RU2552845C2 - Method for nitrate-containing liquid radioactive waste processing - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention refers to agents for nitrate-comprising liquid radioactive waste (LRW) processing and can be used at nuclear power plants and specialised plants conditioning low and mean-active radioactive waste. In the declared method, before hardening in inorganic or polymer matrix material, the nitrate-containing LRW is biodegradable by the enzymatic processes with specially selected consortia of microorganisms with added phosphoric acid and saccharose; the microorganisms recover nitrate ions to molecular nitrogen, whereas a non-radioactive gas phase (nitrogen, carbon dioxide) is dumped. The radioactive nuclides are absorbed by biomass slag formed by the biodegradation and consisting of the microorganisms and their metabolites; phosphoric acid promotes carrying out the denitrification processes and additionally leads to strontium phosphate sedimentation that provides higher strength of the cement matrix.

EFFECT: reducing the amount of nitrate-containing LRW before added to the inorganic (cement, ceramic) or polymer matrix, reducing the amount of long-storage (burial) final product, preventing biodegradation of nitrate-containing LRW as a part of long-storage cement compound.

3 cl, 1 tbl, 2 dwg

Description

The invention relates to the field of environmental protection, in particular to the processing of nitrate-containing liquid radioactive waste, and can be used in nuclear power plants and specialized enterprises, conditioning radioactive waste of low and medium activity.

Nitrates are the main component of liquid radioactive waste (LRW), which are aqueous solutions of inorganic salts (acetate, sulfate, chloride, phosphate, oxalate, sodium bicarbonate, calcium, iron, ammonium) with a total salinity of up to 300-500 g / l and organic liquids ( solvents, extractants, detergents, mineral oils) with a total concentration of up to 2-15 g / l. Each of the salt or organic components present in the LRW composition, depending on the method of their processing, can create certain problems, but the greatest difficulties arise due to the presence of a nitrate anion in the waste composition (usually in the form of sodium nitrate). For example, when bituminized, sodium nitrate, which is inherently an oxidizing agent, can form fire and even explosive mixtures with bitumen at certain concentrations. With high-temperature methods (vitrification), toxic gaseous decomposition products of sodium nitrate (primarily toxic nitrogen dioxide) are formed. When cementing, the strength of the cement stone is very significantly reduced, and the long-term storage of cured nitrate salts of LRW in the cement compound leads to the destruction of the cement matrix due to internal processes of biogenic gas and acid formation [Gorbunova OA The effect of microbiological destruction of the cement matrix on the safety of long-term storage of conditioned radioactive waste // Physics and Chemistry of Materials Processing, 2011, No. 4. - S. 98-106].

When implementing technologies for the deep disposal of LRW into an underground reservoir, there is a risk of the spread of environmentally soluble nitrate ions toxic to the environment beyond the depths of the storage facilities. Nitrate-dependent oxidation of radionuclides adsorbed on rocks is fraught with an increase in the migration ability of transvalent radioactive elements in higher oxidation states [Nazina TN, Safonov AV, Kosareva IM and other Microbiological processes in the deep storage of liquid radioactive waste "North" // Microbiology, 2010, T. 79, 4. - S. 551-561].

In this regard, it seems appropriate to develop a method for processing nitrate-containing LRW, which ensures the decomposition of sodium nitrate during processing while preventing the formation of toxic gaseous products and reducing the volume of LRW subject to further curing.

The curing of nitrate-containing LRW in an inorganic matrix is the most common way to immobilize LRW of low and medium activity. During immobilization, they tend to minimize the volume of the final compound compared to the initial volume of LRW. On average, an increase in volume when included in a cement or ceramic matrix occurs 1.5-3 times, which requires large storage volumes, economic and personnel costs when organizing long-term controlled storage.

To reduce the amount of radioactive waste to be cured, LRW is preliminarily purified from radionuclides.

A known method of processing LRW [RF patent No. 2273066, G21F 9/06, G21F 9/20. A method for processing liquid radioactive waste / State Unitary Enterprise MosNPO Radon, Dmitriev SA, Panteleev VI, Demkin VI, Adamovich DV, Svitsov AA - declared. 09/13/2004, publ. March 27, 2006], which consists in the fact that the initial LRW stream is subjected to sedimentation with the formation of a supernatant and sludge, after which the supernatant is clarified on a mechanical filter to form a filtrate. The filtrate is separated by ultrafiltration to form a concentrate, which is mixed with the initial LRW stream and permeate. In this case, the permeate is subjected to electrodialysis separation into a brine, which is concentrated by the electroosmotic method to obtain a stream of radioactive concentrate and a dilute stream mixed with a permeate stream, and a dialysate, which is subjected to deep desalination to form a deactivated solution. In this case, the filtrate of the mechanical filter is subjected to ion-selective sorption before ultrafiltration, then associative additives are introduced into it. Ultrafiltration is carried out in a forced-turbulent mode, and deep dialysate desalination is carried out in two stages: in the first, by reverse osmosis, and in the second stage, the deactivated solution is subjected to electro-ionization. As a result of purification of LRW from radionuclides, up to 94% of decontaminated water can be discharged into the hydrographic network; the reduction coefficient of the volume of radioactive LRW to be cured is about 90.

