FR2470381A1 - Determining local state of fluid in pressure vessel - using thermoelectric heat detector to measure heat exchange coefft. - Google Patents

Abstract

Process for determining the local state of a fluid medium inside a vessel comprises placing at least 1 heat conducting element in the vessel and heating. The coefft. of thermal exchange with the fluid is measured at least twice, and the values are used to determine the relative state of the fluid. The appts. is also claimed. The appts. can be used for gas and liq. media, partic. in PWR nuclear reactors. Any change occurring in the fluid is easily detected, e.g. prodn. of bubbles or emulsions, or decomposition of water in a reactor vessel.

Description

The object of the present invention is a crc, sold e '.

a device for determining the state of a fluid medium in an enclosure.

More precisely, the present invention relates to the qualitative determination in an enclosure containing a fluid medium, this medium being liable to change as regards the fluids which it contains, this determination involving the thermal conductivity of the various fluids ( gases or liquids) contained in the enclosure.

It is known that in nuclear reactors of the pressurized water type (PiiR), the pressure resistance vessel contains in normal operation exclusively water in liquid form. It is also known that during certain accidental operations of this type of reactor, the origins of which may be various, certain fluids in gaseous form may appear in the original liquid medium. It can mainly be water vapor or gas resulting in particular from the decomposition of the water in the reactor vessel.

 The invention makes it possible to detect in particular the appearance of such a phenomenon. However, it is of course not limited to this type of nuclear reactors, nor even to nuclear reactors or their ancillary components such as pressurizers.

 In general, the invention relates to the detection of local changes of state occurring in a liquid medium, the changes in state consisting in the local appearance of a fluid (liquid or gas) whose thermal conductivity is different. that of the original fluid medium. However, the invention applies more particularly to this detection of the localized appearance of a gas - in an initially liquid medium. Of course, this appearance of gas can be in the aspect of bubble formation or creation of an emulsion.

Because the detection is local, the invention can be applied to two main situations
Either at the start, the entire enclosure is filled with a single fluid and it is a question of detecting the appearance of a second fluid, or at the start there are two distinct fluids and the invention then makes it possible to follow the evolution of the interface between these two fluids.

 Returning to the problem of pressurized water reactors, we know that no special device is provided for detecting the appearance of a gas in the tank.

It can therefore be seen that the invention, according to one of its applications mentioned above, is of particular interest.

 To carry out this localized determination of the state of the fluid medium, a heat conducting element is placed in this fluid medium; this element is heated and the coefficient of heat exchange between this element and the fluid medium is measured. In other words, we analyze the way in which the heat flux is evacuated from the element in the fluid medium. Depending on the nature of the fluid which is in contact with the element, this evacuation is more or less rapid or significant.

 Depending on the application envisaged, the heat source may be internal to the enclosure. This is the case when making this determination in a nuclear reactor which is in operation or which has a certain residual power. In other cases, an external heat source may be used: for example, a device for Electric heating.

 It should be added that this qualitative determination is always made by relative measurement, that is to say by comparison of at least two measurements. Either, these two heat exchange coefficient measurements are made at the same time at two different points of the enclosure, the comparison and processing of these measurements can then make it possible to detect the presence of two different fluids at the two measurement points; either these two measurements are carried out at the same point at two different times. We can then deduce, if this is the case, a change of state over time at the point considered.

It should be noted that in the case of a nuclear reactor and in particular inside the core, it is known that the axial and radial distributions of the temperatures are by no means uniform. When two measurements of exchange coefficient are carried out in two points of the heart, we can therefore be led to "correct" the measurements before comparing it to take into account the distribution of temperatures.

 In fact, a reading will be representative of a change of state if the two or n measurements taken deviate significantly from the temperature distribution curve.

 to determine the local state of the fluid medium in a large enclosure, it is necessary to have a large number of measurement points in this enclosure. This can be achieved by installing a large number of fixed elements in the enclosure. It is also possible to use a smaller number of elements which are moved inside the enclosure in well-marked positions.

By this method, it is possible, in the tank of a PWR, to detect the appearance in water of a liquid vapor emulsion, or of dry vapor or of another gas. In addition, the position of the elements detecting these new states allows to obtain a certain localization.

