FR2995656A1 - Sealing e.g. composite material wall of a propellant tank of a launch vehicle, comprises introducing a heavy gas at a high pressure against an inner face of the wall prior to the introduction of light gas - Google Patents

Abstract

The method comprises introducing a heavy gas (5) having a molecular size strictly greater than a molecular size of a light gas at a high pressure against an inner face (2) of a wall (1) prior to the introduction of the light gas. The wall contains, on the inner face, the light gas at a pressure higher than a pressure (PO) prevailing on an outer face (3) of the wall. The wall is a peripheral wall delimiting a closed enclosure. The wall is a composite material wall with an organic matrix. The wall is a porous wall having pores with a dimension greater than a molecular size of the light gas. The method comprises introducing a heavy gas (5) having a molecular size strictly greater than a molecular size of a light gas at a high pressure against an inner face (2) of a wall (1) prior to the introduction of the light gas. The wall is intended to contain, on the inner face, the light gas at a pressure higher than a pressure (PO) prevailing on an outer face (3) of the wall. The wall is a peripheral wall delimiting a closed enclosure. The wall is a composite material wall with an organic matrix. The wall is a porous wall having pores with a dimension greater than a molecular size of the light gas. The molecular size of the heavy gas is lower than a mean size of the pores of the wall. The pressure and a period of validity of the heavy gas on the inner face depend on a thickness of the wall and the mean size of the pores of the wall. The high pressure of the heavy gas is decreased before the introduction of the light gas. The heavy gas is diazotized. The heavy gas is applied at a high pressure against the inner face of the wall until the heavy gas begins to flee from the outer face of the wall.

Description

The invention relates to a method for sealing a tank made of composite material, in particular a liner-free tank.

Many applications require storing light gases in tanks or at least isolating them from another medium. Thus, space launchers for example generally comprise a pressurized neutral light gas tank such as helium to maintain pressure or push propellant or liquid propellant stored in a second tank.

However, in order to reduce the weight of the tanks they are not equipped with waterproofing liner. This is particularly the case for tanks that serve less than a few hours, for example on a space launcher. Without liner and the helium being a light gas with small molecules, the helium leakage rate through the walls of the tank of composite material is high. Indeed, composite materials generally have many pores through which gas molecules can all the more easily pass that they are small. In addition, the reservoir being filled at high pressures the reservoir walls deform and the pores expand, leaving even more light gas molecules escape.

There are known (see "Ultralight Linerless Composite Tanks for In-Space Applications", Mallick et al., Caltech, Jet Propulsion Laboratory, Sep. 30, 2004, AIAA Space 2004 Conference, San Diego) techniques for improving the imperviousness of such a tank by the design of new composite materials more impermeable and more resistant to microcracks.

However, such materials have yet to develop and no current composite material provides effective impermeability in the absence of liner. The invention therefore aims to overcome these disadvantages by proposing a new method of sealing a wall.

The invention aims in particular to provide such a method for sealing a tank wall composite material without liner.

The invention also aims at providing such a method that does not weigh down the tank, especially for a spatial launch. The object of the invention is to propose such a method which is simple to implement, even on existing reservoirs, that is to say without having to modify them structurally. The invention also aims at providing such a method which is economical. The invention also aims at providing such a non-hazardous process, in particular in which the compounds used are harmless for health. Throughout the text, the term "pore" means a small orifice of any type: for example pores or defects of a material, microcracks, etc. Throughout the text, the term "molecule" and its derivatives are used for stable multi-atomic or monoatomic compounds, especially for multi-atom gases such as nitrogen (N2) or a single atom such as rare gases, for example. example such as helium (He). Throughout the text, the term "molecular size" refers to the average size of a molecule. This size can be defined according to various criteria, such as for example in Van Der Waals radius, in molecular weight, in maximum bulk, etc. The molecular size of certain gases is therefore in fact an atomic size (for example for rare gases He, Ne, Ar, ...). Likewise, the "size" of a pore is determined according to the same measurement method, so that the size of a pore can be compared with the molecular size of a molecule. Throughout the text, the concepts of sealing and impermeability apply to gases and not to liquids. The invention therefore relates to a method of sealing a wall 30 having an inner face and an outer face, said wall being intended to contain, on the inner face, a gas, said light gas, at a pressure, called high pressure. , greater than the pressure prevailing on the outer face, characterized in that, prior to the introduction of the light gas, at least one gas, said heavy gas, of molecular size strictly greater than the molecular size of the light gas is applied to a high pressure against the inner face of the wall.