The disadvantage of this method is the complex hardware and technological design of the process, which requires high capital, operating and personnel costs.

A known method of processing LRW containing radionuclides in ionic and colloidal forms and ballast components of mineral and organic nature in dissolved and suspended states [RF Patent No. 2465666, IPC G21F 9/08. A method of processing radioactive waste / Basiev A.A., Basiev A.G., Seliverstov A.F. - declared. 12/29/2010, publ. 10.27.2012], which consists in the fact that the organic components of LRW are oxidized by supplying ozone with a concentration of more than 0.2 g / l to the waste stream that has previously been filtered on a mesh filter material. The flow of ozonized LRW is directed towards the ozone stream in the bubbles, the mixing chamber is divided by a grid into two sections, the lower of which is fed ozone from the ejector by a closed LRW stream taken from the same section, then separated by a grid and raised in the second section to meet the LRW stream, while LRW flow is directed sequentially through two or more chambers, in the last of which the amount of ozone is selected based on at least 1/3 of the product: ozone concentration, LRW consumption and the stoichiometric coefficient of its reaction with organic matter in LRW, are closed The first LRW stream with ozone is fed from the ejector to the lower section of the mixing chamber tangentially to its perimeter. The invention allows to reduce the volume of radioactive concentrate.

The disadvantage of the method for processing LRW is the use of expensive reagents (ozone) and complex hardware and technological support of the process.

A known method of curing LRW of low and medium levels of activity [RF patent No. 2087043, IPC 6 G21F 9/16. The method of solidification of radioactive waste / P.V. Krivenko, J.V. Skurchinskaya, O.N. Peter and Paul and others - publ. 08/10/1997, Bull. No. 22 (2 hours)].

The method is as follows. LRW (specific activity 10 ⁴ -10 ⁸ Bq / l) is mixed with a clay component (kaolin, bentonite or spondyl clay) at a water-solid ratio of 1.5-3.0. The resulting suspension is mixed with a binder with a water binder ratio of 0.5-1.6. As a binder, hydrated lime or finely ground slag with the addition of 2.5-5 wt.% Clinker or Portland cement, or slag Portland cement is used with a weight ratio of clay component and binder (15-50) :( 50-85). After 28 days of normal moisture storage, monolithic cement samples with high mechanical strength and low leaching rate are obtained.

The disadvantage of this method is the relatively large volume obtained in the form of cement compounds of solid radioactive waste (compared with the initial LRW volume increases by 3-5 times), which requires the creation of additional storage facilities and increases labor and material costs, as well as the total cost of the resulting solid radioactive waste .

A known method of solidification of radioactive waste [Patent RU No. 2451350 C2. The method of immobilization of nuclear waste / FOLECINE SOLETANCHES (FR), l. Martin, J.-J. Haman, V. Bernard. - declared. No.2013030270 / 07 of 07/19/2010, publ. 05/20/2012, bull. No. 14], which consists in improving the matrix based on a sand-mixed mineral composition obtained by mineralizing the biomass of organisms belonging to bacteria, plants, animals, containing 50-80 wt.% Calcium carbonate and 10-30 wt.% Silicon.

The disadvantage of this method is the difficulty in preparing a matrix of constant composition, the use of a significant amount of the initial biomass and energy costs for mineralization, volumetric and complex equipment for producing plant biomass, and the duration of the process of cultivating organisms.

Closest to the proposed method, selected as a prototype, is a method for incorporating LRW containing sodium nitrate into a ceramic matrix [RF Patent No. 2086019, IPC G21F 9/16. A method for incorporating liquid radioactive waste containing sodium nitrate into a ceramic matrix / Moscow Scientific and Production Association "Radon", Soloviev V.A .; Lifanov F.A .; Laschenova T.N. - declared. 05/19/1995, No. 95108144/25, publ. July 27, 1997], which consists in mixing LRW with a total sodium nitrate content of up to 270 g / l with a mineralizer, which is used as ammonium silicofluoride in a ratio to sodium nitrate, as 2.4: 1; a nitrate ion reducing agent, which is used as urea in the ratio to sodium nitrate, as 1.8: 1, decomposing the nitrate anion according to the following scheme:

