Nuclear reactor provided with core support system

Abstract

A nuclear reactor (1) comprises: a vessel (2) closed at the top by a cap (3) and containing a core (4) comprising a bundle of fuel elements (5) and immersed in a primary fluid (F) for cooling the core (4); and a heat exchanger (40) for removing heat from the primary fluid (F) by means of a secondary fluid circulating in the heat exchanger (40); the core (4) is supported by a lower support (90) supporting a lower portion of the core (4) below the active zone of the fuel elements (5) and an upper support (80) supporting an upper portion of the core (4) above the active zone of the fuel elements (5); the upper support (80) is joined to the top cover (3) by a support structure (6) extending from the top cover (3) and has an end element (16) which is open centrally and internally provided with a plurality of clamping elements (14) for vertically supporting and resiliently radially constraining the fuel element (5).

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent application claims the priority of Italian Patent Application No. 102022000012476 filed on June 13, 2022, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to nuclear reactors, in particular to fast nuclear reactors cooled by liquid metal or molten salt. Background Art

[0004] It is known that fast neutron nuclear reactors can be constructed with compact dimensions and with a core that can operate for many years without refueling. This feature allows the construction of plants that are not provided with replacement systems and spent fuel storage pools, with the attendant advantages of (i) reduced plant costs and (ii) reduced nuclear proliferation risks. Once the fuel charge is exhausted, the reactor core will be transported inside the reactor vessel to a centralized facility provided with spent fuel handling means. Obviously, transporting the entire vessel requires maximum compactness.

[0005] In patent application PCT / IB2017 / 052606, a solution is described in which the fuel elements are mechanically supported by means of respective heads which are connected to each other and to an anchoring structure by means of supporting means, the supporting means acting between adjacent fuel elements or between fuel elements located at the periphery of the core and the anchoring structure, the supporting means forming an integral part of the head of the fuel element.

[0006] Patent application PCT / IB2017 / 052609 also describes a solution in which the fuel elements are provided with an expander, characterized by a mechanical coupling of high thermal expansion elements, which are alternately engaged with low thermal expansion elements to amplify the radial expansion of corresponding end elements, which engage with each other when a given temperature is exceeded to space the fuel elements from each other, and operate the radial expansion of the core to widen the core by rotating the fuel elements around corresponding feet, wherein the corresponding heads are spaced from each other.

[0007] These last two patents combined together allow the formation of a support system that functions in a zone subject to reduced neutron radiation and also allows the fuel elements to be spaced apart in case of exceeding the design temperature, so as to shut down the reactor. However, the solution obtained also does not allow the heat exchangers and pumps to be housed in the vertical direction of the core, which is desirable from the point of view of reactor compactness. Summary of the invention

[0008] The object of the present invention is to provide a nuclear reactor, in particular a fast nuclear reactor cooled with liquid metal or molten salt, which overcomes the said disadvantages of the known solutions and has structural and safety advantages.

[0009] The invention therefore relates to a nuclear reactor, in particular a fast nuclear reactor cooled with liquid metal or molten salt, as defined in the appended claim 1 and thanks to the auxiliary features and device configurations of the nuclear reactor in the dependent claims.

[0010] In summary, the invention relates to a nuclear reactor, in particular a fast nuclear reactor, preferably a fast nuclear reactor cooled with heavy liquid metal or molten salt, in which the core is axially and radially constrained in the upper part and radially constrained in the lower part by an upper support supporting the upper part of the core above the active zone of the fuel elements and a lower support supporting the lower part of the core below the active zone of the fuel elements, respectively. At least the upper support is carried by a support structure extending from the top cover of the reactor inside the reactor vessel so as to leave a usable space above the core, which space can accommodate a heat exchanger.

[0011] In a preferred embodiment, a support structure extending from the top cover of the reactor branches into an upper core confinement branch provided with an upper support and a lower core confinement branch provided with a lower support. In general, the support structure looks like a double-bottomed cylindrical body, in which both bottoms are incomplete. The upper and lower bottoms confine the core only at two ends, which are located in a lower neutron flux region because they are away from the active part of the core due to the provision of an upper gas region and a lower gas region.

[0012] In other embodiments, the two branches of the support structure - not integrally connected to each other - are formed by respective separate bodies, both branches being supported by the top cover of the reactor, for example concentrically relative to each other (with the branch carrying the upper support being radially inside the branch carrying the lower support). Alternatively, the upper support is carried by a body extending from below, from the bottom wall of the main vessel of the reactor, while the upper support is carried by a support structure suspended from the top cover of the reactor.

