Patent Number: 
Section: claims

1. A nuclear reactor, comprising:a vessel that houses a core immersed in a primary fluid in the vessel for cooling the core, the core including a bundle of fuel elements that extend along respective parallel longitudinal axes and that are each provided with an active part, a head, and, between the active part and the head, expanders arranged radially in a direction perpendicular to the respective parallel longitudinal axes;wherein the expanders each include low thermal expansion elements, high thermal expansion elements alternating vertically with the low thermal expansion elements such that each high thermal expansion element is vertically interposed between two low thermal expansion elements, a terminal closing element covering the low thermal expansion elements and the high thermal expansion elements, and a fastening member axially securing the low thermal expansion elements and the high thermal expansion elements to the terminal closing element, the low thermal expansion elements being made of a first material having a first thermal expansion coefficient and the high thermal expansion elements being made of a second material having a second thermal expansion coefficient greater than the first thermal expansion coefficient;wherein the low thermal expansion elements alternately engage with the high thermal expansion elements to amplify radial expansion of terminal parts of the expanders which, when a predetermined temperature is exceeded, shift laterally relative to each other to space the fuel elements from one another and radially expand the core;wherein each fuel element of the bundle of fuel elements is provided with a plurality of expanders that project radially from a shaft of the fuel element and are angularly spaced around the respective parallel longitudinal axis of the fuel element; andwherein the low thermal expansion elements are Z-shaped and the high thermal expansion elements are in the shape of a parallelepiped. 2. The nuclear reactor of claim 1, wherein each head of the bundle of fuel elements includes multiple peripheral faces and each fuel element of the bundle of fuel elements is provided with a number of expanders equal to a number of the multiple peripheral faces of the head of the fuel element. 3. The nuclear reactor of claim 1, wherein for each fuel element of the bundle of fuel elements, radial expansion of the expanders flexes the shaft of the fuel element and spaces the active parts of the fuel elements, thereby expanding the core by radially moving respective feet of the fuel elements, positioned at respective lower axial ends of the fuel elements, while respective heads of the fuel elements, positioned at respective upper axial ends of the fuel elements, remain substantially stationary. 4. The nuclear reactor of claim 1, wherein expansion of the expanders spaces the fuel elements, thereby expanding the core by rotation of the fuel elements around respective feet of the fuel elements effective to radially move the respective feet, positioned at respective lower axial ends of the fuel elements, with respective heads of the fuel elements positioned at respective upper axial ends of the fuel elements and spaced from one another; said head of each one of the fuel elements being radially constrained by flexible containment elements. 5. The nuclear reactor of claim 1, wherein each one of the expanders extends perpendicular to the respective parallel longitudinal axis of the respective fuel element. 6. The nuclear reactor of claim 1, wherein each one of the expanders includes a terminal closing element that covers the low thermal expansion elements and the high thermal expansion elements and projects radially to an outside of the low thermal expansion elements and the high thermal expansion elements. 7. The nuclear reactor of claim 1, wherein:for each expander of the plurality of expanders, the shaft of each of the fuel elements includes a radial extension that projects radially from the shaft and has a radially external end and a radially internal end joined to the shaft opposite to the radially external end;the low thermal expansion elements include a first low thermal expansion element having an axially bent radially external terminal part engaged with the radial extension and a radially internal end;the high thermal expansion elements include a first high thermal expansion element having a radially internal end engaged with the radially internal end of the first low thermal expansion element and a radially external end engaged with a radially external terminal part of a second low thermal expansion element. 