Apparatus and method for mechanically testing tubular nuclear fuel cladding sample under accident conditions

Abstract

To provide an apparatus and a method for mechanically testing tubular nuclear fuel cladding sample under accident conditions.SOLUTION: An apparatus and a method for mechanically testing a tubular nuclear fuel cladding sample (1) under accident conditions comprise: a deformable cylindrical core (5) designed to be inserted into the tubular sample (1) so as to be positioned in a mechanical test space (4), the deformable cylindrical core (5) having, around its periphery, a winding (11) of an electrically conductive wire (12), the winding (11) having a first connection end (14A) designed to be electrically connected to one of the compression pistons (2) and a second connection end (14B) designed to be electrically connected to the other compression piston (2); and an electric generator (19) designed to cause an electric current to flow between the two compression pistons (2) and through the winding (11).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to the field of nuclear power, and in particular to the field of plant safety.The present invention relates to diagnostic and testing tools used to analyze the mechanical behavior of components used in nuclear reactors. [Background technology]

[0002] Nuclear reactors typically contain an assembly of tubular rods, called cladding, within which nuclear fuel pellets are stacked. This tubular cladding is where the fission reaction occurs, generating heat and fission products. The tubular cladding forms the first containment barrier against the diffusion of fuel and fission products.

[0003] For safety reasons, it is of utmost importance in nuclear power plants to know the mechanical behavior of the tubular cladding material under specific stresses representative of nominal operating conditions and of specific incidental or accidental conditions. To obtain this information, specific mechanical and thermal tests are carried out in shielded cells on tubular cladding samples from the reactor, and also on non-irradiated cladding samples in conventional laboratories.

[0004] In this context, the present invention aims to provide data on the mechanical behavior (in particular deformation at fracture) of tubular cladding samples under conditions that are of paramount importance, conditions that correspond to those encountered in a nuclear reactor under accident conditions.

[0005] With the aim of reproducing as faithfully as possible the actual nominal operating conditions of nuclear fuel cladding, French patent applications FR 3 041 097 and FR 3 101 421 describe solutions for compressing a medium placed inside a cladding sample in order to subject the cladding to biaxial mechanical stress. However, these devices do not allow mechanical testing under accident conditions, i.e., when temperatures rise very rapidly.

[0006] Other solutions propose combining mechanical testing elements with means to heat the cladding in order to recreate accident conditions. However, these solutions allow for sample temperature rise rates of about 100 to 200 °C per second, which is still far from real accident conditions, where temperatures can rise at rates of up to 1000 °C / s. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] French Patent Application Publication No. 3041097 [Patent Document 2] French Patent Application Publication No. 3101421 Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE INVENTION It is an object of the present invention to improve upon prior art devices and methods. [Means for solving the problem]

[0009] To this end, the present invention relates to an apparatus for mechanically testing tubular nuclear fuel cladding samples under accident conditions, the apparatus comprising two compression pistons that can be moved relative to each other along a clamping axis and that can define a mechanical testing space between them, the compression pistons comprising at least one electrically conductive part, the apparatus further comprising: a deformable cylindrical core designed to be inserted into the tubular nuclear fuel cladding tube sample and placed in the mechanical test volume, the deformable cylindrical core having a winding of electrically conductive wire around it, the winding having a first connection end designed to be electrically connected to one of the compression pistons and a second connection end designed to be electrically connected to the other of the compression pistons; a generator designed to cause an electric current to flow between the two compression pistons and in the windings; It has.

[0010] According to another subject, the invention relates to a method for mechanically testing tubular nuclear fuel cladding samples under accident conditions using a device such as described above, said method comprising the following steps: placing a tubular nuclear fuel cladding tube sample in the test space, inserting the deformable cylindrical core into the tubular nuclear fuel cladding tube sample, and electrically connecting the first connecting end and the second connecting end to corresponding compression pistons; a first sequence in which the two compression pistons are connected to a generator, thereby heating the windings and increasing the temperature of the tubular nuclear fuel cladding sample; a second compression sequence in which the two compression pistons are decoupled from the generator and compress the cylindrical core; Includes:

[0011] The present invention makes it possible to carry out mechanical testing of tubular nuclear fuel cladding samples under accident conditions by means of a moving compression piston which makes it possible to subject the samples to mechanical deformation, whether uniaxial or biaxial, accompanied by a very rapid temperature rise of the sample, which is similar to that under real accident conditions.

