WO2025032459A1

WO2025032459A1 - Fuel element for a nuclear reactor

Info

Publication number: WO2025032459A1
Application number: PCT/IB2024/057505
Authority: WO
Inventors: Luciano Cinotti; Luigi VERNETTI; Andrea Barbensi; Peter James COCHRAN
Original assignee: Newcleo Srl
Current assignee: Newcleo Srl
Priority date: 2023-08-04
Filing date: 2024-08-02
Publication date: 2025-02-13
Anticipated expiration: 2026-02-04
Also published as: IT202300016707A1

Abstract

A fuel element (1) for a nuclear reactor comprises a box structure (3), containing fuel bars (10), and at least one foot (2) and one head (5) positioned at respective opposite longitudinal ends of the box structure (3); the foot (2) being provided with at least one opening (2b) to allow in use reactor core cooling fluid to enter inside the box structure (3); the foot (2) is provided with a shutter member (30) cooperating with a peripheral edge (37b) of the opening (2b) to selectively open and close the opening (2b), so as to allow circulation of cooling fluid within the box structure (3) during reactor operation, and to prevent circulation of cooling fluid and keep the box structure (3) filled with cooling fluid during and after extraction of the fuel element from the reactor core, even in gas environment.

Description

"FUEL ELEMENT FOR A NUCLEAR REACTOR"

Cross-Reference To Related Applications

This Patent Application claims priority from Italian Patent Application No . 102023000016707 filed on August 4 , 2023 , the entire disclosure of which is incorporated herein by reference .

Technical Field

This invention relates to a fuel element for a nuclear reactor, in particular a nuclear reactor cooled by liquid metal or molten salts , equipped with a cooling system to cool the fuel element during refuelling .

Background

During normal operation of a nuclear reactor, for example a nuclear reactor cooled by liquid metal or molten salts , the active part of the fuel elements is fully immersed in the primary cooling fluid inside the hot collector of the reactor, with, potentially, only the respective upper heads of the fuel elements , which are not active , that emerge above the level of cooling fluid in the hot collector . When refuelling, the spent fuel elements are trans ferred from the liquid metal or molten salts bath of the reactor to an auxiliary pool of water or to a storage system cooled with air .

When a fuel element is extracted from the core of the reactor, as heat dissipation through the primary cooling fluid stops , techniques need to be implemented so that the temperature of the metal sheaths that enclose the fuel pellets does not exceed a predetermined temperature beyond which it could incur damage .

Cooling must always be ensured, taking into account any unexpected events as well , such as temporary blockage of the fuel element at each position along the trans fer route .

Another situation to consider is that involving any blockage with the active part of the fuel element partially or wholly extracted from the primary cooling fluid .

The fuel element contains a given number of crowns of fuel bars inside and, i f trans ferred into a gas environment , the bars of the outside crowns are easily cooled by irradiation towards the container box and from this towards the environment outside the box itsel f , which may be the reactor environment or fuel trans fer system container .

The bars of the inner crowns , in contrast , have more di f ficulty in cooling .

Patent application PCT/ IB2017 / 052607 describes a solution that resolves the issue , but requires active ventilation carried out by the fuel trans fer system with all the risks relating to the risk of an electricity outage , and fan stoppage .

Summary

One purpose of this invention i s to provide a fuel element for a nuclear reactor that overcomes the drawbacks highlighted by the known solutions and has construction and safety advantages.

This invention thus relates to a fuel element for a nuclear reactor as defined in the attached claim 1 and, for its auxiliary features, in the dependent claims.

The invention also relates to a nuclear reactor, in particular a nuclear reactor cooled by liquid metal or molten salts, comprising this fuel element, as defined in claim 10.

The fuel element of the invention is, basically, equipped with a cooling system to cool the fuel element during refuelling operations, in a simple, efficient, and totally reliable way.

