various points of the support plate. There consequently takes place a slight deflection of the support plate which in turn produces a deformation of slight amplitude of the stack itself and, as a consequence, gives rise to a not negligible variation in the distance between the axes of the columns.

The present invention has for its object to ensure that the lattice pitch on which the moderator columns are disposed is kept strictly constant irrespective of the deformations of the support structure.

A further object of the invention is to simplify the problems of loading and unloading, so that by reason of the constancy of pitch, the openings of the channels which are formed along the axes of the moderator bricks always remain opposite the openings formed through the reactor vessel so as to permit the introduction of the fuel elements to be stacked in the interior of said channels.

To this end, the support structure in accordance with the invention for the columns of a moderator stack consists of an assembly of adjacent tiles separated by a lateral clearance and disposed side by side in a same horizontal plane, each tile having a polygonal configuration admitting of a vertical axis of symmetry andadapted to support an assembly unit which is formed by a limited number of said columns, and is characterized in that each tile is coupled only to the central column which forms part of the assembly unit carried on said tile and which is disposed along the axis of symmetry of said tile, and is supported on a stationarily fixed bearing surface by pillars I Eithhfii Patented Sept. 14, 1965 which are spaced symmetrically about said axis and the lower portions of which are thermally insulated from said tile.

Apart from this principal arrangement, the support structure calls for the use of various secondary arrange ments which will be discussed in greater detail below and which are especially concerned on the one hand with the manner in which the tiles are arranged with respect to each other as a function of their own configuration and of the transverse cross-section of the moderator bricks which constitute the columns and, on the other hand, with the means employed for the purpose of ensuring hori zontality of each individual tile.

The invention will in any case be more clearly understood from the complementary description which follows below in reference to one particular form of embodiment which is given by way of example without implied limitatron.

In the accompanying drawings:

FIG. 1 is a perspective view of a support tile on which an assembly of moderator columns is supported.

FIG. 2 is a top view of a support tile.

FIG. 3 is a partial view on a smaller scale representing a group of tiles similar to the group of FIG. 1, said tiles being disposed in adjacent relation.

FIG. 4 is a diagrammatic View in partial cross-section along the line IV-IV of FIG. 2. As shown in FIG. 1, the stack is composed of a group of columns 1 each constituted by the superposition of bricks 2 of moderator material such as graphite, for example. Each column is made up in this example of four bricks placed one directly above the other and the transverse cross-section of which is a regular hexagon. The columns ll of the stack are connected together by means of longitudinal keys 3 which engage in grooves or keyways 4 formed along the lateral faces of the bricks 2. The various columns are disposed in adjacent relation while allowance is made between the columns for a small lateral clearance 5, the dimensions of which vary during the operation of the reactor by reason of the swelling or shrinkage of the columns under the action of radiations and temperature differences.

In accordance with the invention, the columns rest on support plates or tiles 6, each tile being designed to support only a small number of columns. (This number is equal to seven in the form of embodiment which is illustrated in the figures.) The said tiles have a polygonal external configuration (namely hexagonal in the example which has been chosen) and are supported on the bottom of the reactor vessel (which has not been illustrated in the drawings) by three pillars such as the pillar 7, the constructional detail of which will be explained precisely below.

As can be seen in FIG. 2, the combined assembly of seven columns which are supported on the tile 6 and which are shown in chain-dotted lines in the drawing constitutes a unit which, in transverse cross-section, does not have the configuration of a regular polygon. In order that identical support tiles may accordingly be employed for the stack, it is found necessary to displace these latter relatively to each other as has been shown in FIG. 3 by a distance which is equal to one third of the length of one side of a tile. There has been provided between two adjacent tiles such as the tiles 6a and 6b a lateral clearance 8 of small width which enables said tiles to expand independently of each other during the operation of the reactor. The relative displacement of three tiles such as the tiles 6a, 6b and 6c thus provides a free space 8a, the dimensions of which are sulficiently small not to cause any hindrance whatsoever to the supporting of the columns themselves. As shown in FIG. 2

there are formed in each tile 6 seven vertical bores 9 each corresponding to a central channel which is formed along the axis of a column in order to accommodate the fuel elements and in order to serve as passages for the flow of coolant.

In accordance with an essential arrangement of the invention, only that column 2,, which is located at the center of the combined assembly of seven columns which are supported on the tile 6 (as shown in FIG. 2) is connected to this latter by means of a spigot ring 10 which is integral with the tile itself and which engages slightly in the interior of the channel which is formed through said column. The underlying principle of this design is that the axis of the column considered is stationary whereas the tile is permitted to expand about said axis. The distance between the axes of the two central columns of two adjacent tiles is therefore constant and, inasmuch as all the other columns are keyed together, they are unable to move or consequently to produce any variation in the lattice pitch. The expansion of the tiles is accompanied only by a slight sliding movement of these latter with respect to the bottom portion of the peripheral columns which are supported on said tiles, the coefiicient of friction of graphite on the metal of which said tiles are made being such as to permit said sliding movement to take place very easily without inducing any harmful stresses in the columns themselves.

In order to dispose the various adjacent tiles in the same horizontal plane, each tile is supported on its three pillars '7 by means of a system which can be adjusted for height. This system consists of a plug 11 having an external surface which is threaded so that said plug can thus be screwed into a tapped hole 12 which extends through the tile 6 opposite each pillar 7. Each plug is provided with two holes 13 so as to permit this latter to be screwed home by means of a pin wrench and is provided in the central portion thereof with an adjusting screw 14 which is adapted to engage inside the top portion 15 of the pillar 7 so as to effect the height-adjustment of the tile 6 by screwing of the plug and by producing action on the screw 14.

