Patent Number: 
Section: claims

1. A fixed cluster for a pressurized water nuclear reactor core comprising:rods to be inserted into guide tubes of a nuclear fuel assembly; anda support for the rods from which the rods extend in a longitudinal direction the longitudinal direction being orientated vertically downwards when the fixed cluster is arranged on the nuclear fuel assembly;the support including:an upper head having a longitudinal center axis;fins extending radially outwards from the upper head;systems for mounting the rods so as to be distributed on the fins; andat least two elements for abutment against the upper plate of the core, the abutment elements each protruding longitudinally from the respective fins, beyond the mounting systems, in a direction vertically upwards when the fixed cluster is arranged on a nuclear fuel assembly, the abutment elements being arranged radially outwards relative to the adjacent mounting systems,wherein each abutment element is arranged radially outwards relative to all of the rods supported by the respective fin the abutment element protrudes longitudinally from. 2. The cluster according to claim 1 wherein the abutment elements are located at radially outer ends of the fins. 3. The cluster according to claim 1 wherein the abutment elements are arranged angularly about the longitudinal center axis in a regular manner. 4. The cluster according to claim 3 wherein the support includes two abutment elements arranged in a diametrically opposed manner relative to the longitudinal center axis. 5. The cluster according to claim 1 wherein at least one portion of the upper head and the fins are integral. 6. The cluster according to claim 1 wherein the mounting systems include members for receiving upper ends of the rods and nuts which are screwed to the upper ends to fix the rods in receiving members. 7. The cluster according to claim 6 wherein the nuts protrude longitudinally from the members in a direction vertically upwards when the fixed cluster is arranged on the nuclear fuel assembly, and wherein the nuts are arranged at various levels along the longitudinal center axis. 8. The cluster according to claim 6 wherein the upper ends of the rods include shanks which extend through the nuts and are welded to the nuts. 9. The cluster according to claim 1 wherein at least one fin includes a passage for receiving an instrument. 10. A core of a pressurized water nuclear reactor comprising:an upper plate;a lower plate; andnuclear fuel assemblies which are arranged between the upper plate and lower plate, the core further comprising fixed clusters and movable clusters which are arranged on respective nuclear fuel assemblies, the fixed clusters each comprising:rods which are intended to be inserted into guide tubes of the respective nuclear fuel assembly;a support for the rods from which the rods extend in a longitudinal direction, the longitudinal direction being vertically downwards when the fixed cluster is arranged on the respective nuclear fuel assembly; andat least one element for longitudinal abutment against the upper plate of the core of the nuclear reactor;wherein at least one of the fixed clusters is a fixed cluster according to claim 1, the abutment elements of the fixed cluster being in vertical abutment against the upper plate around a water passage hole which is provided in the upper plate above the nuclear fuel assembly on which the fixed cluster is arranged. 11. The core according to claim 10 wherein at least one movable cluster comprises:rods which are intended to be inserted into the guide tubes of the respective nuclear fuel assembly; anda rod support from which the rods extend in a longitudinal direction, the longitudinal direction being vertically downwards when the movable cluster is arranged on the respective nuclear fuel assembly;wherein the shapes of the supports for the fixed cluster and the movable cluster are similar. 12. The core according to claim 11 wherein the fixed cluster and the movable cluster are adjacent. 13. An assembly comprising a nuclear fuel assembly and a fixed cluster which is capable of being arranged on the nuclear fuel assembly wherein the fixed cluster is a fixed cluster according to claim 1. 14. The cluster according to claim 1 wherein the abutment elements do not mount any of the rods on the fins. 15. The cluster according to claim 1 wherein the abutment elements are integral with the respective fins, forming rigid and solid elements.