[0013] The upper cylindrical portion of the support structure, located above the upper core confinement bottom, is perforated to allow the passage of cold primary cooling fluid at the exchanger outlet and to direct the cold primary cooling fluid towards the inlet of the core in an annular duct, which is delimited externally by the reactor vessel and internally by the support structure.

[0014] The lower supporting bottom supporting the core internally replicates the outer contour of the core to be accommodated. The upper supporting bottom supporting the core internally comprises a plurality of clamps, preferably six in number for a core with a hexagonal cross section, for accommodating the heads of the fuel elements, which interact with an end element forming part of the upper supporting bottom. The clamps comprise movable elements acting on the heads of the fuel elements by means of elastic urging elements, for example formed by a helical spring or a leaf spring. The heads of the fuel elements internally contain an expanding bimetallic element, which, when the design temperature is exceeded, overcomes the reaction of the elastic element of the clamp and spaced the heads of the fuel elements apart.

[0015] The upper support supporting the core is provided with a cavity and an elastic radial containment system, wherein the cavity is used to accommodate a cam for vertically supporting the fuel element, and the elastic radial containment system is used for the head of the fuel element, and the elastic radial containment system interacts with the bimetallic spacer expander system for the head of the fuel element in the event of an accidental overheating of the core to cause the reactor to stop operating; the lower support structure ends with a hexagonal profile to radially accommodate the feet of the fuel element. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention is described in detail in the following non-limiting embodiments with reference to the figures in the accompanying drawings, in which:

[0017] - Figure 1 is a schematic diagram in longitudinal section of a nuclear reactor according to the invention;

[0018] - Figure 2 yes Figure 1 An enlarged scale view of a detail of a nuclear reactor;

[0019] - Figure 3 is based on Figure 2 A simplified top view of the plane of the center line II and showing in particular the vertical support system of the fuel elements;

[0020] - Figure 4 yes Figure 3 details of the drawings;

[0021] - Figure 5 is based on Figure 2 A simplified partial view from above of the plane of the center line II-II and showing in particular the system for horizontal restraint of the fuel elements;

[0022] - Figure 6 yes Figure 5 Schematic diagram of an alternative to the system shown in ;

[0023] - Figure 7 is based on Figure 2 A simplified partial view from above of the plane of the center line III-III and showing in particular the release device of the elastic compression system of the fuel element;

[0024] - Fig. 8A and Figure 8B They are respectively a simplified perspective view and an exploded partial perspective view of a thermal expander used in the nuclear reactor of the present invention. DETAILED DESCRIPTION

[0025] Reference Figure 1 and Figure 2 , a nuclear reactor 1 comprises a main vessel 2 containing a primary cooling fluid F and closed at the top by a head 3 .

[0026] The container 2 contains inside: a core 4 formed by a plurality of fuel elements 5 and immersed in the primary fluid F; and a support structure 6 supporting the core 4 , the support structure 6 being carried by the top cover 3 .

[0027] For example, but not necessarily, the fuel elements 5 have a hexagonal cross section and are arranged adjacent to each other to form the core 4 as a whole, which in turn has a substantially hexagonal shape.

[0028] In the illustrated preferred embodiment, the support structure 6 is shaped as a double bottom shell and has a substantially cylindrical lateral wall 7 supported by the top cover 3 and an upper bottom 8 and a lower bottom 9 branching radially inside the lateral wall 7 and vertically spaced apart from each other.

[0029] The lateral wall 7 comprises a perforated upper portion 70 positioned above the upper bottom 8 and provided with a plurality of radial through holes 71 allowing the passage of the primary fluid F, and a lower portion 72 positioned below the upper bottom 8 and from which the lower bottom 9 branches.

[0030] The lateral walls 7 and the bottom 9 delimit, together with the container 2 , an annular duct 10 for supplying the core 4 .

[0031] The bottom 8 defines an upper support 80 that supports the core 4, and the bottom 9 defines a lower support 90 that supports the core. Preferably, the support 80 is configured to axially (vertically) support the core 4 and provide elastic radial constraints to the core 4, while the support 90 is configured to define radial constraints of the core 4.

[0032] In particular, the bottom 9 has a central through opening delimited by a peripheral edge 11 having a profile corresponding to the outer periphery of the core 4 so as to form a radial constraint to the core 4 .

