WO2006042914A1

WO2006042914A1 - Method of controlling an element comprising at least two layers including a fissionable layer and corresponding device

Info

Publication number: WO2006042914A1
Application number: PCT/FR2004/002704
Authority: WO
Inventors: Christophe Jarousse
Original assignee: Compagnie Pour L'etude Et La Realisation De Combustibles Atomiques
Priority date: 2004-10-21
Filing date: 2004-10-21
Publication date: 2006-04-27
Also published as: AR051057A1; WO2006042914B1

Abstract

The invention relates to a method of controlling an element comprising a stack of at least two layers (3, 5, 7), whereby at least one (5) of said layers comprises a fissionable material. The inventive method comprises a step consisting in determining the thermal resistance of at least one interface between two layers of the element. The invention can be used, for example, to control uraniferous sources.

Description

A method of controlling an element comprising at least two layers including a fissile layer and corresponding device

The present invention relates to a method for controlling an element of the type comprising a stack of at least two layers, at least one of the layers comprising a fissile material.

The invention applies in particular, but not exclusively, to the control of uranium targets for the production of isotopes by irradiation of the target. These isotopes are then used for example in the medical field. Such a target is described in US-5,615,238.

This target comprises a low enriched uranium sheet forming the layer of fissile material.

This sheet is inserted between two metal tubes, for example aluminum. Such a target therefore comprises on its current section a stack of an aluminum layer, a low enriched uranium layer, and another layer of aluminum.

The uranium sheet does not extend to the ends of the target where the aluminum tubes are in contact with each other.

Uranium foil and inner and outer tubes must be in intimate contact at their interfaces. In other words, the air gaps present at the interfaces between the layers, in particular because of the roughness of the surfaces of the layers, must be sufficiently thin. This is to ensure good evacuation of the heat produced in the uranium sheet during irradiation of the target. Thus, an uncontrolled rise in temperature in the target is avoided. Similarly, the tubes must be in intimate contact at the ends of the target.

To verify that these constraints are met, metallographic sections of the targets are examined in order to measure the thickness of the air gaps present at the interfaces. Such a control has many disadvantages.

First of all, it is a destructive control.

In addition, cutting targets imposes significant safety and security constraints, as it can lead to the production of uranium powder. Then, it is necessary to manage the uraniferous waste of the controlled targets.

Moreover, the analysis of the metallographic sections, and in particular the measurement of the thicknesses of the air gaps at the interfaces, is very sensitive to the conditions of cutting of the targets. Thus, such a method proves unreliable and presents a high degree of uncertainty.

This destructive inspection technique can only be carried out on a statistical sampling of the elements produced.

Finally, such a control can be operated only cold and not under conditions that can be representative of the temperature reached during irradiation of the target.

This lack of reliability and precision involves taking very large safety margins during the thermohydraulic calculations necessary for the development of the safety case for target irradiation. The conditions of future irradiation of the target may be influenced by this lack of precision.

This can lead, at the level of manufacture, to the implementation of severe criteria difficult to achieve.

The object of the invention is to provide a control method which makes it possible more simply to control more reliably and industrially that the stacked layers of an element of the aforementioned type are in intimate contact at their interfaces.

For this purpose, the subject of the invention is a method for controlling an element of the type comprising a stack of at least two layers, at least one of the layers comprising a fissile material, characterized in that the method comprises a step of determining the thermal resistance of at least one interface between two layers of the element.

According to particular embodiments, the method may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination: the determination step is performed by means of computer determination means,

- it includes steps of: - application of a thermal stress on a first face of the element, and

measurement of the evolution of the temperature on a second face of the element, the thermal stress is substantially a pulse of Dirac,

- The method comprises a step of comparing the determined thermal resistance or a magnitude representative of the determined thermal resistance with a threshold value to make a verdict on the compliance of the element. The invention also relates to a device for implementing a method as defined above, characterized in that it comprises means for determining the thermal resistance of at least one interface between two layers of item.

According to particular embodiments, the device may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination:

the determination means are computer means of determination,

it comprises means for applying a thermal load on the first face of the element and means for measuring the evolution of the temperature on the second face of the element,

the means for applying the thermal load are adapted to produce a thermal load substantially in the form of a Dirac pulse; it comprises means for comparing the determined thermal resistance or a magnitude representative of the thermal resistance; determined with a threshold value to make a verdict on the conformity of the element.

The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which: FIG. 1 is a diagrammatic view, in perspective and partially cut away, of a uranium target on which the control method according to the invention can be implemented,

FIG. 2 is a schematic transverse section of the target taken along the plane M-II of FIG. 1;

FIG. 3 is a schematic view illustrating a control device according to the invention on which the target of FIG. 1 is mounted,

FIG. 4 is a schematic perspective and enlarged view of part of the device of FIG. 3, and FIG. 5 is a diagram illustrating the evolution of the temperatures measured and calculated by the control device of FIG. 3.

