OXYGEN POTENTIOMETRIC PROBE, FOR THE MEASUREMENT OF THE OXYGEN CONCENTRATION OF A LIQUID METAL, APPLICATION TO THE MEASUREMENT OF OXYGEN IN LIQUID SODIUM OF A NUCLEAR REACTOR OF TYPE RNR-Na

Abstract

A potentiometric oxygen sensor for measuring the oxygen concentration of a liquid metal, including: a metal tube forming at least one sensor body part; an electrochemical subassembly containing an electrolyte, intended to be in contact with the liquid metal, and a reference electrode contained in the electrolyte, the electrolyte being made of yttrium-doped or calcium-doped hafnia (HfO.sub.2), or of thoria (ThO.sub.2), which is optionally yttrium-doped or calcium-doped, or of yttrium-doped or calcium-doped zirconia (ZrO.sub.2), the reference electrode containing at least one metal and its oxide form at the operating temperature of the sensor, an insert made of a transition metal from group 4 of the Periodic Table or an alloy thereof, arranged between the sensor body part and the electrolyte, the insert being attached to the sensor body part and brazed onto the electrolyte by a brazing joint.

Claims

1. A potentiometric oxygen sensor for measuring the oxygen concentration of a liquid metal, comprising: a metal tube forming at least one sensor body part; an electrochemical subassembly comprising an electrolyte, intended to be in contact with the liquid metal, and a reference electrode contained in the electrolyte, the electrolyte being made of yttrium-doped or calcium-doped hafnia (HfO.sub.2), or of thoria (ThO.sub.2), which is optionally yttrium-doped or calcium-doped, or of yttrium-doped or calcium-doped zirconia (ZrO.sub.2), the reference electrode comprising at least one metal and its oxide form at the operating temperature of the sensor; and an insert made of a transition metal from group 4 of the Periodic Table or an alloy thereof, arranged between the sensor body part and the electrolyte, the insert being attached to the sensor body part and brazed onto the electrolyte by a brazing joint, the coefficient of thermal expansion of the insert being close to the coefficient of thermal expansion of the electrolyte and below the coefficient of thermal expansion of the sensor body part, the stiffness of the insert being higher than the stiffness of the sensor body part.

2. The potentiometric oxygen sensor according to claim 1, further comprising a retaining ring, arranged both around the insert and the sensor body part, the ring being adapted to hold said objects during the production of the brazing joint.

3. The potentiometric oxygen sensor according to claim 2, the retaining ring being made of a material whose coefficient of thermal expansion is close to that of thoria or of hafnia.

4. The potentiometric oxygen sensor according to claim 1, the sensor body comprising two tubular parts, the lower part being the one attached to the electrolyte and the upper part being intended to project outside the liquid metal, the two tubular parts being assembled together by a metallic joint connector, the male part of which is integrally fastened to the end of the lower or upper part of the sensor body and, respectively, the female part of which is integrally fastened to the end of the upper or lower part of the sensor body.

5. The potentiometric oxygen sensor according to claim 1, further comprising an openwork metal sheath arranged around the electrolyte, the openwork sheath being adapted to allow the liquid metal to pass through.

6. The potentiometric oxygen sensor according to claim 5, further comprising a retaining ring, arranged both around the insert and the sensor body part, the ring being adapted to hold said objects during the production of the brazing joint, the openwork sheath being attached to the ring.

7. The potentiometric oxygen sensor according to claim 1, the transition metal of the insert being zirconium (Zr), hafnium (Hf) or titanium (Ti).

8. The potentiometric oxygen sensor according to claim 1, the brazing joint being made of nickel (Ni), copper (Cu) or an alloy thereof (Ni—Cu).

9. The potentiometric oxygen sensor according to claim 1, the sensor body housing a measuring head suitable for measuring the electrical potential difference in the reference electrode and also for measuring the temperature.

10. A measurement method, comprising measuring a concentration of oxygen in liquid sodium or in lead-lithium (Pb—Li) eutectic alloy with the potentiometric oxygen sensor according to claim 1, wherein the sensor body is made of stainless steel, the insert is made of zirconia, and the electrolyte is made of yttrium-doped or calcium-doped hafnia (HfO.sub.2) or of yttrium-doped or calcium-doped thoria (ThO.sub.2).

11. A measurement method, comprising measuring a concentration of oxygen in liquid lead and its alloys with heavy metals with the potentiometric oxygen sensor according to claim 1, wherein the sensor body is made of stainless steel, the insert is made of titanium, and the electrolyte is made of zirconia.

12. A unclear fission reactor cooled with liquid metal comprising at least one potentiometric oxygen sensor according to claim 1.

