SE189244C1 - - Google Patents

Description

Inventor: R Martin Priority Begtird frau August 21, 1958 (France) The present invention relates to moderator stacks in nuclear reactors operating with solid moderator material.

It is known that in large nuclear reactors, which are of solid moderator material and in which the channels extend vertically, the active portion, in which the nuclear reactions are generated, consists of a block with large dimensions of, for example, graphite, which is pierced with a plurality of vertical channels. , in which the refrigerant circulates and where the fuel elements are attached.

The graphite block usually consists of a stack of rods with an almost parallelepipedic shape with almost a square section and with a much greater length than thickness (3-8 times); these rods are formed by pull-fling.

These stacks are practically the only molten structural part in the central part of the reactor and must therefore have the mechanical and geometric properties listed below: the stack must have sufficient stability and mechanical strength for the reactor to be used; these properties must be maintained for such a long time that the structure has time to amortize; the ducts in which the refrigerant circulates and where the elements of fissile material are arranged must maintain a sufficiently good connection in order to be able to maintain a certain cooling of these elements as well as convenient charging and discharging; in front of it no play or any bending should occur, which would cause varnishing, section displacements or clamps; the same conditions must be met with regard to others in the stack: occurring halrunn, which were intended to accommodate control, monitoring, safety and other apparatus, are required for the operation of the reactor.

The suitability of graphite to meet the above conditions is rapidly degraded due to the following factors, which are related to the properties of graphite and the function of the nuclear reactor.

The graphite Or used is an anisotropic material, yam havadaxel is the axis of attraction (longitudinal direction); the coefficient of expansion Or as a rule considerably weaker in this direction than in a direction perpendicular thereto; During operation, the active part of the reactor constitutes the stable for strong heat generation, whereby the graphite is exposed to a room temperature that significantly exceeds the operating temperature. During thift breaks, the graphite cools to room temperature. As a result, the graphite stack is subjected to a sequence of hot and cold conditions corresponding to the operating and standstill periods, respectively, whereby a temperature periodicity is obtained, during which the dimension of the graphite rods varies pseudoperiodically; in an operating reactor, the graphite coated inside the reactor is exposed to all nuclear reaction radiation. Some of these, rather than the fast neutrons, induce positive or negative shape changes (expansions or contractions) in graphite, which shape changes are known as the Wigner effect.

These changes in form are much greater than the conditions conditioned by the temperatures and they can amount to a few percent. Deforme- 2— -. the rings are averded anisotropically, a: The Wigner widening is usually weaker in the direction parallel to the axis of attraction. It varies from one stalk to another in the interior of the stack depending on the temperature and the received radiation dose. It is the bigger the weaker the radiation temperature dr.

The first challenge bars were compact, ie. the graphite rods were stacked against each other in layers, which were arranged horizontally or vertically crosswise or parallel; in such stacks the Wigner deformations occurred ku, mulatively and the external dimensions of the stack could consequently vary to a large extent. Due to the non-uniform nature of the phenomenon, the cartels assumed the following forms, literally in the longitudinal direction of the road, through which the displaceability of the regulating rods and the fuel elements could be defended.

Attempts have been made to eliminate this effelite by constructing stacks in which clearances were provided during the assembly of the graphite rods across the direction of attraction. The distance radian rods are maintained by means of various devices, especially those written in British Patents 784,291, 784,292 and 785,876.

The various proposals, the solutions show hi. a. the following irregularities: a significant waste of material due to the fact that every stay and every hiccup must be required in order for losses to be cut; considerable gas losses between the channels and the gaps due to the play between the pins and the corresponding recesses; a plurality of splices (three per layer), whereby the two above-mentioned imperfections are dyed; risk of vertical retraction of a column as a result of friction clamping; asymmetrical deformations of the profile of the channels, which lead to the engagement between narrowing elements up, phaves and which hero on the anisotropy of the Wigner effect oc-h the different orientation Mr the axis of attraction of the rods and the washers; deformations of the profile of channels for control rods to, due to irregularities, subunits in the degree of Mr Wigner erosion in the washers.

