Device and method for mechanical testing of a tubular sample of nuclear fuel cladding under accident conditions

Abstract

Device and method for mechanical testing of a tubular sample (1) of nuclear fuel cladding under accident conditions, comprising: – a deformable cylindrical core (5) adapted to be arranged in a mechanical test space (4), being inserted into the tubular sample (1), this deformable cylindrical core (5) comprising at its periphery a winding (11) of an electrically conductive wire (12), this winding (11) comprising a first connection end (14A) adapted to be electrically connected to one of the compression pistons (2), and which comprises a second connection end (14B) adapted to be electrically connected to the other compression piston (2); – an electric generator (19) adapted to circulate an electric current between the two compression pistons (2) and in said winding (11). Figure for abstract: Fig.2

Description

Title of the invention: Device and method for mechanical testing of a tubular sample of nuclear fuel cladding under accident conditions Technical field

[0001] The invention relates to the field of nuclear energy and more particularly to the field of installation safety. The invention relates to the diagnostic and testing means used to analyze the mechanical behavior of elements used in nuclear reactors.

[0002] A nuclear reactor typically comprises assemblies of tubular rods, called claddings, in which the nuclear fuel pellets are stacked. These tubular claddings are where the nuclear fission reactions take place, generating heat and fission products. The tubular claddings form the first containment barrier against the dissemination of fuel and fission products.

[0003] For safety reasons, in a nuclear power plant, it is essential to know the mechanical behavior of the tubular cladding material, subjected to certain stresses, representative of nominal service conditions, incidental or specific accidental conditions. To obtain this information, specific mechanical and thermal tests are carried out in shielded cells on samples of tubular cladding from reactors. Tests are also carried out in conventional laboratories on non-irradiated cladding samples.

[0004] The invention aims, in this context, to provide data on the mechanical behavior of a sample of tubular sheath (in particular the deformation at rupture) undergoing conditions representative of those encountered in a reactor under accident conditions, which are the most critical conditions. PRIOR ART

[0005] In order to get as close as possible to the actual nominal operating conditions of nuclear fuel cladding, patent applications FR3041097 and FR3101421 describe solutions for compressing a medium placed inside a cladding sample, with the aim of imposing biaxial mechanical stresses on the cladding. However, these devices do not allow mechanical tests in accident conditions, i.e. when the temperature increases very quickly.

[0006] Other solutions propose to associate the mechanical test elements with means of heating the sheath, so as to approach accident conditions. However, these solutions allow sample temperature rise rates of around 100 to 200°C per second, which nevertheless remain far from real accident conditions, in which the temperature can increase at rates of up to 1,000°C / s. Statement of the invention

[0007] The invention aims to improve the devices and methods of the prior art.

[0008] To this end, the invention relates to a device for mechanical testing of a tubular sample of nuclear fuel cladding under accident conditions, this device comprising two compression pistons movable relative to each other along a clamping axis and defining between them a mechanical test space. In this device, the compression pistons comprise at least one electrically conductive portion. The device further comprises: - a deformable cylindrical core adapted to be arranged in the mechanical test space, by being inserted into said tubular sample of nuclear fuel cladding, this deformable cylindrical core comprising at its periphery a winding of an electrically conductive wire, this winding comprising a first connection end adapted to be electrically connected to one of the compression pistons, and a second connection end adapted to be electrically connected to the other compression piston; - an electric generator adapted to circulate an electric current between the two compression pistons and in said winding.

[0009] According to another object, the invention relates to a method for mechanically testing a tubular sample of nuclear fuel cladding under accident conditions, using a device as described above. This method comprises the following steps: - placing a tubular sample of nuclear fuel cladding in the test space, the deformable cylindrical core being inserted into the tubular sample of nuclear fuel cladding, and said first connection end and second connection end being electrically connected to the corresponding compression piston; - a first sequence of temperature increase of the tubular sample of nuclear fuel cladding, by heating said winding by connecting the two compression pistons to the electric generator; - a second compression sequence in which the two compression pistons are disconnected from the electric generator and compress the cylindrical core.

[0010] The invention makes it possible to carry out mechanical tests on tubular samples of nuclear fuel cladding in accident conditions, using mobile compression pistons allowing the sample to be subjected to mechanical deformations, whether monoaxial or biaxial, with a very rapid rise in temperature of the sample, close to real accident conditions.