13. A nuclear fusion reactor comprising at least one potentiometric oxygen sensor according to claim 1.

14. The potentiometric oxygen sensor according to claim 3, the retaining ring being made of an iron-nickel (Fe—Ni) alloy.

15. The potentiometric oxygen sensor according to claim 6, the openwork sheath being attached to the ring by screwing.

16. The method according to claim 10, wherein the method is operated at a temperature between 250° C. and 450° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] FIG. 1 is a schematic view in longitudinal cross section of a potentiometric oxygen sensor, the figure showing the sensor positioned on and attached to a pipe containing the liquid metal for which the sensor is intended to measure the oxygen concentration.

[0067] FIG. 2 is a graph representing the curve of temperature as a function of time during a brazing step to produce a sensor according to the invention.

[0068] FIG. 3 is an image obtained by scanning electron microscopy, at the interface of a brazing joint made of a nickel alloy and of an electrolyte made of yttrium-doped hafnia of a sensor according to the invention.

[0069] FIG. 4 is also an image obtained by scanning electron microscopy, with magnification relative to the preceding figure, at the interface of a brazing joint made of a nickel alloy and of an electrolyte made of yttrium-doped hafnia of a sensor according to the invention.

DETAILED DESCRIPTION

[0070] It is pointed out here that throughout the present patent application, the terms “lower”, “upper”. “over”, “under”, “inner”, “outer”, “internal” and “external” should be understood with reference to a potentiometric sensor according to the invention in the vertically fixed operating configuration, in longitudinal cross section view along its longitudinal axis of symmetry X.

[0071] FIG. 1 shows a potentiometric oxygen sensor 10 according to the invention, of axisymmetric form about a central axis X.

[0072] As illustrated, this sensor 10 is attached to a wall 20 of a pipe containing a liquid metal (L), typically liquid sodium, under the temperature and pressure conditions encountered in a primary loop of an SFR reactor, the oxygen content of which it is desired to measure.

[0073] This sensor 10 first comprises a tubular sensor body, the lower tube 1 of which is intended during functioning to be immersed in the liquid metal, and the upper tube 9 of which is intended to project outside the liquid metal (L). The tubes 1, 9 of the sensor body are made, for example, of 304L or 316L type stainless steel.

[0074] The lower end of the sensor 1 comprises a container 2 constituting an electrolyte made of yttrium-doped or calcium-doped hafnia (HfO.sub.2), or of thoria (ThO.sub.2), which is optionally yttrium-doped or calcium-doped, or of yttrium-doped or calcium-doped zirconia (ZrO.sub.2). As illustrated, the electrolyte 2 is preferably configured in the form of a pocket.

[0075] The electrolyte contains a material 3 forming a reference electrode. This material 3, which should preferentially be liquid at the operating temperature of the sensor, is made of indium (In) and in its oxide form (In.sub.2O.sub.3), or of bismuth (Bi) and in its oxide form (Bi.sub.2O.sub.3), or of gallium (Ga) and in its oxide form (Ga.sub.2O.sub.3) or of sodium and in its oxide form (Na.sub.2O).

[0076] According to the invention, the sensor 10 comprises an insert 4 made of zirconium (Zr), hafnium (Hf) or titanium (Ti), arranged between the lower tube 1 of the sensor body and the electrolyte 2. This insert 4 is, on the one hand, attached to the tube 1 and, on the other hand, brazed onto the electrolyte 2 by a brazing joint 5.

[0077] As stated hereinbelow, the brazing joint 5 is produced by a brazing filler made of nickel, copper or an alloy thereof (Ni—Cu) in the form of a strip or at least of a wire or of a deposit applied beforehand of the brazing onto the inside diameter of the insert 4.

[0078] To ensure the attachment of the insert 4 to the lower tube 1 of the sensor body, a retaining ring 6 arranged around these two parts 1, 4 is provided. This ring 6 also makes it possible to hold these parts during the production of the brazing joint 5. Preferentially, this ring 6 is made of Fe—Ni or FeNi—Co alloy with coefficients of expansion close to that of the insert 4 and of the electrolyte 2.

[0079] The measuring head 7 of the sensor is housed inside the sensor body 1, 9 and comes into contact with the material 3 forming the reference electrode. This measuring head 7 thus makes it possible to measure the electrical potential difference in the reference electrode 3. Advantageously, it may be envisaged for it also to measure the temperature. The sensitive element(s) of the measuring head are made of molybdenum or electrical wires. Preferably, this (these) sensitive element(s) are housed in a ceramic sheath, such as an alumina sheath, so as to ensure the electrical insulation with the metal tubes 1, 9 of the sensor body.

