Patent Number: 041347919
Section: description

The plate-type nuclear reactor fuel assembly shown in FIG. 1 is generally designated by the reference 1. This fuel assembly is mainly composed of a stack of parallel and vertical plates 2 containing in a manner known per se a series of small plates 3 of nuclear fuel material surrounded by a clad which is formed of thin metal foil 4. Said small plates are first stacked together along the plane of each fuel plate 2, then enclosed between two thin cladding sheets 5 and 6 respectively, said sheets being joined together along their lateral sides by means of metallic strips 7 which are welded to the sheets 5 and 6 in order to form a leak-tight cladding with these latter. These fuel plates 2 are preferably designed in accordance with the above-mentioned arrangements which have already been described and claimed in American Patent Application Ser. No. 484,743 of July 1, 1974. The stack of fuel plates 2 is maintained in position with a predetermined spacing between the successive parallel plates by forming notches 8 in the lateral strips 7 which close the cladding, said notches being intended to permit the engagement of spacing combs 9, the teeth 10 of which define the spacing between said plates. Combs of this type advantageously correspond to the arrangements described in French patent Application No 75 30247 of Oct. 2nd, 1975, in accordance with any one of the alternative embodiments contemplated in the cited Application. If so required, the combs 9 which serve to space the fuel plates 2 in the stack can be so arranged that, instead of engaging in notches 8 in order to ensure that they do not project from the apparent contour of the fuel-plate stack, they are more simply mounted on the lateral edges of these latter as illustrated in FIG. 1a; in this case, said combs are designed in the form of substantially rectangular strips having grooves 9a which permit the engagement of the edges of the fuel plates 2 and which are welded against the cladding of these latter by means of weld fillets 9b. In accordance with the invention, at least a number of the parallel plates of the stack which constitutes the fuel assembly is provided with sleeves 11 which are designed in the form of hollow tubular elements of appreciable length and the height of which is substantially greater than the height of the fuel plates 2 in the exemplified embodiment which is illustrated in FIG. 1. Said sleeves 11 are welded at 12 against portions of fuel plates which are designated respectively by the references 2a and 2b for example, said portions being disposed in the same plane so as to constitute the fuel plate 2. The sleeves thus have two additional lengths 13 and 14 at the top and at the bottom of the fuel assembly. Said additional sleeve lengths are in turn rigidly fixed, especially by welding or any other suitable means of attachment, to two parallel end-pieces 15 and 16 respectively which define the total height of the fuel assembly and make it possible in particular to ensure not only the cohesion of the fuel stack but also the positioning or withdrawal of said fuel assembly in or from the reactor core (not shown). The end-plates 15 and 16 are provided in the usual manner with a series of holes 17 in order to permit the reactor core coolant which usually consists of water under pressure to circulate freely and especially upwards through each fuel assembly. After passing through the bottom end-plate 15, the stream of water then circulates between the fuel plates 2 of the stack, then passes out of the fuel assembly through the top end-plate 16. It is clear from the foregoing that the sleeves 11 thus perform the function of tubular tie-rods between the end-plates 15 and 16. In accordance with a final advantageous arrangement which is known per se, at least a certain number of said sleeves are reserved for the sliding motion and guiding of control rods 18 formed of neutron-absorbing material, said rods being necessary for controlling the neutron flux and making reactivity changes during reactor operation. In the example of construction hereinabove described, the stack of fuel plates 2, the lateral coupling combs 9 and the tubular sleeves 11 which are incorporated with a certain number of said fuel plates form a single-unit structure in conjunction with the end-plates 15 and 16. In other alternative embodiments illustrated in FIGS. 2 and 3, the complete array of fuel plates 2 of the stack can be freely mounted so as to be capable of "floating" with respect to a rigid structure which ensures cohesion of the assembly, with the result that the fuel plates are capable of withstanding the expansions which take place during operation without being subjected to any particular mechanical stresses. To this end, at least a certain number of fuel plates 2 of the stack are rigidly fixed to guide sleeves 19 which are similar to the support sleeves 11 of the previous example and designed to permit insertion of tubular tie-rods 20, the ends of which project from said sleeves 19. Said tubular tie-rods are in turn secured to the end-plates 15 and 16, especially by welding or other mechanical means. The guide sleeves 19 are advantageously provided on the end edges of the fuel plates 2 with projecting portions 21 and 22 which make it possible in the case of the portion 21 formed at the lower ends of the plates to apply these latter against the end-plate 15. The projecting portion 22 which is provided on the top edge of each fuel plate between a sufficient clearance space between this latter and the end-plate 16 to permit maximum expansion of the fuel plates. In another alternative embodiment which is illustrated in FIG. 3, the sleeves 19 do not extend to the full length of the corresponding fuel plates 2 but can be subdivided into a plurality of separate sections disposed in the line of extension of each other and designated in the figure by the references 19, 19b, 19c and 19d. As in the previous embodiment, these sections are traversed by tubular tie-rods such as the tie-rod 20 which are rigidly fixed to the end-plates 15 and 16 in order to ensure cohesion of the fuel assembly. FIG. 4 illustrates a detail improvement made in the construction arrangement of the plate-type fuel assembly in any one of the alternative embodiments shown in FIGS. 1 to 3. This improvement is primarily intended to ensure that the fuel plates 2 are braced in a more effective manner in the stack and particularly in the central region of