Patent Number: 
Section: claims

1. A nuclear reactor, in particular a pool-type nuclear reactor, having a main tank that houses a core, comprising:a bundle of fuel elements and immersed in a primary fluid circulating between the core; andat least one heat exchanger, wherein:the bundle of fuel elements extends along respective parallel longitudinal axes and have respective bottom ends immersed in the primary fluid to constitute the core, andthe bundle of fuel elements include respective service sections, the respective service sections upwardly extending from the respective bottom ends and emerge from the primary fluid. 2. The reactor according to claim 1, wherein the fuel elements are mechanically supported by respective top end heads anchored to a supporting structure of the core. 3. The reactor according to claim 2, wherein the fuel elements hang superiorly from beams of the supporting structure and extend downwards from the beams. 4. The reactor according to claim 3, wherein each beam supports a row of fuel elements which it engages through respective slots, provided with sliding blocks and roller tracks and formed in the heads of the fuel elements. 5. The reactor according to claim 3, wherein the beams are longitudinally slidable by actuators provided with a chain-drive system. 6. The reactor according to claim 5, wherein the supporting structure comprises a first set of beams, which support the fuel elements during normal operation of the reactor, and a second set of beams, which are arranged aligned to respective beams of the first set and intervene one at a time to support in cantilever fashion the fuel elements disengaged from the corresponding beam of the first set when the latter is made to slide outwards to release a fuel element to be replaced. 7. The reactor according to claim 2, wherein the supporting structure is housed within a closing structure that closes superiorly the tank. 8. The reactor according to claim 7, wherein the supporting structure comprises beams arranged through openings made in a lateral wall of the closing structure. 9. The reactor according to claim 7, wherein the closing structure is a multi-level structure comprising a substantially annular bottom structure, set above the tank, an intermediate structure, which extends upwards from the bottom structure, and a central top structure, which closes above the intermediate structure; the supporting structure of the core being positioned above the tank and below the top structure. 10. The reactor according to claim 9, wherein the fuel elements are housed through an opening of the bottom structure, and the top end heads are accessible by handling machines housed in a cell set above the top structure after prior opening of further openings formed on the top structure. 11. The reactor according to claim 10, further comprising safety ducts, installed on respective tops of functional components, which connect a space occupied by covering gas in the tank with the cell and are internally provided with controlled-failure diaphragms, which fail following upon pre-set overpressures. 12. The reactor according to claim 10, wherein the closing structure comprises a roof, which closes the tank, and a top closing element, which substantially face one another and have respective central openings; and a first tubular superelevation portion, which connects the openings, and a second tubular superelevation portion, which extends from the top closing element, said portions housing the fuel elements; the roof being provided with windows for housing functional components of the reactor and the closing element having trapdoors aligned to respective windows. 13. The reactor according to claim 12, wherein a region outside and substantially around the first tubular superelevation portion defines a service room, which is separate from the cell and set in which are respective top heads of the functional components and provided with mobile insulation structures for temporary constitution of a sealed isolated space containing the top heads of the functional components. 14. The reactor according to claim 12, wherein the beams, from which the fuel elements hang, are positioned through openings made in the second superelevation portion and rest on supports. 15. The reactor according to claim 1, wherein the fuel elements are arranged in parallel rows and are mobile in a direction perpendicular to the axes along the rows and, limitedly to the recovery of pre-set gaps, also in a direction transverse to the rows. 16. The reactor according to claim 1, further comprising a substantially tubular separation structure that separates a hot manifold overlying the core from a cold manifold surrounding the hot manifold and radially outer with respect thereto; the separation structure enveloping the bundle of fuel elements for defining at the same time a fuel elements containment structure. 17. The reactor according to claim 16, wherein the separation structure one of:is constituted by a single monolithic piece; andcomprises a plurality of sectors set alongside one another circumferentially and separable from one another to be replaced without removal of the fuel elements. 18. The reactor according to claim 17, wherein in normal operation of the reactor the sectors of the structure are constrained radially by contrast devices co-operating with a tubular superelevation portion of the closing structure that houses the heads of the fuel elements. 19. The reactor according to claim 1, wherein the bundle of fuel elements is surrounded by one or more rings of shielding elements, which are set radially outwards around the bundle of fuel elements. 20. The reactor according to claim 19, further comprising adjustable contrast devices, acting on a radially outer ring of peripheral shielding elements for gripping said elements radially. 21. The reactor according to claim 19, wherein the shielding elements are enclosed by a basically tubular separation structure. 