A high pressure according to the invention is a pressure of greater value than the pressure prevailing on the outer face of the wall, which is generally atmospheric pressure under normal conditions. When the wall is a propellant tank wall for a space launcher, the filling of the tank is carried out on the ground and the pressure on the outer face of the tank is an atmospheric pressure at ground level, then it decreases during launching up to to reach a pressure close to zero in orbit; it is therefore always lower than the atmospheric pressure. Any wall is more or less permeable to gases, and it is even more so in the absence of a specific layer of waterproofing (or liner).

Surprisingly, the inventors have determined that the prior injection at a high pressure of a gas of molecular size larger than the molecular size of a light gas to be contained by the wall significantly reduces the leakage rate. light gas for a fixed period.

In a process according to the invention the heavy gas is introduced into at least a portion of the pores of the wall when it is applied at a high pressure against the inner face of the wall. Indeed, since the heavy gas is introduced at a high pressure greater than the pressure on the outer face, the wall is deformed under the effect of this pressure, so that the pores of the wall expand and accommodate more easily heavy gas molecules. The heavy gas is then trapped in the pores when the pressure is lowered before the introduction of the light gas. When the heavy gas is applied at a high pressure against the inner face, several successive regimes can be observed.

First a regime during which the heavy gas accumulates in the pores of the wall and forms a heavy gas front progressing in the thickness of the wall of the inner face to the outer face. During this accumulation regime, the leakage rate of the heavy gas through the wall is zero. A transient regime follows when the heavy gas front reaches the outer face, with a heavy gas flow through the wall that grows and then stabilizes to give way to a stationary (or permanent) leakage regime, with a quasi-flow rate. constant. A final regime, decreasing flow, may possibly occur if the tank containing the heavy gas at high pressure is not large enough volume with respect to the observation time.

However, with equal pressure difference between the inner and outer faces, the leakage rate of the heavy gas (i.e., the amount of gas escaping through the wall per unit time) is much lower than the leakage rate of a light gas. Indeed, the heavy gas being chosen molecular size larger than the light gas, its progression in pores of the wall, between the inner face and the outer face is slower than that of light gas. The progression in the wall of a heavy gas of large molecular size is slowed down by its size, whereas a light gas is much less so. A process according to the invention thus makes it possible, by introducing heavy gas into the wall, in particular into wall defects such as pores, to significantly reduce or even to cancel the leakage rate of the light gas through the wall for a fixed period. Indeed, during the subsequent introduction of the light gas at a high pressure against the inner face, it exerts said high pressure on the heavy gas molecules trapped in the wall and pushes them to the outer face. Thus, a diffuse front of light gas pushes a thick gas thickness towards the outer face. The thickness of heavy gas in the wall depends in particular on the high pressure to which the heavy gas has been applied against the inner face, the size of the heavy gas molecules, the porosity and the permeability of the wall, and the duration of application of the heavy gas. The rate of progression of the light gas front is limited by the rate of progression of the heavy gas ahead (in the inner-to-outer direction). Thus, thanks to the invention, the heavy gas escapes at a leak rate much lower than the leakage rate of the light gas, so that a smaller amount of gas is released on the outside than if the light gas had was introduced directly against the wall. By means of a process according to the invention it is thus possible to introduce the light gas at a high pressure while maintaining a leak rate close to zero for a predetermined period. In addition, only the heavy gas escapes on the outer face as long as the light gas front has not reached the outer face (near diffusion between the front light gas front and the rear edge of the heavy gas), This is particularly advantageous when the heavy gas is less expensive, but also and above all less harmful and less dangerous than light gas. In a method according to the invention the high pressure of heavy gas can be decreased before a leak rate is detected on the outer face of the wall. Indeed, in a process according to the invention, it is not necessary that the heavy gas has been accumulated in the pores over the entire thickness of the wall. However, the efficiency of the process according to the invention is improved when the heavy gas is accumulated throughout the thickness of the wall. Indeed, the greater the thickness of heavy gas introduced into the wall, and the longer the sealing time of the tank will be important. Therefore, advantageously and according to the invention, the heavy gas is applied at a high pressure against the inner face until the heavy gas begins to leak from the outer face of the wall. The high pressure of heavy gas is therefore advantageously reduced only from the detection of a leakage rate on the outer face. Advantageously, the pressure of heavy gas is reduced to a pressure equivalent to the pressure prevailing on the outer face before the introduction of the light gas. A method according to the invention makes it possible to waterproof a wall vis-à-vis a light gas for a period determined mainly by: the thickness of the wall and the rate of progression of the heavy gas in the wall.

