Patent Number: 047160080
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a fuel assembly in the vessel 2 of a pressurized water nuclear reactor. Fuel assembly 1 is associated with two movable clusters 3 and 4, cluster 3 being a control cluster having neutron absorbing rods and cluster 4 being a spectral shift cluster, of fertile material for example. The rods of clusters 3 and 4 are fixed to respective carriers 5 and 6. The carriers 5 and 6 are housed in respective split guide tubes 7 and 7' and are rectilinearly movable along the axes of the guide tubes. Each carrier 5 or 6 is securely connected to the end of a drive shaft 8 or 9, respectively. The drive shaft 8 is provided with conventional driving means having electromagnetic coils 10 and pawls 10' which may be similar to those described in European Patent Application No. 111,435 and will therefore not be described in detail. The drive shaft 9 is movable inside and along a water tight enclosure or envelope 11 communicating with the vessel 2 at its lower end. Upward movement of shaft 9 may be caused by an upward hydraulic force produced by partially depressurizing the upper part of enclosure 11 by opening a solenoid valve 12; downward gravity movement of the shaft occurs when valve 12 is closed again. Means for catching and locking the upper end of the drive shaft 9 are provided at 13 and will be described in greater detail with reference to FIGS. 4 to 12. The guide tubes 7 and 7' are positioned inside a structure 14 located above the fuel assembly 1 and are held in place by guide plates 15 perpendicular to the axis of the guide structure 14 and located at several levels inside the latter. Referring to FIG. 3, the guide plates 15 are held by studs 16 and a peripheral clearance exists between the plates 15 and an outer casing of the guide structure 14. In the central part of the guide structure 14, three reinforcing tie bars 17 maintain the spacing of the guide plates 15. Referring again to FIG. 1, the carriers 5 and 6 are equipped with pairs of slide blocks 18 which take the forces due to overhang and restrict accidental movements and distortions of the rods of clusters 3 and 4, thereby avoiding wear of the latter due to rubbing contact with the guide plates 15. Referring to FIG. 2, the fuel assembly lattice of a reactor core is illustrated having one fuel assembly out of two provided with a pair of clusters. The guide structures 14 associated with the fuel assemblies 1 are hexagonal in shape. The hatched circles indicate the guide tubes 7' of spectral shift clusters 4 and the unhatched circles indicate the guide tubes 7 of control clusters 3. The core assemblies inserted between those assemblies 1 which are associated with the guide structures 14 of hexagonal shape receive neither absorbing rods nor fertile rods but clusters of plugs (not shown) closing the guide tubes in the assembly for avoiding bypass of the core through the fuel assemblies devoid of clusters. Referring to FIG. 3 again, the carriers 5 and 6 of clusters 3 and 4 are shown in guide structure 14. Four of the walls or facets 19 of the casing of the guide structure 14 are thinner due to recesses in their outer surfaces. The spaces available outside the casing for coolant transfer and circulation are consequently increased. Each cluster 3 or 4 has a plane of symmetry containing the axes of the two carriers 5 and 6 but does not have rotational symmetry relative to the axis of an extension of the respective carrier 5 or 6 which is connected to the associated drive shaft. The clusters 3 and 4, together with carriers 5 and 6, are distributed throughout the space inside the guide structure 14. Referring to FIG. 4, a system for catching and locking the spectral shift control shaft 9 in a high position and for unlocking this shaft is located in the upper part of the water tight enclosure 11. The system includes a hollow cylindrical bush 20 rotatably received in a casing 21 through two ball bearings 22 and 23. The casing 21 is securely connected to the enclosure 11. The lower part 24 of the casing 21 is housed inside the bush 20 and carries a pair of pawls 25, pivotably connected on an axis 26 fixed to the lower part 24. An offset lower finger 27 of each pawl 25 is dimensioned for engagement into a circular groove 28 of the drive shaft 9. Two annular cams 29 and 30 carried by the bush are shaped for rocking the pawls 25 around their axes 26 responsive to rotation of the hollow bush 20 around its axis. Means for rotating bush 20 comprises a set of upper inclined surfaces 31 (FIG. 12); the inclined surfaces 31 co-operate with studs 32 secured to a thimble 33, axially slidable along the axis of the water-tight enclosure 11 by the drive shaft 9 when the latter rises upon opening of the solenoid valve 12. The drive shaft 9 is formed with a chamfered shoulder 34 sized to abut thimble 33 and to force the latter upwards when it is subjected to the upward hydraulic force produced by depressurization of the upper portion of enclosure 11. The thimble 33 is biased downwardly by a spring 35 which exerts a shock-absorbing force on the thimble 33, in the direction opposite to that exerted by the drive shaft 9. The inclined surfaces 31 machined in the rotatable bush are such that axial movement of the studs 32 in either direction with respect to the bush causes rotation of the bush 20 and, consequently, of