[0012] The present invention allows mechanical testing to be carried out at temperature ramp rates of approximately 400 to 800°C per second, which corresponds to temperature ramp rates representative of conditions that tubular nuclear fuel cladding may be subjected to during an accident.

[0013] The device according to the invention may comprise the following additional features, either individually or in combination: the windings are integrated into the material of the deformable cylindrical core; said electrically conductive wire is provided with an electrically insulating sheath; the conductive wire is wound helically on the outer cylindrical surface of the deformable cylindrical core from one end to the other along the clamping axis. The first and second connection ends respectively include first and second conductors passing through a corresponding portion of the compression piston. said compression pistons each comprising a conductive bar designed to be connected to a generator and an insulating cylindrical buffer designed to be placed against said deformable cylindrical core; The deformable cylindrical core comprises an inner cylindrical portion made of a first material, fitted with an outer annular portion made of a second material different from the first material. the yield strength of the inner cylindrical portion is lower than the yield strength of the outer annular portion; the melting point of the outer annular portion is higher than the melting point of the inner cylindrical portion; The device includes jaws designed to clamp the ends of the tubular nuclear fuel cladding sample. the jaws can be separated along the clamping axis;

[0014] The method according to the invention may include the following additional features, either individually or in combination: A first sequence is carried out by heating said winding by Joule heating. A first sequence is carried out by inductive heating of said tubular nuclear fuel cladding sample. [Brief explanation of the drawings]

[0015] Other features and advantages of the present invention will become apparent from the following non-limiting description, which refers to the accompanying drawings. [Figure 1] 1 is a cross-sectional view of a mechanical testing apparatus according to the present invention with a tubular nuclear fuel cladding sample in place. [Figure 2] FIG. 2 is a cross-sectional view of the device of FIG. 1 during mechanical testing. [Figure 3] 3 is a graph showing the temperature increase possible with the device of FIGS. 1 and 2. [Figure 4] 3 is a graph showing the heating rate as a function of temperature level during use of the apparatus of FIGS. 1 and 2.

[0016] Similar and common elements among the various embodiments are designated by the same reference numerals with reference to the figures. DETAILED DESCRIPTION OF THE INVENTION

[0017] Figure 1 shows a schematic cross-section of a mechanical testing device according to the invention, which is intended to carry out mechanical testing of tubular nuclear fuel cladding samples under accident conditions.

[0018] The mechanical testing apparatus is shown ready to perform testing, with a tubular nuclear fuel cladding sample 1 placed within the apparatus and ready to perform testing.

[0019] The mechanical testing apparatus comprises two compression pistons 2 movable relative to each other along a clamping axis 3. The compression pistons 2 define a mechanical testing space 4 between them in which the tubular sample 1 can be mechanically stressed after thermal conditioning.

[0020] The mechanical testing device further comprises a deformable cylindrical core 5. The cylindrical core 5 preferably consists of a ductile material or an assembly of ductile materials so that it exhibits a radial deformation (a local increase in its diameter) when compressed along the clamping axis 3.

[0021] In particular, in this embodiment, the cylindrical core 5 has an inner cylindrical portion 6 made of a material that deforms under low compressive force at the nominal operating temperature. For example, in this case, the inner cylindrical portion 6 is made of highly ductile aluminum with a low yield strength starting from 200°C. Therefore, the force required to compress the cylindrical core 5 is moderate so as not to damage the compression piston 2, and an excessively large pressing means is not required.

[0022] The cylindrical core 5 further comprises an outer annular portion 7, into which the inner cylindrical portion 6 is fitted in this example. Preferably, the outer annular portion 7 is made of a material that can withstand higher temperatures than the inner cylindrical portion 6. For example, the outer annular portion 7 is made of copper, which has a melting point of about 1085°C. At the test temperature, the outer annular portion 7 must have high ductility and deform with low compressive force, which is the case in this example for an aluminum / copper pair. Other suitable material pairs, one with a higher melting point than the other, may also be envisaged.