Each pillar is constituted by a central portion 16 of s eel which is surrounded by a second casing 17, and the space which is formed between the external surface of the central portion 16 and the casing 17 is filled with a suitable layer 13 of heat-insulating material. The bottom of each pillar rests on a bearing surface 19, said bearing surface being in turn supported on the bottom of the reactor vessel by studs of asbestos such as the stud 2t) which are located in the line of extension of the pillars. Since the support plate 19 must be maintained at a temperature which is as constant as possible, said support plate can be heat-insulated with a certain thickness 22 of a suitable material on that side which faces the reactor core, that is to say which is directed towards the til-es 6, whilst the underface of said support plate can be ventilated by means of a circulation of coolant gas through the spaces 21.

The foregoing arrangement accordingly makes it possible to maintain the lower portion of the pillars on a surface which remains cold or at least at a constant temperature, the use of heat-insulating material being especially intended to prevent convection between the bearing surface referred-to above and the heat-transporting gas which circulates in the passages provided in the moderator bricks. The pillars themselves are preferably made of a metal having poor heat conductivity such as stainless steel or hastalloy.

It should finally be noted that, by reason of the small size of the tiles 6, the expansion of these latter is relatively very slight and that by reason of the symmetrical arrangement of the pillars, said expansion is always isotropic about the axis of the tile, or in other words about the axis of the central column of the assembly unit which is supported on said tile. The buckling to which the pillars 7 are subjected is therefore also very slight and within perfectly permissible limits during the operation of the reactor.

By virtue of these arrangements, there is thus formed a support structure which is perfectly stable and which is particularly well adapted to the case of reactors in which substantial temperature differences occur between the ends of the moderator columns, while the expansion of the tiles does not produce any stress in said columns irrespective of the operating conditions and allows the lattice pitch to remain perfectly constant.

It will naturally be understood that the invention is not limited in any respect to the form of embodiment which has been described and which has been given solely by-way of example.

What I claim is:

I. In a nuclear reactor, a vertical stack of solid moderator wherein said stack is made up of adjacent vertical columns connected to each other by a system of longitudinal keys engaged in radial grooves formed in said columns, clearances being provided between adjacent columns and between the keys and the bottom of the grooves in which said keys engage so as not to impair, during the operation of the reactor, the lattice pitch on which said columns are disposed, support structure consisting of an assembly of adjacent tiles separated by a lateral clearance and disposed side by side in a same horizontal plane, each tile having a polygon-sided configuration admitting of a vertical axis of symmetry and adapted to support an assembly unit which is formed by a limited number of said columns, characterized in that each tile is coupled only to the central column which forms part of the assembly unit carried on said tile and which is disposed along the axis of symmetry of said tile, and is supported I on a stationarily fixed bearing surface by pillars which are spaced symmetrically about said axis and the lower portions of which are thermally insulated from said tile.

2. In a nuclear reactor in accordance with claim 1, support structure characterized in that the lower portion of each pillar is embedded in a thickness of heat-insulating material which covers the uppermost face of said stationarily fixed bearing surface while the underface of said bearing surface is cooled by a circulation of gas.

3. In a nuclear reactor in accordance with claim 1, support structure characterized in that the tiles have a side-configuration corresponding to a regular hexagon and are displaced relatively to each other by a distance which is equal to one third of the length of one side of a ti e.

4. In a nuclear reactor in accordance with claim 1, support structure characterized in that the coupling between the pillars and the tiles is carried out by means of a screw system which can be adjusted for height, thereby ensuring horizontality of said tiles at the time of assembly.

References Cited by the Examiner UNITED STATES PATENTS 2,863,815 12/58 Moore et al. l76-29 3,119,746 1/64 Lemesle et al 176-84 X FOREIGN PATENTS 913,818 12/62 Great Britain.

CARL D. Q'UARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner.

Claims (1)

1. IN A NUCLEAR REACTOR, A VERTICAL STACK OF SOLID MODERATOR WHEREIN SAID STACK IS MADE UP OF ADJACENT VERTICAL COLUMNS CONNECTED TO EACH OTHER BY A SYSTEM OF LONGITUDINAL KEYS ENGAGED IN RADIAL GROOVES FORMED IN SAID COLUMNS, CLEARANCES BEING PROVIDED BETWEEN ADJACENT COLUMNS AND BETWEEN THE KEYS AND THE BOTTOM OF THE GROOVES IN WHICH SAID KEYS ENGAGE SO AS NOT TO IMPAIR, DURING THE OPERATION OF THE REACTOR, THE LATTICE PITCH ON WHICH SAID COLUMNS ARE DISPOSED, SUPPORT STRUCTURE CONSISTING OF AN ASSEMBLY OF ADJACENT TILES SEPARATED BY A LATERAL CLEARANCE AND DISPOSED SIDE BY SIDE IN A SAME HORIZONTAL PLANE, EACH TILE HAVING A POLYGON-SIDED CONFIGURATION ADMITTING OF A VERTICAL AXIS OF SYMMETRY AND ADAPTED TO SUPPORT AN ASSEMBLY UNIT WHICH IS FORMED BY A LIMITED NUMBER OF SAID COLUMNS, CHARACTERIZED IN THAT EACH TILE IS COUPLED ONLY TO THE CENTRAL COLUMN WHICH FORMS PART OF THE ASSEMBLY UNIT CARRIED ON SAID TILE AND WHICH IS DISPOSED ALONG THE AXIS OF SYMMETRY OF SAID TILE, AND IS SUPPORTED ON A STATIONARILY FIXED BVEARING SURFACE BY PILLARS WHICH ARE SPACED SYMMETRICALLY ABOUT SAID AXIS AND THE LOWER PORTIONS OF WHICH ARE THERMALLY INSULATED FROM SAID TILE.

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