RU2758464C1 - Method for purification of gas mixture from tritium - Google Patents

Abstract

FIELD: gas mixture purification.SUBSTANCE: invention relates to a method for purifying a gas mixture from tritium, which consists in catalytic oxidation of tritium followed by sorption of the resulting water containing tritium. In this case, an anhydrous salt or lower hydrates of magnesium chloride is used as a sorbent for water vapor containing tritium, after saturation of the sorbent with water vapor containing tritium, magnesium oxide and water are added to it, the components are mixed and held until complete curing, while the components are taken into ratio sufficient to obtain the final crystalline hydrate 3MgO×MgCl2×11H2O.EFFECT: method makes it possible to reduce radiation risks by eliminating the formation of secondary radioactive waste, as well as to simplify the technology for cleaning gases from tritium, including through the use of cheap, efficient and widespread materials.1 cl, 1 ex

Description

The inventive method relates to the field of environmental protection from radioactive contamination and can be used in the purification of gas mixtures from tritium by oxidation followed by sorption of water vapor containing tritium.

Safety assurance when working with tritium at nuclear facilities is based on the principle of comprehensive protection, which makes it possible to reliably separate tritium from process gases. The technological stage of processing gaseous radioactive waste containing tritium must meet the following requirements: waste minimization, safety, isolation from the environment, and economic efficiency.

For laboratory premises in which equipment with local forced exhaust ventilation is installed (cabinets, boxes, chambers, shelters, etc.), under normal operating conditions, the removed contaminated air must be purified from tritium. In accordance with the requirements of the current Sanitary Rules "Radiation safety when working with tritium and its compounds (SP RB-RTS-04)", the air removed by the exhaust, local, repair and emergency ventilation systems should be specially cleaned from: gaseous tritium, tritium oxide, tritium aerosols.

The purification of gases from tritium, based on the oxidation of the latter, followed by the sorption of tritium-containing water, has received wide practical application in laboratory and industrial practice. The oxidation of tritium can be carried out on metal oxides, for example CuO, or directly with gaseous oxygen in the presence of catalysts.

The catalytic purification of gases from tritium followed by adsorption has become the most widespread in practice. This is due to the fact that the processes occur at relatively low temperatures (in the presence of nickel, palladium, and platinum, the interaction takes place already at 20 ° C), the period of effective use of industrially produced catalysts is several years, the method is distinguished by its technical simplicity and high productivity. An additional advantage of the catalyst from tritium gas cleaning is the fact that this method in addition to the conversion of gaseous tritium in the presence or addition of oxygen in a stoichiometric (with respect to H ₂₎ ratio also allows simultaneous capture and tritium oxide present in the cleaned gases. The effectiveness of this method in practice is determined by the catalytic activity of the converter (catalytic unit) and the absorption capacity of the adsorber (adsorption unit). However, the presence of inhibitors (CO, CO ₂ , SO ₂ , etc.) in the purified gas environment significantly reduces or completely blocks the catalytic activity.

The process of oxidation of tritium on metal oxides occurs at a higher temperature of 270-500 ° C, but this method is not sensitive to the presence of impurities in the gas being purified. Also, there is no need to introduce oxygen into the gas environment to be purified, which increases the safety of the work carried out and makes it possible to purify inert gases from tritium.

In adsorbers of oxidizing plants, various zeolites are most often used as a filler (Tritium. LF Belovodsky et al. M .: Energoatomizdat, 1985). The use of this type of material for capturing water vapors containing tritium is due to its good absorption capacity, which provides a low total breakthrough coefficient, together with the possibility of multiple regeneration. However, despite the reusability of zeolites, the tritium-containing water captured by them does not chemically bind, which entails the need for its subsequent isolation and binding into stable matrix materials for safe storage and / or disposal. The complete extraction of water from zeolites is a complex and energy-intensive operation. For example, the regeneration of the widespread NaA zeolite is carried out for 4 hours at a residual pressure of 0.13 Pa and 500 ° C. As a result of capturing desorbed vapors, secondary liquid radioactive waste (LRW) is formed. In the case of water containing tritium, the technological operation resulting in the formation of highly toxic secondary LRW is extremely dangerous for personnel and the environment. The elimination of this operation and direct curing of the adsorber filler after saturation with water containing tritium will reduce the radiation hazard of the work performed, simplify the hardware and the entire technological process.

