Nuclear fuel rod and manufacturing method

Description

[0001] The present invention relates to the field of nuclear fuel rods, in particular for light water reactors.

[0002] Nuclear fuel assemblies used in light water reactors generally include a bundle of nuclear fuel rods, each nuclear fuel rod comprising a cladding containing nuclear fuel, the cladding being formed from a tube closed at each of its two ends by a plug.

[0003] Each cap, for example, is welded to the corresponding end of the tube to create a watertight seal. Various welding techniques can be used to attach each cap to the tube, such as TIG welding (tungsten inert gas welding), laser welding, or electron beam welding.

[0004] The tube and caps are usually metallic. They are, for example, made of zirconium or zirconium alloy, such as Zircaloy.

[0005] In order to protect the cladding from the particularly aggressive environment in which it is located during the operation of the nuclear reactor, it is possible to equip the tube with an external protective coating, for example a protective coating made of chrome or chrome alloy.

[0006] The presence of a protective coating on the tube can negatively affect a weld between the tube and a cap. The resulting weld may be weakened and / or susceptible to corrosion.

[0007] To mitigate this risk, the protective coating is applied to most of the tube's length, but not to the end sections where the caps are welded. This prevents the coating from contaminating the weld between the tube and each cap.

[0008] However, portions of the tube not covered with a protective coating constitute areas of weakness in the tube and the pencil, as they may be more susceptible to wear by force or corrosion, including high-temperature oxidation in accidental regimes.

[0009] US2019 / 139654A1 discloses a nuclear fuel rod containing nuclear fuel pellets contained in a cladding comprising a tube with a coating covering the tube, the tube being closed by caps.

[0010] US5401933A discloses a method of welding an end cap to one end of a control pencil tube, the end cap and the tube being coated.

[0011] One of the goals of the invention is to provide a nuclear fuel rod that is easy to manufacture while being resistant, particularly to corrosion and oxidation at high temperature.

[0012] To this end, the invention proposes a nuclear fuel rod according to claim 1.

[0013] Optional features of the nuclear fuel rod are defined in claims 2 to 6.

[0014] The invention also relates to a method for manufacturing a nuclear fuel rod according to claim 7.

[0015] Optional features of the manufacturing process are defined in claims 8 to 13.

[0016] The invention and its advantages will be better understood upon reading the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, on which: there Figure 1 is a schematic longitudinal cross-sectional view of a nuclear fuel rod; and the Figures 2 And 3 are cross-sectional views illustrating a manufacturing process for a nuclear fuel rod such as that of the Figure 1 .

[0017] There Figure 1 illustrates a nuclear fuel rod 2 intended for use in a light water reactor, in particular a pressurized water reactor (or PWR for "Pressurized Water Reactor") or a boiling water reactor (or BWR for "Boiling Water Reactor"), a "VVER" type reactor, an "RBMK" type reactor, or a heavy water reactor, for example of the "CANDU" type.

[0018] The nuclear fuel rod 2 has the shape of an elongated rod along a central axis A.

[0019] The nuclear fuel rod 2 includes a cladding 4 containing nuclear fuel.

[0020] The sheath 4 comprises a tube 6 and two plugs 8, each plug 8 being welded to a respective end of the tube 6 to seal that end. The tube 6 extends along the central axis A of the nuclear fuel rod 2. The tube 6 preferably has a circular cross-section centered on the central axis A.

[0021] Nuclear fuel, for example, takes the form of a stack of pellets 10 arranged axially inside the tube 6, each pellet 10 containing fissile material. The stack of pellets 10 is also called a "fissile column".

[0022] The nuclear fuel rod 2 optionally includes a spring 12 disposed inside the tube 6, between the stack of pellets 10 and one of the plugs 8, to push the stack of pellets 10 towards the other plug 8. The spring 12 is compressed between the stack of pellets 10 and the plug 8.

[0023] A volume or plenum 14 is provided between the stack of pellets 10 and the plug 8 against which the spring 12 bears. This plenum 14 allows for the storage of gases escaping from the nuclear fuel during reactor operation. The spring 12 is located within the plenum 14.