 As explained above, the method consists in measuring the heat exchange coefficient between the localized element which is heated and the fluid medium which surrounds it. To carry out this measurement which will only be used by comparison with another measurement, it is of course necessary to equip each element with the means for carrying out this measurement, that is to say to deliver a preferably electrical signal the magnitude of which is representative of this exchange coefficient. According to the invention, this measurement is made by giving the measuring element an elongated shape and by placing inside the element two temperature measuring devices located at two different points on the element.

A device which is particularly suitable for this measurement is the apparatus known under the name of "gamma radiation thermometer" or "y-thermometer". These devices are described in particular in French patent applications 77 08657 of 23 March 1977 and 79 05739 of March 6, 1979 filed in the name of the applicant for "Device for measuring local power in a nuclear reactor fuel assembly
It is however important to note that these y-thermometers are described for a very specific use which is to locally measure the quantity of heat produced within the fuel in an assembly of the reactor core. We therefore see that even if we put implemented in this embodiment the same devices, the use of these devices is completely different since it involves measuring the heat exchange coefficient. Nevertheless, one could advantageously refer to these texts for certain structural details.

 However, the measuring elements may not be literally y-thermometers. The measuring element may consist simply of a cylindrical bar of conductive material in which two temperature sensors have been placed in the same section plane of the bar, one of the sensors being arranged along the axis of the bar and the other near its periphery. The value of the difference in measured temperatures is in relative value representative of the coefficient of heat exchange with the fluid medium and therefore representative of the state of the fluid medium.

In any case, the invention will be better understood on reading the following description of several embodiments of the measuring elements, these examples illustrating at the same time the implementation of the method which is the subject of the invention. The description refers to the appended figures in which there is shown
- in FIG. 1, a simplified view in vertical section of a PWR reactor vessel showed possible implantations of the measuring elements
- in FIG. 2, a simplified view of a PWR fuel assembly showing the introduction of measuring devices in the form of a rod comprising several measuring elements inside the guide tubes of the fuel assembly
- In Figure 3, a partial view in vertical section of a first embodiment of the measuring element which corresponds to a so-called "wet" y-thermometer
- In Figure 4, a partial view in vertical section of a second embodiment of the measuring element which corresponds to a so-called "dry" y-thermometer
- In Figure 5, a vertical sectional view of a measuring element corresponding to an individual y-thermometer type "wet", this lt'ment being r.lobile;
- In Figure 6, a vertical sectional view of a measuring element corresponding to an individual y-thermometer of the "dry" type, this element being mobile;
- Figures 7 and 8, views in vertical section of the measuring elements according to embodiments in which the element has the shape of a bar without chamber; and
- In Figure 9, a vertical sectional view of the lower part of a measuring element comprising both y-thermometers type! 'dry "and type" wet ".

 In the following description, we consider the case where the enclosure is a PWR type reactor which normally contains pressurized water. It is therefore a question of detecting a possible appearance of water vapor or of another gas.

Similarly, in this case, there is a heat source which is either the power released by the core, or the residual power of the reactor. However, as already indicated, the invention could be applied to other enclosures containing another liquid. At the same time, it should be noted that in some cases, it may be necessary to use an external heat source to heat the measuring elements.

In FIG. 1, the tank 2 of a nuclear reactor is shown closed by its bottom 4 and its cover 6.

The core 8 with its fuel assemblies 10 has also been shown, the core 8 being supported by the core basket 12. Above the core 8, the upper internal structures 13 have been symbolized for serving as a guide for the control rods .

 This figure shows various possible layouts for the measuring devices according to the invention. The reference D1 represents a measuring device in the form of a rod comprising several successive measuring elements. This rod D1 or rod penetrates through the cover 6 and is introduced into the heart. The reference D2 illustrates a shorter measuring device which passes through the cover and which is only used to carry out a detection at the top of the tank. The reference D3 illustrates a measuring device in the form of a rod which crosses the bottom 4 of the pressure-resistance tank and enters the fuel assemblies.

In the perspective view of FIG. 2, the reference 10 schematically designates a fuel assembly structure comprising in known manner a series of sheathed fuel needles or rods 14 regularly distributed, the geometry of the network according to which these rods are arranged. being maintained by cellular grids 16 mounted in the assembly at regular intervals.