Brief Description of the Drawings

The invention is described in the following nonlimiting embodiment, with reference to the figures in the accompanying drawings in which:

Figure 1 is a schematic view of the whole in longitudinal cross-section of a fuel element for a nuclear reactor in accordance with the invention, shown during normal operation of the reactor;

- Figures 2 and 3 are respective cross-section views along the dashed lines II-II and III-III in Figure 1;

Figures 4 and 5 are enlarged views of respective details of the fuel element in Figure 1, shown during normal operation of the reactor ;

Figures 6 and 7 represent , respectively, details from Figures 4 and 5 during extraction of the fuel element from the reactor ;

Figure 8 shows an enlarged-scale view of the detail shown in Figure 6 ;

Figure 9 shows a variant of the detail in Figure 4 ;

Figures 10 and 11 illustrate an additional variant of the details in Figures 4 and 5 , shown during normal operation of the reactor ;

Figures 12 and 13 represent , respectively, details from Figures 10 and 11 during extraction of the fuel element from the reactor .

Description of Embodiments

In Figure 1 , reference number 1 indicates a fuel element for a nuclear reactor, for example a nuclear reactor cooled by liquid metal or molten salts , equipped with a cooling system to cool the fuel element during refuelling .

The general conf iguration of the reactor is not , in itsel f , part of the invention and is not , therefore , described or illustrated . In general , the reactor comprises a core , formed from multiple fuel elements and immersed in a primary cooling fluid through a hot collector and a cold collector, and one or more heat exchangers that remove heat from the primary cooling fluid via a secondary fluid .

To better understand the invention, Figure 1 indicates the level H of the primary cooling fluid in the reactor under normal operating conditions of the reactor .

The fuel element 1 extends along a longitudinal axis A that is vertical in use and comprises , in succession from the bottom and along the axis A, a lower foot 2 , a box structure 3 , a connection shaft 4 , and an upper head 5 , connected, in succession, to each other .

The head 5 is equipped with at least one gripping housing 6 , shaped so as to be hooked by grippers of a fuel loading and unloading machine , known and not represented for simplicity .

Multiple fuel elements 1 arranged next to each other and parallel to each other constitute the core of the reactor, which is not represented in the figures .

With reference to Figures 2 and 3 as well , the box structure 3 is hollow inside and extends along the axis A, preferably with a hexagonal ( as in the example illustrated) or square cross section .

At the centre of the box structure 3 , an internal box structure 7 is inserted, which is also hollow inside and, for example , also having a hexagonal or square cross section; the box structure 3 and the internal box structure 7 are connected to each other using a pair of radial elements 8 , 9 placed at respective , opposite axial ends opposite of the box structures 3 , 7 .

In the space delimited by the box structure 3 and the internal box structure 7 , multiple fuel bars 10 are contained; these are parallel to each other and to the axis A.

Each fuel bar 10 has an active intermediate portion 11 arranged between a lower portion 12 and an upper portion 13 containing fission gases .

With reference to Figures 4 and 5 as well , the lower radial element 8 is connected to a lower tubular structure

14 , for example a cylindrical one , using a connecting element

15 . The structure 14 proj ects downwards from the foot 2 and is provided with multiple radial through slots 16 .

The foot 2 is hollow and provided with at least one lower opening 2b, formed at one lower axial end of the foot 2 , to enable cooling fluid to enter the foot 2 during use and, thus , to enter the box structure 3 .

Inside the internal box structure 7 there is an actuating rod 17 that extends along the axis A and proj ects , in its lower part , inside the connecting element 15 and the structure 14 too and, in its upper part , inside the shaft 4 and a lower hollow portion 18 of the head 5 too .

The actuating rod 17 is firmly connected to a hollow element 19 in its upper part ; this element is equipped with a threaded internal cavity 20 and is provided with at least one abutment portion 21 radially proj ecting from a lateral , external surface of the hollow element 19 ; the abutment portion 21 slides axially and is angularly blocked in a seat 21b formed inside the lower portion 18 of the head 5 . The abutment portion 21 normally ( in the reactor' s normal operating condition) rests on an internal , lower shoulder of the seat 21b .