Figure 1 illustrates a uranium target 1 for the production of radioactive isotopes, for example for use in the medical field.

This substantially cylindrical target 1 comprises: an inner tube 3 made of metal, for example aluminum,

two uranium sheets 5 arranged around the inner tube 3, each of these sheets extending over an angle of about 180 ° around the longitudinal axis A of the target 1, and

an outer tube 7 made of aluminum. The inner tube 3 has a flange 8 projecting radially at each of these ends. These two flanges 8 delimit a housing in which the sheets 5 are arranged.

It will be observed that the fissile layer formed by the sheets 5 is discontinuous, since an angular space 9 (FIG. 2) is provided between the longitudinal edges 10 facing the sheets 5. The tubes 3 and 7 have, for example, thicknesses of approximately 3 mm.

The sheets 5 have for example a thickness of 125 microns. In order to avoid the interaction between the uranium sheets 5 and the tubes 3 and 7, the sheets 5 are individually wrapped in thin sheets of nickel, for example 15 μm thick.

Thus, for each sheet of uranium 5, a first portion 11 of the corresponding nickel sheet is interposed between the inner tube 3 and the 5, and a second portion 13 of the nickel sheet is disposed between the sheet 5 and the outer tube 7.

To achieve the target 1, the sheets 5, previously wrapped in their nickel sheets, were introduced between the flanges 8 of the inner tube 3. The inner tube 3 was then introduced into the tube 7.

A tool was then inserted into the inner tube 3 to cold plastically deform both the inner tube 3 and elastically the outer tube 7, then the ends of the tubes 3 and 7 were welded.

The plastic deformation of the tube 3 and the elastic deformation of the tube 7 ensure on the one hand that the uranium sheets 5, the parts 11 and 13 of the nickel sheets, and the two layers formed by the tubes 3 and 7 are in intimate contact at their interfaces, and secondly that the tubes 3 and 7 are in close contact with the rims 8, that is to say at the longitudinal ends of the target 1. To ensure that the blades of air present at the interfaces between the sheets 5, the parts 11 and 13 of the nickel sheets, and the tubes 3 and 7 are effectively thin, that is to say that there is a sufficient contact between the sheets 5, the parts 11 and 13 and the tubes 3 and 7, the control device 15 illustrated in FIG. 3 is used. This control device 15 comprises:

means 17 for producing a thermal load on the inner face 19 (FIG 1) of the target 1,

means 21 (FIG 4) for measuring the temperature on the outer face 23 (FIG 1) of the target 1, and 25 for determining the thermal resistances R of the interfaces between the different layers of the target 1.

The device 15 also comprises a base 27 extended upwards by a foot 29 supporting a plate 31.

The means 17 for producing a thermal load comprise an electroluminescent device such as a flash lamp.

This flash lamp 17 comprises a tube sandwiched between two sections 33 of electrical connection. The sections 33 are spaced from one another and aligned. They are carried by the plate 31. As can be seen in FIG. 4, the ends facing the sections 33 are provided with rings 35 for positioning and holding the target 1.

When the target 1 is mounted on the control device 15, as illustrated in FIG. 4, the target 1 connects between the two sections 33 and is held in this position by the rings 35. the target 1 then surrounds the lamp flash 17.

The rings 35 are movable relative to the sections 33 and can more precisely slide along the sections 33 and turn around the sections 33.

The flash lamp 17 is connected, via the sections 33, to a source 37 of power supply, which is for example itself connected to the local electrical network.

The flash lamp 17 is adapted to produce thermal pulses of the Dirac type, that is to say that it produces almost instantaneous energy discharges.

The means 21 for measuring the temperature comprise a thermocouple which is disposed in an opening 39 formed in the plate 31 between the two ends facing the sections 33. The foot 29 comprises means 30 for moving the thermocouple 21 in order to apply it locally against the outer face 23 of the target 1.

These displacement means 30 are shown diagrammatically in FIG. 3. Thanks to the possibility of displacement of the rings 35, it is possible to modify the point of contact of the outer face 23 of the target 1 where the temperature is measured.

The thermocouple 21 is connected to the determination means 25 which are formed for example by a computer which itself comprises one or more processors, storage means, input and output means and possibly display means.

In the example shown, it is a microcomputer 25 which comprises in particular a screen 41, a central unit 43 and a keyboard 45. This computer 25 is further connected to the power source 37 of the flashlamp 17 to control its operation, i.e. the production of Dirac-type thermal pulses.