[0033] The bottom 8 has a central through opening delimited by an end element 16 which internally comprises a plurality of clamps 14 for vertical support and elastic radial restraint of the fuel element 5 .

[0034] Advantageously, in order to reduce neutron damage of the bottom 8 and the bottom 9, the bottom 8 and therefore the support 80 act on the upper part of the core 4 situated above the active zone 12 of the fuel elements 5, and the bottom 9 and the support 90 act on the lower part of the core 4 situated below the active zone 12 of the fuel elements 5. In fact, outside the active zone 12 of the fuel elements 5, the neutron flux is lower, also because the opposite ends of the fuel rods contain gas and no fuel.

[0035] Also refer to Figures 3 to 5 The clamping pieces 14, six in number for example for a core 4 having a hexagonal cross section, are movably supported by the end elements 16 and can be moved to radially clamp the heads 15 of the fuel elements 5. The clamping pieces 14 act on the heads 15 of the fuel elements 5 by means of a push element 18, which is provided with an elastic member 19, such as a spring, in particular a helical spring, and an element 20 for pre-tensioning the elastic member 19.

[0036] The clamp 14 vertically supports the fuel elements 5 of the outermost crown of the core 4 by means of the cavity 21, on which the cams 22 of the fuel elements 5 engage. As described in patent application PCT / IB2017 / 052606, the fuel elements 5 of the outermost crown in turn use the same cam system 22 to vertically fix the fuel elements 5 belonging to the innermost crown.

[0037] The clamp 14 is forced against the head 15 of the fuel element 5 by means of a pusher element 18 and is guided by the compression of the clamp 14 through one or more cylindrical bodies 23 or bodies 23 of other shapes, which slide in corresponding housings 24 of the end element 16, which are in hydraulic communication with the primary fluid F by means of calibrated holes 25. The calibrated holes 25 connect the interior of the housing 24 with the bulk of the primary fluid F in order to allow the core 4 to expand and also define a damping system for possible seismic shocks.

[0038] exist Figure 6 , an alternative to the clamping member 14 is shown, in which the pushing member 18 pressed on the head 15 of the fuel element 5 is provided with an elastic member 26 in the form of a leaf spring, which bends when horizontally engaged by the force generated by the pushing of the head 15 of the fuel element 5 and the ends of the elastic member 26 slide along the corresponding support members 27.

[0039] Figure 7The figure shows a detail of the clamping piece 14, wherein a hole 28 of suitable shape (e.g. rectangular) is made in the central part and at the radial ends. Inside the hole 28, a release cam 29 is arranged, which is integral with the end element 16 and is operated by an operating shaft 30, and which can be rotated to move the clamping piece 14 inside the hole 28, thereby selectively reducing / increasing the compression effect exerted by the clamping piece 14 on the head 15 of the fuel element 5.

[0040] Reference Fig. 8A , Figure 8B Each fuel element 5 is provided on its head 15 with a plurality of heat expanders 31 (for example, six heat expanders when the fuel element 5 has a hexagonal cross-section) positioned on the corresponding sides of the fuel element 5, similar to the case shown in patent PCT / IB2017 / 052609.

[0041] Each thermal expander 31 (for example, each of the six thermal expanders belonging to each head 15 of the fuel element 5) is formed by alternating first elements "a" with a high thermal expansion coefficient with second elements "b" with a low thermal expansion coefficient; in particular, the thermal expander 31 includes a central expander element 32 with a larger thermal expansion, which is hinged on the opposite side to a corresponding lateral expander element 33 with a smaller thermal expansion, which is in turn connected to other elements with a larger thermal expansion, and so on, ending with a corresponding box-shaped element 34, which completes the whole and is hooked together due to the joint 35.

[0042] This type of thermal expander is already known from PCT / IB2017 / 052609, in which the spacing is produced by deformation of the fuel element. In addition to what has already been provided in PCT / IB2017 / 052609, the thermal expander 31 has a return joint 36 which also allows the thermal expander 31 to be radially retracted to ensure expansion according to temperature.

[0043] The nuclear reactor 1 then comprises a heat exchanger 40 ( Figure 1 ), the heat exchanger 40 is used to remove heat from the primary fluid F by means of a secondary fluid circulating in the heat exchanger 40. According to the invention, the heat exchanger 40 is arranged centrally above the core 4. In fact, thanks to the support structure 6, the space above the core 4 can be used to accommodate the heat exchanger 40.