[0023] Compression pistons 2 are arranged on either side of the cylindrical core 5 and clamp it so that the cylindrical core 5 is compressed axially (ie along the clamping axis 3).

[0024] Preferably, the portion of the compression piston 2 designed to contact the cylindrical core 5 has approximately the same diameter as the cylindrical core 5 .

[0025] In this example, the compression piston 2 comprises an electrically conductive bar 8 and an electrically insulating cylindrical buffer 9. The bar 8 is made, for example, of metal. The diameter of the cylindrical buffer 9 is substantially equal to the diameter of the cylindrical core 5. In a variant, the cylindrical buffer 9 may be electrically conductive and may include an insulating coating on its surface in contact with the cylindrical core 5.

[0026] The mechanical testing device may further comprise means for positioning the tubular sample 1. In this example, these means consist of jaws 10 designed to be attached to each end of the tubular sample 1, for example by clamping, screwing, welding or other fastening means.

[0027] Thus, in addition to holding the tubular sample 1, the jaws 10 also allow the tubular sample 1 to participate in mechanical testing by clamping or axially stretching it, if biaxial mechanical testing is desired.

[0028] To conduct the test, a tubular sample 1 is prepared from a new (for property testing) or used fuel cladding tube. Nuclear reactors conventionally include a fuel cladding tube having a length of about 4 meters and a diameter of about 1 centimeter. The tubular sample 1 can be prepared by cutting the fuel cladding tube. In one example of this embodiment, the tubular sample 1 thus prepared has a length of about 3 cm (along the clamping axis 3 in FIG. 1 ) for a cylindrical core 5 having a length of about 14 mm.

[0029] The tubular sample 1 is placed in a mechanical testing space 4 and a cylindrical core 5 is inserted into the tubular sample 1. In this example, jaws 10 are further attached to both ends of the tubular sample 1.

[0030] The cylindrical core 5 further has a winding 11 formed therearound by a conductive wire 12. In this example, the wire 12 is wound helically around the outer periphery of the cylindrical core 5. In this way, the wire 12 is wound helically from one end to the other of the cylindrical core 5, i.e., over the entire length along the clamping axis 3.

[0031] In this example, the wire 12 is integrated into the material of the outer cylindrical surface of the cylindrical core 5, as shown in Figure 1. In this example, the cylindrical core 5 comprises an inner cylindrical portion 6 and an outer annular portion 7, and the wire 12 is embedded in the material of the outer annular portion 7.

[0032] Thus, the winding 11 forms a coil wound around the cylindrical core 5 .

[0033] Other configurations for forming such a coil with the windings 11 are possible. The wire 12 may be arranged, for example, in a helical groove formed on the outer surface of the cylindrical core 5 by an additive manufacturing or processing method. Furthermore, the wire 12 may be wound directly on the cylindrical core 5. The wire 12 may have a circular cross section, as in the example of FIG. 1, or may have, for example, a rectangular or flat cross section. The wire 12 may be, for example, a conductive strip wound on the surface of the outer cylindrical surface of the cylindrical core 5. The wire 12 may also be in the form of a sheet or coating on the outer surface of the cylindrical core 5.

[0034] The wire 12 may further comprise an electrically insulating sheath 13. The sheath 13 is designed to withstand high temperatures and is produced, for example, by a ceramic coating of the wire 12.

[0035] The coil formed by the winding 11 has a first end turn 11A and an opposite second end turn 11B, which are electrically connected to one side of the compression piston 2 by connecting ends 14A, 14B.

[0036] In this example, the first connection end 14A of the winding 11 has a first conductor 15A that passes through the cylindrical buffer 9 and is electrically connected to the conductive bar 8.

[0037] Similarly, at the other end of the winding 11, a second connection end 14B of the winding 11 has a second conductor 15B that passes through the cylindrical buffer 9 and is electrically connected to the corresponding bar 8.

[0038] This allows an electric circuit to be established between the two compression pistons 2 via the windings 11 forming a coil. Figure 1 shows schematically a generator 19 connected in this way to the two compression pistons 2. A switch 20 is arranged to show a schematic closed state of the circuit, whereby an appropriate alternating or direct current (see first and second embodiments below) flows through the windings 11, increasing the temperature of the tubular sample 1.

