SU1716574A1 - Method for cleaning chlorine-containing gases from radioactive aerosol particles of actinide - Google Patents

Abstract

Use: production and regeneration of nuclear fuel based on actinides with dividing isotopes on molten metal chloride salts. SUMMARY OF THE INVENTION: Pre-purification, gas dehydration, and fine purification are carried out by precipitation of radioactive aerosol particles during condensation of chlorine from waste gases at low temperatures and reduced pressure with recycling to the process and discharging the purified gases to the atmosphere. 2 hp ff, 1 ill. 5 fc

Description

The invention relates to nuclear energy, mainly to the technology of production and regeneration of nuclear fuel based on actinides with dividing isotopes using salt melts of metal chlorides, and can be used to purify chlorine-containing waste gases from radioactive aerosol particles. .

There is a known method for neutralizing waste chlorine-containing gases by passing them through molten alkali metal hydroxide at 500-700 ° C. The barbaty gas is melted through the melt, gaseous chlorine is absorbed, and at the same time, aerosol particles containing radioactive nuclides are captured, and the reaction products, alkali metal chlorides, can be used as the solvent salt in the head unit, the chlorine electrolyzer.

This method requires complex equipment, especially with significant amounts of processed gases. Due to the high temperature and aggressiveness of the alkali melt and the chlorides formed, the equipment has a short service life - it is unreliable in operation. Individual units require frequent replacement, after which they are classified as radioactive waste.

Work with solid alkali metal hydroxides, and, in particular, with

O (L

VI

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their melts represent a significant hazard, are difficult to mechanize and automate.

The method has limited application, since the amount of alkali metal chloride formed exceeds its need for use as a solvent salt.

By bubbling the aerosol through the melt, it is difficult to achieve high degrees of purification from aerosol particles, which are required in this case, t, e. additional cleaning is required.

In practice, the required degree of gas purification is achieved by stepwise cleaning, preliminary and fine. It is caused by the fact that fine filters effectively operate at low loads with a mass concentration of aerosol particles and by volume concentration of radioactive substances in the exhaust gases. Therefore, the pre-treatment stage is selected such that the maximum amount of the mass and radioactive fractions of aerosol particles is extracted from the exhaust gases, and the pre-treatment operation can be combined with the preparation of gas for fine cleaning, for example, with drying.

However, the known method cannot provide fine purification, but only partially provides a preliminary one, since the gases passing through the alkali melt contain a large mass of aerosol particles, which are chemically aggressive and hygroscopic.

The known method consists in that waste chlorine-containing gases, including radioactive aerosol particles, formed during the purging of technological devices with gases (or creating a vacuum in the devices) with radioactive substances in them, are subjected to pre-treatment, passing through a packed, dish-shaped or other an absorber irrigated with an absorption solution, for example a 5% solution of soda ash, and after moisture removal in the refrigerator (or some other method), fine cleaning is carried out; ka through the filter Petr new.

This method allows the first stage pre-treatment (in the absorber) to purify the gas from the main amount of contaminants simultaneously from chlorine gas). In a dehumidification operation, a gas is prepared for cleaning on a Petr nova cloth filter. At the second stage (fine cleaning) on the filter from the fabric of Petr Nova they catch the residual amount of aerosol contamination. After accumulation

The radioactive dust used filter from fabric Petr nova is removed to waste.

The disadvantages of this method are low degree of purification of flue gases at both stages of purification and in sum, low reliability - with sudden influxes to purification of gases with a higher concentration of radioactive substances

0 to increase emissions into the atmosphere. In addition, chlorine is not disposed of, but dumped into waste, used filters from Petr nov fabric create radioactive waste.

