Patent Number: 
Section: claims

1. An absorber cluster for a pressurized water nuclear reactor, comprising:a cluster of absorber rods, each absorber rod comprising a tubular sheath and an absorber column formed by a stack of neutron absorbing elements received within the tubular sheath, each of the neutron absorbing elements extending along a longitudinal axis and having a cross-sectional structure determined by a cross-sectional geometry, a composition and a porosity of the neutron absorbing element, the absorber columns containing at least two absorber assemblies and each of the absorber assemblies consists of a plurality of the neutron absorbing elements having a same cross-sectional structure, the absorber rods being configured for being movable vertically during normal operation of the pressurized water nuclear reactor in order adjust a reactivity of the core of the pressurized water nuclear reactor,the cluster comprising at least a first absorber assembly of the absorber assemblies and a second absorber assembly of the absorber assemblies, the neutron absorbing elements respectively of the first absorber assembly and the second absorber assembly being formed from a same material or a same combination of materials selected from the group of neutron absorbing materials consisting of a first europium hafnate, a second europium hafnate, a first samarium hafnate, a second samarium hafnate, hafnium carbide and samarium hexaboride,the first europium hafnate and the second europium hafnate being two defined compounds each formed by a reaction between hafnium oxide and europium oxide, the first europium hafnate and the second europium hafnate having different compositions,the first samarium hafnate and the second samarium hafnate being two defined compounds each formed by a reaction between hafnium oxide and samarium oxide, the first samarium hafnate and the second samarium hafnate having different compositions,the cross-sectional structure of the neutron absorbing elements of the first absorber assembly being different from the cross-sectional structure of the neutron absorbing elements of the second absorber assembly before a first use of the absorber cluster in the pressurized water nuclear reactor. 2. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the first absorber assembly and the second absorber assembly are formed from the first samarium hafnate, and the first samarium hafnate has a molar content of samarium oxide of between 8% and 20%. 3. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the first absorber assembly and the second absorber assembly are formed from the first europium hafnate, and the first europium hafnate has a molar content of europium oxide of between 43% and 54%. 4. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the absorber assemblies are made from the first europium hafnate and/or second hafnate europium. 5. The absorber cluster according to claim 1, wherein the neutron absorbing elements of the first absorber assembly are made exclusively of neutron absorbing materials of the group consisting of the first samarium hafnate, the second samarium hafnate, the first europium hafnate and second europium hafnate, while the neutron absorbing elements of the second absorber assembly are made exclusively of hafnium carbide. 6. The absorber cluster according to claim 5, wherein the neutron absorbing elements of the first absorber assembly and the neutron absorbing elements of the second absorber assembly are disposed in distinct absorber rods. 7. The absorber cluster according to claim 6, wherein the absorber rods include as many absorber rods receiving neutron absorbing elements of the first absorber assembly as absorber rods receiving the neutron absorbing elements of the second absorber assembly. 8. The absorber cluster according to claim 5, wherein the neutron absorbing elements of the first absorber assembly and the neutron absorbing elements of the second absorber assembly are arranged in distinct absorber rods which are adjacent in pairs. 9. The absorber cluster according to claim 5, wherein the absorber assemblies of the absorber cluster are exclusively the first absorber assembly and the second absorber assembly. 10. The absorber cluster according to claim 5, wherein the absorber assemblies include a third absorber assembly comprising a plurality of the neutron absorbing elements made exclusively of boron carbide, exclusively of samarium hexaboride, or exclusively of materials of the group consisting of the first europium hafnate and the second europium hafnate. 11. The absorber cluster according to claim 10, wherein the neutron absorbing elements of the third absorber assembly are arranged in the absorber rods receiving the neutron absorbing elements of the first absorber assembly and the second absorber assembly. 12. The absorber cluster according to claim 10, wherein the neutron absorbing elements of the third absorber assembly are arranged in the absorber rods receiving the neutron absorbing elements of the first absorber assembly and the second absorber assembly in the upper part of the absorber rods. 13. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the first absorber assembly and the second absorber assembly contain a plurality of different materials from among the group of materials. 14. The absorber cluster according to claim 1, wherein at least one of the absorber columns exclusively contains neutron absorbing elements of a same absorber assembly. 15. The absorber cluster according to claim 1, wherein the neutron absorbing elements of the first absorber assembly and the second absorber assembly are made from the same material or the same combination of materials of the group of materials, and differ in the geometry of their cross-section and/or porosity rate. 16. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the absorber assemblies contain exclusively materials of the group consisting of the first europium hafnate and the second europium hafnate. 17. The absorber cluster according to claim 1, wherein at least one of the absorber assemblies has a plurality of the neutron absorbing elements having a multilayer cross-sectional structure comprising a plurality of layers each made of a material or a combination of materials from among the group of materials, wherein each of the neutron absorbing element having a multilayer cross-sectional structure comprises at least two layers of distinct compositions and/or distinct porosity rates. 18. The absorber cluster according to claim 1, wherein at least one of the absorber columns contains the neutron absorbing elements of the first absorber assembly and the neutron absorbing elements of the second absorber assembly that are distributed along the absorber column in a repeated pattern. 19. The absorber cluster according to claim 1, wherein the first europium hafnate, the second europium hafnate, the first samarium hafnate, the second samarium hafnate, the hafnium carbide and/or the samarium hexaboride contained in the neutron absorbing elements of the first absorber assembly and the second absorber assembly are in cubic crystalline phase. 20. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the first absorber assembly and the second absorber assembly contain a single material of the group of materials. 21. The absorber cluster according to claim 1, wherein at least one absorber columns contains neutron absorbing elements of at least two distinct absorber assemblies. 22. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the first absorber assembly and the second absorber assembly are formed from the second samarium hafnate, and the second samarium hafnate has a molar content of samarium oxide of between 50% and 60%. 23. The absorber cluster according to claim 1, wherein the neutron absorbing elements of at least one of the first absorber assembly and the second absorber assembly are formed from the second europium hafnate, and the second europium hafnate has a molar content of europium oxide of between 19% and 26%. 24. An absorber rod for a pressurized water nuclear reactor, comprising:a tubular sheath and an absorber column formed by a stack of neutron absorbing elements received within the sheath, the absorber column containing at least two neutron absorbing elements, each neutron absorbing element being made of a material or a combination of materials selected from a group of neutron absorbing materials consisting of first europium hafnate, a second europium hafnate, a first samarium hafnate, a second samarium hafnate, hafnium carbide, and samarium hexaboride, the neutron absorbing elements differing in their cross-sectional structure before first use of the absorber rod in the pressurize water nuclear reactor, the first europium hafnate and the second europium hafnate being two defined compounds each formed by a reaction between hafnium oxide and europium oxide and having different compositions, the first samarium hafnate and the second samarium hafnate being two defined compounds each formed by a reaction between hafnium oxide and samarium oxide and having different compositions,the absorber rod being configured for being movable vertically during normal operation of the pressurized water nuclear reactor in order adjust a reactivity of the core of the pressurized water nuclear reactor. 25. An absorber cluster for a pressurized water nuclear reactor, comprising:a cluster of absorber rods, each absorber rod comprising a tubular sheath and an absorber column formed by a stack of neutron absorbing elements received within the tubular sheath, each of the neutron absorbing elements extending along a longitudinal axis and having a cross-sectional structure determined by a cross-sectional geometry, a composition and a porosity of the neutron absorbing element, the absorber columns containing at least two absorber assemblies and each of the absorber assemblies consists of a plurality of the neutron absorbing elements having a same cross-sectional structure,the cluster comprising at least a first absorber assembly of the absorber assemblies and a second absorber assembly of the absorber assemblies, the neutron absorbing elements respectively of the first absorber assembly and the second absorber assembly being formed from a same material or a same combination of materials selected from the group of neutron absorbing materials consisting of a first europium hafnate, a second europium hafnate, a first samarium hafnate, a second samarium hafnate, hafnium carbide and samarium hexaboride,the first europium hafnate and the second europium hafnate being two defined compounds each formed by a reaction between hafnium oxide and europium oxide, the first europium hafnate and the second europium hafnate having different compositions;the first samarium hafnate and the second samarium hafnate being two defined compounds each formed by a reaction between hafnium oxide and samarium oxide, the first samarium hafnate and the second samarium hafnate having different compositions,the cross-sectional structure of the neutron absorbing elements of the first absorber assembly being different from the cross-sectional structure of the neutron absorbing elements of the second absorber assembly before first use of the absorber cluster in the pressurized water nuclear reactorwherein the neutron absorbing elements of the first absorber assembly are made exclusively of neutron absorbing materials of the group consisting of the first samarium hafnate, the second samarium hafnate, the first europium hafnate and the second europium hafnate, while the neutron absorbing elements of the second absorber assembly are made exclusively of hafnium carbide.