Pressurized-water-cooled nuclear reactor with compact steam generators

1. A pressurized-water-cooled nuclear reactor, comprising a pressurized tank, which houses a core, a hot header over the core, and a cold header surrounding the hot header and separated from the hot header by a separation structure and in which primary water constituting a core cooling fluid circulates; the reactor comprising at least one steam generator, extending along a vertical axis and positioned in the cold header and in which secondary water circulates for removing heat from the primary water; the steam generator comprising a plurality of heat-exchange tubes in which the secondary water circulates and which have respective substantially plane spiral portions set on top of one another in a number of levels to form a substantially annular tube bundle delimiting a substantially cylindrical central zone, which houses a supply duct in which the pressurized primary water coming from the hot header flows downwards; wherein each spiral portion is constituted by a plurality of substantially concentric turns set inside one another about the axis and which lie substantially in a common plane perpendicular to the axis; and wherein each tube comprises a delivery branch that connects a radially external end of the spiral portion to a water-delivery manifold, and a return branch that connects a radially internal end of the portion to a steam-return manifold; and wherein the delivery branches extend in a prevalently vertical direction substantially parallel to the axis, and are set circumferentially alongside one another and are radially external to the annular tube bundles; and the return branches extend in a prevalently vertical direction substantially parallel to the axis, and are set circumferentially alongside one another and radially inside the annular tube bundles; and wherein set within the tube bundle is a substantially cylindrical ferrule provided with first calibrated holes and delimiting inside the supply duct.

2. A reactor according to claim 1 , wherein the steam generator extends along an axis, and the spiral portion of each tube is wound about the axis and is formed by a plurality of substantially concentric turns that lie in a common plane substantially perpendicular to the axis.

3. A reactor according to claim 2 , wherein set on each level is the spiral portion of just one tube, or else set on each level are the spiral portions of two or more tubes, the turns of the different tubes of the level being inserted in one another in such a way that the n-th turn of one tube is followed by the n-th turn of the next tube and so forth.

4. A reactor according to claim 1 , wherein the steam generator comprises a water-delivery manifold and a steam-return manifold, housed within the steam generator and connected to a water-inlet duct and to a steam-outlet duct respectively, the steam-outlet duct being set substantially coaxial inside the water-inlet duct.

5. A reactor according to claim 4 , wherein the manifolds are hydraulically separated and set inside a single manifold body connected to a single nozzle formed on a wall of the pressurized tank.

6. A reactor according to claim 5 , wherein the steam generator is supported vertically by the tank substantially at the height of a horizontal middle plane of the nozzle.

7. A reactor according to claim 5 , wherein the manifold body is joined to the nozzle in a releasable way by a gripping element and gaskets, the gripping element being provided with adjustment elements for adjusting circumferentially the gripping force between the manifold body and the nozzle.

8. A reactor according to claim 5 , wherein the manifold body has a tubular inlet portion adjacent and consecutive to a closed-bottom hollow terminal portion; a dismantleable tubular element being inserted in the tubular portion for defining an annular space that constitutes the water-delivery manifold; the steam-return manifold being made in the closed-bottom hollow terminal portion.

9. A reactor according to claim 4 , wherein the manifolds are set in a central area of the steam generator, and the steam generator comprises two tube bundles set on top of one another on opposite sides of the manifolds.

10. A reactor according to claim 4 , wherein the water-delivery manifold is provided with a pressure-loss regulation device for regulation of heat-exchange tubes inlet local pressure losses.

11. A reactor according to claim 10 , wherein the pressure-loss regulation device comprises a pressure loss generator element, that is arranged inside the water-delivery manifold, is substantially cylindrical, and is provided with second small calibrated holes facing respective mouths of the heat-exchange tubes for determining a pre-set pressure loss located at inlet to each tube so as to stabilize boiling in the tubes in parallel.

12. A reactor according to claim 11 , wherein an axisymmetrical mobile element is axially slidably arranged inside the pressure loss generator element, provided with third small calibrated holes facing the second calibrated holes of the pressure loss generator element and acting on which are pusher elements for varying the position of the mobile element with respect to the element between a position of alignment, in which the second holes are substantially aligned with the third holes, and a staggered position, in which the second holes are staggered with respect to the third holes.

13. A reactor according to claim 1 , wherein the steam generator comprises a supporting structure releasably connected to the wall of the tank.

14. A reactor according to claim 13 , wherein the supporting structure has pins inserted in respective seats formed on brackets that project radially from an internal surface of the pressurized tank.

15. A reactor according to claim 1 , wherein the spiral portions are rigidly supported in a plurality of circumferential positions by respective supports radially and angularly spaced apart from one another.

16. A reactor according to claim 1 , wherein the steam generator comprises an adjustable system for pre-compression of the tube bundle.

17. A reactor according to claim 16 , wherein the adjustable pre-compression system is a mechanical system in which the spiral portions are packed tight and compressed by pre-tensioned elastic pusher elements acting on the spiral portions and/or on supports of the spiral portions for gripping the tube bundle axially.

18. A reactor according to claim 1 , wherein a plurality of steam generators are set circumferentially alongside one another around the structure and intercept the entire flow of primary water flowing between the hot header and the cold header.

19. A reactor according to claim 18 , wherein the steam generators are provided with respective perforated top plates for enabling inlet of the primary water inside the steam generators and which are set alongside one another to constitute a horizontal hydraulic separation structure, which is part of the separation structure between the hot header and the cold header.

20. A pressurized-water-cooled nuclear reactor, comprising a pressurized tank, which houses a core, a hot header over the core, and a cold header surrounding the hot header and separated from the hot header by a separation structure and in which primary water constituting a core cooling fluid circulates; the reactor comprising at least one compact steam generator, extending along a vertical axis and positioned in the cold header and in which secondary water circulates for removing heat from the primary water; the steam generator comprising a plurality of heat-exchange tubes in which the secondary water circulates and which have respective substantially plane spiral portions set on top of one another in a number of levels to form a substantially annular tube bundle delimiting a substantially cylindrical central zone, which houses a supply duct in which the pressurized primary water coming from the hot header flows downwards; wherein the spiral portions are rigidly supported in a plurality of circumferential positions by respective supports radially and angularly spaced apart from one another; wherein each spiral portion is constituted by a plurality of substantially concentric turns set inside one another about the axis and which lie substantially in a common plane perpendicular to the axis; and wherein set within the tube bundle is a substantially cylindrical ferrule provided with first calibrated holes and delimiting inside the supply duct.

21. A pressurized-water-cooled nuclear reactor, comprising a pressurized tank, which houses a core, a hot header over the core, and a cold header surrounding the hot header and separated from the hot header by a separation structure and in which primary water constituting a core cooling fluid circulates; the reactor comprising at least one compact steam generator, extending along a vertical axis and positioned in the cold header and in which secondary water circulates for removing heat from the primary water; the steam generator comprising a plurality of heat-exchange tubes in which the secondary water circulates and which have respective substantially plane spiral portions set on top of one another in a number of levels to form a substantially annular tube bundle delimiting a substantially cylindrical central zone, which houses a supply duct in which the pressurized primary water coming from the hot header flows downwards; wherein the steam generator comprises an adjustable system for pre-compression of the tube bundle; wherein each spiral portion is constituted by a plurality of substantially concentric turns set inside one another about the axis and which lie substantially in a common plane perpendicular to the axis; and wherein set within the tube bundle is a substantially cylindrical ferrule provided with first calibrated holes and delimiting inside the supply duct.