[0024] As illustrated on the Figures 2 And 3 which illustrate the welding of a plug 8 to one end of the tube 6, the tube 6 is fitted with a tube liner 16.

[0025] Tube 6 has an internal surface 6B, facing inwards towards the inside of tube 6, and an external surface 6A facing outwards towards tube 6.

[0026] The tube coating 16 covers the external surface 6A of the tube 6 to protect it from the external environment. During operation, the tube coating 16 is in contact with the environment.

[0027] Tube 6, for example, is made from a zirconium-based material.

[0028] In this context, a zirconium-based material refers to a material made of pure zirconium or a zirconium-based alloy. A pure zirconium material is a material comprising at least 99% zirconium by weight.

[0029] A zirconium-based alloy is an alloy comprising at least 95% zirconium by weight.

[0030] In one embodiment, the zirconium-based material of tube 6 is a quaternary zirconium alloy whose composition comprises, by weight, 0.8 to 1.8% niobium, 0.2 to 0.6% tin, and 0.02 to 0.4% iron, the remainder being zirconium and unavoidable impurities. Tube 6, for example, has a thickness between 0.4 mm and 1 mm. The thickness of tube 6 is the distance between the inner surface 6B and the outer surface 6A of tube 6.

[0031] The coating of tube 16 is thin and, for example, has a thickness strictly less than that of tube 6.

[0032] The coating of tube 16, for example, has a thickness between 5 µm and 25 µm, in particular between 10 µm and 20 µm. The thickness of the coating of tube 16 is taken perpendicular to the external surface 6A of tube 6, i.e. radially with respect to the central axis A of tube 6.

[0033] The tube 16 coating is made of a chrome-based material.

[0034] In the present context, a chromium-based material refers to a material made of pure chromium or a chromium-based alloy.

[0035] A pure chromium material is a material comprising at least 99% chromium by weight. A chromium-based alloy is an alloy comprising at least 85% chromium by weight.

[0036] In one embodiment example, the chromium-based material is a chromium-based alloy chosen from: a binary chromium-aluminum alloy, a binary chromium-nitrogen alloy and a binary chromium-titanium alloy.

[0037] The tube coating 16 comprises a single layer made of chrome-based material or several superimposed layers made of chrome-based material, preferably with the same chrome-based material.

[0038] The multi-layered structure of the tube 16 coating results, for example, from the deposition process used to deposit the tube 16 coating onto the tube 6, especially if the deposition is carried out in several passes.

[0039] The tube 16 coating extends all the way along tube 6, from one end of tube 6 to the other.

[0040] In the preferred embodiment, the coating of tube 16 is continuous. It completely covers the external surface 6A of tube 6.

[0041] Each cap 8 includes an insertion portion 18 inserted into the tube 6 for fixing the cap 8 to the corresponding end of the tube 6, and an exposed portion 20 remaining outside the tube 6 once the cap 8 is fixed to the corresponding end of the tube 6.

[0042] Each stopper 8 has an annular surface 22 that bears against the end of the tube 6 when the stopper 8 is positioned at the end of the tube 6. The annular surface 22 is centered on the central axis A. The annular surface 22 is located at the junction between the insertion portion 18 and the exposed portion 20. As illustrated in the Figure 2 , the annular surface 22 is for example frustoconical, widening from the insertion part 18 towards the exposed part 20.

[0043] Each cap 8, for example, is made from a zirconium-based material.

[0044] In one embodiment, the zirconium-based material of each cap 8 is a quaternary zirconium alloy whose composition includes, by weight, 0.8 to 1.8% niobium, 0.2 to 0.6% tin and 0.02 to 0.4% iron, the remainder being zirconium and the inevitable impurities.

[0045] In one example of the embodiment, tube 6 and each cap 8 are made of the same material.

[0046] Each cap 8 of the nuclear fuel rod 2 is fitted with a cap coating 24. The cap coating 24 extends over the exposed part 20 of the cap 8.

