Patent ID: 12241858

DETAILED DESCRIPTION

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.

FIG.1shows a potentiometric oxygen sensor10according to the invention, of axisymmetric form about a central axis X.

As illustrated, this sensor10is attached to a wall20of 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.

This sensor10first comprises a tubular sensor body, the lower tube1of which is intended during functioning to be immersed in the liquid metal, and the upper tube9of which is intended to project outside the liquid metal (L). The tubes1,9of the sensor body are made, for example, of 304L or 316L type stainless steel.

The lower end of the sensor1comprises a container2constituting an electrolyte made of yttrium-doped or calcium-doped hafnia (HfO2), or of thoria (ThO2), which is optionally yttrium-doped or calcium-doped, or of yttrium-doped or calcium-doped zirconia (ZrO2). As illustrated, the electrolyte2is preferably configured in the form of a pocket.

The electrolyte contains a material3forming a reference electrode. This material3, which should preferentially be liquid at the operating temperature of the sensor, is made of indium (In) and in its oxide form (In2O3), or of bismuth (Bi) and in its oxide form (Bi2O3), or of gallium (Ga) and in its oxide form (Ga2O3) or of sodium and in its oxide form (Na2O).

According to the invention, the sensor10comprises an insert4made of zirconium (Zr), hafnium (Hf) or titanium (Ti), arranged between the lower tube1of the sensor body and the electrolyte2. This insert4is, on the one hand, attached to the tube1and, on the other hand, brazed onto the electrolyte2by a brazing joint5.

As stated hereinbelow, the brazing joint5is 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 insert4.

To ensure the attachment of the insert4to the lower tube1of the sensor body, a retaining ring6arranged around these two parts1,4is provided. This ring6also makes it possible to hold these parts during the production of the brazing joint5. Preferentially, this ring6is made of Fe—Ni or FeNi—Co alloy with coefficients of expansion close to that of the insert4and of the electrolyte2.

The measuring head7of the sensor is housed inside the sensor body1,9and comes into contact with the material3forming the reference electrode. This measuring head7thus makes it possible to measure the electrical potential difference in the reference electrode3. 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 tubes1,9of the sensor body.

The two tubes1,9of the sensor body are assembled together by means of a metallic joint connector8. As illustrated, this metallic joint connector8is envisaged to be arranged in the liquid metal (L). This connector, preferably made of stainless steel, with a metallic joint8, 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.

In the example illustrated, the male part80of the connector8is welded to the upper end of the lower tube1and the female part81is welded to the lower end of the upper tube9of the sensor body. The reverse arrangement may, of course, be envisaged.

An openwork metal sheath11, in the form of an end cap which allows the liquid metal to pass through, is screwed onto the retaining ring6, being arranged around the electrolyte2. This openwork sheath11enables, on the one hand, protection of the electrolyte2during 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 sheath11is made, for example, of 304L or 316L type stainless steel.

To ensure leaktight attachment of the sensor during functioning to the pipe20, a fixing flange12welded to the upper end of the tube9of the sensor body is attached by screwing to a fixing flange22of the pipe20. In order to ensure leaktightness, a metallic O-ring22is arranged in the fixing flange22of the pipe. The fixing flanges12,22are made, for example, of 304L or 316L type stainless steel.

A metal connector13is screwed onto the top of the fixing flange12so as also to hold by screwing a connector14of the measuring head7from which the electrical measuring wires lead toward an electrical connection of a high-impedance voltmeter.

The various successive steps of the process for manufacturing a potentiometric sensor10according to the invention which has just been described are now described.

Step a/: the brazing filler5is placed in contact inside the insert4. The brazing filler5is 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 insert4.

Step b/: fitting of the container forming the electrolyte2into the insert4is performed.

Step c/: the insert4and the lower tube9of the sensor body are attached together, by means of the retaining ring6.

Step d/: brazing is then performed between the electrolyte2and the insert4according to the following process.

A heat treatment above the melting point of the lowest-melting eutectic of the system consisting of the insert4material and the brazing filler5is first performed, so as to melt the latter, which, after cooling, forms the brazing joint5.

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.

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).

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 insert4—brazing filler). This makes it possible to enrich the liquid present at the interface with transition metal.

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.

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.