NaNO ₃ + 3CO (NH ₂ ) ₂ + O ₂ = NaOH + 2N ₂ + 3CO ₂ + 3NH ₃ + H ₂ O;

and ceramic-forming material, which is used as bentonite, a mixture of tripoli and aluminum hydroxide, as well as loam in the ratio to sodium nitrate, as 4.8: 1. The mixture is dehydrated to a residual moisture content of not more than 10 wt.% At a temperature of not more than 100 ° C. Then the mixture is heated at a temperature of 100-180 ° C for 6-8 hours, then at a temperature of 180-900 ° C for at least 4 hours, fired at 900 ° C for at least 1 hour and cooled.

The disadvantages of the prototype is the relatively large volume obtained in the form of ceramic compounds of solid radioactive waste (compared with the original LRW volume increases by 1.5-2.5 times), the need for high-temperature methods, which is not only costly, but also requires additional measures to ensure radiation safety and additional gas purification equipment, the filters of which, trapping gases with radionuclides of volatile ¹³⁷ Cs, also then require processing.

The technical task of this invention is to reduce the volume of nitrate-containing LRW before incorporating them into an inorganic (cement, ceramic) or polymer matrix; a corresponding preliminary reduction in the volume of the final product subject to long-term storage (disposal); in the prevention of biodegradation of nitrate-containing LRW in the composition of the cement compound during long-term storage; in reducing the cost of the processing and storage of nitrate-containing LRW.

This problem is solved due to the fact that nitrate-containing LRW before curing in an inorganic or polymeric matrix material is subjected to biodegradation due to enzymatic processes by specially selected consortia of microorganisms with the addition of phosphoric acid and sucrose, while microorganisms restore nitrate ions to molecular nitrogen, non-radioactive gas phase nitrogen, carbon dioxide) is discharged into the atmosphere, which leads to a decrease in volume by 2-5 times; radionuclides are sorbed by the biomass slag formed after biodegradation, consisting of the microorganisms themselves and their metabolic products; phosphoric acid contributes to the microbiological processes of denitrification and additionally leads to the formation of a precipitate of strontium phosphate, which increases the strength of the cement matrix.

A distinctive feature of this method is a decrease in the volume of nitrate-containing LRW to be cured by 2-10 times due to the processes of biodegradation of nitrate ions.

A distinctive feature of this method is the use of a specially selected consortium of natural non-pathogenic bacteria of the genus Pseudomonas, which have increased radio resistance and are characteristic of deep LRW storage facilities.

A distinctive feature of this method is the sorption of radionuclides of nitrate-containing LRW in the biomass slag formed after biodegradation, as well as the formation of biomass proteins and lipids in the slag, which can act as plasticizing additives during curing.

Biomass slag formed during the processing of nitrate-containing LRW by the proposed method, which is a mixture of biomass with sorbed radionuclides and bacterial metabolic products, is not only not antagonistic to the inorganic or polymer matrix material, but also contains components similar to plasticizing additives that improve the curing parameters ( improved mixing, fluidity, setting time of the final compound).

The inventive method is implemented as follows.

Nitrate-containing LRW with a nitrate ion concentration of not more than 12 g / l are mixed with a sucrose solution until the sucrose: nitrate concentration ratio is 1: 1. The solution is fed into a hermetically sealed metal device with a stirrer and with a gas purge system. Phosphoric acid acts as a titrant to maintain the pH within 7–8, which plays an additional role as a buffer system and a strontium phosphate salting out agent. A suspension of denitrification cells containing at least 10 ⁷ cells / cm ³ is supplied to the device. The process is carried out for at least 2 days at a temperature of from 20 to 30 ° C until the concentration of nitrate ions decreases below 45 mg / L. After denitration, part of the drain is drained, the biomass is filtered and subjected to heat treatment from 25 to 90 ° C for 1 to 3 days inversely with temperature for drying, after which it is solidified into an inorganic or polymer matrix and buried.

The essence and features of the claimed invention are further illustrated by the drawings, which show the following.

In FIG. 1 - kinetics of nitrate ion consumption by the bacterial community DN1;

In FIG. 2 - the proportion of sorption of radionuclides by cultures of microorganisms, where:

1 - bacterial community DN1;

2 - bacterial community DN2;

3 - bacterial community DN3.

The implementation of the method is confirmed by the following examples.