[0039] The connection between the connection ends 14A, 14B and the compression piston 2 can be established using any means such as connectors, electrical contacts, etc. The connection can also be established by soldering or crimping after the assembly is in place.

[0040] Starting with this assembly of FIG. 1, the tubular sample 1 is placed in a mechanical testing apparatus, and with the windings 11 electrically connected to the compression piston 2, the mechanical testing method can be carried out.

[0041] In the first sequence, called the temperature rise sequence (shown in FIG. 1, roughly with switch 20 closed), the tubular sample 1 is first subjected to thermal stress caused by a rapid increase in the temperature of its wall due to the windings 11.

[0042] According to a first embodiment, the winding 11 is heated by Joule heating, and as its temperature increases it heats the wall of the tubular sample 1. In this case, the wire 12 has an electrical resistance compatible with the Joule heating device, and a high direct current (e.g., tens of amperes) is applied between the two compression pistons 2. In one example embodiment, the wire 12 has a diameter of about 0.5 to 1 mm.

[0043] According to a second embodiment, the winding 11 heats the wall of the tubular sample 1 by induction. In this case, a current, for example a high frequency alternating current, designed to generate induction phenomena in the tubular sample is applied between the two compression pistons 2.

[0044] These two embodiments can be combined, for example, by driving the generator 19 alternately in Joule heating mode (high intensity) and in induction heating mode (high frequency AC).

[0045] In a second sequence, called the compression sequence (shown in FIG. 2, roughly with switch 20 open), as soon as the tubular sample 1 has undergone the desired temperature increase, the compression pistons 2 are pressed together and compress the cylindrical core 5. The compression pistons 2 are moved, for example, by a press.

[0046] Figure 2 shows the result of this compression sequence.

[0047] The cylindrical core 5 deforms radially, increasing its diameter and, similar to expanded nuclear fuel, stressing the walls of the tubular sample 1. This deformation, on the one hand, brings the tubular sample 1 to a high temperature and, on the other hand, takes place while this heating is also taking place at a high rate, thereby affecting the mechanical properties of the tubular sample 1 in a manner that is close to a real accident situation.

[0048] The present invention is adapted for biaxial mechanical testing, in which the compression pistons 2 are biased towards each other while the jaws 10 are also biased away from each other, so that the tubular sample 1 is not only deformed radially but also axially by being stretched between the two jaws 10.

[0049] In the second sequence, the current flowing through the windings 11 is interrupted, once the tubular sample 1 has already reached the desired temperature. During compression of the cylindrical core 5, the windings 11 and the connection ends 14A, 14B may be damaged or destroyed without affecting the mechanical test. In the event of such destruction, the cylindrical core 5 becomes a consumable part that must be replaced after each mechanical test.

[0050] 3 and 4 relate to an example of a mechanical test carried out using a tubular sample 1 having a length of 3 cm, a cylindrical core 5 having a diameter adjusted to the inner diameter of the tubular sample 1 having a length of 14 mm, and a wire 12 having a diameter of 0.8 mm. In this example, the wire 12 is made of copper-nickel. The test in this example is carried out by heating the winding 11 by Joule heating. In this example, the tubular sample 1 could be heated to a temperature of about 800°C during the first sequence at a heating rate of more than 500°C / s.

[0051] Figure 3 shows both the temperature in °C (curve 16 and the y-axis values ​​on the left side of the graph) and the heating rate in °C / s (curve 17 and the y-axis values ​​on the right side of the graph) measured on tubular sample 1 as a function of time (x-axis).

[0052] First, the tubular sample 1 was heated to 350°C to replicate the operating temperature under the nominal operating conditions of a nuclear reactor. After reaching a plateau at 350°C, a high current was passed through the wire 12, which was rapidly heated by Joule heating (in 131.5 seconds), and the current was cut off when the tubular sample 1 reached 800°C. After the current was cut off, the temperature of the tubular sample 1 rose to 870°C due to residual heat.

[0053] The wire diameter in this example is 0.8 mm, which represents a good compromise between a small diameter (if the wire 12 diameter is too small, the wire 12 may melt before heating the tubular sample 1) and a diameter that is too large to form the windings 11.

