RU2412494C1 - Method of decontaminating low-mineralised low-activity wastes in field conditions - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the technology of decontaminating liquid radioactive wastes using a membrane-sorption method in field conditions. The method of decontaminating low-mineralised low-activity liquid radioactive wastes is realised through cleaning on mechanical and ultra-filters, desalination on reverse-osmosis filters and post-cleaning on ionite filters with post-evaporation of the formed radioactive concentrates until salt saturation at temperature lower than 100°C in a container and subsequent addition of the saturated salt concentrates to portland cement. Also, wastes are cleaned from radio nuclides on filters using selective sorbents with protection from ionising radiation. Post-evaporation of concentrates from reverse-osmosis filters is carried out at atmospheric pressure in a container. The condensate from the evaporation apparatus is returned to the ultra-filters. The concentrate from the ultra-filters is returned to the mechanical filters. Initial radioactive concentrates with beta-radiating nuclides specific activity greater than 0.1 MBq/kg and/or radioactive concentrates with specific activity greater than 0.1 MBq/kg before evaporation are directed to the selective sorbent filters.

EFFECT: more efficient removal of radionuclides even without using selective sorbent filters.

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Description

The invention relates to a technology for the neutralization of liquid radioactive waste (LRW) by membrane-sorption methods in the field.

During the operation of nuclear power plants and other nuclear facilities, in addition to the formation of reagent LRW (deactivating, washing, regenerating solutions, etc.), characterized by increased salinity and radioactivity, significant volumes of low-mineralized natural waters are contaminated with radionuclides to concentrations exceeding the permissible levels by only 3-4 orders of magnitude. Such waste is often generated at facilities that do not have their own water treatment plants, i.e. requiring the use of mobile (transported) installations.

LRW generated in large enterprises, such as nuclear power plants, are neutralized in stationary industrial installations of special water treatment (SVO). The main method used in these plants is cleaning with mechanical filters, evaporators operating at low overpressure, and ion-exchange filters, followed by curing of the radioactive concentrates by bitumen or cementing [A. Nikiforov. and other neutralization of liquid radioactive waste. - M .: Energoatomizdat, 1985, p. 54-55].

The disadvantage of this method is that with a high degree of purification from radionuclides (for real nuclear waste, the degree of purification is on average 10 ⁴ times with LRW concentration up to 150-200 g / l) evaporation at temperatures above 100 ° C is a highly energy-intensive process (per 1 m ^{3 of} solution is consumed up to 1 t of steam), which limits its use in the field on mobile (mobile) plants and makes it unprofitable when processing small volumes of low-mineralized low-active LRW. In addition, in this technological scheme, when evaporated in an alkaline medium (pH 10-11), maintained in LRW to transfer iodine radionuclides in non-volatile form, a high degree of purification from the bulk of the salts is accompanied by significant contamination of the distillate with ammonia, which sharply increases the load on ion-exchange filters .

When the bottom residue is added (with a salinity of 150-200 g / l) in order to reduce the volume of radioactive waste to the salt saturation limit of solutions (400-500 g / l), the degree of purification from radionuclides decreases and the hardness salts are intensively released on the heating surfaces of the evaporators, reducing their heat transfer and making it difficult to operate. This requires periodic acid flushing of devices with the addition of HNO ₃ (evaporation in acidic mode at pH ~ 3), which additionally increases the salt content and, therefore, the volume of landfill waste. Moreover, cementing NPP concentrates with increasing salt content of waste from 200 g / l to 400 g / l requires a decrease in mortar-cement ratio (from 0.6-0.7 to 0.3-0.4), i.e. double increase in cement consumption. As a result, the volume of the cured product sent for disposal is practically not reduced. Moreover, the leachability of radionuclides from cement blocks with high salinity increases and, therefore, their environmental safety decreases [I. Sobolev. and others. Disposal of radioactive waste at centralized points. - M .: Energoatomizdat, 1983, p.40-45].