Example 1

Selected cultures of bacteria of the genera Pseudomonas (bacterial community DN1) were cultured for 7 days on a nutrient medium composition, g / l: NaNO ₃ - 1.5; sucrose - 1.5; NH ₄ Cl - 1 g / l; NaCl - 0.8 g / l; MgSO ₄ - 0.1 g / l; Na ₂ SO ₄ - 0.1 g / l, under anaerobic conditions, at a neutral pH, temperature 20 ± 1 ° C. Every 24 hours, the concentration of nitrate ions in the medium was analyzed using a calibrated ion-selective electrode. The results presented in FIG. 1 indicate that the concentration of nitrate ions after biodegradation for 16 hours decreases to values less than 45 mg / l, i.e. up to MPC values for drinking water.

Example 2

Biodegradation was carried out analogously to example 1. On Wednesday, radionuclides of thorium, uranium, cesium, strontium were added in concentrations typical for nitrate-containing radioactive waste ( ¹³⁷ Cs 1 · 10 ³ Bq / l, ⁹⁰ Sr 1 · 10 ³ Bq / l, Th, ²³³ U 1 · 10 ² Bq / l). After biodegradation and subsequent dehydration of biomass slag in the sample, the content of radionuclides was determined. The results (figure 2) indicate that Th sorption was almost 100%, U - up to 85%, Sr - up to 50%, Cs - on average 10%.

Example 3

Biodegradation of nitrate-containing LRW (total mineralization of 100 g / l, nitrate content of 10 g / l, pH = 9.0, specific beta activity of 10 ⁴ Bq / l) was carried out analogously to example 1.

After 15 days of biodegradation, part of the formed biomass sludge was dehydrated by evaporation at a temperature of 65 ° C, and then mixed with a cement mortar prepared on the basis of Port400 cement grade M400 D0 at a water-cement ratio W / C = 0.7, where B is the mass of water in the cement mortar , g; C is the mass of Portland cement, g. Another part of the biomass sludge of a similar composition was not dehydrated for comparison, but was mixed with dry Portland cement, taken at W / C = 0.7. In the resulting cement mortars, the spreadability was determined using an AzNII cone, delamination in a measuring cylindrical vessel with a capacity of 20 cm ³ for 1.5 hours at rest. From cement mortars using collapsible forms, 2 × 2 × 2 cm cubes were prepared, kept in air-wet conditions for up to 28 days, after which the compressive strength was determined. The results are presented in table. one

The data table. 1 confirm that cement mortars based on nitrate-containing LRW that have previously undergone biodegradation have the flowability required for workability and, upon hardening, satisfy the requirements of GOST R 51883-2002 in strength. Thus, when implementing the proposed method, the technical problem is solved to reduce the volume of the final product to be disposed of, and to improve the cementation parameters of nitrate-containing LRW (to increase the spreadability when the regulated compressive strength is achieved).

Thus, the advantages of the invention are to reduce the cost of the processing and storage of nitrate-containing LRW. The inventive method allows you to:

- reduce the volume of nitrate-containing LRW before incorporating them into an inorganic or polymer matrix using low-cost biodegradation processes;

- reduce the duration of subsequent radiation-hazardous operations, the volume of mixers (respectively, the volume of decontaminating water) during curing of slag LRW biomass, less than the initial volume of LRW;

- reduce the need for inorganic or polymeric matrix material for curing;

- to reduce the volume of the final cured product, subject to long-term storage (burial), due to the greater degree of inclusion of nitrate-containing LRW biomass into the slag compound;

- to prevent undesirable destruction processes of the matrix material during the biodegradation of nitrate-containing LRW as part of the compound during long-term storage and to increase the reliability of localization of radionuclides in an inorganic or polymer matrix.

Claims (3)

1. A method of processing nitrate-containing liquid radioactive waste, characterized in that liquid nitrate-containing radioactive waste with a nitrate ion content of up to 12 g / l is first biodegradable by enzymatic processes by consortia of denitrifying microorganisms for at least 2 days at a temperature of from 20 to 30 ° C with the addition of sucrose as a carbon source in the ratio of sucrose: nitrate = 1: 1 in an airtight container providing anaerobic conditions, equipped with a gas blower and forced mixing iem, wherein the microorganisms nitrate ions are reduced to molecular nitrogen nonradioactive discharged into the atmosphere, and adsorb radionuclides, and proceeds bioremediation transvalentnyh radioactive metals, after which the resulting biomass solidified slag inorganic or polymeric matrix material. 2. The method according to claim 1, characterized in that as a consortium of microorganisms, natural non-pathogenic Pseudomonas genus bacteria with increased radioresistance are used, characteristic of underground ecosystems containing nitrate-containing liquid radioactive waste. 3. The method according to claim 1, characterized in that the biomass slag formed for draining non-radioactive water is additionally dehydrated at a temperature of 25 to 90 ° C. for 1 to 3 days inversely with temperature.

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