[0054] Curve 17 shows the evolution of the heating rate, which is the most important parameter. The highest heating rate reached is above 600 °C / s and is reached at a temperature of 600 °C. From this temperature, radiation from the outer surface of the tubular sample 1 seems to become significant and the heating rate decreases. Then, when the current is switched off, the heating rate drops sharply to 800 °C.

[0055] Figure 4 illustrates this same phenomenon of the change in heating rate. The graph in Figure 4 shows the change in heating rate (°C / s) as a function of the temperature reached (°C).

[0056] The heating rate is approximately 400°C / s to 800°C / s, and the average value over the temperature range of 500°C to 800°C (dotted line portion 18) is 540°C / s.

[0057] Variations can be made. In particular, the jaws 10 required for biaxial testing are optional if such biaxial testing is not desired. The jaws 10 can also consist of any other mechanical means designed to stretch the tubular sample 1 axially.

[0058] The winding 11 may have a winding profile other than the helical profile described above. In particular when operating by Joule heating, the shape of the circumference of the cylindrical core 5 may be described.

Claims

1. An apparatus for mechanically testing tubular nuclear fuel cladding samples (1) under accident conditions, comprising: The invention comprises two compression pistons (2) that are movable relative to each other along a clamping axis (3) and that define a mechanical test space (4) between them, the compression pistons (2) comprising at least one electrically conductive portion; a deformable cylindrical core (5) designed to be inserted into the tubular nuclear fuel cladding tube sample (1) and placed in the mechanical test space (4), the deformable cylindrical core (5) having a winding (11) of an electrically conductive wire (12) around it, the winding (11) having a first connection end (14A) designed to be electrically connected to one of the compression pistons (2) and a second connection end (14B) designed to be electrically connected to the other compression piston (2); a generator (19) designed to pass an electric current between the two compression pistons (2) and in the windings (11); having Device.

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

3. 3. Device according to claim 1 or 2, characterized in that the electrically conductive wire (12) is provided with an electrically insulating sheath (13).

4. 4. The device according to claim 1, wherein the conductive wire (12) is wound helically on the outer cylindrical surface of the deformable cylindrical core (5) from one end to the other along the clamping axis (3).

5. 5. The device according to claim 1, wherein the first connection end (14A) and the second connection end (14B) respectively comprise a first conductor (15A) and a second conductor (15B) passing through a corresponding part of the compression piston (2).

6. 6. The device according to any one of claims 1 to 5, characterized in that the compression pistons (2) each comprise a conductive bar (8) designed to be connected to the generator (19) and an insulating cylindrical buffer (9) designed to be placed opposite the deformable cylindrical core (5).

7. 7. The device according to any one of claims 1 to 6, characterized in that the deformable cylindrical core (5) comprises an inner cylindrical portion (6) made of a first material, fitted with an outer annular portion (7) made of a second material different from the first material.

8. 8. The device according to claim 7, wherein the yield strength of the inner cylindrical portion (6) is lower than the yield strength of the outer annular portion (7).

9. 9. Device according to claim 7 or 8, characterized in that the melting point of the outer annular part (7) is higher than the melting point of the inner cylindrical part (6).

10. An apparatus according to any one of claims 1 to 9, characterized in that it comprises jaws (10) designed to clamp the ends of the tubular nuclear fuel cladding sample (1).

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

12. A method for mechanically testing a tubular nuclear fuel cladding sample under accident conditions using an apparatus according to any one of claims 1 to 11, comprising the steps of: - placing a tubular nuclear fuel cladding tube sample (1) in the test space (4), inserting the deformable cylindrical core (5) into the tubular nuclear fuel cladding tube sample (1), and electrically connecting the first connecting end (14A) and the second connecting end (14B) to corresponding compression pistons (2); a first sequence in which the two compression pistons (2) are connected to a generator (19) to heat the windings (11) and thereby increase the temperature of the tubular nuclear fuel cladding sample (1); - a second compression sequence, which decouples the two compression pistons (2) from the generator (19) and compresses the cylindrical core (5); A method comprising:

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

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

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