RU2342720C1 - Method of treating liquid radioactive wastes - Google Patents

Abstract

FIELD: physics; nuclear technology.

SUBSTANCE: present invention pertains to environmental conservation, and more specifically to treatment of liquid radioactive wastes. The method of treating liquid radioactive wastes involves elutriation of the initial stream of liquid radioactive waste with formation of a supernatant fluid and sludge. The supernatant fluid is defecated on a mechanical filter with formation of a filtrate. The filter is subjected to ion-selective sorption after deep desalination in two stages. The first stage involves reverse osmosis with formation of streams of intermediate concentrate and deactivated solution. Before elutriation, the liquid radioactive wastes undergo preliminary filtration on filters with a bed made from sypron and granular polypropylene, capable of separating oil, oil-products and alpha-radionuclides from liquid radioactive wastes. After elutriation, the supernatant fluid is subjected to successive mechanical filtration on sand and carbon filters, with formation of a filtrate. This filtrate is undergoes deep desalination through reverse osmosis. After the first stage, the deactivated solution undergoes ion-selective sorption, and then pH correction on a limestone filter. At the second deep desalination stage, the intermediate concentrate undergoes pre-concentration through reverse osmosis with formation of a concentrate with salt content of 100-150 g/l, which is taken for further conditioning, and a permeate, which is taken back to the first stage of deep desalination. The invention allows for concentrating liquid radioactive wastes, containing oil-products and radio nuclides Cs¹³⁷, Sr⁹⁰, and Pu²³⁹, as well as obtaining very pure permeate with neutral pH and reducing secondary liquid and solid radioactive wastes.

EFFECT: obtaining a very pure permeate with neutral pH, reduced secondary liquid and solid radioactive wastes.

1 cl, 1 dwg, 1 ex

Description

The inventive method relates to the field of environmental protection, and more specifically to the field of processing liquid radioactive waste (LRW). The invention can be used in the processing of low-level LRW with a high content of suspensions, surface-active substances (surfactants), oil products (oils) and salinity up to 10 g / l, which are formed as a result of the activity of radiochemical industries. Mostly in radioactive waste processing facilities.

In practice, integrated technological schemes for processing LRW are widely used. For example, there is a method for complex processing of LRW (Patent RU 211894. BI No. 26 of 09.20.1998), which consists in the step-by-step processing of LRW containing cesium and strontium radionuclides in the following sequence: first, LRW is fed to the pre-treatment stage, which includes blocks of mechanical cleaning, ultrafiltration, then they are passed through a selective inorganic sorbent based on ferrocyanides of transition metals of copper, nickel, cobalt and a porous inorganic carrier, then they are subjected to desalination and concentrates NIJ one of the following methods: distillation, a two-step, wherein in the first step using reverse osmosis or electro-demineralization, the second stage distillation or electroosmotic concentration. The filtrate after the stage of desalination is subjected to sorption purification on ion exchange.

The disadvantages of this method are:

- when cleaning LRW containing oils and dissolved organics, membrane poisoning will occur, since LRW does not undergo preliminary purification from oil products and dissolved organics;

- the use of ion exchange for desalination, which is accompanied by the introduction of a large amount of chemical reagents into the LRW stream during the regeneration of ion exchange resins, resulting in the formation of secondary liquid and solid radioactive waste.

The closest technical solution to the proposed (prototype) is a method of complex processing of LRW (Patent RU 2273066. BI No. 9 of 03/27/2006), which consists in the fact that the initial stream of LRW is subjected to sedimentation with the formation of a supernatant and sludge, the supernatant clarify on a mechanical filter with the formation of a filtrate. Before ultrafiltration, the filtrate is subjected to ion selective sorption, then associative additives are introduced into it, converting the molecular and ionic forms of the remaining radionuclides into colloidal and aggregative forms, while ultrafiltration is carried out in a forced-turbulent mode. Next, the filtrate is sent to ultrafiltration. The concentrate is mixed with the initial LRW stream, and the permeate is subjected to electrodialysis separation into brine and dialysate. concentrate and diluent stream mixed with permeate stream. The dialysate is subjected to deep desalination with the formation of a deactivated solution, and the deep desalination of the dialysate is carried out in two stages: in the first reverse osmosis with the formation of a stream of an intermediate concentrate, which is mixed with brine, and a deactivated solution, and in the second stage, the deactivated solution is subjected to electrodeionization.

The disadvantages of this method are:

- the impossibility of processing LRW containing surfactants, oils and dissolved organic compounds, since LRW does not undergo preliminary purification from oil products and dissolved organics.

- permeate at the outlet of the reverse osmosis unit, as a rule, has a pH in the range of 3-4.5. Such water cannot be used for recycled water supply or for discharge to the ground, since the pH of the deactivated solution is not adjusted.

The technical result consists in concentrating LRW containing oil products and radionuclides Cs ¹³⁷ , Sr ⁹⁰ and Pu ²³⁹ , as well as obtaining a deeply purified permeate with a neutral pH and reducing secondary liquid and solid radioactive waste.