There is a known method for purifying helium from impurities of hydrogen isotopes (patent RU 2323157, С01В 23/00, publ. 04/27/2008), according to which the initial gaseous mixture is fed into a sorption unit with porous nanodispersed carbon to absorb hydrogen isotopes. At the same time, helium is removed from sorption block. The temperature in the sorption block is maintained at 77-80 K, after the removal of helium, the temperature in the sorption block is raised to room temperature for desorption of hydrogen isotopes, having previously connected it to a vessel for collecting hydrogen isotopes, which contains a hydride-forming sorbent. The method makes it possible to purify helium from impurities of hydrogen isotopes with simultaneous extraction and trapping of tritium.

The disadvantages of this method are:

- complex hardware equipment of the installation for the implementation of the method and the need to ensure the tightness of all units and connections in a wide temperature range;

- the possibility of separating only a gas mixture of inert helium and hydrogen isotopes, and the presence of foreign gases will not allow the separation process to be fully implemented;

- to ensure the selectivity of the sorption capacity of the sorbent, it is necessary to maintain it at a low temperature in a narrow permissible range.

A known method and device for purifying air from gaseous tritium and its concentration in a constant volume of water (patent RU 2635809, G21F 9/02, publ. 16.11.2017), according to which tritium is oxidized inside a hermetically sealed chamber at high temperatures (1000-1300 ° C) in a hydrogen-oxygen flame, the gas mixture for which comes from a suitable generator. After processing, the resulting mixture of air and water vapor containing tritium is removed from the chamber and cooled, passing through the refrigerator, and condensed water accumulates in a water filter. The air purified from tritium is pumped back to the plants and technological lines. Water containing tritium from the water filter is supplied in parts to the generator, where it is converted into a hydrogen-oxygen mixture. The resulting gas is used to oxidize tritium from the next portion of air. Multiple repetition of the cycle allows increasing the concentration of tritium in the condensed water.

The disadvantages of this method include the following:

- oxidation of tritium occurs at a high (over 1000 ° C) temperature and at the same time an explosive gas mixture is used at certain concentrations, which imposes increased requirements on the reliability of the installation;

- to obtain a hydrogen-oxygen mixture, electrolysis is used, and the combustion products are cooled to condense water vapor from them, which entails high energy costs;

- as a result, secondary radioactive waste water with a high tritium content is formed, which must be converted into a solid state, which requires additional technological operations and equipment;

- the installation used to implement the method is technologically complex.

Closest to the claimed method is a method for reducing the concentration of hydrogen isotopes in a gas environment (patent RU 2243148, С01В 3/56, publ. 27.12.2004). The authors of this invention propose to use biologically active soil with a moisture content of 0.5-20 wt. %. Biologically active soil is understood as containing microorganisms, including the so-called hydrogen bacteria capable of oxidizing hydrogen isotopes. The use of this method makes it possible to eliminate the need to remove moisture from the used catalyst material, to carry out cleaning at room temperature, and also to reduce the cost of the method due to the use of available and cheap material.

Among the disadvantages of the known method, the following can be noted:

- biologically active soil used as a sorbent retains its catalytic ability up to a moisture content of no more than 20%;

- the stability of the shown results of the catalytic activity of each specific soil sample in relation to the oxidation of hydrogen isotopes depends on its temperature, humidity, sampling depth, on preliminary chemical and physical treatment;

- the experimental use of the method for air purification from tritium, known from the description, took place at a temperature of 15 to 22 ° C, and it is possible to assume that at higher or lower temperatures, the catalytic activity of the soil may significantly decrease or completely stop;

- during long-term storage, in order to preserve the original properties, it is necessary to create special climatic conditions for the soil;

- after use, the soil contaminated with tritium passes into the category of secondary radioactive waste, and in order to prevent the dispersion of radionuclides, it is necessary to carry out additional measures for its immobilization in the matrix material and packaging.

The objective of the claimed invention is to reduce radiation risks by eliminating the formation of secondary radioactive waste, and to simplify the technology for cleaning gases from tritium, including through the use of cheap, effective and widespread materials.

The technical result achieved by using the proposed method is as follows:

- the formation of secondary radioactive waste is excluded, which excludes the technologically complex and environmentally hazardous stage of regeneration of the sorbent from radioactive water vapors;

- simplification of the technological and hardware design of the process of catalytic oxidation of radioactive gases;

- low partial pressure of water vapor above the surface of the sorbent allows its safe reloading without the use of boxes with an inert atmosphere;

- the sorbent used is already one of the initial components of the resulting final matrix, suitable for long-term storage and / or disposal, and there is no need to use an intermediate sorbent, which, after the end of its operation, goes into the category of secondary radioactive waste;

- the absorption capacity of the sorbent is retained when it absorbs water vapor up to 50% of the initial weight.