 The bundle of pencils 14 comprises, at a suitably determined location in the network, a protective tube 18, allowing the introduction inside of it of the measuring device D, the latter being placed in this tube 18 by its lower part or its upper part and making it possible to carry out a local measurement of the state of the fluid medium at different levels in the assembly, these levels being identified in the drawing by a series of arrows 20.

 We will now describe several embodiments of the measuring devices according to the invention. However, since these devices make extensive use of the structure of y-thermometers which is well known, in particular in the light of the two patent applications already cited, these devices will not be described in detail, but we will especially focus on explain the particular operation within the framework of the invention.

In Figure 3, there is shown a measuring device similar to a wet type y-tnermometer. The device consists of a cylindrical rod 30 of small diameter and great length made of a material which is a good conductor of heat and electricity, preferably made of a metal, for example stainless steel, or of a suitable conductive alloy or ceramic. , and a tube 32 in the protective glove of the rod 30 threaded into this tube.

 This elongated rod 30 thus capable of extending over the entire height of the outer protective tube 18 in the fuel assembly 8, once it is placed in this tube 32 and in line with the areas where the measurements are to be made, reduced section portions 34 surrounded by sealed annular chambers 36 delimited between these portions 34, the internal wall of the tube 32 and the portions 38 of larger diameter.

 According to the invention also, the cylindrical rod 30 has a longitudinal axial channel 40 'extending over the entire height of this rod and in which is disposed a set of differential thermocouples such as 42, each of these thermocouples being associated with one of the measurement zones identified by the arrows 20 (FIG. 2) along the length of the tube 18, so that a hot junction 44 of thermocoup is substantially arranged in the middle of the height of each part 34 of reduced section and a junction cold 46 beyond the end of the corresponding chamber 36, in the part 38 of the rod 30 of larger section.

 During normal operation of the reactor, that is to say when it contains only water under liquid fore, the differential thermocouple gives an indication representative of the local power. I1 operates as a conventional y-thermometer. If it appears in the space between the guide tube 18 and the outer tube 32 of the device for measuring the vapor or a gas, there is a change in the heat exchange, in particular at the part of the diameter more important 38. The temperature difference measured by the thermocouple 42 (welds 44 and 46) is then modified. There is therefore a modification of the electrical signal delivered by the thermocouple which thus indicates that there has been a modification of the state of the fluid medium at the level of the measurement element considered. Of course, this measurement could also have been compared to the measurement made by a measuring element which remains surrounded by water.

 In Figure 4, there is shown another embodiment of the device corresponding to a dry type y-ternometer. This device is identical to that of FIG. 3, but it does not include the tube 32. Consequently, there remain the reduced diameter portions 34 but there is no longer, strictly speaking, chamber 36 since these chambers are filled with the heat transfer liquid.

If water fills the space between the rod 30 and the guide tube 18, the temperature difference detected by the differential thermocouple 42 is practically zero. There is therefore practically no electrical signal. If on the contrary, water vapor or a gas enters the space between the rod 30 and the tube 18 at the welds 44 and 46, there is a modification of the heat exchange coefficient and it is collected at the outlet from the differential thermocouple a signal which is representative of the temperature difference at the level of the two welds and therefore of the change of state of the fluid medium.

In FIG. 9, a hybrid measuring device has been represented, corresponding in its upper part to the "wet" type and in its lower part to the "dry" type.

The upper part 30a of the bar 30 is provided with chambers 36a corresponding to the measurement elements according to the wet type with the external tube 32. On the contrary, in the lower part 30b of the bar 30, there are only parts of reduced diameter 34b corresponding to the "dry" type.

The advantage of this hybrid structure is as follows: the upper part 30a is housed in the fuel assemblies 10. It therefore gives normal operation indications on the local power according to the conventional operating mode of y-thermometers. On the contrary, the lower part of the device operates only as a device for detecting the state of the fluid medium. This can in particular be used to detect complete dewatering of the heart.

In the previous embodiments, each measuring device is in the form of an elongated bar which in fact comprises several successive measuring elements, each measuring element consisting of a portion of reduced diameter and a differential thermocouple. These devices are permanently placed in the reactor.