To achieve this technical result, a method for processing liquid radioactive waste (LRW) is proposed, which consists in the fact that the initial stream of LRW is sedimented to form a supernatant and sludge, the supernatant is clarified on a mechanical filter to form a filtrate, the filtrate is subjected to ion selective sorption after deep desalination in two stages: at the first stage, reverse osmosis with the formation of flows of an intermediate concentrate and a deactivated solution, while before settling LRAW is subjected to preliminary filtration on filters loaded with sipron and granular polypropylene, which are able to separate oils, oil products and alpha radionuclides from LRW, and after settling, the supernatant is subjected to sequential mechanical filtration on sand and carbon filters, with the formation of a filtrate, which is subjected to deep desalination by reverse osmosis, and after the first stage, the deactivated solution is subjected to ion-selective sorption, and then the pH is adjusted to limestone filter, in addition, in the second stage of deep desalination, the intermediate concentrate is subjected to postconcentration by reverse osmosis with the formation of a concentrate with a salt content of 100-150 g / l, which is sent for further conditioning, and permeate, which is sent again to the first stage of deep desalination.

Distinctive features of the proposed method is that before settling LRW is subjected to preliminary filtration on filters loaded with sipron and granular polypropylene, with the ability to separate oils, oil products and alpha radionuclides from LRW, and after settling, the supernatant is subjected to sequential mechanical filtration on sand and coal filters, with the formation of a filtrate, which is subjected to deep desalination by reverse osmosis, while after the first stage it is deactivated the solution is subjected to ion-selective sorption, and then the pH is adjusted on a limestone filter, in addition, in the second stage of deep desalination, the intermediate concentrate is subjected to postconcentration by reverse osmosis to form a concentrate with a salinity of 100-150 g / l, which is sent for further conditioning, and permeate, which sent again to the first stage of deep desalination.

Fundamentally, all industrial methods for processing LRW consist in the maximum possible concentration of radioactive components by isolating water from the LRW in which the content of radionuclides does not exceed the maximum permissible concentrations. The membrane installations used in most modern technological schemes for LRW treatment allow to concentrate LRW substantially and to obtain deeply desalinated clean water suitable for discharge to the ground or reuse in engineering networks. The main problem that arises when using membrane technologies (reverse osmosis, electrodialysis, ultrafiltration) is the restrictions on the content of organic substances (surfactants) and oil products (oils) in LRW. These impurities significantly limit the use of membrane technology, because at the same time, the service life of the membranes is significantly reduced, and the amount of secondary liquid and solid radioactive waste increases. Filtration of LRW on filters from sipron and granular polypropylene allows the removal of oils, surfactants and other undissolved oil products from LRW due to the formation of a thin film on the surface of sipron and polypropylene. Sequential filtration on a sand filter and activated carbon BAU allows you to clean LRW from fine particles and dissolved organic impurities due to their sorption on activated carbon. Since permeate after reverse osmosis contains small concentrations of Cs ¹³⁷ ions (up to 1 × 10 ² Bq / L), ion-selective purification of permeate after reverse osmosis concentration of LRW is provided, which allows one to obtain at the output a solution that is deeply purified from radionuclides Pu ²³⁹ , Sr ⁹⁰ , Cs ¹³⁷ . Carrying out pre-concentration of the intermediate concentrate can reduce the number of secondary LRW with a coefficient of 50-100, while the permeate is sent again to desalination to the first stage. In addition, this allows you to get a concentrate with a salinity of 100-150 g / l. Neutralization of pH peremeat by filtering it on limestone allows you to get a solution with a pH of 6.5-8 at the outlet, which fully complies with sanitary standards for discharge into the hydrographic network.

The drawing shows a block diagram of a method for processing LRW, where:

1 - filter with loading from sipron; 2 - filter loaded from granular polypropylene, equipped with an oil separation device; 3 - intermediate tank-settler; 4 - sand filter; 5 - carbon filter; 6 - an intermediate container for adding antiscalant and, if necessary, adjusting the pH; 7 - the first stage of reverse osmosis; 8 - the second stage of reverse osmosis; 9 - filter with inorganic ion-selective sorbent; 10 - filter with crushed limestone to adjust the pH; 11 - receiving tank for purified water, H1 and H2 - pumps.

Example. The proposed method for LRW processing was used for LRW processing in the preliminary tests of the LRW processing station at GUP Mos NPO Radon. LRW of the following composition was supplied for processing: suspension - 150 mg / l, oil products - 20 mg / l, surfactant - 10 mg / l, salinity - 1500 mg / l. The specific Σα activity was 7 × 10 ² Bq / L, the specific Σβ activity was 1 × 10 ⁴ Bq / L.