The present invention relates to a new arrangement of a stack of rods of solid moderator material for a nuclear reactor, which stack is not exposed to previously unserved units and which in addition offers a number of other advantages.

The stack according to the invention, which comprises adjacent vertical columns, which consist of superimposed prismatic blocks and through which each longitudinally central channel extends, can be characterized in that for straightening the axes of the atic column independently sprints in the form of passfills with parallel surfaces. inserted in crowds, which Ore occupied funnel towards and in the middle of the edges for the upper and lower planes of contact between blocks placed on top of each other and that for maintaining the condition between the pins prismatic and / or cylindrical transverse rules Oro arranged along two. at right angles to each other in grooves in the corresponding spirits of the blocks, the skis and the grooves being so deep that the pins and crossbars do not extend into the central channels in the blocks.

The pins according to the invention can have a circular, polygonal or mixed circumferential contour. In a night monster with square meshes, it turns out that a group of four sprints affects ten narrowing rods. The depth of the grooves accommodating the springs is limited to a minimum value ph sal: lard set that a slight rotation of the rods around the shaft does not stiffen according to the torque of the sprints.

The sprints can consist of graphite. The direction of application should in some cases be parallel to the surfaces of such salt that the play between these surfaces and the grooves is not irradiated. The smallest possible thickness was used, which is compatible with the requirement of resistance to shear pans. The irradiation in fact causes a certain thickness increase and results in a light rating of the diameter of the stack.

The sprints can also be made of more resistant materials, for example beryllium; in delta cases they may be of lesser thickness. Since the elimination of the heryl glue Or less On of, graphite, the irritation of the play between the sprint and the crowd is reduced by this tjoekleks reduction.

The center plane of the springs accommodating the springs extends through the axis of the channel. They can be perpendicular to the star surfaces' side surfaces or tilt with two. 0 three degrees in relation to these. In the latter case, clearance is limited, a plane through non-aligned planes, whereby the neutron migration is limited. The directions of orientation, the rods of the rods arranged one on top of the other, cross each other suitably for the same reason, g.

The transverse rules according to the invention can consist of graphite; in delta cases their axis should run parallel to the direction of least Wigner expansion, which alit most often coincides with the direction of attraction. In addition, it is impossible to place only three parallel half-grooves at the top of the pit one star and three towards half-grooves perpendicularly half-grooves in the foot of the rod supported by the first-mentioned star; on the delta way, you board inboard large size Indian perpendicular to each other.

Since these elements are small, graphite manufacturing methods can be applied, which lead to a Mg degree of Wigner expansion. You can also add graphite with not too high purity due to the relative volume of the parts.

It is advantageous to remember a small gap between the Indian crossbars and the corresponding cavities in order to insulate the bars from this heat in relation to the large, gas-cooled rods circulating in the channels. The one in the transverse rules through the neutron bombardment only produced the possibility of keeping them at a higher temperature, which has the consequence that their mechanical resistance is hated and the degree of Wigner expansion is reduced.

Delta results are obtained without s-duration in that the transverse rules are thus phased out so that they only show suitable thickened ledges at the spirits. Gaps are lined between the abutment parts of the two-sided rules and the cavities with such salt that adjacent rods have the opportunity for lateral expansion and vertical displacement.

The transverse rules can be made of any other material, soul or imperceptible or only slightly exposed to the 'Wigner effect' and which exhibit acceptable mechanical and nuclear reaction properties. Due to the insignificant volume of the cells, it is possible to use expensive materials, for example beryllium.

Part Or it is common for the control rods to be moved in channels, which Oro 11pp: tag = in the gravity center for squares, which Oro formed by each mesh in the night pattern. In the embodiment according to the invention, such a channel would come into conflict with the transverse rules. This responsiveness is avoided by providing the crossbars with a break at this stall and bringing them to rest against a ring, such as a beryllium horse, the thickness of which is such that the required resistance to elevation by compression is obtained. Beryllium, in fact, has a small coefficient of thermal expansion and a small coefficient of expansion. neutron capture selection.

