FR3014861A1 - HYDROGEN TRAP, RESULTING FROM RADIOLYSIS REACTIONS - Google Patents

Abstract

This hydrogen trap, which can be used in particular for the absorption of this gas produced in containers for storing or transporting nuclear organic waste, comprises an active product, divided into powder form and coated in a selective filtration matrix, presenting itself in the form of a thin polymer membrane.

Description

The subject of the invention is a hydrogen trap derived from radiolysis and / or thermolysis reactions. Nuclear facilities produce certain wastes, which may include very diverse objects and materials, such as filters, scrap metal, rubble, glasses, etc. There are also organic materials based on cellulose, such as paper, wood, cotton, and plastics (packaging covers, boots, gloves, etc.). All these wastes constitute materials, either activated during their stay in nuclear installations, or contaminated by radioelement powder, including uranium or plutonium, when they are used in these installations, these radioelements being able to emit particles a, 13, there. Under the effect of the irradiation generated by these radioelements, the materials constituting the waste can degrade and release gaseous compounds, the nature and quantity of which depends on their composition, their temperature and the type of radiation; there may be mentioned, among them, corrosive gases, such as HCI, and flammable gases, such as hydrogen gas H2 or carbon monoxide CO.

This phenomenon is called radiolysis. The production of such flammable gases is problematic when the waste is confined in a closed enclosure of reduced volume, called containment enclosure, since they can quickly reach a high concentration without the possibility of evacuation. In the case of hydrogen, the flammability threshold in air is around 4%. When the concentration exceeds this threshold, a source of heat or a spark may be sufficient to ignite the gas mixture or to produce a violent deflagration. It can be noted that this threshold of flammability is three times higher in the case of CO carbon monoxide, which consequently has a substantially reduced risk compared to that of hydrogen.

The present invention therefore relates to a device for trapping hydrogen, produced by thermal and / or radiolytic degradation, which could be arranged in a containment enclosure in which the waste has been placed, such an enclosure possibly consisting of a packaging intended for transportation or storage of radioactive material. There are materials able to form compounds with hydrogen and thus to trap this gas in solid form, which will be called thereafter active means or "getters"; there are among them metal oxides or metal hydrides, based on ZrCo, Zr2Fe, or Zr-Ti-V, for example. The trapping of hydrogen, carried out by adsorption then absorption on these active means, is however inhibited in the presence of certain gases produced at the same time, which therefore constitute passivants of the active means, and among which mention may be made of carbon CO, as well as some corrosive gases like HCI mentioned above. It is specified that the passivation of the active means is essentially at the surface. It is generally difficult to predict the respective concentrations of the different gases and thus to prevent the risk of surface passivation of the active means. Traps have therefore been developed in which the active means are separated from the hydrogen-containing atmosphere and the other gases by selective filters, which ideally allow hydrogen to pass only. In the French patent application document registered under number 11 51136, the active means are thus arranged in a container, one side of which is equipped with the selective filter and which is also hermetically sealed. The hydrogen passes through the filter and is then trapped by the active means, which maintains a hydrogen partial pressure difference between the inside and the outside of the container, able to continuously reduce the partial pressure of hydrogen produced in the atmosphere. pregnant, while other gases are held out of the container. This device is itself effective, but it has the disadvantage that the filter may crack when it is constituted by a thin layer of microporous silica, the latter being fragile, which then ruins the effectiveness of the device. In other designs, the active product is present in a state embedded in a massive matrix, which provides selective filtration. No. 6,262,328 A and WO 2010/066811 A, in which the matrix is respectively based on glass or cement. The active product is dispersed in the matrix in a divided state, so as to increase its surface of adsorption or absorption of hydrogen. The matrix must be chosen permeable to hydrogen, but not to other gases and in particular to passivants, which imposes a well chosen porosity and which can be difficult to obtain. But these traps are often not very effective anyway, since hydrogen has to traverse a very variable matrix thickness to reach a particle of active products. The kinetics of hydrogen scavenging, by diffusion through the matrix, can be very slow, and the portion of the active means which is located the most inside the matrix can even be useless if it is not reached by hydrogen. The consequence is that a large amount of active product must be used. The hydrogen trap of the present invention, which thus performs the selective filtration, of gases having a kinetic diameter greater than hydrogen, is distinguished from the traps where the active product is embedded in a filter matrix in two aspects: the matrix is of polymer; and it is in the form of a membrane. The polymers have the ability to be easily produced in membranes of a determined and uniform thickness. It should be recognized, however, that many polymers, such as polycarbonates or polysulfones, would not withstand the conditions encountered in radiolytically or thermolysed waste containment enclosures because of the temperature and degradation of ionizing radiation. radiolysis products. Polymers which are sufficiently resistant and which, in addition, have the desired characteristics of filtration selectivity, are nevertheless constituted by the family of polyimides and therefore constitute a preferred embodiment of the invention.