The initial LRW flow enters filter 1 loaded with sipron and filter 2 loaded with granular polypropylene. Due to the physical properties of the materials, oil products in the form of a thin film and large suspensions, on which the bulk of alpha-nuclides, for example, Pu ^239, are retained on them. The total alpha activity of Pu ²³⁹ after this stage of purification is reduced to 3.1 × 10 ¹ Bq / L.

Next, the filtrate enters the intermediate tank-settler 3, where there is sedimentation of LRW on the sludge and clarified part. The sludge is sent for further conditioning, and the clarified part is sent for sequential filtration on sand and charcoal filters using pump H1. On sand filter 4, finer suspensions and the remaining amount of alpha-nuclides are retained. On a carbon filter 5 loaded with activated carbon of the BAU brand, LRW is purified from dissolved organic compounds. The concentration of petroleum products in the filtrate after the carbon filter is less than 0.03 mg / L.

The filtrate after mechanical cleaning is sent to a receiving tank 6 for acidification with nitric acid. In the process of acidification, the destruction of hardness salts occurs, which prevents sedimentation in the membrane elements and extends their service life. Then LRW pump H2 is sent to a two-stage reverse osmosis system 7.8, where deep desalination of LRW and purification from radionuclides Cs ¹³⁷ and Sr ^{90 take place} . After the first stage, permeate is formed - a deactivated solution and an intermediate concentrate. The intermediate concentrate has a salinity of 10-15 g / l is sent to the second stage of pre-concentration 8, after which it has a salinity of 100-150 g / l. These are the optimal concentration limits, since with a lower salt content the amount of secondary LRW increases, and with a higher salt content the load on the membranes significantly increases and their service life decreases. The concentrate after the second stage is sent for further conditioning. After the second stage, permeate is sent again to the first stage of reverse osmosis.

The deactivated solution after the first stage of deep desalination of LRW has a salt content of 50-100 mg / L, specific Σα activity of Pu ²³⁹ decreased to 2.7 × 10 ^-1 7 × 10 ² Bq / L, specific Σβ activity to 2.1 × 10 ² Bq / l 1 × 10 ⁴ Bq / l. Then it is sent to selective sorption for purification of Cs ¹³⁷ radionuclides. As the filtering material in the filter 9, the inorganic sorbent Phoenix based on nickel ferrocyanides is used.

After ion-selective purification 9, the filtrate is sent to adjust the pH in the filter 10 loaded with crushed limestone. In the process of deep desalination of LRW at the reverse osmosis installation, the permeate - deactivated solution has a pH of 3-5, such water cannot be discharged into the hydrographic network. After adjusting the pH on the limestone, the deactivated solution has a pH of 7-8, which corresponds to sanitary standards.

As a result, the final solution has a specific radioactivity for β-radionuclides (Sr ⁹⁰ ) of 0.7 × 10 ¹ Bq / l, for α-radionuclides (Pu ²³⁹ ) of 1.8 × 10 ^-1 , specific activity for Cs ¹³⁷ was 0, 2 × 10 ^-1 , the concentration of suspensions was 1 mg / l, surfactant - 0.06 mg / l, petroleum products - 0.03 mg / l, which does not exceed the maximum permissible concentration for discharge.

Thus, the LRW deactivation coefficient for β-radionuclides was 1.4 × 10 ³ , for α-radionuclides - 3.8 × 10 ³ Bq / l. The degree of purification of petroleum products was 3.3 × 10 ² . In comparison with the prototype, this value would not exceed 1 × 10 ¹ .

The degree of concentration was 70. According to the prototype, in the absence of filters with sipron, granular polypropylene and activated carbon, the concentration of LRW would be 10-20, since the solution would have to add regeneration solutions for washing the membrane elements, and the amount of secondary solid radioactive waste in the form of spent membranes would increase by 2 times.

The proposed method can be carried out on an industrial scale on standard equipment.

Currently, this scheme is being implemented in the State Unitary Enterprise Mos NPO Radon within the framework of the approved project for the construction of a LRW Processing Station (Building No. 101). The productivity of the station is 10 m ³ / hour.

Claims (1)

A method of processing liquid radioactive waste, namely, that the initial LRW stream is sedimented to form a supernatant and sludge, the supernatant is clarified on a mechanical filter to form a filtrate, the filtrate is subjected to ion selective sorption after deep desalination in two stages: in the first stage, reverse osmosis with the formation of flows of an intermediate concentrate and decontaminated solution, characterized in that before settling, the liquid radioactive waste is preliminarily final filtration on filters loaded with sipron and granular polypropylene, capable of separating oils, oil products and alpha radionuclides from liquid radioactive waste, and after settling, the supernatant is subjected to sequential mechanical filtration on sand and carbon filters to form a filtrate, which is subjected to deep desalination by reverse osmosis, and after the first stage, the deactivated solution is subjected to ion-selective sorption, and then adjust the pH to In addition, at the second stage of deep desalination, the intermediate concentrate is subjected to postconcentration by reverse osmosis to form a concentrate with a salt content of 100-150 g / l, which is sent for further conditioning, and permeate, which is sent again to the first stage of deep desalination.