Said ring rests on a ledge in the graphite and a clearance Or arranged between the ring and the narcissistic rods for absorbing the thermal expansion and due to the Wigner effect.

The compensation zone obtained according to the invention may comprise the entire stack, but it may also be restricted to the zone in which the Wigner expansion is strongest. One can also imagine zones in which the games between the rods have different values depending on the irradiation and the temperature. The transition from a compensated zone to an uncompensated zone does not offer any responsibilities.

The entire stack according to the invention rests on a flat metal substrate, which metal pieces in the form of half springs are placed on. The substrate Or is provided with a slide corresponding to the ducts for the supply of gas. A screen element made in one piece with the substrate supports the industry elements and allows regulation of the coolant supply. The whole assembly is heated together by means of the elastic buckles, for example those described in French patent specification 1,190,048.

When grinding a stack with a hexagonal, centered pattern, which Or is composed of prismatic rods with a hexagonal bottom surface, which Oro drilled along the shaft, the transverse bars must of course be made of a material which Or is indistinguishable or join. Hasty for the Wigner effect in two mutually perpendicular directions; it thereby assumes a triangular shape and abuts against radial pins, which occur in a number of six habitual graphite rods.

The advantages achieved by the invention are numerous: The number of splice planes is reduced to a customary layer instead of one of some previously used devices. The risk of loss of cooling gas is thus reduced to a corresponding extent.

Via, the same sections for the graphite rods Or the contact surface between two superimposed rods stoma on has devices, which comprise counting discs. As a result, greater stability is obtained.

The kfincla assembly with the help of tap-pairs and slides entails a significant material loss; these compounds also constituted an additional loss (in the absence of tatnin.gs tiles). The elimination of such seals may in part reduce material losses by removal.

The transverse rules according to the invention occupy a much smaller volume of discs or washers with the same function. Dot Or is thus able to produce them from a material with higher mechanical half-strength and less likelihood of the Wigner effect, for example non-nuclear grant or beryllium.

By thermal insulation by means of a gas shield (of the same character as the cooling medium), the crossbars can be lined at a higher temperature on the rods themselves. This heating causes it. the advantage that the mechanistic properties of the crossbeams are improved and the Wigner effect is reduced in the event that the crossbeams consist of graphite.

The coefficient of friction between graphite and graphite Or fog kand, cid one works in a dry atmosphere and during irradiation. By means of the invention, the antiquities and the moment of uncertainty associated with these frictions can be avoided.

The columns in the Bar Concern are completely independent of each other, which is an extraordinary advantage. Oin in a device with briekor as a result of a particularly strong local heating or column is displaced vertically to a greater extent On one or flora adjacent columns, it may occur that after this .uppyarmnins, ie. after expansion, the column cannot return to its normal lap due to a clamping between adjacent rods. This phenomenon, sometimes referred to as vertical creep, is a reason for paint loss.

With the plant according to the invention, this risk is eliminated. If in fact one of the rods is lifted, the distance between the columns remains constant and stayser "returns by itself to its lap regardless of the coefficient of friction. Losses of cooling gas are avoided on the delta. It becomes possible to reduce it usually in the form of casting plates on The elastic spanners ", which secure the entire assembly of the unit, can be agoras, lighter.

The continuity and centering of the channels is maintained in all circumstances and the profile of the channels remains circular.

The channels for control rods remain usable at all times, since no lateral displacement of one layer of rods in relation to another is not possible.

The washers are also manufactured in a simple, salt-free manner and the material losses are reduced to a very high load value. The bar surfaces, which do not include any protrusions, are produced in a single working step. Since the work is carried out without interruption, a flat surface is obtained on the elements and consequently improved density.

The mounting elements (discs and crossbars) are easy to manufacture and the number of element types only amounts to Ire within the usual parts of the stack (rods, pins and crossbars). The crossed arrangement of superimposed rods is obtained on simple salt pawn rotation of a normal rod.