The choice of a membrane for coating the active product makes it possible at the same time not to excessively increase the filtering thicknesses and to widely uniformize this thickness. The consequence is a more efficient use of the active means. Indeed, a matrix in the form of a membrane makes it possible, for a given quantity of active means, to substantially increase the active surface area of the trap relative to the case of a massive filtering matrix of the prior art. The thickness of the matrix that the hydrogen must pass through to reach the active means is thus substantially reduced in the case of a matrix in the form of a membrane. Among certain favorable characteristics, the membrane may have a thickness of less than 100 μm, preferably of approximately 50 μm. Moreover, the active product and the matrix are in the highest possible mass ratio, preferably at least 1/3 and, more preferably, at least 2/3. Thus, the higher the amount of active product embedded in the membrane, the higher the gravimetric entrapment capacities.

Among the favorable features, it is also noted that the active product can be divided into particles having a major dimension of less than 40 μm. Advantageously, the particles are distributed homogeneously in the matrix. The invention will now be described with reference to the figures: - Figure 1 is a view of the structure of the trap; FIG. 2 is a possible embodiment and FIG. 3 is a graph showing the efficiency of said embodiment. In this particular embodiment, the active product is a Zr2Fe alloy, which has been ground manually, to obtain a metal powder with a particle size of 36 .mu.m. This powder was then added to a 2% polymeric solution of Matrimid 5218, which is the trade name of a polyimide variety. A 50 μm thick layer was then cast before being dried. The mass ratio was 2/3. The structure shown in Figure 1, where the scale indication is 100 μm, was obtained.

One embodiment of the trap is shown in FIG. 2, in which the membrane bears the reference 1. It is engaged in a receptacle 2 having at least one opening and which is formed of walls that are either continuous or discontinuous. The membrane 1 can be arranged by making folds, gadroons, a spiral, etc., to avoid excessive clutter. FIG. 3 compares the quantity of hydrogen absorbed by the active product as a function of time according to whether the active product is uncoated (upper curve A), or on the contrary coated (lower curve B) according to the embodiment detailed above. . It is thus shown that in the presence of the CO passivator, the invention according to the embodiment makes it possible to substantially increase the quantity of hydrogen trapped by the active means, the quantity of active means being identical for the two curves. The gaseous mixture was composed of 95% H 2 and 5% CO at a temperature of 25 ° C and a hydrogen partial pressure of 3 bar at the start. In this FIG. 3, the time t is indicated in days, on the abscissa, and the quantity Q of absorbed hydrogen is indicated as a percentage of mass of the active product on the ordinates. Polyimides are preferred because of their resistance to the high temperatures encountered in the containers of the nuclear waste, their resistance to irradiation and their property of separation between, in particular, hydrogen and carbon monoxide CO. In addition, a polyimide membrane can be freely deformed to accommodate any container, or any location in which it is to be placed. The thickness of the membrane is preferably small enough to facilitate the diffusion kinetics of the gas to the active material, but since the thin membranes can tear easily, a thickness of 50 μm has been chosen for this mode of as a compromise. Small particles of active product, the largest dimension of which is between 20 and 40 μm, for example, give better absorption possibilities.

Claims (7)

CLAIMS1) A hydrogen trap for use in conjunction with ionizing radiation emitting products, comprising an active product for trapping hydrogen and a matrix, coating the active product, for selective filtration of gases of higher kinetic diameters than hydrogen, characterized in that the matrix is a polymer membrane. 2) A hydrogen trap according to claim 1, characterized in that the polymer is a polyimide. 3) A hydrogen trap according to any one of claims 1 or 2, characterized in that it comprises an open container in which the membrane is arranged in a pleated state. 4) Hydrogen trap according to any one of claims 1 to 3, characterized in that the active product and the matrix are in a mass ratio greater than 1/3. 5) A hydrogen trap according to claim 4, characterized in that the active product and the matrix are in a mass ratio greater than 2/3. 6) Hydrogen trap according to any one of the preceding claims, characterized in that the active product is divided into particles. 7) hydrogen trap according to claim 6, characterized in that the particles have a major dimension of less than 40 .mu.m.