the annular cams 29 and 30. Openings 36 formed in the fixed casing 21 permit longitudinal movement of the studs 32 while preventing their rotation about the shaft axis. Referring to FIG. 4, the drive shaft 9 is illustrated in an intermediate position while it is lifted by the upward force f due to opening of the solenoid valve 12. The groove 28 is still below the level of pawls 25. The latter are open and permit the drive shaft 9 to move freely. The bush 20 remains in its rest position since the studs 32 are stationary. When the chamfered shoulder 34 abuts thimble 33 and starts raising the thimble and the studs 32, the latter move along part 37 (FIG. 12) of the inclined surfaces or ramps 31 and the bush 20 begins to rotate. When the studs 32 have run along the whole length of the part 37, the locking means is as shown in FIG. 5. The pawls 25 have been closed by cam 30 and the finger 27 have lodged in the circular groove 28 formed in the drive shaft 9. Reference can be made to FIGS. 8, 9, 10, and 11 for a more complete understanding of operation of cams 29 and 30 to open or close the pawls 25. FIG. 8 is a section along B--B of FIG. 5 and illustrates the profile of the upper cam 29, while FIG. 9, which is a section along C--C of FIG. 5, shows the profile of the lower cam 30. In FIG. 11, the profiles of both cams are superimposed, making it possible to understand which cam acts on the pawls 25 upon each rotational movement of the rotatable bush 20. FIG. 10 is a section along D--D of FIG. 5 and shows the pawls 25 and their axes 26 in casing 21. When the drive shaft 9 is in the position illustrated in FIG. 5, the solenoid valve 12 is turned off. Then the shaft 9 moves down slowly, since the fall of shaft 9 requires that pressurized water flows along a restricted flow path consisting of the narrow clearance between shaft 9 and enclosure 11. The drive shaft 9 is stopped when its location with respect to the axes 26 of the pawls 25 is as shown in FIG. 6; it is then locked in a "high" position. The movement of the studs 32 between the locations of FIGS. 5 and 6 occurs along ramp 38 and ends when the slides are at the level identified as P6 in FIG. 12. The shaft is then locked (the levels shown respectively as P4, P5 and P7 in FIG. 12 correspond to the positions in FIGS. 4, 5 and 7). When it is desired to move down the cluster 4 for inserting it into fuel assembly 1, in order to shift the neutron spectrum during the first part of the operating cycle, for example, the solenoid valve 12 is temporarily opened again. The drive shaft 9 is forced up by the upward force produced by partial depressurization in the enclosure 11. The chamfered shoulder of drive shaft 9 again abuts thimble 33. The stud 32 again moves along the ramp 39 and rotates the rotatable bush 20. The cam 29 comes to bear on the upper part of the pawls 25 which finally is in an open position as shown in FIG. 7. In FIG. 12, the studs 32 are in the region shown as P7. The solenoid valve 12 is then closed in order to cause fall of the drive shaft 9. The device is once again in the position of FIG. 4, the studs 32 having moved along ramp 40. References P4, P5, P6 and P7, correspnding to those shown in FIG. 12, have been entered in Figure 11. The position of the cams 29 and 30 relative to the pawls 25 in each position to which reference is made in FIG. 12 can thus be seen. The control device of the invention makes it possible to insert completely the spectral shift clusters 4 during part of the core operating cycle, and to keep cluster 4 locked in a "high" position during the balance of the operating cycle, while the control clusters 3 can be inserted at any depth into the core. The motions of clusters 3 and 4 are completely mutually independent of each other. For instance, the cluster 3 may be inserted slightly into the assembly 1 without inserting the rods of cluster 4 at all into the fuel assembly 1. The control device according to the invention is not bulky since it has only two drive shafts 8 and 9 only. All the spectral shift rods forming the cluster 4 are handled by a single drive device (9, 13) and all control rods forming the cluster 3 also are moved by a single control device (8, 10, 10'). The locations of the control devices on the cover of vessel 2 form a uniform polygonal network which is compatible with the ancillary equipment required for installing, constructing and cooling the devices. Moreover, the outer profiles of each of the guide structures 14 are such that they permit horizontal coolant transfer and exchange without excessive head loss towards radial outlets distributed over the vessel. The invention is not restricted to the specific embodiment which has been described by way of example; numerous modifications are possible. For instance the means for catching and locking the drive shaft of the spectral shift clusters could, of course, be modified and incorporate, for example, only an axially slidable bush acting directly on the pawls. Slide blocks 18 could be carried not only by the carrier 5 or 6, but also by other parts. It could also consist of a first slide block placed on the carrier 5 or 6 and a second slide block placed on the drive shaft 8 or 9 which extends the carrier 5 or 6. Finally, the clusters 4 of the second set could be clusters other than spectral shift clusters, for example clusters of "passive" or inert rods.