FR2994860A1 - PASSIVE TRITIUM SAMPLING DEVICE - Google Patents

Abstract

The invention relates to a device (100) for extracting tritium present in a gaseous environment (E), comprising: - a reservoir (110) in fluid communication with the gaseous environment, and - a quantity of adsorbent material (120) disposed within said reservoir (110) adapted to adsorb tritium contained in the gaseous environment, the device comprising at least one fluid communication opening (131) between the reservoir and the gaseous environment, the device being characterized in that: - the geometry of the fluid communication opening (131) is adapted, depending on the duration of the sampling, so that the amount of adsorbent material is not saturated at the end of the sampling, and, the adsorbent material is shaped so as to have a surface of exchange of revolution with the gaseous environment.

Description

FIELD OF THE INVENTION The invention relates to the study and monitoring of radionuclides in the environment, and in particular devices for trapping and measuring a quantity of tritium contained in the air.

STATE OF THE ART Tritium is the radioactive isotope of hydrogen comprising three nucleons (a proton and two neutrons). It is represented by the chemical symbol T, and occurs in the environment in three chemical forms: - Tritiated water, HTO, forms the most abundant, - Tritium gas, HT, - Organic tritium, noted OBT, which can appear following environmental exchanges or metabolic reactions of living beings. As the amount of tritium in the environment has increased with the development of the use of nuclear energy, there is a growing need to analyze its concentration and behavior, particularly in the atmosphere. Tritium trapping devices contained in the air are already known. In particular, devices known as "bubblers", an example of which is illustrated schematically in FIG. 1, comprise pots filled with water 11 in which an air stream F is injected at a very low flow rate - of the order of 30 liters per cubic meter. hour on average - over a period of several days. The tritium contained in the air is trapped by exchange in the water contained in the different pots 11. It is then analyzed to obtain an indication of the average activity of tritium in the air during the sampling period.

This device nevertheless has disadvantages. First, it is an "active" device, that is to say that requires a permanent supply of energy, which represents a material constraint. This device is also expensive and requires maintenance interventions. In addition, the amount of tritium contained in the water vapor initially present in the air is greatly diluted since it is mixed with the volume of water contained in the pots, this dilution limiting the sensitivity of the measurement. Another type of tritium trapping device has been developed, illustrated schematically in FIG. 2, consisting in condensing the water vapor contained in the air, in order to recover the tritium present in the air in the form of tritiated water. This device 20 is an open system comprising a duct 21 bathed in the ambient air, represented in the form of an air flow F, in which circulates a gas g liquefied at a negative temperature, and on which the water vapor present is condensed. in the air in contact with the pipe. The device further comprises a ventilation 22 to ensure the renewal of ambient air near the pipe. At the end of sampling, the condensed water Ec can be recovered to analyze the quantity of tritium that it contains. This type of device makes it possible to take air samples over shorter periods (less than one hour), and therefore does not make it possible to obtain an average value of the quantity of tritium present in the air over periods of the order of the week or more. This device has the disadvantages of being an active system, expensive to use and difficult to implement. Another tritium sampling device is still known with reference to FIG. This device is passive, that is to say it does not require any source of energy. It comprises a reservoir 31 inside which is placed a cylinder of adsorbent material 32, adapted to take tritium contained in the air by adsorption. However, this device does not have the same efficiency of sampling tritium throughout the duration of the sampling. Indeed, the adsorbent material quickly reaches, after a period of three to four days, to a state of saturation in the surface layers of the absorbent material, in contact with the environment. This prevents the results from being exploited to derive an average activity of tritium in the environment during a sampling period exceeding the saturation threshold, typically 15 days. PRESENTATION OF THE INVENTION The object of the invention is to overcome at least one of the disadvantages mentioned above.

An object of the invention is thus to propose a passive device for sampling tritium in a gaseous environment, making it possible to obtain a value of the average activity of tritium throughout the sampling. Another object of the invention is to provide an economical tritium sampling device, easy and fast to deploy in an environment.