The rate of progression of the heavy gas in the wall, for its part, depends on the nature of the wall (size and shape of the pores, material of the wall, etc. therefore porosity and permeability to the gas of the wall), the nature of the heavy gas (average molecular size, physicochemical reactivity with the material of the wall, ...), and the value of the high pressure applied on the inner face. The inventors have thus determined that the appropriate choice of heavy gas made it possible, for certain walls of composite material, to achieve improved impermeability for periods of time ranging from a few hours to a few days depending on the wall, the pressure conditions and the gas. used, ...

However, in many applications, waterproofing a wall for a determined period of time is sufficient. This is particularly the case in propellant or rocket propellant applications for launchers whose duration of use does not exceed a few hours including in particular the filling time of the tank, the duration of launching and the duration of setting in orbit .

A method according to the invention is therefore particularly advantageous on space launchers because the absence of liner on the wall makes it possible to guarantee a much lower launching weight, while benefiting from impermeability that is simple to achieve and effective for the duration of the period. use of the tanks.

In addition, the invention thus allows the use of a light gas, not a heavy gas, to pressurize tanks, which again provides an advantage in terms of total weight at launch. A method according to the invention is also simple to implement because a device for pressurizing a gas against the inner face necessarily already exists to be able to introduce the light gas against the wall. It is therefore simply a question of adding a treatment step with a heavy gas. Advantageously, a method according to the invention is applied to a peripheral wall delimiting a closed enclosure. The wall is in particular a tank wall. In particular, a process according to the invention is advantageously applied to a wall made of organic matrix composite material.

Indeed, these materials have pores and they are therefore particularly permeable to light gases such as helium. More generally, a method according to the invention is therefore advantageously used when the wall is a porous wall having pores which, in the percolating network in which they participate, all have a size greater than the molecular size of the light gas. . In addition, advantageously and according to the invention, the molecular size of at least one heavy gas is less than the average pore size of the porous wall. In fact, the heavy gas must be chosen so that it can at least be introduced into the pores of the wall. If the heavy gas could not enter it while the light gas can, the heavy gas would not fulfill its role of waterproofing by filling the pores of the wall and the leakage rate of the light gas would be as important as without the heavy gas treatment of the invention. The choice of the molecular size of the heavy gas is therefore essential in the efficiency of a process according to the invention. The heavy gas is therefore also chosen to be able to enter the wall without being significantly rejected when the pressure of heavy gas is reduced (before the introduction of the light gas). If the amount of heavy gas accumulated in the wall is not sufficient, there is a risk that the diffuse front of heavy gas will extend beyond the inner and outer faces prematurely and thus a risk of gas mixing. light and heavy gas in the tank, but also a drop in the effectiveness of the waterproofing obtained by the invention. The molecular size of the heavy gas must therefore be chosen so that it can be introduced into the pores of the wall, but also so that it can be braked there. In particular, the heavy gas advantageously has a size between the average pore size at equal pressure between the inner face and the outer face of the wall, and the average pore size at high pressure on the inside face and at a pressure much lower than this high pressure on the outer face. Indeed, the wall expands under the effect of a pressure difference between inner face and outer face and therefore in these conditions the pores expand, allowing the heavy gas to enter and stay there trapped when the pressure difference decreases.

Advantageously and according to the invention, several heavy gases of distinct molecular size greater than the molecular size of the light gas can be successively introduced at a high pressure against the inner face, so as to be introduced into pores of different sizes. The various heavy gases are advantageously introduced successively in descending order of molecular size. Thus, the heaviest heavy gas is introduced into the larger pores, then the next in descending order of size is introduced into the smaller pores, and so on. All the pores are thus filled with heavy gases whose size is particularly close to the size of each pore, so that their rate of progression in the thickness of the wall is even smaller. The successive introduction or not of several heavy gases thus makes it possible to increase the duration of waterproofing of the wall. Alternatively, the heavy gases can be introduced simultaneously in order to reduce the number of pressurization cycles and depressurization of the wall which result in the increase of the number of microcracks and the weakening of the wall. Advantageously and according to the invention, the pressure and the duration of application of at least one heavy gas on the inner face depend on the thickness of the wall, the porosity of the wall (therefore in particular the average size pores of the wall), and gas permeability of the wall. Indeed, the greater the thickness (e) of the wall is and the longer the duration beyond which the heavy gas passes completely through the wall and begins to be released on the outer face is important. In addition, the higher the permeability (k) wall, the faster the gas passes through the wall.