[0011] The invention makes it possible to carry out mechanical tests with a temperature rise rate of the order of 400 to 800°C per second, which corresponds to a temperature rise rate representative of the conditions to which nuclear fuel tubular cladding may be subjected in the event of an accident.

[0012] The device according to the invention may include the following additional characteristics, alone or in combination:

[0013] - said winding is integrated into the material of the deformable cylindrical core;

[0014] - the electrically conductive wire is provided with an electrically insulating sheath;

[0015] - the electrically conductive wire is wound helically on one side cy external lindic of the deformable cylindrical core, from one end to the other of the deformable cylindrical core along the clamping axis;

[0016] - the first connection end and the second connection end respectively comprise a first conductor and a second conductor each passing through a portion of the corresponding compression piston;

[0017] - the compression pistons each comprise an electrically operated bar conductor, adapted to be connected to the electric generator, and an electrically insulating cylindrical pad adapted to be disposed against the deformable cylindrical core;

[0018] - the deformable cylindrical core comprises an internal cylindrical portion of a first material, fitted into an external annular portion of a second material, different from the first material;

[0019] - the elastic limit of the internal cylindrical portion is less than the limit elastic of the external annular portion;

[0020] - the melting temperature of the outer annular portion is higher than the tem melting temperature of the internal cylindrical portion;

[0021] - the device comprises jaws adapted to fix the ends of said sample nuclear fuel cladding tubular;

[0022] - the jaws are movable in spacing along the clamping axis.

[0023] The method according to the invention may include the following additional characteristics, alone or in combination:

[0024] - the first sequence is carried out by heating said winding by Joule effect;

[0025] - the first sequence is carried out by induction heating of the sample nuclear fuel cladding tubular. PRESENTATION OF FIGURES

[0026] Other characteristics and advantages of the invention will emerge from the description non-limiting which follows, with reference to the attached drawings in which:

[0027] - [Fig.l] is a sectional view of a mechanical testing device according to the invention, with a tubular sample of nuclear fuel cladding in place;

[0028] - [Fig.2] is a sectional view of the device of [Fig.l], during testing mechanics;

[0029] - [Fig.3] is a graph illustrating the temperature rise permitted by the device of figures 1 and 2;

[0030] - [Fig.4] is a graph illustrating the heating rate as a function of the level of temperature, when implementing the device of figures 1 and 2.

[0031] Elements similar and common to the various embodiments bear the same reference numbers in the figures. DETAILED DESCRIPTION

[0032] [Fig.l] is a schematic sectional view of a mechanical testing device according to the invention. This device is intended for carrying out mechanical tests on a tubular sample of nuclear fuel cladding under accident conditions.

[0033] The mechanical test device is illustrated in a configuration where it is ready to perform a test, with a tubular sample 1 of nuclear fuel cladding installed in the device and ready to be tested.

[0034] The mechanical testing device comprises two compression pistons 2 movable relative to each other along a clamping axis 3. The compression pistons 2 define between them a mechanical testing space 4 in which the tubular sample 1 can be placed under mechanical stress after thermal conditioning.

[0035] The mechanical testing device further comprises a cylindrical core 5 which is deformable. The cylindrical core 5 is preferably made of a ductile material or an assembly of ductile materials, so as to exhibit radial deformation (local increase in its diameter) when it is compressed along the clamping axis 3.

[0036] Particularly advantageously, in the present example, the cylindrical core 5 comprises an internal cylindrical portion 6 made of a material which deforms at low compressive force at nominal operating temperatures. For example, the internal cylindrical portion 6 is here made of aluminum, which is very ductile with a low elastic limit from 200°C. The force required to compress the cylindrical core 5 thus remains moderate so as not to damage the compression pistons 2, and does not require excessively substantial pressing means.

[0037] The cylindrical core 5 further comprises, in this example, an external annular portion 7, in which the internal cylindrical portion 6 is fitted. Preferably, the external annular portion 7 is made of a material that can withstand a higher temperature than the internal cylindrical portion 6. For example, the annular portion external 7 is made of copper, which has a melting point of around 1085 °C. At the test temperature, the external annular portion 7 must have high ductility and must deform at low compressive force, which is the case in this example with the aluminum / copper pair. Any other suitable pair of materials, one of which has a higher melting point than the other, can be considered.

[0038] The compression pistons 2 are arranged on either side of the cylindrical core 5 so that their clamping is adapted to compress axially (i.e. along the clamping axis 3) the cylindrical core 5.