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

Hafnia and thoria are particularly stable ceramics that are very difficult to reduce in comparison with other ceramics such as Al2O3or ZrO2. It was notably observed, unexpectedly, that zirconium reduces these ceramics and that the oxygen obtained from this reduction dissolves in the brazing5, and also possibly a little in the insert4.

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.

The absence of a layer of oxide of the group 4 transition metal at the interface with the electrolyte2is ensured by sufficient dilution of this metal in the brazing element5and 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 electrolyte2and that the oxygen is dissolved in a large amount of the joint filler due to the presence of the insert.

To illustrate the brazing according to this step d/, an electrolyte2made of yttrium-doped hafnia is produced and is brazed with an insert made of zirconium4.

The electrolyte made of yttrium-doped hafnia2is a pocket with a tubular part having an outside diameter of 10 mm.

The zirconium insert4has a tubular part with an outside diameter of 12.5 mm.

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.

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

The thermal cycle performed for the brazing is shown in the graph illustrated inFIG.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 T1may 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.

FIGS.3and4show the interface obtained between the resulting brazing joint5and the electrolyte2.

Observation of the structure of this bonding zone shows the absence of interface cracks. The inner tube electrolyte2made of yttrium-doped hafnia, the brazing joint5and the outer insert4made of zirconium are seen, from right to left inFIG.3and from left to right inFIG.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.

Step e/: once the brazing is finished, the electrolyte2undergoes reoxidation by circulating a slightly oxidizing gas, for example <1% of O2in argon, at a temperature of between 500 and 800° C.

Step f/: the openwork sheath11is attached by screwing onto the retaining ring6.

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

Step h/: once the leakage test has been passed, the material3, i.e. the metal and its oxide form, forming the reference electrode is introduced into the bottom of the pocket2by passing it inside the lower tube1of the sensor body.

Step i/: the fixing flange12is then welded onto the upper tube9of the sensor body.

Step j/: the upper tube9is assembled with the lower tube1of the sensor body by means of the connector, the leaktightness being achieved by means of the metallic joint of the connector8.

Step k/: finally, the measuring head7is introduced into the sensor body1,9, the leaktightness being achieved by means of the screwed connector13at the end of the upper tube9of the sensor body.

The installation and functioning of a potentiometric sensor10according to the invention that has just been described are performed as follows.

Step 1/: the sensor10is introduced into an empty pipe20, i.e. a pipe containing no liquid metal, the leaktightness being achieved at the flange of the pipe12,21by means of the joint22.

Step 2/: the temperature of the pipe20is raised beyond the melting point of the liquid metal.

Step 3/: once this melting point has been exceeded, the pipe20is filled with liquid metal (L).

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

Step 5/: a potential measurement is taken with a high-impedance potentiometer between the measuring head7and the emerging part of the upper tube9of the sensor body, and a temperature measurement is taken on the thermocouple of the measuring head7.

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

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

The potentiometric oxygen sensor according to the invention may be used for measuring 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.

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

[1] L. Brissonneau, “New considerations on the kinetics of mass transfer in sodium fast reactors: An attempt to consider irradiation effects and low temperature corrosion”, Journal of Nuclear Materials, 423 (2012), pp 67-78.[2] Mason, L., N. S. Morrison, and C. M. Robertson. “The monitoring of oxygen, hydrogen and carbon in the sodium circuits of the PFR. inLiquid Metal Engineering and Technology”.1984. Oxford.[3] Osterhout, M. M. “Operating experience with on-line meters at experimental breeder reactor II(EBR II). inLIMET Liquid Metal Technology”.1980. Richland, USA, J. M. Dahlke.[4] Fouletier, J. and V. Ghetta, “Potentiometric sensors for high temperature liquids, inMaterials Issues for Generation IV Systems”, V. Ghetta, Editor. 2008, Springer Science. pages 445-459.[5] Jayaraman, V., Gnanasekaran, T., 2016. “Review—Evolution of the Development of In-Sodium Oxygen Sensor and Its Present Status”. J. Electrochem. Soc. 163, B395-B402.[6] Roy, J. C. and B. E. Bugbee, “Electrochemical oxygen sensor for measurement of oxygen in liquid sodium”. Nuclear Technology 1978. 39: pages 216-218.