8. A nuclear reactor, comprising:a vessel that houses a core immersed in a primary fluid in the vessel for cooling the core, the core including a bundle of fuel elements that extend along respective parallel longitudinal axes and that are each provided with an active part, a head, and, between the active part and the head, expanders arranged radially in a direction perpendicular to the respective parallel longitudinal axes;wherein the expanders each include low thermal expansion elements, high thermal expansion elements alternating vertically with the low thermal expansion elements such that each high thermal expansion element is vertically interposed between two low thermal expansion elements, a terminal closing element covering the low thermal expansion elements and the high thermal expansion elements, and a fastening member axially securing the low thermal expansion elements and the high thermal expansion elements to the terminal closing element, the low thermal expansion elements being made of a first material having a first thermal expansion coefficient and the high thermal expansion elements being made of a second material having a second thermal expansion coefficient greater than the first thermal expansion coefficient;wherein the low thermal expansion elements alternately engage with the high thermal expansion elements to amplify radial expansion of terminal parts of the expanders which, when a predetermined temperature is exceeded, shift laterally relative to each other to space the fuel elements from one another and radially expand the core;wherein each fuel element of the bundle of fuel elements is provided with a plurality of expanders that project radially from a shaft of the fuel element and are angularly spaced around the respective parallel longitudinal axis of the fuel element; andwherein each one of the expanders includes a terminal closing element that covers the low thermal expansion elements and the high thermal expansion elements and projects radially to an outside of the low thermal expansion elements and the high thermal expansion elements. 9. The nuclear reactor of claim 8, wherein the low thermal expansion elements are Z-shaped and the high thermal expansion elements are in the shape of a parallelepiped. 10. The nuclear reactor of claim 8, wherein:for each expander of the plurality of expanders, the shaft of each of the fuel elements includes a radial extension that projects radially from the shaft and has a radially external end and a radially internal end joined to the shaft opposite to the radially external end;the low thermal expansion elements include a first low thermal expansion element having an axially bent radially external terminal part engaged with the radial extension and a radially internal end;the high thermal expansion elements include a first high thermal expansion element having a radially internal end engaged with the radially internal end of the first low thermal expansion element and a radially external end engaged with a radially external terminal part of a second low thermal expansion element. 11. A nuclear reactor, comprising:a vessel that houses a core immersed in a primary fluid in the vessel for cooling the core, the core including a bundle of fuel elements that extend along respective parallel longitudinal axes and that are each provided with an active part, a head, and, between the active part and the head, expanders arranged radially in a direction perpendicular to the respective parallel longitudinal axes;wherein the expanders each include low thermal expansion elements, high thermal expansion elements alternating vertically with the low thermal expansion elements such that each high thermal expansion element is vertically interposed between two low thermal expansion elements, a terminal closing element covering the low thermal expansion elements and the high thermal expansion elements, and a fastening member axially securing the low thermal expansion elements and the high thermal expansion elements to the terminal closing element, the low thermal expansion elements being made of a first material having a first thermal expansion coefficient and the high thermal expansion elements being made of a second material having a second thermal expansion coefficient greater than the first thermal expansion coefficient;wherein the low thermal expansion elements alternately engage with the high thermal expansion elements to amplify radial expansion of terminal parts of the expanders which, when a predetermined temperature is exceeded, shift laterally relative to each other to space the fuel elements from one another and radially expand the core;wherein each fuel element of the bundle of fuel elements is provided with a plurality of expanders that project radially from a shaft of the fuel element and are angularly spaced around the respective parallel longitudinal axis of the fuel element;wherein for each expander of the plurality of expanders, the shaft of each of the fuel elements includes a radial extension that projects radially from the shaft and has a radially external end and a radially internal end joined to the shaft opposite to the radially external end;wherein the low thermal expansion elements include a first low thermal expansion element having an axially bent radially external terminal part engaged with the radial extension and a radially internal end;wherein the high thermal expansion elements include a first high thermal expansion element having a radially internal end engaged with the radially internal end of the first low thermal expansion element and a radially external end engaged with a radially external terminal part of a second low thermal expansion element. 12. The nuclear reactor of claim 11, wherein the low thermal expansion elements are Z-shaped and the high thermal expansion elements are in the shape of a parallelepiped. 13. The nuclear reactor of claim 11, wherein each one of the expanders includes a terminal closing element that covers the low thermal expansion elements and the high thermal expansion elements and projects radially to an outside of the low thermal expansion elements and the high thermal expansion elements.