The head 5 then has an upper portion 22 that houses an actuating element 23 that extends downwards along the axis A and comprises a threaded shaft 24 that engages the threading of the internal cavity 20 of the hollow element 19 and interacts with an upper end-stroke element 26 and an elastic compensation member 27 , in turn attached, below, by a restriction on a cavity 28 formed on the upper portion 22 of the head 5 .

In the upper part axially along the axis A, the actuating element 23 has an actuation cavity 29 , wherein a tool or another handling member for rotating the actuating element 23 around the axis A can be inserted .

In the lower part , the actuating rod 17 supports , inside the structure 14 , a shutter member 30 , for example using a threaded extension 31 of the actuating rod 17 that engages a tightening bolt 32 .

The shutter member 30 is configured to selectively open and close the opening 2b of the foot 2 , so as to allow the circulation of the cooling fluid inside the box structure 3 during the operation of the reactor, and prevent the circulation of cooling fluid and keep the box structure 3 full of cooling f luid during and after the extraction of the fuel element from the reactor' s core , including in a gas environment .

According to a preferred solution in the context of this invention, the actuating rod 17 is hollow and sealed in its lower part ( i . e . it is closed at an axial lower end by a bottom wall 17b ) and is provided with at least one radial lower hole 33 and at least one radial upper hole 34 formed so they pass through a lateral wal l of the actuating rod 17 and placed at a level slightly below and, respectively, at a level above the level H of the free surface of the cooling fluid of the reactor ( in the reactor ' s normal operating conditions ) .

The shaft 4 is also , preferably, equipped with radial through holes 35 .

Before loading the fuel element 1 in the reactor , the abutment portion 21 of the hollow element 19 , connected to the actuating rod 17 , is located abutting the lower portion 18 of the head 5 and correspondingly the shutter member 30 is located in an open position, wherein the shutter member 30 is located inside the structure 14 at one lower end of the structure 14 and axially spaced below the opening 2b .

According to a preferred solution, in the context of this invention, the portion of the structure 14 between the shutter member 30 (when this is in the open position) and a level H above the slots 16 is filled with cooling fluid in the solid state .

Then, during the step for loading the fuel element 1 in the reactor and gradually immersing it in the primary cooling fluid, which is liquid and hot , the primary, solid cooling fluid contained in the structure 14 melts and enables the gradual filling of the box structure 3 and the gradual wetting of the fuel element 1 to the level H . When the lower holes 33 of the actuating rod 17 are located at a level below the level H of the primary cooling fluid of the reactor, the interior of the actuating rod 17 i s also filled with primary cooling fluid .

When the reactor is operating, the fuel bars 10 are completely immersed in the primary cooling fluid that circulates inside the box structure 3 and between the fuel bars 10 entering from the foot 2 of the fuel element 1 .

With reference to Figures 6- 8 too , before extracting the spent fuel element 1 from the core with actuating means (which are known and not described in the context of this invention) that are engaged, for example , in the actuation cavity 29 of the actuating element 23 , the threaded shaft 24 is rotated to raise the actuating rod 17 , engaging the threading 25 of the hollow element 19 . The abutment portion 21 prevents the rotation of the hollow element 19 and, therefore , raises the actuating rod 17 when the threaded shaft 24 is rotated .

The stroke of the actuating rod 17 stops when the shutter member 30 comes into contact with a kni fe shaped portion 37 of the structure 14 , provided with a peripheral edge 37b that delimits the opening 2b .

The shutter member 30 is preferably provided with an annular metal gas ket 38 , for example one made of gold, which cooperates with the kni fe shaped portion 37 .

Another rotation of the actuating element 23 , up to a predetermined rotation torque , compresses the elastic compensation member 27 to ensure the maintenance of a contact force between the kni fe shaped portion 37 of the structure 14 and the gasket 38 placed on the shutter member 30 .

In this way, the fuel element is provided with a cooling system 40 , comprising, in particular, the hollow box structure 3 , the shutter member 30 , the opening 2b formed in the foot 2 , and communicating with the interior of the box structure 3 , the actuating rod 17 , which moves the shutter member 30 , as well as , preferably, the other components described above .