In this respect, the invention proposes a device for sampling tritium present in a gaseous environment, comprising: a reservoir in fluid communication with the gaseous environment, and a quantity of adsorbent material disposed inside said reservoir, adapted to adsorb the tritium contained in the gaseous environment, the device comprising at least one fluid communication opening between the reservoir and the gaseous environment, the device being characterized in that: - the geometry of the communication opening of the fluid is adapted, depending on the duration of the sampling, so that the amount of adsorbent material is not saturated at the end of the sampling, and, the adsorbent material is shaped so as to have a surface of exchange of revolution with the gaseous environment.

Advantageously, but optionally, the tritium sampling device according to the invention may further comprise at least one of the following characteristics: the surface of the opening or the cumulative surface of the fluid communication openings is less than 400 mm 2 for a 10-day collection, and 200 mm2 for a 20-day collection. - The one or more communication openings are formed in a lid or a membrane attached to said tank. the membrane is a hydrophilic membrane. - The device comprises a circular lid attached to the reservoir, the fluid communication openings being circular and arranged in an arrangement selected from the following group: o openings arranged at the vertices of a regular polygon centered on the center of the lid, o openings arranged at the vertices of a regular polygon centered on the center of the lid, and an opening in the center of the lid, o openings arranged at the vertices of a regular polygon centered on the center of the lid, and in the middle of the sides of the polygon, o openings arranged at the vertices of a regular polygon centered on the center of the cover, in the middle of the sides of the polygon, and in the center of the cover, o openings arranged at the vertices of a regular polygon centered on the center of the cover, and on at least one diagonal of the polygon so that the openings of said diagonal are regularly distributed, o openings arranged at the vertices of a poly regular gone centered on the center of the lid, in the middle of the sides of the polygon, and on at least one diagonal of the polygon so that the openings of said diagonal are evenly distributed, o openings arranged at the vertices of a regular polygon centered on the center cover, in the middle of the sides of the polygon, in the center of the lid, and on at least one diagonal of the polygon so that the openings of said diagonal are evenly distributed, o spirally arranged openings, o randomly distributed openings on the lid. - The reservoir comprises symmetrical side walls around an axis of the reservoir, and the exchange surface of the adsorbent material is of revolution about said axis. the exchange surface of the adsorbent material is cylindrical, frustoconical or conical. - The device further comprises a permeable wall disposed within the reservoir, the adsorbent material being disposed between the side walls of the reservoir and the permeable wall so that said permeable wall defines the exchange surface between the adsorbent material and the gaseous environment. the permeable wall is a grid. the absorbent material is chosen from the following group: silica gel, molecular sieve, zeolite, activated carbon.

The invention also proposes a method of sampling tritium in a gaseous environment, comprising the steps of: - determining a desired sampling time, - selecting a geometry of communication openings of a sampling device according to one of the preceding claims depending on the desired sampling time, and arrange the device in the gaseous environment, and - maintain the device in the gaseous environment for the duration of the sampling. BRIEF DESCRIPTION OF THE FIGURES Other characteristics, objects and advantages of the present invention will appear on reading the detailed description which follows, with reference to the appended figures, given by way of non-limiting examples and in which: FIG. 1 , already described, schematically shows a tritium trapping device of the prior art called bubbler, - Figure 2, already described, schematically shows a tritium trapping device of the prior art by condensation. FIG. 3, already described, schematically represents a passive tritium sampling device of the prior art. - Figures 4a, 4b and 4c show cross-sectional views of three embodiments of a tritium sampling device according to the invention. - Figures 5a to 5e show several embodiments of lids of a sampling device according to the invention. - Figure 6 shows the mass of water adsorbed by a device according to the invention for the duration of sampling.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION FIGS. 4a and 4b show embodiments of a device 100 for the passive sampling of tritium in a gaseous environment E. Such a device comprises a tank 110, in communication of fluid with the environment E, so that once the device placed in the environment E, a part E 'thereof is inside the tank. In the tank 110 is placed a quantity of adsorbent material 120. The adsorbent material preferably comprises silica gel, but alternatively may comprise zeolite or activated carbon.