[0047] In a preferred embodiment, the cap coating 24 is continuous. For example, it completely covers the exposed part 20.

[0048] The cap coating 24 preferably comes to the outer edge of the annular surface 22.

[0049] The cap coating 24, for example, is made of a chrome-based material.

[0050] In one embodiment, the chromium-based material of the cap coating 24 is a chromium-based alloy selected from: a binary chromium-aluminum alloy, a binary chromium-nitrogen alloy and a binary chromium-titanium alloy.

[0051] The cap coating 24 of each cap 8 fitted with a cap coating 24 is distinct from the tube coating 16.

[0052] In one embodiment, the tube coating 16 and the cap coating 24 of each cap 8 are made of the same material.

[0053] When a cap 8 is fitted with a cap liner 24, the latter is provided so that the tube liner 16 and the cap liner 24 are joined when the cap 8 is disposed at the end of the tube 6.

[0054] In particular, the cap coating 24 extends over the exposed part 20 so as to be contiguous with the tube coating 16 when the cap 8 is disposed at the end of the tube 6.

[0055] The insertion part 18, and where applicable the annular surface 22, are devoid of cap coating 24. The cap coating 24 does not cover the insertion part 18 and, where applicable, the annular surface 22.

[0056] Preferably, each plug 8 is fixed, and in particular welded to the tube 6 without the presence of a eutectic at the junction between the tube 6 and the plug 8, between the material of the tube 6, the material of the tube liner 16, the material of the plug 8 and, where applicable, the material of a plug liner 24 covering the plug 8.

[0057] In particular, when tube 6 and / or cap 8 is / are made of zirconium-based material and tube cladding 16 and / or any cap cladding 24 is / are made of chromium-based material, nuclear fuel rod 2 is devoid of chromium-zirconium (Cr-Zr) eutectic at the junction between tube 6 and cap 8.

[0058] The presence of a eutectic at the junction between tube 6 and plug 8, in particular a chromium-zirconium (Cr-Zr) eutectic, would be likely to weaken the nuclear fuel rod 2 and reduce its resistance to oxidation, particularly at high temperature (typically above 350°C).

[0059] As illustrated on the Figures 2 And 3 , a manufacturing process for the nuclear fuel rod 2 includes supplying the tube 6 coated with its tube coating 16 and a cap 8 optionally coated with a cap coating 24 ( Figure 2 ), then the fixing of the cap 8 to one end of the tube 6, the fixing being carried out by electric resistance welding or "resistance butt welding" ( Figure 3 ).

[0060] For the resistance welding of the plug 8 onto the tube 6, the plug 8 is positioned at the end of the tube 6, and an electric current is applied between the tube 6 and the plug 8 so as to weld the plug 8 to the tube 6. During the passage of the electric current, the tube 6 and the plug 8 heat up by Joule effect so that their contact surfaces weld together under the combined effect of a welding force and the heating.

[0061] More specifically in the present case, the stopper 8 is positioned at the end of the tube 6 so that its annular surface 22 bears axially against the end of the tube 6, and then the electric current is applied (see Figure 3 ).

[0062] Electric resistance welding is performed without the addition of material.

[0063] Preferably, the fastening is achieved by resistance butt welding.

[0064] For fixing each end of the plug 8 to the tube 6 by electric resistance welding, the plug 8 is positioned at the end of the tube 6, the plug 8 and the tube 6 being pressed against each other with a determined pressure force (Arrows F on the Figure 3 ), an electric current is applied between tube 6 and plug 8 while maintaining the pressure force, so as to weld plug 8 to tube 6.

[0065] More specifically in the present case, the stopper 8 is positioned at the end of the tube 6 so that its annular surface 22 bears axially against the end of the tube 6 under the determined pressure force, which is maintained during the application of the electric current (see Figure 3 ).