In Figures 5 and 6, there are shown two embodiments of the measuring device that could be described as individual. Indeed, these devices each have only one measurement element, these devices being moved in the guide tubes 18. These two measurement elements in fact correspond to a half-length of chamber according to the embodiments shown respectively in FIGS. 3 and 4.

 In the embodiment according to FIG. 5 which corresponds to the “wet11” type, there is a base 50 extended by a rod 52 of reduced diameter. The rod 52 is surrounded by a wall 54 in the form of a glove finger fixed to its base on the base 50. This enclosure thus defines a sealed chamber 56 filled with gas. Inside the bar, there is a differential thenoocouple 58 whose cold junction 60 is housed in the base 50 and whose hot weld 62 is housed near the end of the rod 52. In addition, there is in the base 50 a simple thermocouple 64 for determining the absolute temperature. This assembly is fixed to the end of displacement means which may consist of a pole 66 or in cables. We can thus move the measuring device inside the guide tubes 18 to perform measurements at several points.

 In Figure 6, there is shown a measuring device similar to that shown in Figure 5 but in its "dry" version. The only difference lies in the fact that the tube 54 in the form of a thimble has been removed.

 In the previous examples, the measuring devices are in the form of a thermal conductive bar having at least one portion of reduced diameter and comprising as many differential thermocouples as there are portions of reduced diameter, the two welds or junctions of the differential thermocouple being arranged along the axis of the bar. Referring to Figures 7 and 8, we will describe two embodiments of measuring devices comprising a bar of cylindrical shape, that is to say without portions of reduced diameter.

According to the embodiment of Figure 7, it is also a y-thermometer. Each thermocouple provided with its hot junction and its cold junction is housed in a cavity 72 formed in the solid rod 70. This cavity 72 comprises a central part 72a of relatively reduced length, arranged along the axis of the rod 70, a part 72b disposed at the periphery of the rod 70 as best seen in FIG. 7 and an inclined connection part 72c connecting the central part 72a and the peripheral part 72b.

Inside this cavity is housed a set of conductors forming a differential thermocouple. This thermocouple assembly bears the general reference 74. It suffices to indicate that the hot weld 74b is housed in the peripheral part 72b of the cavity 7r, while 3v welded fuoii, 74a is preferably housed at the upper end of the central part 72a of the cavity 72. It is understood that thus by measuring the radial gradient between a cold weld 72a and the corresponding hot weld 74b, a temperature difference measurement is obtained which can be converted into a measurement of the state of the medium fluid for the zone Zi considered.

 In all the embodiments shown in FIGS. 3 to 7 and 9, the use of different types of couplings comprising the two welds or junctions has been described. It should however be noted that in the particular use of these y-thermometers according to the invention, one could also use two simple thermocouples.

 FIG. 8 shows a new embodiment of the device for measuring the state of the fluid medium. It consists of a cylindrical bar 80 which comprises a succession of measurement zones Z. each corresponding to a measurement element. In each measurement zone, there is in the same cross section of the bar a first simple thermocouple 82 whose junction is arranged along the axis of the bar and a second thermocouple 84 whose junction is located in the vicinity of the periphery of the bar 80. The difference in temperature recorded by these two thermocouples is representative in relative value of the value of the heat exchange coefficient between the bar and the surrounding fluid medium.

It goes without saying that in a complete embodiment, the measurement devices are associated with electronic or electrical circuits to process electrical signals delivered by the thermocouples and to apply corrective coefficients when comparing these measurements to possibly take into account of the known temperature gradient prevailing in the enclosure.

 I1 must be added in the case where the measuring device is in the form of a bar or a rod comprising several measuring elements, it may be advantageous to thermally separate the different measuring elements. This can be achieved by separating the bar portions corresponding to each measuring element by an insulating washer or by connecting the different measuring elements by a cable or chain.

 It is important to emphasize that the measurement devices according to the invention can be calibrated before they are placed in the enclosure. To do this, simply apply a heat source corresponding to the in situ operation; to place them successively in fluid environments corresponding to the different cases which may arise in situ and to measure the values of the electrical signals corresponding to these different environments.