[0039] Preferably, the portion of the compression pistons 2 which is adapted to come into contact with the cylindrical core 5 is substantially of the same diameter as the cylindrical core 5.

[0040] In the present example, the compression pistons 2 comprise a bar 8, which is electrically conductive, and a cylindrical pad 9 which is electrically insulating. The bar 8 is for example metallic. The diameter of the cylindrical pad 9 is substantially equal to the diameter of the cylindrical core 5. Alternatively, the cylindrical pad 9 may also be conductive and comprise an insulating coating on its face in contact with the cylindrical core 5.

[0041] The mechanical testing device may further comprise means for positioning the tubular sample 1. In this example, these means consist of jaws 10 which are adapted to be secured to each end of the tubular sample 1, for example by clamping, screwing, welding, or any other fixing means.

[0042] The jaws 10 thus make it possible, in addition to holding the tubular sample 1, to participate in the mechanical test by fixing or axially stretching the tubular sample 1 if a biaxial mechanical test is desired.

[0043] For the implementation of a test, the tubular sample 1 is prepared from a new (for a characterization test) or used fuel cladding. Nuclear reactors conventionally comprise fuel claddings with a length of the order of 4 meters and a diameter of the order of 1 centimeter. The tubular sample 1 can be prepared by cutting a fuel cladding. In an exemplary embodiment, the tubular sample 1 thus prepared has a length of the order of 3 cm (along the clamping axis 3 in [Fig.l]), for a cylindrical core 5 with a length of the order of 14 mm.

[0044] The tubular sample 1 is placed in the mechanical test space 4, and the cylindrical core 5 is inserted into the tubular sample 1. In this example, the jaws 10 are further secured to each end of the tubular sample 1.

[0045] The cylindrical core 5 further comprises on its periphery a winding 11 made of an electrically conductive wire 12. In this example, the wire 12 is wound in a spiral at the external periphery of the cylindrical core 5. The wire 12 is thus wound in helical manner from one end to the other of the cylindrical core 5, that is to say over its entire length along the clamping axis 3.

[0046] In this example, the wire 12 is embedded in the material of the outer cylindrical face of the cylindrical core 5 as illustrated in [Fig.l]. In this example, with the cylindrical core 5 consisting of an inner cylindrical portion 6 and an outer annular portion 7, the wire 12 is embedded in the material of the outer annular portion 7.

[0047] The winding 11 thus forms a coil wound on the cylindrical core 5.

[0048] Alternative arrangements may be envisaged for forming such a coil with the winding 11. The wire 12 may for example be arranged in a helical groove made on the external surface of the cylindrical core 5, using an additive manufacturing or machining process. The wire 12 may also be wound directly onto the cylindrical core 5. The wire 12 may be of circular section, as in the example of [Fig.l], or for example of rectangular or flattened section. The wire 12 may for example be a conductive strip wound on the surface of the external cylindrical face of the cylindrical core 5. The wire 12 may also have the form of a sheet or a coating on the external surface of the cylindrical core 5.

[0049] The wire 12 may further be provided with an electrically insulating sheath 13. The sheath 13 is adapted to withstand high temperatures and is for example produced by a ceramic coating of the wire 12.

[0050] The coil constituted by the winding 11 has a first end turn 11A as well as a second end turn 11B opposite. Each of these end turns 11A, 11B is electrically connected to one of the compression pistons 2 by a connection end 14A, 14B.

[0051] In the present example, the first connection end 14A of the winding 11 comprises a first conductor 15A which passes through the cylindrical buffer 9 and which is electrically connected to the corresponding bar 8, which is conductive.

[0052] Similarly, at the other end of the winding 11, the second connection end 14B of the winding 11 comprises a second conductor 15B, which passes through the cylindrical buffer 9 and is electrically connected to the corresponding bar 8.

[0053] An electrical circuit can thus be established between the two compression pistons 2, through the winding 11 forming a coil. [Fig.l] schematically illustrates an electrical generator 19 thus connected to the two compression pistons 2. A switch 20 schematizes the closing of the circuit so that a suitable alternating or direct current (see first and second embodiment below) flows in the winding 11 to cause a rise in temperature of the tubular sample 1.

[0054] The connection of the connection ends 14A, 14B with the compression pistons 2 can be made by any means, such as connectors, electrical contacts, etc. It is also possible to make this connection by welding or crimping once the assembly is in place.