Thanks to the cooling system 40 , as a result of the operations described above , it is possible to extract the fuel element 1 from the reactor core ensuring the cooling fluid remains inside the box structure 3 up to its upper end edge 39 ( open) and inside the actuating rod 17 up to the level of its lower holes 33 .

The convective motions and conduction of the cooling fluid between the fuel bars 10 enables the trans fer of the decay heat towards the box structure 3 and internal box structure 7 . The box structure 3 is , in turn, cooled via natural convection of the fluid wherein it is immersed and also by irradiation towards the external environment when this is a gas environment . Convective motions of the cooling fluid and conduction trans fer heat from the internal box structure 7 to the actuating rod 17 and, via conduction, to its internal cooling fluid that , via convective motions , can trans fer part of the heat to its upper part above the edge 39 of the box structure 3 before then trans ferring it to the outside environment with similar methods to those of the box structure 3 .

The cooling of the foot 2 of the fuel element 1 will cause the cooling fluid contained in it to freeze , thus preventing any loss .

Figure 9 shows a variant in which a box structure 41 extending the fuel element 1 is connected to the lower part of the foot 2 ; the box structure 41 extends below the foot 2 along the axis A extending the box structure 3 and may have various functions , such as that of removing the core supply area from the active part of the core itsel f . In this case , the whole volume inside the box structure 41 can also be filled with cooling fluid before assembling the core .

Figures 10 and 11 illustrate a variant of the components shown in Figures 5 and 9 , wherein the shutter member 30 is located, during normal reactor operation, in a higher position compared to when extracting the fuel element 1 from the reactor core .

While in the embodiments previously described the shutter member 30 moves upwards to close the opening 2b, in this variant the shutter member moves downwards to close the opening 2b . The shutter member 30 always cooperates with a kni fe shaped portion 37 of the lower tubular structure 14 ; the structure 14 also proj ects , in this case , downwards from the foot 2 but is positioned above the opening 2b ( in other words , the peripheral edge 37b that delimits the opening 2b is positioned at a lower end of the structure 14 ) ; the kni fe shaped portion 37 is defined by an internal narrowing of the structure 14 .

The radial element 8 , in particular its central internal portion 42 , consists of an upper safety abutment for the axial movement of the shutter member 30 .

In this embodiment too , below the foot 2 and, in particular, below the structure 14 equipped with the opening 2b, the extension box structure 41 may be extended .

In this variant , when extracting the fuel element , the actuating rod 17 is loaded with compression by an elastic compensation member 43 so as to be able to exert a compression force on the kni fe shaped portion 37 via the shutter member 30 .

Once the fuel element 1 has been extracted from the reactor, the pressure exerted by the cooling fluid contained in the fuel element 1 contributes to the tightening between the shutter member 30 and the kni fe shaped portion 37 .

The above reveals clear advantages of this invention : the cooling system proposed is completely passive ; the convective motions of the liquid cooling fluid enable the trans fer of heat to the surfaces that exchange with the outside (box structure 3 and any upper part of the actuating rod 17 with limited di f ferences in temperature compared to the fuel bars 10 ) ; the availability of a cooling system along the whole axial profile of the active portion 11 of the fuel bars 10 makes it possible to cool the active part of the element even in a hypothetical accident in which the fuel element 1 remains blocked during refuelling in a position in which only the active portion 11 of the fuel bars 10 , or only a portion of it , emerges above the level H of the cooling fluid;

- in the medium and long term, after extraction from the reactor with the decreasing of the decay power of the fuel element 1 , the cooling fluid contained in it will tend to solidi fy, thus contributing to the structural stability of the element itsel f , during any transportation steps ;

- when loading the reactor, the delayed entry of cooling fluid inside the fuel element 1 , due to the liquefaction of the solid cooling fluid, prevents entry inside the foot 2 of any material that may be floating on the surface of the reactor cooling fluid;

- in the variant with the shutter placed in the lower part during refuelling, the pressure exerted by the cooling fluid contained inside the fuel element contributes to the tightening force between the shutter and kni fe shaped portion .