This material, when it is in contact with the gaseous environment E 'inside the device 100, makes it possible to adsorb the tritium contained therein. The device 100 advantageously comprises a membrane or a cover 130 attached to the tank 110, in which are formed one or more communication openings 131, allowing the communication of fluid between the environment E and the tank 110. This fluid communication can also be achieved by the use of a porous membrane (not shown) placed on the reservoir, allowing gaseous exchange between the reservoir and the environment E.

The Applicant has found, surprisingly, that a phenomenon of saturation of the adsorbent material during the sampling can be avoided by adapting the geometry of the communication opening or openings 131, both in cumulated total surface, in position or disposition openings. Thus, for a sampling time of about 10 days, the total area of the communication opening 131 between the device and the environment is preferably less than 600 mm 2, and very advantageously less than 200 mm 2. Referring to Figure 5, there are shown preferred embodiments of arrangements of a plurality of communication openings 131 on a cover 130 attached to the tank 110.

The cover 130 is preferably circular, and the reservoir 110 advantageously has symmetrical side walls 111 around an axis X-X of the reservoir, the cover being centered on said axis. The tank 110 may be a solid of revolution about said axis. In the case of a single communication opening 131 on the cover 130, this opening is preferably circular and centered on the X-X axis, as shown in FIG. 5a. If several communication openings 131 are provided, the openings are advantageously circular and positioned at the top of a regular polygon centered on the X-X axis, like the pentagon shown in FIG. 5b.

An additional opening may also be in the center of the cover 130 as in Fig. Sc. Additional openings may also be located at the midpoints of one or more sides of the polygon. In Figure 5d, the polygon is a square, all sides have an opening in the middle.

Additional openings may also be provided on one or more diagonals of the polygon so that the openings of said diagonal are evenly distributed, as is the case in Figure 5e. The openings can also be randomly or spirally distributed on the lid. Advantageously, all the openings arranged in a cover 130 are circular and of the same diameter. The number of openings and their diameter are adapted to obtain a cumulative surface area of less than 400 mm 2, for example, in the case of a sampling lasting between 2 and 10 days.

In the case of a sample of 20 days, the cumulative surface area of the openings is advantageously less than 200 mm 2, preferably less than 190 mm 2. In the case where the sampling device is to be used for a shorter duration, for example less than 2 days, the cumulative surface area of the openings may be greater than 400 mm 2, advantageously greater than 1000 mm 2. In addition, and returning to Figures 4a and 4b, the Applicant has also found, surprisingly, that the adsorbent material is shaped in the tank so as to have a surface of exchange of revolution with the gaseous environment E 'present in the tank, helps to avoid a state of saturation of the adsorbent. Preferably, the material has a surface of exchange of revolution about the axis X-X of the reservoir, this exchange surface may be cylindrical, as in Figure 4a, or conical, as in Figure 4b. The exchange surface may also have a cone section shape, as in FIG. 4c.

To maintain the shape of the adsorbent material, preferably a permeable wall 140 is used, pressed against the adsorbent material so that it is held between the walls of the reservoir and the wall. The shape of the exchange surface of the adsorbent material with the environment E 'is thus defined by the permeable wall 140.

In the case where the adsorbent material chosen is granular, such as for example silica gel, the wall 140 is advantageously a grid whose characteristic size of the holes is smaller than the characteristic grain size of the adsorbent material.

According to a preferred embodiment of the tritium sampling device, use is made of adsorbent material of molecular sieve type (for example zeolite) disposed in a frustoconical geometry such as that of Figure 4c. The cumulative surface of the openings 131 of the lid is equal to 250 mm 2.