To solve this problem and achieve the technical result, a method of purifying a gas mixture from tritium is claimed, which consists in the oxidation of tritium followed by sorption of the resulting water containing tritium, in which, according to the invention, anhydrous salt or lower hydrates are used as a sorbent of water vapor containing tritium magnesium chloride, after saturation of the sorbent with water vapor containing tritium, magnesium oxide and water are added to it, the components are mixed and kept until complete curing, while the components are taken in a ratio sufficient to obtain the final crystalline hydrate 3MgO × MgCl ₂ × 11H ₂ O.

In the claimed method, anhydrous salt or lower hydrates of magnesium chloride are used as a sorbent of tritium-containing water obtained by catalytic oxidation of tritium in the gas mixture. The inventive method makes it possible to effectively capture the vapors of radioactive water formed during the oxidation of gaseous tritium. In this case, the sorbent that binds tritium-containing water is at the same time one of the initial components of the matrix material for its curing (or for curing tritium-containing water). This approach makes it possible to simplify the technological process of gas purification from tritium, eliminating the stage of sorbent regeneration. Repeated use of known sorbents leads to a decrease in their absorption capacity. After the end of the planned operation, the structure of materials of known sorbents contains non-removable residues of radioactive components, which translates them into the category of secondary radioactive waste. When water vapor containing tritium, anhydrous salt, or lower hydrates of magnesium chloride is used as a sorbent, additional processing of the sorbent is not required and secondary radioactive waste is not formed. In the course of the research, it was found that the residual air humidity above the surface of the proposed sorbent does not exceed 0.001 g / m ³ . This parameter remains unchanged during the entire time of use of the sorbent batch. After saturation with vapor, the sorbent in the absorption unit is replaced, which guarantees a constant level of efficiency of the gas purification process. With the experimental blowing of the surface of the 2-aqueous magnesium chloride powder with air with an initial moisture content of 33-75%, at a temperature of 25 ° C, the water vapor absorption rate remains practically constant until the initial sorbent mass increases by 50%. The widely used 6-aqueous magnesium chloride is used as a starting material for obtaining the inventive sorbent. Dehydration of the initial component occurs at temperatures up to 200 ° C and can be realized without the use of expensive technologically sophisticated equipment. Low partial pressure of water vapor above the surface of the sorbent - allows its safe reloading, transportation, storage and mixing with other components to obtain a compound suitable for long-term storage or disposal.

By the combination of the factors considered, it can be argued that the use of oxidation of tritium for the purification of a gas mixture from tritium, followed by the sorption of the obtained radioactive water using anhydrous salt or lower hydrates of magnesium chloride, and the addition of magnesium oxide and water to a sorbent saturated with vapors of tritium-containing water until it is completely curing, allows you to achieve the specified technical result.

The inventive method for purifying a gas mixture from tritium is carried out in the following sequence. The gas mixture, in which tritium is present, enters the oxidation unit of the plant, where tritium is oxidized to tritium-containing water on a catalyst or metal oxide. Vapors of tritium-containing water enter the sorption unit, where the sorbent is located - anhydrous salt or lower hydrates of magnesium chloride. After saturation of the sorbent with bunks of tritium-containing water, magnesium oxide and water are added to it in a ratio sufficient to obtain the final crystalline hydrate 3MgO × MgCl ₂ × 11H ₂ O. The components are mixed and kept until complete curing into a stable matrix material. Thus, as a result, the gaseous medium is purified from tritium and at the same time the resulting tritium-containing water is imprisoned in a stable matrix.

An example of a specific implementation.