There is a method of neutralizing low-saline low-active water in the field at the installation, including cleaning on mechanical and ultrafilters, desalination on reverse osmosis filters and post-treatment on ion exchangers with curing of the resulting radioactive concentrates by inclusion in slag Portland cement [Epimakhov V.N., Oleinik M.S. A method of neutralizing low-mineralized low-level waste in the field. RF patent No. 2144708, bull. No. 2, 2000].

The main disadvantage of this method is that with cost-effective process parameters (pressure up to 7 MPa) concentration of LRW is achieved only to a salinity of not more than 50 g / l. In order to achieve a salinity of 200 g / l in the concentrate, the pressure in the reverse osmosis apparatus should be increased to more than 20 MPa, which is almost impossible due to the insufficient strength of the reverse osmosis membranes. Moreover, with increasing concentration of LRW, their degree of purification from radionuclides decreases. In addition, with increasing salt content of the concentrate, the release of hardness salts on reverse osmosis membranes also increases, which requires preliminary reagent softening of the solutions before reverse osmosis, in particular, by treatment with spent ion exchangers. All this leads to the production of 3-4 large volumes of concentrates supplied for curing in comparison with SVO at nuclear power plants.

A known method for the neutralization of low-mineralized low-level water in the field, including purification on mechanical and ultrafilters, desalination on reverse osmosis filters, purification on ion-exchange filters, additional evaporation of the resulting radioactive concentrates on a rotor-film vacuum concentrator to saturation with salts, followed by inclusion of saturated salt port in Epimakhov V.N., Oleinik M.S., Pankina E.B. and others. The method of neutralizing low-mineralized low-level waste in the field. RF patent No. 2195726, bull. No. 36, 2002]. By its technological essence and the achieved result, this method is the closest to the claimed one and is selected as a prototype.

The main disadvantage of this method is that a vacuum vacuum is required to carry out the evaporation process at a temperature below 100 ° C. So, rotary-film vacuum evaporators operate mainly at a temperature of 60-80 ° C [Maksin V.I., Vakhnin I.G., Standritchuk O.Z. and other Rotary film evaporator. // Chemistry and water technology, 1989, v.11, No. 1, p. 78-80], which corresponds to a vacuum of 0.2-0.5 from atmospheric. In accordance with this, the design of the evaporator is complicated (a pump and special vacuum equipment are required) and its metal consumption increases (up to a ton), which makes it difficult to use in the field. In addition, such a deep concentration of low-mineralized (less than 1 g / l) low-level waste to saturation in salts (depending on the salt composition up to 400-500 g / l) can lead to an increase in the specific activity of salt concentrates to values of more than 1 MBq / kg beta-emitting radionuclides, i.e. to the formation of medium-level waste [Sanitary rules for the management of radioactive waste (SPORO-2002). M .: Ministry of Health, 2002, p.10], further handling of which in the field will be difficult (protection and reprocessing by special methods are required [Nuclear Energy and the Environment. Issue 1. M .: AINF 539, 1981]). If there are surfactants and other organic impurities in evaporated LRW and other organic impurities, including petroleum products, such a device with a developed evaporating surface (film) as a result of foam entrainment causes significant condensate contamination. Moreover, even if the condensate of the evaporator is directed to reverse osmosis filters in accordance with this method, high molecular weight contaminants (primarily oil products) will interfere with the operation of reverse osmosis elements.

The problem solved by this invention is to simplify the hardware design of the evaporation process, increase the efficiency of LRW treatment in conditions of contamination of surfactants and oil products and increase the safety of staff.

