Patent Number: 047160080
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a device for control of the core of a nuclear reactor by means of clusters with diverse functions. such as control clusters and spectral shift clusters. A method which provides for better utilization of nuclear fuel and reduced uranium fuel costs involves shifting the neutron energy spectrum in the core of the reactor from "soft" to "hard" during the early phase of the operating cycle or core life. For this purpose, it has been proposed, for example, during the first part of the reactor operating cycle, to introduce into the core the rod clusters of material which preferentially absorbs slow neutrons (typically fertile material which can be converted into fissile material under the action of the slow neutrons, such as depleted uranium). A hardening of the neutron spectrum is thus produced both by the reduction in the volume of the moderator in the core and by absorption of low-energy neutrons. In a second part of the reactor operating cycle, the rod clusters which exhibit non-fission slow neutron absorption are withdrawn from the reactor core and the fissile material in the nuclear fuel contained in the assemblies during the first phase is consumed. Reference may be made, for example, to European Patent Application Nos. 108,019 and 108,020. In such reactors, joint use is then made of clusters of neutron-absorbing rods, which may be inserted to a greater or lesser extent into the assemblies forming the reactor core to control the latter, and of spectral shift clusters which are completely inserted into the fuel assemblies forming the reactor core during the first part of the operating cycle. In particular, the control clusters and the spectral shift clusters may be associated with the same reactor core assemblies, with a common motorized drive. European Patent Application No. 111,435 describes a device incorporating control clusters and spectral shift clusters which are coaxial, each with a rotational symmetry of distribution of the rods in the fuel assembly with which they are associated. Movement of the clusters is produced by two coaxial control shafts rectilinearly moveable along a fluid-tight enclosure and equipped with means for locking the shafts in a plurality of positions. Each shaft connected to a control cluster is actuated by conventional means comprising electromagnetic coils and pawls. On the other hand, each shaft for controlling a spectral shift cluster, which is mounted coaxially inside the shaft of the control cluster, comprises a piston slidably accomodated in the control shaft and upwardly moved upon opening of a valve for pressure/release in the upper part of the fluid tight enclosure along which the control shafts are movable. In its uppermost position, the spectral shift shaft can be coupled to the control cluster shaft by fingers which engage in a groove in the spectral shift shaft. While such a fine-control device is generally satisfactory, it nevertheless has the disadvantage that the motions of the shafts are not entirely independent, since the spectral shift shaft is subject to the motions of the control shaft. The spectral shift clusters are consequently partly inserted even when insertion is not desirable. French Pat. No. 2,168,564 discloses fuel assemblies associated with guide/tubes along which a plurality of cluster control shafts slide individually. The rods of these clusters are distributed with a rotational symmetry in relation to the rod distribution pattern in the fuel assembly. While such a device provides complete uncoupling in guidance and movement of the various control shafts, as well as a uniform distribution of the rods in the fuel assembly, it has the disadvantage of being very bulky due in particular to the requirement for control and guidance means situated inside and outside the vessel and associated with each of the control shafts. For a description of hydraulic and electromechanical drive means for such shafts, reference may be had to French Pat. No. 2,232,820. SUMMARY OF THE INVENTION It is an object of the invention to overcome the above-mentioned disadvantage, and it is a more specific object to provide a device for fine control of the core of a nuclear reactor by means of clusters with diverse functions, which device is of reduced bulk and in which the shafts for controlling the clusters and the associated guiding means are completely independent of each other. For that purpose, a device for fine control of the core of a nuclear reactor having a vessel containing pressurized fluid (typically pressurized water) and a core immersed in the fluid and incorporating a plurality of vertically arranged fuel assemblies comprises a first set of clusters arranged to be guided and inserted more or less deeply into some of said fuel assemblies and a second set of clusters arranged to be guided and inserted completely into some of said fuel assemblies during part only of the core operating cycle. Each of the clusters consists of a group of rods arranged parallel to each other, arranged for being movable vertically along the assemblies and inside a guide structure for guiding the rods. Each cluster has an upper carrier movable inside and along a guide tube positioned inside the guide structure. The carrier is fast with a vertically movable drive shaft. Means for moving the shafts controlling the clusters of the first set may be conventional and comprise electromagnetic coils and pawls. The means for moving the drive shafts of the clusters of the second set are hydraulic. Such drive shafts each move in a fluid-tight enclosure communicating with the vessel and which may be partially depressurized at the upper end thereof. Means are provided for individually locking the shafts driving the clusters of the second set in a higher position and for unlocking them. According to one aspect of the invention, one fuel assembly out of every two is associated with two unsymmetrical clusters, namely a cluster of the first set and a cluster of the second set. The guide tubes of the two clusters are arranged symmetrically relative to the axis of a same guide structure associated with the fuel assembly. The means for guiding and moving the cluster carriers are independent of each other. The guide structures associated with fuel assemblies are typically polygonal in shape. According to a particular feature of the invention, the assembly formed by the carrier and the drive shaft of each cluster is equipped with slide blocks for guiding the said assembly inside the guide tube in which it moves. According to another feature of the invention, the guide tubes are held in position inside the guide structure by guide plates or partitions which are perpendicular to the axis of the guide structure and equipped with at least one vertical spacer rod extending along the axis of the guide structure. In a particular embodiment of the invention, the means for locking each drive shaft associated with a cluster of the second set in a "high" position and for unlocking the shaft comprises: two pawls pivotably connected to a casing fixed inside the enclosure and capable of engaging in a groove in the control shaft. a rotatable bush formed with two cams, one for opening the pawls and the other for closing the pawls, and means for rotating the rotatable bush responsive to axial movement of the drive shaft. Preferably, the means for turning the rotatable bush when the control shaft moves axially comprises a thimble movable axially, equipped with two studs cooperating with inclined abutting surfaces on the rotatable bush and a shoulder which a chamfer of the control shaft abuts when the latter moves upwards.