[0080] The two tubes 1, 9 of the sensor body are assembled together by means of a metallic joint connector 8. As illustrated, this metallic joint connector 8 is envisaged to be arranged in the liquid metal (L). This connector, preferably made of stainless steel, with a metallic joint 8, preferably made of copper or nickel, advantageously makes it possible to perform a brazing leaktightness test. This test is performed, for example, by connecting a helium leakage detector onto the connector. A vacuum is produced in the sensor body by means of the detector pump, and helium is then injected outside the sensor. In the event of leakage, helium penetrates into the sensor body and is sucked toward the detector counter. Care is obviously taken to have very good leaktightness at the connection onto the sensor body, so as not to generate an artificial leak.

[0081] In the example illustrated, the male part 80 of the connector 8 is welded to the upper end of the lower tube 1 and the female part 81 is welded to the lower end of the upper tube 9 of the sensor body. The reverse arrangement may, of course, be envisaged.

[0082] An openwork metal sheath 11, in the form of an end cap which allows the liquid metal to pass through, is screwed onto the retaining ring 6, being arranged around the electrolyte 2. This openwork sheath 11 enables, on the one hand, protection of the electrolyte 2 during the handling of the sensor and, on the other hand, prevents the dispersion of shards in the liquid metal in the possible event of breakage. The openwork sheath 11 is made, for example, of 304L or 316L type stainless steel.

[0083] To ensure leaktight attachment of the sensor during functioning to the pipe 20, a fixing flange 12 welded to the upper end of the tube 9 of the sensor body is attached by screwing to a fixing flange 22 of the pipe 20. In order to ensure leaktightness, a metallic O-ring 22 is arranged in the fixing flange 22 of the pipe. The fixing flanges 12, 22 are made, for example, of 304L or 316L type stainless steel.

[0084] A metal connector 13 is screwed onto the top of the fixing flange 12 so as also to hold by screwing a connector 14 of the measuring head 7 from which the electrical measuring wires lead toward an electrical connection of a high-impedance voltmeter.

[0085] The various successive steps of the process for manufacturing a potentiometric sensor 10 according to the invention which has just been described are now described.

[0086] Step a/: the brazing filler 5 is placed in contact inside the insert 4. The brazing filler 5 is made in the form of a strip and/or at least of a wire made of nickel, copper or an alloy thereof (Ni—Cu) if it is not already present as a deposit on the insert 4.

[0087] Step b/: fitting of the container forming the electrolyte 2 into the insert 4 is performed.

[0088] Step c/: the insert 4 and the lower tube 9 of the sensor body are attached together, by means of the retaining ring 6.

[0089] Step d/: brazing is then performed between the electrolyte 2 and the insert 4 according to the following process.

[0090] A heat treatment above the melting point of the lowest-melting eutectic of the system consisting of the insert 4 material and the brazing filler 5 is first performed, so as to melt the latter, which, after cooling, forms the brazing joint 5.

[0091] The brazing thermal cycle successively includes: a temperature rise, a steady stage at the brazing temperature (“high” stage) and a cooling ramp down to a temperature below the melting point of the brazing. Preferably, the cooling is performed down to room temperature. The term “room temperature” means a temperature of the order of 20 to 25° C.

[0092] The steady stage at the brazing temperature is, for example, of the order of about 10 minutes (for example from 10 minutes to 30 minutes).

[0093] The brazing temperature is below the melting points of the materials to be assembled. More particularly, it is above the theoretical temperature of the lowest-melting eutectic (transition metal of the insert 4—brazing filler). This makes it possible to enrich the liquid present at the interface with transition metal.

[0094] Advantageously, brazing is performed at a moderate temperature to limit the thermomechanical stresses due to the cooling after the assembly cycle. The assembly produced may be used up to temperatures of the order of 900° C.

[0095] Preferably, advantageously, the steady-stage temperature is at least 40° C. above the eutectic formation temperature. For example, for a brazing filler made of pure nickel, a steady stage at about 1000° C. will be chosen, and for a brazing filler made of copper, a steady-stage temperature of about 930° C. will be chosen.

[0096] The brazing is preferably performed in an oxygen-free environment, for example by brazing under secondary vacuum (for example at a total pressure of 10.sup.5 mbar) or under an oxygen-purified neutral gas.

[0097] Hafnia and thoria are particularly stable ceramics that are very difficult to reduce in comparison with other ceramics such as Al.sub.2O.sub.3 or ZrO.sub.2. It was notably observed, unexpectedly, that zirconium reduces these ceramics and that the oxygen obtained from this reduction dissolves in the brazing 5, and also possibly a little in the insert 4.

[0098] Zirconium is not only an active element that is capable of partially reducing a ceramic at elevated temperature, but also makes it possible to obtain a brazing composition that is capable of forming, for example, with nickel, copper and iron eutectics below 1000° C.