[0044] Advantageously, then, the heat exchanger 40 is an annular exchanger which houses inside it a pump 41 for circulating the primary fluid F in the nuclear reactor 1 .

[0045] In this way, the heat exchanger 40 is positioned centrally above the core 4 inside the perforated upper part 70 of the support structure 6, the perforated upper part 70 being provided with radial through holes 71 to allow the primary fluid F leaving the heat exchanger 40 to pass through and to guide the primary fluid F in the annular duct 10 towards the inlet of the core 4, the annular duct 10 being delimited externally by the container 2 and internally by the support structure 6.

[0046] Finally, it should be understood that other modifications and variations may be made to the nuclear reactor described and illustrated herein without departing from the scope of the appended claims.

Claims

1. A nuclear reactor (1), comprising: a container (2) closed at the top by a cover (3) and containing a core (4) comprising a bundle of fuel elements (5) and immersed in a primary fluid (F) for cooling the core (4); and a heat exchanger (40) for removing heat from the primary fluid (F) by means of a secondary fluid circulating in the heat exchanger (40); The core (4) is supported by a lower support (90) and an upper support (80), wherein the lower support (90) supports the lower portion of the core (4) located below the active zone (12) of the fuel element (5), and the upper support (80) supports the upper portion of the core (4) located above the active zone (12) of the fuel element (5); the nuclear reactor (1) is characterized in that the upper support (80) is connected to the top cover (3) via a support structure (6) extending from the top cover (3) and has an end element (16), wherein the end element (16) is open in the center and is provided with a plurality of clamping members (14) inside for vertically supporting and elastically radially constraining the fuel element (5).

2. The nuclear reactor according to claim 1, wherein: The support structure (6) is shaped as a double bottom shell and includes a generally cylindrical lateral wall (7) supported by the top cover (3) and an upper bottom (8) and a lower bottom (9) extending radially inward from the lateral wall (7) and vertically spaced apart from each other; the lower bottom (9) has a central opening defined by a peripheral edge (11), and the lower bottom (9) defines the lower support (90) for radially restraining the fuel elements (5) of the core (4); the upper bottom (8) is provided with the end element (16), and the end element (16) internally includes a plurality of the clamping members (14) for vertically supporting and elastically radially restraining the fuel elements (5).

3. The nuclear reactor according to claim 1 or 2, wherein: The heat exchanger (40) is centrally positioned above the core (4) inside an upper part (70), the upper part (70) being positioned above the upper support (80) and being provided with radial through holes (71) to allow the passage of a primary fluid (F) leaving the heat exchanger (40) and to guide the primary fluid (F) in an annular duct (10) towards the inlet of the core (4), the annular duct (10) being bounded externally by the container (2) and internally by the support structure (6).

4. A nuclear reactor according to any one of the preceding claims, wherein: The clamp (14) is supported by the end element (16) via a respective body (23) and vertically supports the fuel element (5) via a respective cavity (21) engaged by a respective cam (22) which is part of the respective fuel element (5).

5. The nuclear reactor according to claim 4, wherein: The fuel element (5) has a respective upper head (15) which is positioned above the active zone (12) of the fuel element (5) and the clamp (14) radially compresses the head (15) by means of a pushing element (18) operated by elastic members (19, 26).

6. The nuclear reactor according to claim 4 or 5, wherein: The body (23) is housed in a corresponding housing (24) provided with calibrated holes (25) which connect the interior of the housing (24) with the primary fluid (F) and allow the core (4) to expand, and also define a damping system for possible seismic loads.

7. The nuclear reactor according to one of claims 4 to 6, wherein: Each fuel element (5) is provided with a plurality of thermal expanders (31) arranged on the respective sides of the fuel element (5) on the head (15) of each of the fuel elements (5); each thermal expander (31) comprises first elements (a) with a high thermal expansion coefficient alternating with second elements (b) with a low thermal expansion coefficient; the elements (a, b) of the thermal expander (31) are hinged to each other by a return joint (36), which also ensures a return to the initial position when the temperature drops.

8. A nuclear reactor according to any one of the preceding claims, wherein: The end element (16) has a release cam (29) cooperating with a corresponding clamping member (14); each release cam (29) is operated by an operating shaft (30) to rotate and push the clamping member (14), thereby selectively reducing and increasing the compression effect applied by the clamping member (14) on the head (15) of the fuel element (5).

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