Patent Number: 040381366
Section: description

A first embodiment of the false grid is shown respectively in elevation and in cross-section in FIG. 2. In this first alternative form of construction, the false grid 16 is constituted by a stack of metallic layers such as the layer 18. In the particular example shown in the figure, the desired thickness is obtained by stacking seven layers. The layers are maintained in relative positional relation by means of tie-bolts such as 20. The bottom end-connectors of the elements 22 which constitute the lateral shield 14 are inserted in the false grid by means of bores 24 which are formed in said false grid and are obviously disposed so as to have a pitch equal to that of the elements forming the shield system. Two types of bottom end-connectors can be considered for the lateral shield elements: the type shown at 22a and the type shown at 22b. In the case of the element 22a, the end-connector is not provided with a shouldered portion. The bore 24 into which is inserted the end-connector of the element 22a is constituted by superposed bores 26 formed in each layer 18. The bottom layer 18' is provided with a bore 26' of smaller diameter which locks the shield element in position while permitting the coolant sodium to pass through. In the case of the alternative form of construction of the lateral shield element 22b, this latter is provided with a shouldered portion 28. It is therefore no longer necessary to provide the bottom layer 18' with a bore of smaller diameter. Stellited portions are also shown at the level of the shoulder 28 of the element 22b and at the level of the bottom layer 18'. An alternative form of construction of the false grid is shown in FIG. 3 along the same plane of section. In this alternative design, the false grid is again formed by means of a plurality of superposed layers 30a. However, instead of being contiguous as in the example illustrated in FIG. 2, said layers are spaced at a short distance from each other by means of washers such as 32 which are introduced around tie-bolts 34 and interposed between two consecutive layers. The advantage of this alternative form of construction lies in the fact that it is no longer necessary to grind the entire face of each plate 30. It is in fact only necessary to machine the plates 30 at the level of the washers 32. Moreover, the lateral shield elements are supplied with sodium not only through the bottom end-connector but also through the gaps 38 formed between two consecutive plates. So far as the remainder of the structure is concerned, exactly the same elements are again shown in the figure. The bottom end-connector of the shield element is introduced into a bore 40 which extends right through the false grid. Said bore results from the superposed arrangement of bores such as 42 which are formed in each plate 30. There are again shown the two arrangements which were already illustrated in FIG. 2 and which correspond to the two alternative designs of the bottom end-connectors of the lateral shield elements as designated respectively by the references 22a and 22b. In the first alternative embodiment (22a), the end-connector is not provided with a shouldered portion and the bottom layer 30' has a bore 42' of smaller diameter, the end-connector which is applied against the shouldered portion being formed by the portion which projects from the plate 30'; on the contrary, in the alternative embodiment corresponding to the element 22b, this latter has a shouldered portion 44 and no provision is made for the bottom plate 30'. There are again shown exactly the same stellited surfaces as in the alternative embodiment shown in FIG. 2. However, it must be pointed out that, at the level of the gaps formed between two consecutive layers 30, the bottom end-connector of the lateral shield element is provided with an orifice such as 46 through which the coolant sodium is premitted to pass. There are therefore two sodium inlets, namely through the bore 42' of the bottom plate 30' and through the orifices 46 of the bottom end-connector of the lateral shield element. The top view of FIG. 4 shows one half of the false grid, said grid being clearly symmetrical with respect to the axis which limits the half-view. The false grid has the shape of a ring and the line which limits its internal contour has exactly the same shape as the external contour of the reactor core. Since the false grid is slightly higher than the diagrid, said grid ensures peripheral side restraint for the base of the reactor core. As already stated in the description with reference to the previous figures, the false grid is constituted by a plurality of metallic layers 18. In the example shown in FIG. 4, each layer is constituted by six juxtaposed plates 18. To take the example of FIG. 2, there are seven contiguous layers of plates. In order to ensure that the plates of one and the same layer are rigidly coupled together, the plates of a given layer are angularly displaced with respect to the plates of the layer beneath. Thus, the plate 18a.sub.1 of the top layer covers one-half of the plate 18b.sub.1 and 18b.sub.2 of the second layer. Since the plates of the different layers are connected to each other by means of the tie-bolts, the relative angular displacement of the plates of two successive layers ensures rigid interassembly of the plates which constitute the same layer without entailing any need for a mechanical connection between the edges of two adjacent plates of any one layer. In addition, the bores 26 for the insertion of the bottom end-connectors of the lateral shield elements are represented diagrammatically by intersections and the bearing members 48 on which the false grid is supported on the diagrid are represented by circles in broken outline. This form of construction of the false grid offers a large number of advantages over those of the prior art. In particular, it no longer requires complicated machining. Accurate boring and milling operations have in fact been replaced by a simple boring operation for the introduction of the bottom end-connectors of the lateral shield elements. There are no longer any spacer members between the top plate and the bottom plate, with the result that the problem of angular positioning of the end-connectors of the shield elements no longer arises. A better peripheral side restraint is provided for the reactor core by virtue of sectors which are interconnected by means of plates arranged in overlapping relation. Finally, a higher degree of strength and rigidity of the entire structure is obtained.