[0066] As illustrated on the Figure 3 , electric resistance welding is carried out for example using a welding machine 26 comprising gripping members 28, one provided for gripping a plug 8 and the other for gripping the tube 6 near the end of the tube 6 on which the said plug 8 is fixed, the gripping members 28 being provided for applying the tube 6 and the plug 8 against each other, preferably under a determined pressure force, the welding machine 26 comprising an electrical supply 30 for applying the current between the tube 6 and the plug 8 via the gripping members 28.

[0067] Resistance welding is performed by applying a direct current, an alternating current, or a pulsed current. Preferably, resistance welding is performed using an alternating current. The use of a direct current or a pulsed current (obtained, for example, by discharging capacitors) is also possible.

[0068] Preferably, the electrical resistance welding of each plug 8 is carried out by applying an electric current with a current intensity between 10 kA and 20 kA.

[0069] Preferably, the electric resistance welding of each plug 8 is carried out under a pressure force of between 200 daN and 400 daN, preferably between 250 daN and 350 daN.

[0070] Preferably, the electrical resistance welding of each plug 8 is carried out by applying an electric current for a duration of between 10 ms and 30 ms.

[0071] Preferably, the electrical resistance welding of each plug 8 is carried out so that the temperature of the tube 6 and the plug 8 at their interface is between 1300 °C and 1600 °C.

[0072] Advantageously, electric resistance welding is carried out in such a way that, after welding, the tube 6 and the cap 8 have a weld bead whose dimensional characteristics are controlled and managed.

[0073] In other words, after welding, tube 6 and plug 8 have a shape compatible with the structural elements of the fuel assembly.

[0074] This is achieved by controlling, for example, the type of current applied (direct current, alternating current or pulsed current), the amperage of the current, the duration of current application and the pressure force used to press the tube 6 and the plug 8 against each other, and possibly maintained during the application of the electric current, as well as the nature and geometry of the gripping elements 28.

[0075] The ranges of values ​​indicated above, particularly when taken in combination, make it possible to respect the desired temperature range and thus obtain electrical resistance welding.

[0076] Controlling the dimensions of the weld bead along the weld bead avoids having to carry out a weld bead rework operation, in particular by a mechanical or chemical process with material removal, for example by deburring and polishing, and allows the coating of tube 16 to be preserved, and, where applicable, the coating of cap 24.

[0077] Thus, it is possible to obtain a pencil whose tube 6 is fitted with a tube 16 coating over the entire length of the tube 6, from one end to the other of the tube 6. Where appropriate, when each cap 8 is itself fitted with a cap 8 coating, the sheath 4 as a whole (i.e. the tube 6 and each cap 8) is thus coated and effectively protected from the external environment.

[0078] In particular, the cap coating 24 of each cap 8 fitted with a cap coating 24 and the tube coating 16 are joined at the interface between the tube 6 and the cap 8.

[0079] In an example embodiment in which the tube coating 16 and the cap coating 24 are continuous, and, after welding, the cap coating 24 and the tube coating 16 together form a continuous coating, extending continuously over the tube 6 and the cap 8, in particular at the interface between the tube 6 and the cap 8.

[0080] When the tube coating 16 is continuous and each plug 8 has a continuous plug coating 24, the tube coating 16 and the plug coatings 24 form a sheath coating continuously covering the sheath 4, extending all the way along the tube 6 and over the plugs 8, with continuity of the coating at the interface between the tube 6 and each plug 8.

[0081] Each cap 8 fitted with a cap coating 24 is welded onto the tube 6 without affecting the shape of the interface between the tube 6 and the cap 8, and optionally without reworking the interface between the tube 6 and the cap 8, in particular by a mechanical or chemical process with material removal.

[0082] In addition, the manufacturing process allows each cap 8 to be fixed to the tube 6 without the formation of a eutectic at the junction between the tube 6 and the cap 8, between the material of the tube 6, the material of the tube coating 16, the material of the cap 8 and, where applicable, the material of a cap coating 24 covering the cap 8.