The evolution of the temperature can be modeled in an element comprising several layers stacked and subjected to such a thermal pulse.

Such modeling is described in the article "Diffusion in composite layers with automatic solutions of the eigenvalue problem" by MD Mikhailov, MN Ozisik and NL Vulchanov published in "International Journal of Heat and Mass Transfer" of 1983, volume 26, No. 8, pages 1131-1141.

For a one-dimensional element (variation of the temperature only according to the thickness), the temperature is worth at the thickness x and the time t:

T _k (x, t) = AT / ON X _k (β _n , x) e- ⁿ (1) With an element consisting of m layers: m * f (βn) = Σ Pk Cp _k JF _k (x) X _k (βn X) dX (2) k = l xk-1 xk N (β _n ) = Σ Pk Cpk 1 ^ (βn, X) dX (3)

X _t (P _n , X) = A ,, (M "" shm [w " ⁽ _k ^X _n ^k e T _k j ^)] + A _t (β _n ) ^άnK sin '^ [w _kn ^~ e _k γj KW _k , _n . £ Ja. _k (4)

The values of the eigenfunctions A _n and of the roots β _n are the solutions of a transcendental equation that can be solved by the method of

"Sign-Count". These solutions are a function of the thermal resistances R of the interfaces and the exchange coefficient h between the faces of the element and the external atmosphere.

The calculated temperature is thus a function of the following parameters: λ _n : thermal conductivity of the n-layer, p Cp _n : heat density of the _n- layer: thickness of the n-layer, between the abscissa x _n- i and X _n , R _n : Thermal resistance of the interface between the n layer and the layer n + 1, h: exchange coefficient with the outside.

If we consider that the energy pulse arrives on the layer m, then the thermogram in x = 0 is written:

Equation (5) shows that the temperature is directly proportional to the energy Q provided by the flash lamp 17. It is therefore possible to work with standard thermograms:

The advantage of using the normalized temperatures T * is that it is no longer necessary to know the energy of the thermal pulse or the accuracy of the thermocouple 21 used.

In the example considered, the target 1 can be modeled as a one-dimensional element comprising a stack of three layers, namely respectively the inner tube 3, the uranium sheets 5 and the outer tube 7.

To evaluate the intimacy of the contact at the interfaces between the sheets 5 and the tubes 3 and 7, the device 25 determines the actual thermal resistances R at these interfaces. For this, the control device 25 determines, thanks to the thermocouple 21, the thermogram, that is to say the evolution over time, of the actual normal temperature on the outer face 23 of the target 1 following the production a thermal pulse on the inner face 19 of the target 1. Such a thermogram corresponds to the curve 47 in Figure 5. Next, the device 25 minimizes, in the least squares sense, the gap

F between the measured normal temperature T _m * and a calculated normal temperature T *:

F = Σ [τmi * -Ti * (R _n , h, e _n , λ _n , pCp _n )] ²

Where n is the number of times the temperatures are compared (between 1000 and 3000 instants). A classical algorithm then looks for the values of the exchange coefficient h and the thermal resistances R at the interfaces from initial values. At each iteration h and R take a new value with h = h + Δh, and R = R. + ΔR

The iterations are stopped when the increment ΔR is less than 10 ^"7 m ² .K / W for all the resistors and when the increment Δh is less than 1 W / m ² .K for the exchange coefficient. is the precision of the calculation.

The curve 49 in FIG. 5 corresponds to the thermogram calculated for the value of the thermal resistances R at the two interfaces which allow the best coincidence with the measured thermogram 47.

Note that this determination is made assuming that the resistors R are the same at both interfaces.

Curve 51 in FIG. 5 corresponds to the difference between curve 49 and curve 47. It can be seen that curve 51 is centered on ordinate 0, except for the first moments.

This poor coincidence originally between the curves 47 and 49 is due to the fact that the thermal pulse produced by the flash lamp 17 is not a perfect Dirac, but has a duration of about 5 ms. This bad coincidence is also due to a problem of convergence of the calculation to obtain the thermogram 49 in the first moments. It is also possible to envisage a measurement error due to electromagnetic disturbances generated by the electrical discharge producing the thermal pulse.

Whatever the case, the correspondence between the measured thermogram 47 and the calculated thermogram 49 is very satisfactory.

It makes it possible to determine accurately the thermal resistances R at the interfaces between the sheets 5 and the tubes 3 and 7, because the thermograms used are very sensitive to the values of the thermal resistances R. In the example shown, these resistors R are about

1, 13.10 ^"4 m ² .K / W with an error of 0.07 10 ^" * m ² .K / W.