22. The reactor according to claim 21, wherein during normal operation of the reactor the outermost peripheral shielding elements are separated from the separation structure by a gap which, during refueling operations, is shared between the fuel elements of the rows that come under the aforesaid shielding elements, reducing mutual constriction thereof and, consequently, the force of extraction of the fuel elements belonging to those rows. 23. The reactor according to claim 19, wherein, in normal operation of the reactor, the peripheral shielding elements are held grouped together by contrast devices which interact between the shielding elements and a tubular superelevation portion of the closing structure that houses the heads of the fuel elements. 24. The reactor according to claim 19, wherein, in normal operation of the reactor, the radially external peripheral shielding elements are kept grouped together at the bottom by contrast elements having the shape of one of an eccentric cam or wedge, co-operating with a contour structure that surrounds the shielding elements. 25. The reactor according to claim 19, wherein the shielding elements carry, on a number of levels, elastic foils, set along the axis, which with the gap open project laterally to contact a radially internal contact contour of the separation structure, and with the gap closed are completely housed in respective seats of the shielding elements. 26. The reactor according to claim 19, wherein the shielding elements have a general shape substantially similar to that of the fuel elements, and some shielding elements house, in respective top parts, auxiliary gas flows treatment devices. 27. The reactor according to claim 1, wherein the core comprises nuclear reaction control absorbers, which are axially slidably inserted within the fuel elements and are directly moved by actuation mechanisms positioned within the fuel elements. 28. The reactor according to claim 27, wherein the fuel elements comprise absorbers, provided with respective absorbent parts which are positioned at least one of above the respective bottom ends and through the respective bottom ends, and are movable to be selectively introduced into the core and extracted from the core, the respective absorbent parts one of sliding within respective central tubes of the fuel elements to position themselves above the core, sliding within respective conduits to position the respective absorbent parts inside the core. 29. The reactor according to claim 27, wherein the absorbers are brought into a position of intervention by one of: gravitational thrust of masses of material following de-energization of anchorage electromagnets, and a motor-driven internal screw-external screw coupling. 30. The reactor according to claim 1, wherein the fuel elements comprise a conduit, which extends underneath each respective bottom end and inside which are positioned absorbers, provided with an absorbent part positioned underneath each respective bottom end for being selectively introduced into the core and extracted from the core, sliding within the conduit to position the absorbent part inside the core. 31. The reactor according to claim 30, wherein the absorbers are brought into a position of intervention by one of: hydrostatic thrust of primary fluid following de-energization of anchorage electromagnets, and a motor-driven internal screw-external screw coupling. 32. The reactor according to claim 1, wherein the instrumentation for control of the core is directly installed on the fuel elements. 33. The reactor according to claim 32, wherein the cables of the instrumentation for control of the core are set along the fuel elements up to the respective top end heads, and are connected electrically to conductors installed on the supporting structure via electrical sliding contacts for accompanying displacements of the supporting structure and of the fuel elements. 34. The reactor according to claim 1, wherein each respective bottom end of each fuel element comprises a bundle of bars set alongside one another, having internal compartments containing fissile material; the compartments being connected, via ducts for evacuation of fission gases, to an expansion space, and via a further duct that ascends the entire fuel element, to a valve that can be connected, at the level of the head of the element, to an emptying and pressure-control system. 35. The reactor according to claim 1, wherein the service section of each fuel element comprises a central tube, set along the axis, and a perimetric structure with a polygonal cross section set around the central tube; the structure having a series of one of local interruptions or restrictions along the axis, defined by respective cross section variations to constitute respective free zones; side-by-side zones of the fuel elements communicating with one another to form free lattice spaces arranged in horizontal layers. 36. The reactor according to claim 35, wherein the fuel elements have first bottom zones set immediately above the respective bottom ends and constituting a space of hydraulic connection between the outlet of the primary fluid from the core and delivery ducts connected to primary fluid circulation pumps. 37. The reactor according to claim 35, wherein the fuel elements have at least one of:zones arranged at a free surface of the primary fluid in the tank; andzones spaced apart from one another along the axis above the free surface of the primary fluid and occupied by gas, which define stratified gas spaces for communication between the elements. 38. The reactor according to claim 1, wherein the bundle of fuel elements is provided with a gas-treatment system, comprising a plurality of stratified spaces defined at the pre-set levels along the fuel elements and in which a gas circulates, and circulation, heat exchangers, and/or filtering devices, which operate on the gas and are hydraulically connected to the stratified spaces and housed in respective top parts of shielding elements set around the bundle of fuel elements. 39. The reactor according to claim 1, wherein each fuel element is constituted by two or more sections coupled in succession along the axis via releasable couplings in such a way that the sections are separable from one another for recovery and reuse or disposal of the individual sections.