[0083] In particular, when the tube 6 and / or the cap 8 is / are made of a zirconium-based material and the tube coating 16 and / or any cap coating 24 is / are made of a chromium-based material, the nuclear fuel rod 2 is devoid of chromium-zirconium (Cr-Zr) eutectic at the junction between the tube 6 and the cap 8. Preferably, the two caps 8 are fixed to the tube 6 as shown above to form the nuclear fuel rod 2.

[0084] The fixing of the plugs 8, the insertion of the nuclear fuel, here in the form of pellets 10, and possibly the insertion of the spring 12, are carried out sequentially.

[0085] For example, one of the plugs 8 is fixed onto the tube 6, then the nuclear fuel and possibly the spring 12 are inserted into the tube 6, then the other plug 8 is fixed onto the tube 6.

[0086] Preferably, the first plug 8 fixed is the plug 8 located on the opposite side of the spring 12. Then the nuclear fuel and then the spring 12 are inserted into the tube 6, and then the other plug 8 is fixed onto the tube 6.

[0087] Optionally, additional elements are inserted into the tube 6. One additional element that can be inserted into the tube 6 is a tubular spacer intended to be located between the column of pellets 10 and the plug 8 located at the end of the tube 6 opposite to that where the spring 12 is located. It allows in particular to form a second plenum at the end of the pencil opposite the spring 12.

[0088] In operation, the nuclear fuel rod 2 inserted into a nuclear fuel assembly is oriented substantially vertically in the core of the nuclear reactor.

[0089] One of the 8 plugs, called the upper plug 8, is intended to be located at the top, the other of the 8 plugs, called the lower plug 8, being intended to be located at the bottom.

[0090] The upper cap 8 is the one that is adjacent to the spring 12. When a tubular wedge is provided, it is adjacent to the lower cap 8.

[0091] As seen on the Figure 1 , one or each of the plugs 8 advantageously includes a grasping member 32 intended to be able to grasp the nuclear fuel rod, for example to extract it from the nuclear fuel assembly. On the Figure 1 , each of the 8 plugs has such a gripping device 32.

[0092] Thus, preferably, the manufacturing process includes first attaching the lower plug 8 to one end of the tube 6, then inserting the nuclear fuel and spring 12 inside the tube 6, and then attaching the upper plug 8 to the other end of the tube.

[0093] During the manufacturing process, the tube 6 is fitted with the tube coating 16 before each cap 8 is fitted onto the tube 6, and each cap 8 fitted with a cap coating 24 is fitted with this cap coating 24 before being fitted onto the tube 6.

[0094] Although each cap 8 is then welded onto the tube 6, it is possible to obtain a continuous coating of the tube 6 and of each cap 8 coated with a cap coating 24, the tube coating 16 and the cap coating 24 being contiguous.

[0095] The manufacturing process includes obtaining tube 6, for example by a pilgrim step rolling process.

[0096] The manufacturing process includes depositing the tube coating 16 onto the tube 6, and where appropriate depositing a cap coating 24 onto each cap 8 equipped with such a coating 8.

[0097] The coating of the tube 16 and / or each cap 24 is applied, for example, by physical vapor deposition, in particular by sputtering, and even more specifically by magnetron sputtering. This results in a durable coating for the tube 16.

[0098] The physical vapor deposition by magnetron sputtering of the tube coating 16 and / or each cap coating 24 is carried out for example by one of the following techniques or a combination of at least two of the following techniques: direct current magnetron sputtering, pulsed direct current magnetron sputtering, high power impulse magnetron sputtering (HiPIMS or HPPMS), bipolar magnetron sputtering (MSB), dual magnetron sputtering (DMS), unbalanced magnetron sputtering (UBM).

[0099] The nuclear fuel rod 2 is not limited to the embodiment indicated above.

[0100] In the method of implementation of Figures 1 à 3 , each cap 8 is fitted with a cap cover 24 covering the exposed part 20 of the cap 8.

[0101] Alternatively, only one of the two plugs 8 is fitted with a plug coating 24. In this case, it could be either the lower or upper plug 8. The lower and upper plugs 8 are not subjected to exactly the same thermal, chemical, and neutron stresses, so it may be advantageous to coat only one of the two plugs 8.