In addition, the higher the high pressure (P1) applied to the inner face, the faster the transmission of heavy gas through the wall. The duration (Δt) during which a high pressure (P1) is to be applied on the inside face of the wall thus takes the form of: ## EQU1 ## where E represents the porosity of the wall . We can refer to "A new quasi-steady method to measure gas permeability of weakly permeable porous media", Jannot and Lasseux, Review of Scientific Instruments, 83, 015113 (2012), for the calculation method of At and the methods of permeability measurement of a wall. In practice, it is advantageously expected to detect a significant leak (close to a permanent leakage regime) of heavy gas on the outer face to ensure that the wall is saturated with heavy gas. Furthermore, advantageously, a process according to the invention is also characterized in that it comprises a step during which the high pressure of heavy gas is reduced before the introduction of the light gas. In particular, advantageously and according to the invention, the pressure of heavy gas against the wall is reduced to the pressure prevailing on the outer face. At the present time the propellant reservoir walls are generally made of carbon fiber composite material and organic bismaleimide resin matrix ("BMI", Cytec 5250-2-40% 3KHS-5H-285-600) and fiber reinforcements. woven carbon (T300). The heavy gas is advantageously and according to the invention chosen to be neutral with respect to the material of the wall, neutral vis-à-vis the light gas, and neutral vis-à-vis any other compound to be stored against the wall. As part of a preparation for launch, the propellant and propellant tanks are previously pressurized with helium which maintains propellant or propellant in the liquid state under the effect of high pressure high . A process according to the invention is therefore advantageously carried out in which the light gas is helium and in which at least one heavy gas is dinitrogen. Indeed, dinitrogen and helium are neutral with each other, neutral with respect to most propellants and propellants, and neutral with the composite materials commonly used for propellant tanks. The molecular size of the dinitrogen is significantly greater than the molecular size of the helium, so that the nitrous oxide, which is harmless for health and therefore abundantly available at low cost, is advantageously used as a heavy gas. The invention also relates to a process characterized in combination by all or some of the characteristics mentioned above or below. Other objects, features and advantages of the invention will become apparent on reading the following non-limiting description which refers to the appended figures in which: FIG. 1 is a diagrammatic sectional representation of a wall; at a time of a first step of an embodiment of a method according to the invention, - Figure 2 is a schematic sectional representation of a wall 20 according to Figure 1, at a first moment of a second step of an embodiment of a method according to the invention, - Figure 3 is a schematic sectional representation of a wall according to Figures 1 and 2, at a second time of the second step of An embodiment of a process according to the invention, FIG. 4 is a representative diagram of the leakage rates of light gas through a wall when the invention is not applied and when a process according to the invention is applied And the heavy gas leak rate through said wall when said process according to the invention is applied. In a process according to the invention as represented in FIGS. 1 to 3, it is sought to improve the impermeability to a light gas such as helium from a liquid propellant tank wall 1 or liquid launcher rocket propellant. . Helium is used to maintain pressure on propellant / propellant to keep it liquid. The propellant / propellant pressurization can be carried out with other propellant-neutral gases such as nitrous oxide for example, but thanks to a helium pressurization, the launcher benefits the particularly low density of helium to be generally lighter. Said wall 1 is made of composite materials. It is more particularly made of bismaleimide resin organic matrix composite material ("BMI", Cytec 5250-2-40% 3KHS-5H-285-600) and woven carbon fiber reinforcements (T300).

The wall has a thickness of 2.1 millimeters between an outer face 3 forming the outer surface of the reservoir and an inner face 2 forming the inner surface of the reservoir for receiving fluids, in particular helium and propellant / propellant under pressure. In a first step of a process according to the invention - to improve the sealing of such a wall - is introduced into the tank a heavy gas 5 at a high pressure P1 greater than the pressure PO applying to the outer face 3. The heavy gas chosen in this particular embodiment is the dinitrogen of substantially cylindrical shape approximately 420 picometers in length (twice the van der Waals radius of the nitrogen to which is added the triple bond length nitrogen) and 310 picometers in diameter (van der Waals diameter of nitrogen), ie an average size, all dimensions combined of approximately 347 picometers (average of the three spatial dimensions). Indeed, the heavy and light gases in this type of space launcher tanks application are also chosen neutral vis-à-vis the compounds contained in such tanks (hydrazines, oxygen peroxide, ...). So that the introduction of dinitrogen in the tank allows at the same time to clean the tank of the compounds it contained (air for example). The dinitrogen, under the effect of the differential pressure between the inner face 2 and the outer face 3 tends to get into the pores of the wall and migrate into the wall from the inner face 2 to the outer face 3 .