Claims (14)

RESELL ATIONS  1. Method for determining the local state of a fluid medium inside an enclosure (2) characterized in that at least one conductive element is placed inside the enclosure (2) heat (D1, D2, D3), in that said element is heated, in that the coefficient of heat exchange between said element and the fluid medium is measured, in that at least two defined measurements are carried out previously, and in that at least these two measurements are compared in order to deduce therefrom the localized relative state of the fluid medium corresponding to these measurements.  2. Method according to claim 1, characterized in that the two measurements are made at two different times with the same element (D1, D2, D3) which remains stationary.  3. Method according to claim 1, characterized in that the two measurements are made at two different times with the same element (52) which is moved in the enclosure.  4. Method according to claim 1, characterized in that the two measurements are made at the same time with two different elements (D1, D2, D3) occupying different positions. 5. Application of the method according to any one of claims 1 to 4 to the control of the fluid medium in the pressure resistance tank (2) of a nuclear reactor of the pressurized water type.  6. Device for determining the local state of a fluid medium in an enclosure, characterized in that it comprises - at least one element which is a good conductor of heat (30, 50, 70)  suitable for being introduced into said enclosure (2), -means for heating the said element or elements, -measuring means (42, 74) associated with said or each  of said elements for measuring the exchange coefficient between  the associated element (30, 50, 70) and said surrounding fluid medium said element; and means external to said enclosure (2) for treating  said measurements and deduce therefrom the localized relative state of said fluid medium. 7. Device according to claim 6, characterized in that said measuring means for each element consist of two thermometric sensors (42,74) placed at different points in said element. 8. Device according to claim 6, characterized in that said measuring means for each measuring element consist of a differential thermocouple (42, 74).  9. Device according to claim 8, characterized in that it consists of a cylindrical bar of elongated shape (30) made of a material which is a good conductor of heat, said bar (30) comprising a succession of portions of enlarged diameter (38 ) and of reduced diameter portions (34), into a tube (32) surrounding said bar (30) and defining at the level of the reduced diameter portions of the sealed annular chambers (36), and in a plurality of differential thermocouples (42) housed in said bar (30), the two junctions of each thermocouple being respectively arranged at the right of a chamber (44) and at the right of a portion of enlarged diameter (46), each measuring element consisting of a thermocouple (42 ), an annular chamber (36) and an enlarged diameter portion (38).  10. Device according to claim 8, characterized in that it consists of a cylindrical bar (30) of elongated shape, said bar (30) comprising a succession of portions of reduced diameter (34) and portions of enlarged diameter (38 ) and in a plurality of differential thermocouples (42) housed in said bar (30), the two junctions (44 and 46) of each thermocouple being respectively arranged at the right of a portion of enlarged diameter (38) and at the right of a reduced diameter portion (34), each measurement element consisting of a thermocouple (42), a reduced diameter portion (34) and an enlarged diameter portion (38).  11. Device according to claim 8, characterized in that it comprises a cylindrical base (50) extended by a rod (52) of reduced diameter compared to that of said base (50), these two parts being made in one material that is good conductor of heat, and a differential thermocouple (58), one junction (60) of which is housed in said base and the other (62) of which is housed in said rod (52) near its free end  12.Device according to claim 8, characterized in that it comprises a cylindrical base (50) extended by a rod (52J in diameter due to that of the base (50) a tube (54) in the form of a finger glove surrounding said rod (52) and defining with the base (50) a sealed chamber (56) around said rod (52) and a differential thermocouple (58), a junction (60) of which is housed in said base ( 50) and the other junction (62) of which is housed in said rod (52) near its free end. 13. Device according to claim 8, characterized in that it comprises a rod (70) of cylindrical shape made of a material which is a good conductor of heat, said rod (70) being pierced with cavities (72) capable of receiving thermocouples differentials (74), each cavity (72) corresponding to a measuring element comprising an axial part (72a) containing a junction (74a) of said thermocouple (74) and a peripheral part (72b) containing the second junction 174bu of said thermocouple (74 ). 14. Device according to claim 7, characterized in that it comprises a rod (80) of cylindrical shape made of a material which is a good conductor of heat, said rod (80) constituting a succession of measuring elements Zi, said rod (80) comprising in each measuring element Zi two thermocouples arranged substantially in the same cross section of the rod (80), one of the thermocouples (82) being located in the axis of said rod (80), the other ( 84) being disposed near the periphery of said rod 80.