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These deficiencies are due to the fact that incomplete absorption usually occurs in irrigated nozzles, disc or other type of absorbers

0 fractions of highly dispersed aerosol particles (with a diameter equal to or less than 1 micron). Under the condition of low specific activity of aerosol particles, this does not have a significant effect on the quality

5 purification of waste gases, as may-. Owing to the fraction of highly dispersed particles, aerosols are usually small, but the omission of these particles becomes unacceptably large when they include transuranic

0 elements of high specific activity. Due to the same high specific radioactivity, aerosol containing transuranic elements, when cleaning it in the absorber, insignificant amounts

The 5 transuranium elements in the water droplets leaving the absorber with gases are carried. great radioactivity. Petro nov, va fabric has a low capacity and, being saturated with radioactive aerosol particles

0 with high specific activity, leads to a decrease in the degree of gas purification. In addition, the low capacity of the fabric Petra Novova makes it unreliable when there are sudden arrivals for cleaning gases from large

5 concentration of radioactive substances. Petr Novaya fabric filters are not suitable for non-waste technology.

The closest to the proposed is the purification method, which consists in

0 that the waste chlorine-containing gases, including radioactive aerosol particles formed during the purging of technological devices with gases (or the creation of a vacuum in the devices) with

5 of them are radioactive substances, purified by humidifying or treating with water vapor, followed by cyclone purification, after which they condense moisture from the gas in the refrigerator-moisture separator or alternatively heat the gas above the dew point temperature. Fine cleaning is carried out by passing gas through a filter with a Petr Nova cloth.

Pre-cleaning can be carried out by moistening or by treatment with water vapor, followed by purification on an ultrathin glass fiber filter.

This method allows for pre-cleaning by moistening or treating with water vapor to enlarge the aerosol particles by condensing water vapor on them. For the aerosol particles so enlarged in such a way, the deposition conditions are improved by inertial forces. Aerosol particles adhere better to the wetted fibers of the filter or to the film of water flowing down the cyclone wall than to dry surfaces, this is especially important when the gases contain water-soluble salt particles. Liquid draining from cyclones or fiber filters is collected in collection tanks for liquid waste. The dehumidification stage is necessary for the preparation of gases for fine cleaning - for filtration through Petr fabric, since this fabric has low efficiency in filtering moist gases. At this stage with moisture, some of the contamination entrapped with water vapor particles is also precipitated. At the stage of fine cleaning, on the fabric of Petr Novova, they catch an octgus amount of aerosol particles. After the accumulation of radioactive substances, the used filter with cloth Petr nova is discarded.

The disadvantages of this method are the low degree of purification of flue gases at each of the stages of purification and in the amount; low reliability - in case of sudden intake of gases with a high concentration of radioactive substances for purification, they may be released into the atmosphere. In addition, chlorine is not disposed of - it forms waste; frequent replacements of filters made of petrova fabric create radioactive waste.

The purpose of the invention is to increase the degree and reliability of cleaning waste chlorine-containing gases while simultaneously disposing of chlorine gas from waste gases and reducing the amount of radioactive waste.

The goal is achieved by the fact that the waste chlorine-containing gases, contaminated with nuclides of actinides, after pre-trust cleaning and drying, are subjected to fine cleaning by precipitation of radioactive aerosol particles during condensation of chlorine from the exhaust gases by cooling first in the temperature range from -34 to

100 ° C and then in the range from -162 to -196 ° C at atmospheric or reduced pressure.

Purified from radioactive aerosol particles and from chlorine, the exhaust gases, after checking for all contaminants and after the control filtration, are discharged into the atmosphere, and the condensed chlorine is recycled into the process.

Preliminary purification operations — waste chlorine-containing gases contaminated with actinide nuclides (pre-treatment may include treatment with water vapor, followed by passing through a cyclone or fiber filter to remove the dirty condensate, or it may consist in passing the gas through a bulk filter with a granular packing with decreasing gas size), it is necessary to prepare gases for fine cleaning, since the exhaust gases have a large amount of contamination (up to (5-10) 5 Bq / m3 and this does not allow for one stage bring waste gases to the required condition. In addition, salt sublimates, the relative content of which in the exhaust gases reaches 30 g / m, in the absence of pre-treatment clog the communications and chlorine condensation apparatuses.