Advantageously, the dinitrogen is introduced at a high pressure P1 of about 6 bars, for an atmospheric pressure PO 3 on the outside face 3. A nitrous front is formed in the wall, which moves at a speed V 1 depending on the size of the pores of the wall, the size of the dinitrogen and the pressure difference between P1 and P0. It should be noted that other phenomena could be at the origin of the slow movement of the dinitrogen through the wall, for example a physico-chemical reactivity between wall material and dinitrogen. Once the nitrogen begins to leak on the outer face, the pressure on the inner face is reduced to atmospheric pressure PO and helium is introduced into the reservoir. In a second step of a process according to the invention, the light gas 4 - here helium - is introduced into the tank at a high pressure P2 which may or may not be equal to P1, P2 being in any case greater than at PO. The helium 4 thus begins to enter the wall 1 by the inner face 3 and forms a second gas front in the wall which pushes the heavy gas towards the outer face. However, thanks to the invention, the helium front passes through the wall at a rate V 2 which depends on the pore size of the wall, the size of the dinitrogen and the pressure difference between P2 and P0. The helium leak velocity V2 is therefore limited by the large-sized nitrogen which slows the progression through the wall, and thereby also slows the progression of helium 4, blocked behind the nitrous oxide layer. However, as shown in FIG. 3, the beneficial effect of a method according to the invention lasts only for a definite period of time. Indeed, even at a reduced speed V2, the helium finally eject by the outer face all the dinitrogen that was contained in the wall. The helium therefore begins to cross the wall at a speed V3 of its own. It is also possible that the dinitrogen diffuses into the helium and is partially rejected by the inner face, hence the important choice of the heavy gas used.

However, the speed V3 depends only on the size of the pores of the wall 1, the size of the helium 4 and the pressure difference between P2 and P0. V3 is therefore much higher than V2. Indeed, the helium has a diameter of about 280 picometers (Van Der Waals diameter) much lower than the average size of about 347 picometers for the dinitrogen (420x310x310 picometers), and it is therefore much less restrained in the pores of the wall. The leak rate is therefore also much higher. In laboratory tests, the inventors were able to measure the coefficient of permeability of a wall of constant thickness of 2.1 mm in bismaleimide resin organic matrix composite material ("BMI", Cytec 5250-2-40% 3KHS-5H-285-600) and woven carbon fiber reinforcements (T300) to obtain the results taught in FIG. 4. Said wall is made from a stack of eight layers of woven carbon fibers (T300) impregnated with BMI and all oriented in the same direction (0 ° warp direction relative to each other). The stack of prepregs is then autoclaved to obtain a 2.1 mm thick sample. The laboratory tests consisted in closing two half-gas chambers on a sample of such a wall. The sample was stretched 45 MPa in the sample plane, in the "warp" direction of the prepregs, and at a constant temperature of 40 degrees Celsius. In the first half-chamber a constant pressure of 6 bar of gas (helium or nitrogen) was maintained, while the second half-chamber was at atmospheric pressure at the beginning of each test. Measuring the pressure over time in the second half chamber determines the rate of gas leakage through the wall sample. In Figure 4 are shown gas leak rates (ordinate) through a wall of the type shown in Figures 1 to 3 as a function of time (abscissa). Curve 6 represents the rate of leakage of helium over time through a wall that has not undergone a process according to the invention. The helium is introduced into the tank from the moment 0. It is found that the helium starts to leak very early and reaches very quickly, after a mean time (t0-0) of about 30 hours, a permanent leakage regime with a high f0 leakage rate of 1.48.10-8 ms-1 (or cubic meters, per second, per square meter of wall area).

Curves 7 and 8, on the other hand, are representative of the leakage rate respectively of dinitrogen and helium through a wall treated according to a process according to the invention. Curve 7 represents the rate of leakage of nitrogen through the wall over time, while curve 8 represents the rate of helium leakage through the wall over time.