[0055] From this setup of [Fig. 1], the tubular sample 1 being placed in the mechanical testing device, with the winding 11 electrically connected to the compression pistons 2, the mechanical testing method can be implemented.

[0056] In a first sequence, called temperature rise (illustrated in [Fig.l], with the schematic switch 20 closed), the tubular sample 1 will first undergo a thermal stress caused by a rapid rise in the temperature of its wall thanks to the winding 11.

[0057] According to a first embodiment, the winding 11 heats by the Joule effect and heats the wall of the tubular sample 1 when it rises in temperature. In this case, the wire 12 has an electrical resistance suitable for a Joule effect heating device, and a strong direct current (for example several tens of amperes) is applied between the two compression pistons 2. In an exemplary embodiment, the wire 12 has a diameter of the order of 0.5 to 1 mm.

[0058] According to a second embodiment, the winding 11 heats the wall of the tubular sample 1 by induction. In this case, a current suitable for creating an induction phenomenon in the tubular sample is applied between the two compression pistons 2, for example a high-frequency alternating current.

[0059] These two embodiments can be combined, for example by controlling the electric generator 19 alternately in a Joule effect heating mode (with high intensity) and in an induction heating mode (by a high frequency alternating current).

[0060] In a second sequence, called compression (illustrated in [Fig.2], with the schematic switch 20 open), as soon as the tubular sample 1 has undergone the desired temperature rise, the compression pistons 2 are then urged together and compress the cylindrical core 5. The compression pistons 2 are for example moved by a press.

[0061] [Fig.2] illustrates the result of this compression sequence.

[0062] The cylindrical core 5 deforms radially and its diameter increases, thus stressing the walls of the tubular sample 1 as would the expanded nuclear fuel. This deformation takes place while on the one hand the tubular sample 1 has been brought to a high temperature, and on the other hand this heating has also been carried out at a high speed, which influences the mechanical characteristics of the tubular sample 1 in a manner close to real accident conditions.

[0063] The invention is compatible with a biaxial mechanical test in which, while the compression pistons 2 are forced towards each other, the jaws 10 are further stressed away from each other so that in addition to being deformed radially, the tubular sample 1 is also deformed axially by being stretched between the two jaws 10.

[0064] In this second sequence, the tubular sample 1 having already been brought to the desired temperature, the current passing through the winding 11 is cut off. During the compression of the cylindrical core 5, it is possible that the winding 11 is damaged, or even destroyed, as well as the connection ends 14A, 14B, without affecting the mechanical test. In such a case of destruction, the cylindrical core 5 is then a consumable part, renewed at each mechanical test.

[0065] Figures 3 and 4 relate to an example of a mechanical test carried out with a tubular sample 1 3 cm long, a cylindrical core 5 14 mm long and with a diameter adjusted to the internal diameter of the tubular sample 1, and a wire 12 0.8 mm in diameter. The wire 12 in this example is made of copper-nickel. This example of a test is carried out with heating of the winding 11 by the Joule effect. In this example, the tubular sample 1 could be heated during the first sequence to a temperature of the order of 800 °C, with heating rates greater than 500 °C / s.

[0066] [Fig.3] illustrates on the one hand the temperature in °C measured on the tubular sample 1 (curve 16 and values ​​on the ordinate scale on the left of the graph) and on the other hand the heating rate in °C / s (curve 17 and values ​​on the ordinate scale on the right of the graph), and this as a function of time in seconds (on the abscissa).

[0067] Initially, the tubular sample 1 was heated to 350°C in order to reproduce its operating temperature under the nominal operating conditions of the nuclear reactor. After a plateau at 350°C, a strong current is passed (at 131.5 seconds) in the wire 12 which heats up rapidly by the Joule effect, then the current is cut off when the tubular sample 1 reaches 800°C. After the current is cut off, a residual rise in temperature brings the tubular sample 1 up to 870°C.

[0068] The wire diameter of 0.8 mm in this example represents a good compromise between a small diameter (if the diameter of the wire 12 is too small, the wire 12 may melt before heating the tubular sample 1) and a diameter that is too large, which would not allow the winding 11 to be produced.

[0069] Curve 17 shows the evolution of the heating rate, a parameter of prime importance. The highest heating rate reached is greater than 600 °C / s and is reached at a temperature of 600 °C. From this temperature, the radiation from the external surface of the tubular sample 1 seems to become significant, and the heating rate decreases. The heating rate then drops abruptly to 800 °C, when the current is cut off.