The operation of drying the gas, for example, passing the gas over granules (2–3 mm in diameter) of NaA zeolite at -30 ° C (chlorine does not condense at normal temperature at normal atmospheric pressure) is necessary to reduce the corrosivity of the gas and to ensure the processability.

At the initial stage of fine purification, chlorine-containing waste gases are cooled to a temperature of from -34 to -100 ° С. When cooled, chlorine condenses in the form of liquid droplets both on the cold surfaces of the condenser and on the surfaces of radioactive aerosol particles, enlarged. The coarse aerosol particles, along with chlorine deposited on the surfaces of the condenser, are entrained on these surfaces and adhere to them, draining with the total mass of liquefied chlorine.

In the process of chlorine condensation, the gas is purified from radioactive aerosol particles at least as many times. what time and from chlorine. If, for example, the initial volume content of chlorine in the gas was 50% (50.7 kPa of partial pressure), then after cooling to (100 ° C) the chlorine content in the gas is reduced to its partial pressure of 2 kPa, which is 2% in volume parts (main

the amount goes to liquid). From here you can calculate the cleaning coefficient in the first stage of fine cleaning.

Co.

Thus, all condensable gases: oxygen, nitrogen, argon, containing chlorine in an amount of 2-3% are cleared of radioactive contamination at least a specified number of times.

In the next stage of fine purification, the gases (containing chlorine to 2 vol.%) Are cooled to a temperature of from -162 to -196 ° С. When cooled, chlorine condenses in solid form both on the cold surfaces of the condenser and on the surfaces of radioactive aerosol particles, enlarged. The coarse aerosol particles are co-precipitated together with chlorine on the cold surfaces of the condenser, while the sessified radionuclides of the actinic ions are in the matrix of solid chlorine structures.

In the process of low-temperature condensation of chlorine, the gas is purified from radioactive aerosol particles at least as many times as it is from chlorine. The gas enters the low-temperature condenser with a volume content of chlorine equal to% and a partial pressure of 2 kPa, and comes out with a partial pressure of chlorine 1.33-10 6 Pa (at 84.4 K); from here you can calculate the cleaning coefficient in the second stage of fine cleaning:

2-10 in

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1.33-10

This operation also ensures the removal of chlorine from the exhaust gases to their content within the limits of the maximum permissible concentrations, since the upper limit of the temperature range-162 ° C corresponds to the temperature at which the equilibrium content of chlorine vapor in the exhaust gases at normal pressure is 1 mg / m3 (MPC 1 mg / m. The lower limit of the temperature range -196 ° C is determined by the boiling point of liquid nitrogen.

In the gas cleaning of radioactive waste gases containing highly toxic actinide nuclides, take into account the possibility of situations of sudden large volumes of waste gases or large concentrations of radionuclides, the so-called peak loads or emissions.

In these cases, the flue gases are heated to -1006С. pure chlorine in the amount of up to 3 vol.% is introduced into the exhaust gases, which corresponds to the equilibrium chlorine content in the gas phase at a temperature of -90

to -100 ° C, and again cooled in a condenser to a temperature of from-162 to-196 ° C. At the same time, residual amounts of radioactive contamination are precipitated from gases together with solid chlorine

The proposed method of cleaning allows to reach the coefficient at the stage of fine cleaning.

DoMi6 (Ko / where Sn and C

M (.

0, respectively, the initial and final radionuclide concentrations in the exhaust gases),

The reliability of cleaning the waste gases from radionuclides of actinides and from chlorine is ensured by the fact that for these processes it is possible to use high-capacity devices for radioactive contamination due to the simplicity of the cleaning operations, the possibility of

0 use of simple device designs. Using the method of additional cleaning increases the reliability of cleaning. There are no overpressures in the process, no pumps.