It should be noted that the curves 6 and 8 correspond to the same tank wall, subjected to the same helium pressure. Thus, it can be seen on the curve 6 that, since the dinitrogen was introduced at the instant 0, it reaches a permanent leakage regime later, after an average duration of about 48 hours. In addition, the leakage rate of about 0.58 × 10 -8 ms -1 of steady-state nitrogen is much less (about two-and-a-half times) than the leakage rate f 0, due to the lower rate of progression of the dinitrogen through the wall. In a process according to the invention, the helium is introduced as soon as dinitrogen is detected on the outer face 3 of the wall, advantageously as soon as the permanent leakage rate of the dinitrogen is reached from the instant tl . It can be seen from curve 8 that helium does not start to leak through the wall until much later, when the leakage rate of the dinitrogen begins to decrease. Indeed, the plateau of the curve 7 at a quasi- permanent rate of diazote fl ux corresponds to a progressive ejection of the dinitrogen which was contained in the wall by the helium which exerts a pressure on the dinitrogen and by a re-diffusion on the inner face of the wall. As soon as the first molecules of the helium front arrive on the outer face, and while there are still nitrous molecules in the wall, the two types of gas are ejected simultaneously, which corresponds to a decrease in the leakage rate. of dinitrogen and a growth of helium leakage rate (crossing of curves 7 and 8). At time t2, all the dinitrogen contained in the wall has been ejected, and the helium leakage rate f0 is found.

It is therefore observed that the duration (t241) of approximately 71 hours between the introduction of helium into the reservoir and the attainment of a helium leakage rate f0 is much greater than the average duration (t0-0). about 30 hours. A method according to the invention therefore makes it possible to reduce the total volume of gas lost between time 0 and time t2, and in particular to significantly reduce the volume of light gas lost during this time. The duration of 71 hours before recovering a high helium leak rate is largely sufficient for short-duration helium storage applications. Thus, if the reservoir is pressurized with a light gas for a period of a few hours, much less than this duration (t2-0), a process in accordance with the invention makes it possible to very significantly reduce helium leakage. through the wall, in favor of a nitrous leak that is absolutely harmless, non-dangerous and moreover inexpensive. The invention can be the subject of many other embodiments not shown. In particular, if a method according to the invention is advantageously applied to tank walls, it can be applied to other types of walls, for example walls interposed in pipes. Many gases can be used, individually or in combination, as a heavy gas. They are advantageously chosen according to their physico-chemical properties and those of the wall to be sealed. In particular one can seek to waterproof a wall intended to contain other gases than helium using different gases of the dinitrogen. However, the inert gases and especially non-reactive with the light gas are logically preferred as heavy gases.

In addition nothing prevents the application of a method according to the invention when the light gas is in fact a mixture of light gases, that is to say of small gas. In addition, some heavy gas molecules remain theoretically locked in the wall, so that the final leakage rate of the light gas is theoretically slightly lower than the leakage rate of the same light gas under the same conditions, without the object treatment. the invention.

Claims (9)

CLAIMS1 / - A method of sealing a wall (1) having an inner face (2) and an outer face (3), said wall (1) being intended to contain, on the inner face (2), a gas, said light gas (4), at a pressure, called high pressure, greater than a pressure (P0) prevailing on the outer face (3), characterized in that, prior to the introduction of the light gas (4), at least a gas, said heavy gas (5), of molecular size strictly greater than the molecular size of the light gas (4) is applied at a high pressure against the inner face (2) of the wall (1). 2 / - Method according to claim 1, characterized in that the wall (1) is a peripheral wall defining a closed enclosure. 3 / - Method according to one of claims 1 or 2, characterized in that the wall (1) is a wall of organic matrix composite material. 4 / - Method according to one of claims 1 to 3, characterized in that the wall (1) is a porous wall having pores larger than the molecular size of the light gas (4). 5 / - Method according to claim 4, characterized in that the molecular size of at least one heavy gas (5) is less than the average pore size of the wall (1). 6 / - Method according to one of claims 4 or 5, characterized in that the pressure and the duration of application of at least one heavy gas (5) on the inner face (2) depend on the thickness of the wall (1) and the average pore size of the wall. 7 / - Method according to one of claims 1 to 6, characterized in that the high pressure of heavy gas (5) is decreased before the introduction of the light gas (4). 8 / - Method according to one of claims 1 to 7, characterized in that at least one heavy gas (5) is the dinitrogen. 9 / - Method according to one of claims 1 to 8, characterized in that the light gas (4) is helium.10 / - Method according to one of claims 1 to 9, characterized in that the heavy gas (5) is applied at a high pressure against the inner face (2) of the wall (1) until the heavy gas begins to leak from the outer face (3) of the wall.