[0070] [Fig.4] illustrates this same phenomenon of the evolution of the heating rate. The graph in [Fig.4] represents the evolution of the heating rate in °C / s in function of the temperature in °C reached.

[0071] The heating rate is about 400°C / s to 800°C, and the average value (dotted segment 18) over the temperature range 500°C - 800°C is 540°C / s.

[0072] Alternative embodiments may be implemented. In particular, the jaws 10, which are necessary for a biaxial test, are optional if such a biaxial test is not desired. The jaws 10 may also be constituted by any other mechanical means suitable for axially stretching the tubular sample 1.

[0073] The winding 11 may also have other winding contours than a helical contour as described previously. It may describe shapes around the cylindrical core 5, especially if it acts by the Joule effect.

Claims

Claims

1. Device for mechanical testing of a tubular sample (1) of nuclear fuel cladding under accident conditions, this device comprising two compression pistons (2) movable relative to each other along a clamping axis (3) and defining between them a mechanical test space (4), this device being characterized in that the compression pistons (2) comprise at least one electrically conductive portion, and in that it comprises: - a deformable cylindrical core (5) adapted to be arranged in the mechanical test space (4), by being inserted into said tubular sample (1) of nuclear fuel cladding, this deformable cylindrical core (5) comprising at its periphery a winding (11) of an electrically conductive wire (12), this winding (11) comprising a first connection end (14A) adapted to be electrically connected to one of the compression pistons (2),and a second connection end (14B) adapted to be electrically connected to the other compression piston (2); - an electric generator (19) adapted to circulate an electric current between the two compression pistons (2) and in said winding (11).,

2. Device according to claim 1, characterized in that said winding (11) is integrated into the material of the deformable cylindrical core (5).

3. Device according to one of the preceding claims, characterized in that the electrically conductive wire (12) is provided with an electrically insulating sheath (13).

4. Device according to one of the preceding claims, characterized in that the electrically conductive wire (12) is wound helically on an external cylindrical face of the deformable cylindrical core (5), from one end to the other of the deformable cylindrical core (5) along the clamping axis (3).

5. Device according to one of the preceding claims, characterized in that the first connection end (14A) and the second connection end (14B) respectively comprise a first conductor (15A) and a second conductor (15B) each passing through a portion of the corresponding compression piston (2).

6. Device according to one of the preceding claims, characterized in that that the compression pistons (2) each comprise an electrically conductive bar (8), adapted to be connected to the electric generator (19), and an electrically insulating cylindrical pad (9) adapted to be placed against the deformable cylindrical core (5).

7. Device according to one of the preceding claims, characterized in that the deformable cylindrical core (5) comprises an internal cylindrical portion (6) of a first material, fitted into an external annular portion (7) of a second material, different from the first material.

8. Device according to claim 7, characterized in that the elastic limit of the internal cylindrical portion (6) is lower than the elastic limit of the external annular portion (7).

9. Device according to one of claims 7 or 8, characterized in that the melting temperature of the external annular portion (7) is higher than the melting temperature of the internal cylindrical portion (6).

10. Device according to one of the preceding claims, characterized in that it comprises jaws (10) adapted to fix the ends of said tubular sample (1) of nuclear fuel cladding.

11. Device according to claim 10, characterized in that the jaws (10) are movable in spacing along the clamping axis (3).

12. Method for mechanical testing of a tubular sample of nuclear fuel cladding under accident conditions, implementing a device according to one of claims 1 to 11, characterized in that it comprises the following steps: - placing a tubular sample (1) of nuclear fuel cladding in the test space (4), the deformable cylindrical core (5) being inserted into the tubular sample (1) of nuclear fuel cladding, and said first connection end (14A) and second connection end (14B) being electrically connected to the corresponding compression piston (2); - a first sequence of temperature increase of the tubular sample (1) of nuclear fuel cladding, by heating said winding (11) by connecting the two compression pistons (2) to the electric generator (19);- a second compression sequence in which the two compression pistons (2) are disconnected from the electric generator (19) and compress the cylindrical core (5).;

13. Method according to claim 12, characterized in that the first sequence is carried out by heating said winding (11) by Joule effect.

14. Method according to claim 12 or 13, characterized in that the first sequence is carried out by induction heating of the tubular sample (1) of nuclear fuel cladding.

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