5 for pumping liquids and gases, there are no high temperatures at which the equipment is corroded and destroyed. In this case, low temperatures contribute to the durability of the apparatus.

0 condensation. Devices can be tuned by temperature to the specified injectivity, i.e. in case of sudden arrival of large quantities of flue gases and / or having a large

5 pollution The proposed method allows the surplus to be recycled without compromising the quality of cleaning.

Condensed chlorine is recycled to the process directly from the apparatus or after pre-pressure in steel cylinders.

The simultaneous reduction of radioactive waste is ensured by the fact that the implementation of the proposed method reduces the amount of radioactive waste generated - there are no spent filters that form radioactive waste. In addition, since the proposed method uses for cleaning fine large-capacity apparatuses for contaminating radionuclides, this allows replacing the preliminary wet cleaning with another, forming solid waste, an example passing through a shaken bulk filter, which allows dispose of trapped compounds of radionuclides and therefore forms less waste, and in a compact solid form. The radionuclides deposited in the fine-cleaning apparatus do not accumulate in them, since they are returned to the process with the liquid chlorine.

The drawing shows a hardware-technological scheme of the method implementation.

In the purification-condensation mode, the chlorine-containing waste gases are fed to the bulk filter 1, periodically shaken by means of inductor 2, which is activated by a pressure differential sensor (not shown). After pre-cleaning. In the bulk filter, waste gases are fed to the adsorption column 3 with zeolite 4 granules, with an electric heater coil 5 and heat insulator 6. From the upper part of the adsorption column, the exhaust gases are fed to the first condenser 7, in which chlorine condenses into a liquid together with radioactive aerosol particles and collect in the lower part (8 - liquid chlorine) of the condenser .- in the collector 9. From the first condenser, exhaust gases with chlorine residues and residual amount of radioactive contamination are fed into Ora condenser 10, where chlorine coprecipitated in solid form together with the radionuclides on the cooled surfaces of the condenser. After the second condenser, the exhaust gases (if necessary) are fed with chlorine (1.5–3 vol.%), Heated to –100 ° С in a heat exchanger 11 and fed to the second additional condenser 12. Purified to condition in the second additional condenser The leading gases are passed through the heat exchanger-heat exchanger 13 and discharged into the atmosphere through the control filter 14.

Liquid nitrogen is fed in countercurrent with respect to the direction of movement of the exhaust gases. Liquid nitrogen through the pipeline is poured sequentially into the heat exchangers 15 of the second additional and 16 second capacitors. Air is fed to the heat exchanger-evaporator housing 17 of the second condenser to evaporate liquid nitrogen. Cold nitrogen vapor in the electric heater 18 is heated to -100 ° C and fed from above to the internal heat exchanger tube 19 of the first condenser. Then, the nitrogen in the annular space of this heat exchanger is removed from the top and, after heating, in an electric heating unit 20, is fed from above into the heat exchanger 21 of the adsorption column. The gaseous nitrogen from the heat exchanger of the adsorption tower, after checking for contamination (not shown), is discharged into the atmosphere.

In the mode of evaporation and supply of gaseous chlorine, the head unit opens the corresponding valve on the first condenser or cylinder 22, 5 drained air is fed through valve 23 and control valve 24 sets the flow rate of liquid chlorine to the evaporator 25. From the chlorine evaporator through the trap filter 26 is fed to head unit.

0 PRI me R. Under actual conditions, chlorine-containing waste gases have the composition,%: chlorine 50; oxygen 5-25; nitrogen 0-25; argon 0 - 25; carbon dioxide and water vapor 1; while gases include suspended

5 aerosol particles of sodium chloride, cesium, iron, aluminum, magnesium, silicon, nickel, chromium and alpha-radioactive nuclides of fissile materials with a total concentration of 2-30 g / m3. Waste consumption

0 gases 0.2 - 0.6 m3 / h.