The resistances R in each of these interfaces correspond to two blades of air on each side of each of the parts 11 and 13 of the sheets Nickel. The values determined are much greater than the thermal resistances of parts 11 and 13 alone which are of the order of 2.5. 10 ^"7 m ² .K / W.

We can therefore neglect the contributions to the thermal resistances provided by the parts 11 and 13 of the nickel sheets. It is then possible to convert the thermal resistances R determined in air gap thickness which leads to a thickness of 1, 7 microns for each air space, each interface comprising a blade of air on both sides of the corresponding part 11 or 13 of the nickel sheet.

Thus, the target 1 comprises four blades of air of 1, 7 microns to the right of the temperature measurement point, which point is opposite one of the sheets 5.

It is therefore possible to determine the intimacy of the contact between the layers 3, 5, and 7.

Then, for example, it is possible to compare the cumulative thicknesses of the air gaps, which are representative of the determined thermal resistances R, with a predetermined threshold value to make a verdict on the conformity of the target 1 with respect to the intimacy of the contact. at the interfaces between the sheets 5 and the tubes 3 and 7.

Rather than comparing the thicknesses of the air blades to a threshold value, the determined thermal resistance R can be compared with a predetermined threshold value in order to render the verdict.

By moving the rings 35, it is also possible to have the thermocouple 21 facing the longitudinal ends of the target 1, that is to say facing the flanges 8 of the inner tube 3. It is then possible to measure the thermal resistance R of the interface between the tubes 3 and 7. Here again, we have seen a very good correspondence between the thermograms measured and calculated. It was thus possible to determine in an example that the thermal resistance in this zone there was 5,3.10 ^'WK / W, a value which can be converted in thickness corresponding air knife.

The same measurements and calculations can also be performed at the spaces 9 situated between the longitudinal edges 10 of the sheets 5.

However, the measurements and calculations carried out in these zones are less precise, since the heat tends to be evacuated preferentially at through uranium 5 sheets which have better thermal conductivity than the air gaps present in the spaces 9.

The method described above thus makes it possible to control in a simple and precise manner the interfaces between the different layers of the target 1. In addition, this method is a non-destructive control method, which can be implemented at the scale industrial.

In addition, it will be observed that the control method can be implemented either at room temperature or at a temperature, for example to approach the conditions of use of the target 1. Such a hot measurement, carried out for example at 200 ⁰ C, reduces uncertainty margins and provides even more reliable control.

It will also be observed that this method can provide either air knife thicknesses or thermal resistances R interfaces between the different layers, these values being used directly by thermohydraulic simulation software to verify that the targets 1 produced satisfy the safety constraints. Quantities representative of the thermal resistances R, other than the thicknesses of the blades of air, can be used to render a verdict on the targets 1.

More generally, the method described above can be used to control other elements 1 than uranium targets, their forms being other than cylindrical, for example planar. In general, such an element 1 will comprise at least two layers, one of which comprises a fissile material. For example, it may be a research reactor fuel element. More generally, the method will determine the thermal resistance of at least the interface between two layers. In certain variants, the method may determine the overall thermal resistance of element 1.

Other thermal stresses that pulses can also be considered.

Claims

A method for controlling an element (1) of the type comprising a stack of at least two layers (3, 5, 7), at least one (5) of the layers comprising a fissile material, characterized in that the method comprises a step of determining the thermal resistance of at least one interface between two layers of the element.

2. Method according to claim 1, characterized in that the determining step is performed by means of computing means (25) determination.

3. Method according to claim 1 or 2, characterized in that it comprises steps of:

- applying a thermal stress on a first face (19) of the element (1), and

measuring the evolution of the temperature on a second face (23) of the element (1).

4. Method according to claim 3, characterized in that the thermal bias is substantially a Dirac pulse.

5. Method according to claim 3 or 4, characterized in that the method comprises a step of comparing the determined thermal resistance or a magnitude representative of the determined thermal resistance with a threshold value to make a verdict on the conformity of the element (1).

6. Device for implementing a method according to one of the preceding claims, characterized in that it comprises means (25) for determining the thermal resistance of at least one interface between two layers (3, 5, 7) of the element (1).

7. Device according to claim 6, characterized in that the determining means are computing means (25) for determining.

8. Device according to claim 6 or 7, characterized in that it comprises means (17) for applying a thermal stress on the first face (19) of the element (1) and means (21). measuring the evolution of the temperature on the second face (23) of the element (1).

9. Device according to claim 8, characterized in that the means (25) for applying the thermal stress are adapted to produce a thermal stress substantially in the form of a Dirac pulse.

10. Device according to claim 6 or 7, characterized in that it comprises means (25) for comparing the determined thermal resistance or a magnitude representative of the determined thermal resistance with a threshold value to make a verdict on the conformity of the element (1).