these latter, especially in order to prevent vibrations or deformations of said fuel plates under the action of the flow of coolant. To this end, the fuel plates 2 of the stack are provided in addition to the lateral combs with elongated slots 23 which are located at intervals in the surface of said fuel plates and oriented either parallel or at right angles to the axial direction of the fuel assembly, especially in zones in which there is no fuel material. Said slots 23 permit the engagement of transverse spacers 24 formed of flat lugs 25 which are joined to each other by means of cylindrical-rod elements 26. The spacing width of said lugs corresponds to the desired distance between the fuel plates whilst the rod elements represent the thickness of the fuel plates themselves. As can be seen from FIG. 4, the special shape of said transverse spacers is such as to permit the insertion of these latter in the stack of fuel plates 2 through the elongated slots 23. Once the spacers have been positioned, they can be rotated through an angle of 90.degree. so as to bring the plane of the spacing lugs 25 in a direction which is substantially perpendicular to that of the slots, with the result that the spacers are locked in position. As already mentioned in the foregoing, provision is made for special plates between a certain number of clad fuel plates of a given stack. It is recalled that these special plates have tubular sleeves which serve either to maintain a rigidly spaced relationship between the end-plates of the fuel assembly in the vertical direction or to guide hollow tie-rods which perform a similar function, at least a certain number of said tubular sleeves or tie-rods being intended to permit displacement of reactor control rods in sliding motion. By means of this arrangement, conventional fuel assemblies and especially assemblies of the cylindrical fuel-pencil type can accordingly be replaced in a reactor core by fuel assemblies of the plate type mentioned above without entailing any need to modify the other reactor core structures and in particular the fuel-handling means and control-rod drive mechanisms. In FIG. 5, there have been shown in adjacent relation a fuel assembly 1 comprising fuel plates 2 as described in the foregoing with reference to any one of FIGS. 1 to 3 and a conventional fuel assembly 27 constituted by a cluster of cylindrical fuel pencils 28 which are maintained in position by means of spacer grids 29. In particular, a fuel assembly of this type can be as disclosed and claimed in American Pat. No. 3,954,560 of Dec. 11, 1972 as mentioned earlier. In such a case it is apparent that the coolant which is usually circulated upwards through said fuel assemblies in the direction of the arrows f flows through these latter at a rate which may not be wholly uniform. In particular, the presence of spacer grids 29 in the fuel-pencil assembly 27 causes outward deflection of the flow of fluid and is consequently liable to result in excessive cooling of the fuel plates 2 of the adjacent fuel assembly 1. It is therefore necessary to overcome this disadvantage and to permit substantially identical cooling of both fuel assemblies. In accordance with a particular arrangement of the invention, the plate-type fuel assembly 1 is accordingly provided with sheet-metal strips of suitable height which are intended to form a screen and are designated respectively by the references 30 and 31. Said strips are preferably welded on the external sides of the fuel assembly 1 in the vicinity of the spacer grids of the adjacent fuel-pencil assembly and against the lateral combs 9. In another alternative embodiment which is illustrated in FIG. 6, there is again shown a plate-type fuel assembly 1 placed next to a fuel-pencil assembly 27. In this case, the screen plates 32 are joined to the combs 9 in such a manner as to extend over the entire distance between two successive combs within the fuel assembly in order to ensure that the fuel plates 2 are completely isolated from the coolant flow within the other fuel assembly. Finally, FIGS. 7 to 9 illustrate further arrangements which are advantageously carried into effect in the plate-type assembly under consideration with a view to equalizing the flow between these fuel plates and to ensuring more efficient cooling of this latter. In particular and as illustrated in FIG. 7, the fuel assembly can comprise means for producing turbulence in the fluid flow, these means being constituted by thin metallic cross-strips 33 which extend in a direction parallel to the plane of the fuel plates. Said cross-strips 33 are welded or secured by any suitable mechanical means against the coupling combs 9 which ensure relative spacing and interconnection of the fuel plates 2 of the stack. Vertical slots 34 are formed in at least one edge of each cross-strip and the portions of cross-strips which have thus been cut-out as designated respectively and successively by the references 35 and 36 may or may not be folded-back with respect to the plane of the corresponding cross-strips. In the example illustrated in FIG. 7, the portions 35 thus remain in the plane of said cross-strips whilst the adjacent portions 36 are all bent-back on the same side. On the other hand, in the alternative embodiment which is illustrated in FIG. 8, the successive portions 35 and 36 are all intended to be bent-back alternately on each side of the plane of the cross-strips 23. Finally, in the alternative embodiment illustrated in FIG. 9, the cross-strips which are mounted between the fuel plates of the assembly and designated by the reference 37 have a wavy profile when looking from above, the successive portions 38 and 39 formed by cutting-out one edge of each cross-strip being folded-back on one and the same side of this latter. There is thus formed a plate-type fuel assembly which can be directly substituted within a reactor core for a conventional fuel-pencil assembly without entailing any particular modification of the reactor core structures which are associated with these fuel assemblies. In particular, the control rods can be permitted to pass directly through the fuel assembly itself by means of the sleeves which are added to the fuel plates. It is worthy of note that the construction of the sleeve-type fuel plates can be carried out by means of any suitable method and especially by continuous or non-continuous welding of the sleeves to the flat cladding sheets, either by spot-welding or by mechanical assembly.