Exhaust gases of the indicated composition are fed for preliminary cleaning to a shaken bulk filter, which is a tank in which a granular packing is placed in separate sections with decreasing size along the gas flow. The bulk filter is periodically shaken (as it is filled with sublimates) with a given acceleration so that

0 time, high filtration efficiency is maintained.

After the pre-cleaning stage, the gas is passed through an adsorption tower cooled to -30 ° C. Adsorption5 per column is a tank filled with granular zeolite of the type NaA (in the form of cylinders 2-4 mm in diameter and 4-5 mm high) in an amount of 5 kg, equipped with a heat exchanger for cooling,

0 heater for water desorption and heat insulation outside. Exit after the operation. In the adsorption column, the gas contains a small admixture of water that no longer affects the corrosion resistance of the equipment.

At the next stage, the gas is cooled to a temperature of from -34 to -100 ° C in the first condenser, equipped with a heat exchanger through which liquid vapors circulate.

0 nitrogen. In the first condenser, aerosol particles contaminated with radionuclides precipitate together with condensing chlorine and drain into a collector that occupies the lower part of the condenser.

5 Thus, the gas leaving after this stage of processing contains only nitrogen, oxygen, argon and chlorine, corresponding to the pressure of its saturated vapor at a condensation temperature (1.5% by volume at -100 ° C), and

the amount of radioactive contamination in the gas is reduced by about 25 times.

At the next, low-temperature, purification stage, the gas is cooled to a temperature of from -162 to -196 ° С in the second condenser, equipped with a heat exchanger into which liquid nitrogen is poured. In the second condenser, aerosol particles contaminated with radionuclides co-precipitate with chlorine, which condenses 10 on the cooled surfaces of the apparatus in a solid form. During co-precipitation, aerosol particles contaminated with actinide radionuclides appear in the matrix of solid chlorine structures. Practically the chlorine content in the exhaust gases after the second condenser (at -189 ° С and the total pressure of 90.8 kPa; the theoretical chlorine content / corresponding to the equilibrium partial pressure of vapor 1, Pa, 20 is kg / m3) was vO, 5 MPC i.e. 0.5 mg / m3. The coefficient of gas cleaning from radioactive contamination at this stage of purification is l, and the total for the last two low-temperature stages is 25 | 06 and more.

The effluent from this treatment step contains nitrogen, oxygen and argon. When operating in the temperature range when oxygen and argon are condensed into a liquid, 30 of them are removed from the bottom of the condenser (gas inlet in the upper part) together with non-condensable gases and evaporated in a heat exchanger with cold recovery.

If the exhaust gases include excessive pollution or if the gases are cleaned at a high flow rate, then the last stage of purification should be repeated to bring the exhaust gases to the necessary conditions. For this, the waste gases are sent to the heat exchanger, where they are heated to -100 ° C (the cold is disposed of), then 1.5 to 3% by volume of chlorine is introduced into the gases and cooled again to a temperature of -162 to -196 ° С.45

Thus, the required degree of purification of flue gases from aerosol particles including actinides including radionuclides is achieved.

The need for an additional purification stage 50 with the introduction of chlorine gas is also due to the fact that during the recycling of condensed chlorine into the production process, it must be constantly fed, as part of the chlorine, although insignificant, is consumed or lost.

Since the gas is cooled in steps, the movement of the refrigerant is organized in counterflow, thereby reducing its consumption. Liquid nitrogen is fed into the internal cavity of the heat exchanger of the second condenser, from where it evaporates, cooling off the exhaust gases to condense chlorine in a solid form. The heat contained in the exhaust gases after the first condenser is not enough to evaporate liquid nitrogen in the right quantity to ensure the operation of the first condenser, therefore the heat exchanger of the second condenser is equipped with an air evaporator of the tube type in the tube, one end of the outer tube is plugged, and this end is inserted into the heat exchanger. Air with a regulated flow is supplied to the inner tube of the evaporator — air serves as a coolant for evaporating liquid nitrogen.