1. Checking the performance of the proposed scheme for cleaning gaseous media from water vapor.

_{Magnesium chloride 6-aqueous (analytical grade) MgCl 2} × 6H ₂ O according to GOST 4209-77 was used as the initial component for the preparation of the sorbent of water vapor. Despite the fact that 6-aqueous magnesium chloride hydrate also has a sorption capacity up to the transition to solution, the presence of water in it, exceeding the ratio of MgCl ₂ × 6H ₂ O, negatively affects its operational characteristics. Powder crystals begin to stick together with the formation of solid agglomerates, and then softening to a pasty state. In this regard, the efficiency of the sorbent was considered in the range of hydrates from MgCl ₂ × 2H ₂ O to MgCl ₂ × 6H ₂ O. The anhydrous salt of MgCl ₂ is obtained at a high temperature in an atmosphere of circulating HCl. Despite the fact that there are industrial plants for the production of anhydrous magnesium chloride, it is not safe to obtain it in laboratory practice. Therefore, for experiments on the development of the proposed method used MgCl ₂ × 2H ₂ O. The process of dehydration of magnesium chloride should be carried out stepwise. With a sharp heating of the higher hydrates MgCl ₂ and difficulties in removing the released water, the hydrate melts in the vapors of the released crystallization water and dissociates with the formation of insoluble hydroxide and the release of hydrogen chloride. Dewatering hydrate used electronic hygrometer with a halogen heater, maintaining temperature accuracy ± 1 ° C, the sample mass measurement error of ± 0.01 A weighed sample of the starting MgCl ₂ × 6H ₂ O 10.00 g weight heated to a temperature of 181 ± 1 ° C and kept to constant weight for ~ 30 minutes. At this temperature, according to the literature [Kramar L.Ya. etc. Compositions based on magnesia binder, not prone to cracking during operation], MgCl ₂ × 6H ₂ O loses four water molecules to form MgCl ₂ × 2H ₂ O. After dehydration, the weight of the sorbent sample was 6.45 g.

Further, the obtained sample of the sorbent was placed with the sorption unit of the laboratory installation for cleaning the gaseous medium of the technological line from hydrogen isotopes. The unit is equipped with a converter based on copper oxide, in which the processes of copper reduction and oxidation of hydrogen isotopes (including tritium) to water vapor occur in parallel. At the end of the pumping of the gaseous medium through the purification unit, the sorbent was removed and weighed. As a result, the sample absorbed 3.23 g of water (~ 50%). After that, the sample was placed in a plastic container, 5.91 g of MgO and 4.11 g of water were added. The resulting mixture was mixed and sealed. After 24 hours, a sample of the resulting simulated compound was a solid, monolithic material, without cracks and delamination. There were no traces of free moisture on the walls of the sealed container, which indicates the complete binding of water to a stable crystalline hydrate 3MgO × MgCl ₂ × 11H ₂ O - the main component of magnesia hydraulic binder.

2. Assessment of the main kinetic parameters of the water vapor sorption process.

The main characteristics of the drying agent:

- the minimum residual water vapor content above its surface;

- the percentage of absorbed water relative to the initial mass;

- the efficiency of binding water vapors under various conditions.

The measured residual air humidity above the surface of the obtained sorbent did not exceed 0.001 g / m ³ . The kinetics of the binding of water vapor by the sorbent was determined by measuring the mass of the sample when it was blown with air of known humidity. A sorbent sample weighing 3 g was spread evenly over the bottom of an aluminum saucer, which was placed on an analytical balance. From above it was covered with a plastic dome with two fittings for the inlet and outlet of humidified air. The resulting structure was connected by pipelines to a closed loop with a membrane pump with a capacity of 10 l / min and a container with a saturated solution to maintain the required constant humidity of the forced air flow. To maintain a constant humidity of the air flow, it was driven over a mirror of saturated solutions of the following substances:

- 75% at 25 ° С - sodium chloride (NaCl);

- 50% at 25 ° С - calcium nitric acid 4-water (Ca (NO ₃ ) ₂ × 4H ₂ O);

- 33% at 25 ° С - 6-aqueous magnesium chloride (MgCl ₂ × 6H ₂ O).

The total volume of the closed loop was ~ 1.2 liters, which provided 8-fold air exchange over the sample surface per minute. Blowing was carried out until the moment when the sample binds water in the amount of 50% of its original weight. The average rate of water sorption per 1 g of sorbent was:

- 0.09 g × min / g at air flow humidity of 75% and 50%;

- 0.05 g × min / g at a humidity of the air flow of 33%.

Measurements of the residual air humidity above the sorbent surface after it binds water in an amount of 50% of the initial weight showed that this parameter did not change.

The tests carried out confirm the achievement of the claimed technical result. For the preparation of the sorbent, widely used laboratory equipment and starting materials were used. Practical measurements have shown a low partial pressure of water vapor over the surface of the sorbent and its stable absorption capacity at various humidity indicators of the gaseous medium. The results of testing the technology of curing the used sorbent showed the absence of conditions for the formation of secondary radioactive waste.

Claims (1)

A method for purifying a gas mixture from tritium, which consists in the catalytic oxidation of tritium followed by sorption of the resulting water containing tritium, characterized in that anhydrous salt or lower hydrates of magnesium chloride is used as a sorbent for water vapor containing tritium, after saturation of the sorbent with water vapor containing tritium, magnesium oxide and water are added to it, the components are mixed and kept until complete curing, while the components are taken in a ratio sufficient to obtain the final crystalline hydrate 3MgO × MgCl ₂ × 11H ₂ O.