Lastly, it is understood that modi fications and variations can be made to the fuel element as described and illustrated herein without departing from the scope of the accompanying claims .

Claims

1. A fuel element (1) for a nuclear reactor, comprising a box structure (3) , containing fuel bars (10) , and at least one foot (2) and one head (5) positioned at respective opposite longitudinal ends of the box structure (3) ; the foot (2) being provided with at least one opening (2b) to allow in use reactor core cooling fluid to enter inside the box structure (3) ; characterized in that the foot (2) is provided with a shutter member (30) cooperating with a peripheral edge (37b) of the opening (2b) to selectively open and close the opening (2b) , so as to allow circulation of cooling fluid within the box structure (3) during reactor operation, and to prevent circulation of cooling fluid and keep the box structure (3) filled with cooling fluid during and after extraction of the fuel element from the reactor core, even in gas environment.

2. The fuel element according to claim 1, wherein the shutter member (30) is controlled by an actuating rod (17) which extends from the foot (2) to the head (5) of the fuel element (1) and is operated at the head (5) of the fuel element ( 1 ) .

3. The fuel element according to claim 2, wherein the actuating rod (17) of the shutter member (30) is in turn operated by a threaded coupling between a hollow element (19) integral to the actuating rod (17) and a threaded shaft (24) joined to an actuating element (23) which is put into rotation by means of an actuation cavity (29) .

4. The fuel element according to claim 2 or 3, wherein the actuating rod (17) is internally hollow and closed at a lower axial end and provided with one or more radial through- holes (33) positioned in use below the level (H) of the reactor cooling fluid, so as to allow the cooling fluid to internally fill the actuating rod (17) and contribute to the cooling of the fuel rods (10) closer to the centre of the box structure (3) , where a hollow internal box structure (7) is positioned, due to convective motions of the cooling fluid contained in the actuating rod (17) .

5. The fuel element according to any one of the preceding claims, wherein the opening (2b) is formed in a lower tubular structure (14) , for example cylindrical, of the foot (2) ; and the shutter member (30) is movable between an open position, which the shutter member (30) assumes during operation of the reactor to allow normal circulation of primary fluid in the box structure (3) through the opening (2b) and in which the shutter member (30) is positioned below or above and axially spaced from the opening (2b) ; and a closed position, which the shutter member (30) assumes before extraction of the fuel element (1) from the reactor to maintain cooling fluid trapped inside the box structure (3) and in which the shutter member (30) is in contact with a knife shaped portion (37) of said lower tubular structure (14) .

6. The fuel element according to claim 5, wherein the lower tubular structure (14) projects below the opening (2b) and is provided with radial through slots (16) ; and in the open position the shutter member (30) is positioned below and axially spaced apart from said slots (16) ; and in the closed position the shutter member (30) is positioned above said slots (16) .

7. The fuel element according to claim 5, wherein the lower tubular structure (14) is positioned above the opening (2b) ; and in the open position the shutter member (30) is positioned above and spaced apart from the knife shaped portion (37) of the lower tubular structure (14) , defined by an internal narrowing of the lower tubular structure (14) ; and in the closed position the shutter member (30) is positioned above and in contact with the knife shaped portion (37) on which the shutter member (30) is forced and pressed by the actuating rod (17) .

8. The fuel element according to any one of the preceding claims, wherein the shutter member (30) comprises an annular gasket (38) cooperating in contact with the peripheral edge (37b) of the opening (2b) to realize a fluid sealing coupling; and wherein said fluid sealing coupling is subject to continuous predetermined tightening by means of an elastic compensation member (27) that maintains a predetermined contact force between the gasket (38) of the shutter member (30) and the peripheral edge (37b) of the opening (2b) .

9. The fuel element according to claim 8, wherein the elastic compensation member (27) is compressed by further actuation of a manoeuvring element (23) that also actuates the shutter member (30) after the shutter member (30) has gone into contact against the peripheral edge (37b) .

10. Nuclear reactor, in particular nuclear reactor cooled by liquid metal or molten salts, comprising at least one fuel element according to any one of the preceding claims .