A pair of liquid nitrogen from the second condenser is passed through an electric heater, where they are heated to -100 ° C and fed to the heat exchanger of the first condenser. At the outlet of the heat exchanger of the first condenser, vapors of liquid nitrogen have a temperature not higher than -40 ° C. Then a pair of liquid nitrogen is served in the heat exchanger of the adsorption column, pre-by: reheating to the required temperature in the electric heater. At the outlet of the heat exchanger of the adsorption column, vapors of liquid nitrogen have a temperature of + 10 ° C. Nitrogen gas, which is formed as a result of cooling the exhaust gases, does not require purification and can be disposed of or released into the atmosphere.

Chlorine own pressure (or other gas) when heated is pumped into another separate cylinder. It is advisable to have two gas cleaning chains with low-temperature trapping of aerosol particles. In this case, chlorine is fed into the head apparatus also from the condenser, eliminating the operation of overpressure (the other gas cleaning chain at this time works in the cleaning-condensation mode). Thus, the mode of operation of the gas cleaning chain is periodic, the duration of cycles is 100 hours.

Chlorine collected during the final stage of condensation is also reused. For this, the condenser is heated with air through an evaporator heat exchanger, converting solid chlorine to a liquid state, and transferring the liquid chlorine to the first condenser.

The return of recycled chlorine to the production cycle is carried out as follows. .

Liquid chlorine from the bottom part of the collector (cylinder) - for this, the cylinder collections are provided with a rod - served at a specified flow rate in the upper part of the flow

the evaporator, to the bottom of which a removable trap is attached with a filter, behind which the outlet is located. Liquid chlorine, the stack along the inner wall of the evaporator, evaporates and then its pipe is fed to the head unit. Such a chlorine supply method does not allow anywhere (in a condenser) in an uncontrolled place to accumulate a radioactive contaminant, and chlorine contaminated with actinoid radio-nuclides is returned to the production cycle.

With long-term use in the adsorption column at the stage of drying the zeolite, it is saturated with water and its ability to dry the gas decreases. This ability is restored by heating the zeolite to 250–300 ° C. Water vapors (containing a small amount of chlorine from both the volume of the leaked gas and partially adsorbed by the zeolite) can be directed by means of a carrier gas (air, nitrogen) to the wet chlorine trapping. The amount of chlorine that requires neutralization in this operation is less than 1% of the total amount of condensed chlorine in mass fractions.

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Claims (3)

1. A method for purifying chlorine-containing gases from radioactive aerosol particles of actinides, including pre-treatment, gas dehydration and fine purification, which is so as to increase the degree and reliability of cleaning the exhaust gases while simultaneously disposing of chlorine from the exhaust gases and reducing the amount of radioactive waste, fine cleaning is carried out by precipitation of radioactive aerosol particles during condensation of chlorine from the exhaust gases by cooling to temperatures below -34 ° C at atmospheric or reduced pressure, after Chlorine is recycled to the process, and the purified waste gases are discharged to the atmosphere. 2. A method according to claim 1, 1 and 2, in that the condensation of chlorine gas from the exhaust gases is first carried out in the temperature range from -34 ° C to -100 ° C, and S then in the range from -162 ° to -19b ° C. - g 3. The method according to claim 2, wherein the additional gases are heated to -100 ° C, if necessary, for additional purification, introduce into the exhaust gases pure chlorine in an amount up to 3% by volume . share and re-cooled in a condenser to a temperature of from -162 to -19b ° C. jj air air zhuh I I L otkh. that zm to the atmosphere ± Ш hedgehog. air - emptying mode / nitrogen vapor t OTXV pots but hedgehog. air mode emptying chlorine gas in head apparatus spirit ZHUH AI / pairs azoI L ta -190BC 18 waste gases f - mode CLEANING liquid chlorine in - emptying mode