The invention is explained in more detail with reference: of the accompanying drawing, which shows an embodiment of the invention, to which the invention, however, is limited, and which Fig. 1 is a schematic perspective view with the parts in particular of an assembly according to the invention and Figs. 2 and 3 show in long section along. the line X — X in Fig. 3 resp. cross-section along the line Y-Y in Fig. 2 extended by a channel at height with a connection according to the invention. In that in Fig. 2 oeb. 3, the channel serves to receive a control rod. Fig. 1 shows that the stack draws a vertical stack with a square division. It consists of graphite rods 1, which Aro are provided with an axial bore in the longitudinal direction of the rods, whereby a vertical channel 2 is formed.

If it is assumed that the underlying layer is in its lase, the application of a stay in the overlying layer takes place in accordance with the invention pd. following salt: The four springs or wedges 3 are located in their layers in the crowds 4. These springs actually provide a first connection between the lower rod and its neighbors. They have a slightly smaller diameter than the space formed so that four rods are joined together, thereby allowing irregularities in the vertical displacement.

The transverse grooves 5 are arranged in two different levels on such a salt that they form a square 'Minster'. They are arranged in grooves 6, which are formed by joining together two adjacent, ph lamp strip salt cut rods. Each transverse rule is in its spirit in contact with two pins 3, each of which forces can be transmitted (Fig. 1).

At each proportion, each transverse rule 5 is provided with a thickening 7, whereby the transverse rule is thermally insulated against naroransing rods. These spirits must be flat or preferably somewhat convex in such a way that they allow a relative vertical displacement between the columns; in some cases the springs 3 provided with light concave profile portions.Partial of importance to emphasize, that the supporting surfaces of these transverse rules are placed with. quite large play on such salt that they allow an approaching radian columns without a clamping can occur in saraband with an extension on the grand of Wigner Figures 2 and 3 show a simple solution to the problem of making it possible to implement the control rods. As is known, the control rods are arranged at the stalls where four columns are met. To ensure the connection of the channel at the height of the transverse bars, a metal tube 8 with small absorbent shapes (eg of beryllium) fitted under pressure; le shorter crossbars abut at 9 towards the rudder.

On the tube, circular chamfers 10 are applied to such a salt that different expansion of the graphite or beryllium channels is allowed without an interruption occurring, which could impede the free passage of the control rods.

Claims (6)

Patent claim: A stack of solid moderator material for a nuclear reactor, the stack comprising adjacent vertical columns, VII consisting of ph spaced prismatic blocks and through which each longitudinal central channel, jug & ray, extends to allow the axes of each column to independently sprint in the form of lozenges with parallel surfaces are inserted in groups, which are occupied perpendicular to and in the middle of the edges of the upper and lower herring planes between superimposed blocks, and that to maintain the distance between the springs are prismatic and / or cylindrical transverse bars. arranged along two mutually perpendicular directions in grooves at corresponding spirits of the black, the grooves and grooves being so deep that the pins and crossbars do not extend into the central channel blocks. 2. A stack according to claim 1, characterized in that the pins, which are arranged in groups, the depth of which is reduced to the least possible extent so that a slight rotation of the blocks around its axis does not have any bending moment on the pins, have circles, polygonal or mixed lion ride, that each group of four sprints has a tie with ten adjacent blocks and that clearances are arranged 1 sideways and vertically around the sprints in such a way that the Wigner expansion of the blocks is not hindered. 3. A stack according to claim 1 or 2, characterized in that only eleven parallel half-pairs are received at the top of a block and two parallel half-pairs, which extend perpendicular to the first-mentioned track, are taken up in the lower spirit of the block supported by the first-mentioned block, whereby habit Mutual disturbance between. crossbars extending at right angles to each other are avoided. A pile according to any one of the dangers of patent claims, characterized in that the transverse bars consist of graphite and their axis is parallel to the direction of least Wigner expansion. 5. A stack according to any one of the preceding patent claims, characterized in that control rods are arranged at the stalls where four moderator block columns are met, whereby the continuity of the channel in line with the transverse bars is arranged with a row of beryllium arranged under a compression chamber in a cylindrical in said rods and wherein the transverse rules at this stall are divided into two halves, which are designed so that they abut against the outer side surface of said rods. Request publications: Patents from the United Kingdom 784 291, 784 292, 785 87 6.