[0102] When at least one of the plugs 8 is uncoated (or "uncoated"), each uncoated plug 8 is fixed to the tube 6, and in particular welded to the tube 6, without affecting the shape of the interface between the tube 6 and the plug 8, and optionally without reworking the interface between the tube 6 and the plug 8, in particular by a mechanical or chemical process with material removal.

[0103] In the method of implementation of Figures 1 à 3 , the nuclear fuel is planned in the form of a stack of 10 pellets containing fissile material.

[0104] Alternatively, the nuclear fuel is provided in another form, for example in powder form.

Claims

1. A nuclear fuel rod comprising nuclear fuel contained in a cladding (4), the cladding (4) comprising a tube (6) and two plugs (8), the tube (6) extending along a central axis (A) and having two ends, each plug (8) being attached to a corresponding end of the tube (6) by sealing the end thereof, the tube (6) being covered by a tube coating (16), wherein the tube coating (16) extends over the entire length of the tube (16), from one end of the tube (6) to the other, characterized in that one of the plugs (8) or each plug (8) is at least partially covered by a plug coating (24), the tube coating (16) and the plug coating (24) are made of the same material, each plug (8), whether coated or uncoated, is welded onto the tube (6) without affecting the shape of the interface between the tube (6) and the plug (8), and the plug coating (24) of each plug (8) provided with a plug coating (24) and the tube coating (16) are contiguous at the interface between the tube (6) and the plug (8) .

2. The nuclear fuel rod according to claim 1, wherein each plug (8), whether coated or uncoated, is welded onto the tube (6) without reworking the interface between the tube (6) and the plug (8), in particular, by a mechanical or chemical process involving removal of material.

3. The nuclear fuel rod according to claim 1 or 2, wherein the tube (6) is made of a zirconium based material.

4. The nuclear fuel rod according to any one of the preceding claims, wherein each plug (8) is made of a zirconium based material.

5. The nuclear fuel rod according to any one of the preceding claims, wherein the tube coating (16) is made of a chromium based material.

6. The nuclear fuel rod according to any one of the preceding claims, wherein each plug is attached onto the tube without the presence of a eutectic at the junction between the tube and plug between the tube material, the tube coating material, the plug material and, where appropriate, the material of a plug coating covering the plug.

7. A manufacturing method for a nuclear fuel rod according to any one of the preceding claims, comprising the production of the tube (6) and of the plugs (8), one of the plugs (8) or each plug (8) being coated with a plug coating (24), and the welding by electric resistance welding of at least one of the plugs (8) or of each plug (8) to the corresponding end of the tube (6), without affecting the shape of the interface between the tube (6) and the plug (8).

8. The manufacturing method according to claim 7, wherein each plug (8) is welded onto the tube (6) by electric resistance butt welding.

9. The manufacturing method according to claim 7 or claim 8, wherein the electric resistance welding of at least one of the plugs (8) or of each plug (8) is carried out with a current comprised between 10 kA and 20 kA.

10. The manufacturing method according to any one of claims 7 to 9, wherein the welding by electric resistance of at least one of the plugs (8) or of each plug (8) is carried out by applying the plug (8) against the corresponding end of the tube (6) under a pressing force comprised between 200 daN and 400 daN, preferentially between 250 daN and 350 daN.

11. The manufacturing method according to any one of claims 7 to 10, wherein the electric resistance welding of at least one of the plugs (8) or of each plug (8) is carried out by applying an electric current for a length of time comprised between 10 ms and 30 ms.

12. The manufacturing method according to any one of claims 7 to 11, carried out without reworking the interface between the tube (6) and each plug (8) welded onto the tube (6) by a mechanical or chemical process involving removal of material.

13. The manufacturing method according to any one of claims 7 to 12, wherein each plug (8) is welded onto the tube (6), without forming a eutectic at the junction between the tube (6) and the plug (8), between the material of the tube (6), the material of the tube coating (16), the material of the plug (8), and, where appropriate, the material of a plug coating (24) covering the plug (8).