[0099] The absence of a layer of oxide of the group 4 transition metal at the interface with the electrolyte 2 is ensured by sufficient dilution of this metal in the brazing element 5 and the insufficient time during the brazing cycle to form this layer. Thus, relative to the conventional reactive brazing processes, this layer is not formed due to the fact that the brazing filler is not in direct contact with the electrolyte 2 and that the oxygen is dissolved in a large amount of the joint filler due to the presence of the insert.

[0100] To illustrate the brazing according to this step d/, an electrolyte 2 made of yttrium-doped hafnia is produced and is brazed with an insert made of zirconium 4.

[0101] The electrolyte made of yttrium-doped hafnia 2 is a pocket with a tubular part having an outside diameter of 10 mm.

[0102] The zirconium insert 4 has a tubular part with an outside diameter of 12.5 mm.

[0103] The brazing filler is introduced in the form of a wire 0.45 mm in diameter and 7 mm long. It is an Ni201 wire.

[0104] The brazing filler is placed at the two ends of the brazing zone (a turn of wire at each end, introduced into a groove).

[0105] The thermal cycle performed for the brazing is shown in the graph illustrated in FIG. 2. In this cycle, the temperature increase is stopped at a steady stage, just below the eutectic temperature (Te), to homogenize the temperature, for example for 30 minutes at 900° C. Typically, the homogenization temperature T1 may be below Te−20° C. The steady stage may be from 10 to 30 minutes at a brazing temperature, T2, equal to Te+40° C.

[0106] FIGS. 3 and 4 show the interface obtained between the resulting brazing joint 5 and the electrolyte 2.

[0107] Observation of the structure of this bonding zone shows the absence of interface cracks. The inner tube electrolyte 2 made of yttrium-doped hafnia, the brazing joint 5 and the outer insert 4 made of zirconium are seen, from right to left in FIG. 3 and from left to right in FIG. 4. A strong reserve of pure zirconium is observed close to the interface. Sufficient dilution of the zirconium tube in the brazing which forms during the steady stage at high temperature, and also the greater attraction for oxygen of zirconium than of nickel, result in this configuration, which proved to be favorable for an absence of cracks at the brazing/electrolyte interface.

[0108] Step e/: once the brazing is finished, the electrolyte 2 undergoes reoxidation by circulating a slightly oxidizing gas, for example <1% of O.sub.2 in argon, at a temperature of between 500 and 800° C.

[0109] Step f/: the openwork sheath 11 is attached by screwing onto the retaining ring 6.

[0110] Step g/: in order to check the leaktightness of the sensor assembly, a helium leakage test is performed.

[0111] Step h/: once the leakage test has been passed, the material 3, i.e. the metal and its oxide form, forming the reference electrode is introduced into the bottom of the pocket 2 by passing it inside the lower tube 1 of the sensor body.

[0112] Step i/: the fixing flange 12 is then welded onto the upper tube 9 of the sensor body.

[0113] Step j/: the upper tube 9 is assembled with the lower tube 1 of the sensor body by means of the connector, the leaktightness being achieved by means of the metallic joint of the connector 8.

[0114] Step k/: finally, the measuring head 7 is introduced into the sensor body 1, 9, the leaktightness being achieved by means of the screwed connector 13 at the end of the upper tube 9 of the sensor body.

[0115] The installation and functioning of a potentiometric sensor 10 according to the invention that has just been described are performed as follows.

[0116] Step 1/: the sensor 10 is introduced into an empty pipe 20, i.e. a pipe containing no liquid metal, the leaktightness being achieved at the flange of the pipe 12, 21 by means of the joint 22.

[0117] Step 2/: the temperature of the pipe 20 is raised beyond the melting point of the liquid metal.

[0118] Step 3/: once this melting point has been exceeded, the pipe 20 is filled with liquid metal (L).

[0119] Step 4/: the liquid metal is then raised to the desired temperature.

[0120] Step 5/: a potential measurement is taken with a high-impedance potentiometer between the measuring head 7 and the emerging part of the upper tube 9 of the sensor body, and a temperature measurement is taken on the thermocouple of the measuring head 7.

[0121] Step 6/: the oxygen activity in the liquid metal (L) can then be deduced from the Nernst law, recalled in the preamble.

[0122] Other variants and improvements may be applied without, however, departing from the scope of the invention.

[0123] The potentiometric oxygen sensor according to the invention may be used for meastuing the oxygen content of a liquid metal, which may be sodium (Na) or a sodium-potassium (Na—K) alloy, or lead (Pb), or a lead-bismuth (Pb—Bi) alloy or a lead-lithium (Pb—Li) alloy.

[0124] The invention is not limited to the examples that have just been described; features of the illustrated examples may notably be combined together within variants not illustrated.

LIST OF CITED DOCUMENTS

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