The results obtained with this embodiment are illustrated in FIG. 6. It can be seen that they make it possible to obtain linear measurements during a sampling period of 20 days (the linearity being deduced from the mass of the adsorbent material throughout the sample), at a temperature of 22 ° C. and with a relative humidity of 50%. To carry out the sampling, a desired sampling time is first determined, for example 2, 10 or 15 days. For this sampling period, the geometry of the communication apertures 131 (in arrangement and cumulative surface area) is selected as indicated above, and a shape is chosen for the adsorbent material (cone, cone section or cylinder). Then, the device is placed in the environment for the duration of the sampling. At the end of the sampling, the adsorbent material did not reach saturation, but on the contrary adsorbed linearly the tritium present in the environment. It is therefore possible to deduce from the total amount of tritium adsorbed during the sampling an average amount of tritium present in the environment during this sampling.

Claims (11)

REVENDICATIONS1. Tritium sampling device (100) present in a gaseous environment, comprising: - a reservoir (110) in fluid communication with the gaseous environment, and - a quantity of adsorbent material (120) disposed inside said reservoir , adapted to adsorb the tritium contained in the gaseous environment, the device (100) comprising at least one fluid communication opening (131) between the reservoir and the gaseous environment, the device being characterized in that: - the geometry the fluid communication opening (131) is adapted, depending on the duration of the sampling, so that the amount of adsorbent material is not saturated at the end of the sampling, and, - the adsorbent material (120) is shaped so as to have a surface of exchange of revolution with the gaseous environment. The tritium sampling device (100) according to claim 1, wherein the area of the opening or the cumulative area of the fluid communication openings (131) is less than 400 mm 2 for a 10 day sample, and 200 mm2 for a 20-day collection. 3. Device (100) for sampling tritium according to one of the preceding claims, the or the communication openings (131) are formed in a cover (130) or a membrane attached to said reservoir (110). 4. Device (100) for sampling according to the preceding claim, wherein the membrane is a hydrophilic membrane. 5. Device (100) for sampling according to one of the preceding claims, comprising a circular lid attached to the reservoir (110), the fluid communication openings (131) being circular and arranged in a disposition selected from the following group: - apertures arranged at the vertices of a regular polygon centered on the center of the lid, - apertures arranged at the vertices of a regular polygon centered on the center of the lid, and an opening in the center of the lid, - apertures arranged at the vertices of a regular polygon centered on the center of the lid, and in the middle of the sides of the polygon, - apertures arranged at the vertices of a regular polygon centered on the center of the lid, in the middle of the sides of the polygon, and in the center of the lid, - openings arranged at the vertices of a regular polygon centered on the center of the lid, and on at least one diagonal of the polygon so that the openings of said diagonal are regularly distributed, - apertures arranged at the vertices of a regular polygon centered on the center of the cover, in the middle of the sides of the polygon, and on at least one diagonal of the polygon so that the openings of said diagonal are regularly distributed, - openings arranged at the vertices of a regular polygon centered on the center of the lid, in the middle of the sides of the polygon, in the center of the lid, and on at least one diagonal of the polygon so that the openings of said diagonal are regularly distributed, - openings arranged in spiral, - randomly distributed openings on the cover (130). 6. Device (100) for sampling tritium according to claim 1, wherein the reservoir comprises symmetrical side walls around an axis of the reservoir (110), and the exchange surface of the adsorbent material is of revolution about said axis . The tritium sampling device (100) according to claim 6, wherein the exchange surface of the adsorbent material (120) is cylindrical, frustoconical or conical. 8. Device (100) for sampling tritium according to one of claims 6 or 7, further comprising a permeable wall (140) disposed inside reservoir (110), the adsorbent material (120) being disposed between the walls side of the reservoir (110) and the permeable wall so that said permeable wall (140) defines the exchange surface between the adsorbent material and the gaseous environment. 9. Device (100) for sampling tritium according to the preceding claim, wherein the permeable wall (140) is a grid. 10. Tritium sampling device (100) according to one of the preceding claims, wherein the absorbent material (120) is selected from the following group: silica gel, molecular sieve, zeolite, activated carbon. 11. A method of sampling tritium in a gaseous environment, comprising the steps of: - Determining a desired sampling time, - Selecting a geometry of communication openings (131) of a sampling device (100) according to the one of the preceding claims depending on the desired sampling time, and arrange the device in the gaseous environment, and - maintain the device in the gaseous environment for the duration of the sampling.