To achieve this technical result, in a method for the disposal of low-mineralized low-level liquid wastes in the field, including cleaning on mechanical and ultrafilters, desalination on reverse osmosis filters, and post-treatment on ion exchange filters with additional evaporation of the resulting radioactive concentrates to saturation in salts at a temperature of less than 100 ° C and subsequent inclusion saturated salt concentrates in Portland cement, according to the invention additionally purify the waste from radionuclide on filters with selective sorbents with protection against ionizing radiation, and the evaporation of concentrates of reverse osmosis filters is carried out at atmospheric pressure in a container designed for subsequent cementing and disposal of waste, and the condensate of the evaporator is returned to ultrafilters, the ultrafiltered concentrate is returned to mechanical filters, and to selective filters sorbents send the initial radioactive waste when their specific activity for beta-emitting nuclides is more than 0.1 MBq and / or radioactivity overt concentrates before evaporation under their specific activity of more than 0.1 MBq.

Carrying out the additional evaporation of radioactive concentrates at a temperature of less than 100 ° C at atmospheric pressure ensures the process of water evaporation without boiling, which prevents aerosol and foam removal of contaminants into the condensate (vapor purification factors not lower than 10 ⁵ ) and allows for additional evaporation directly in containers intended for further cementing and disposal of waste, which greatly simplifies the hardware design of the process, reduces the possibility of emergency situations and increases the safety of maintenance living staff [Sivosh K., Lovash D., Litpak L. et al. Evaporation of liquid radioactive waste at a temperature below the boiling point. - In: Researches in the field of neutralization of liquid, solid and gaseous radioactive wastes and decontamination of contaminated surfaces. - Materials of the IV scientific and technical conference CMEA. - M .: Atomizdat, 1978, issue 1, p. 34-40]. The return of condensate from additional evaporation to ultrafilters and the return of ultrafilter concentrate to mechanical filters provides purification from high molecular weight contaminants, including oil products, which enter the condensate due to molecular entrainment, which creates favorable conditions for highly efficient operation of reverse osmosis and ionite filters. Reverse osmosis filters usually provide a concentration of low salt (less than 1 g / l) waste at least 10 times, i.e. when the initial specific activity for beta-emitting nuclides is more than 0.1 MBq, the formation of medium-level waste (more than 1 MBq) is possible, which avoids their preliminary purification on filters of selective sorbents. The salt content of reverse osmosis concentrates, as a rule, does not exceed 50 g / l and, with their specific activity for beta-emitting nuclides, more than 0.1 MBq / kg in the process of adding to salt saturation (depending on the salt composition, up to 400-500 g / l) the formation of medium-level waste (more than 1 MBq / kg) is possible, which avoids purification before selective sorbents are added to the filters. Since selective sorbents are not subject to regeneration, their filters can immediately be placed in concrete protection (filter containers) and, after their capacity is exhausted, go directly to this disposal for burial, which ensures radiation safety of maintenance personnel. Depending on the salt composition of the curable concentrates, they are included in Portland cement of the corresponding brand, including slag Portland cement [Quality of compounds formed by cementing liquid radioactive waste of low and medium levels of activity. - Technical requirements. - RD 95 10497-93. M .: Minatom of the Russian Federation, 1993].

The proposed method is illustrated in the drawing, which shows a diagram of a device for implementing the inventive method.

The method is as follows.

Low-mineralized (up to 1 g / l dry solids) low-level (less than 1 MBq / kg), mainly bicarbonate chloride-sulfate, liquid wastes are directed at a specific activity for beta-emitting nuclides of more than 0.1 MBq / kg, first to selective filters according to radionuclides of sorbents with protection against ionizing radiation, and with a specific activity of less than 0.1 MBq / kg directly to mechanical and ultrafilters for cleaning from suspensions and oil products. Then the waste is fed for desalination to reverse osmosis filters and for purification to ion exchange filters (saline content of the filtrate is less than 10 mg / l and specific activity is not more than 5 Bq / kg). The concentrate formed during reverse osmosis purification with a salt content of not more than 50 g / l is sent at a specific activity for beta-emitting nuclides of more than 0.1 MBq / kg, first to filters with sorbents selective for radionuclides with protection against ionizing radiation, and when the specific activity is less than 0, 1 MBq / kg directly for additional evaporation at atmospheric pressure and a temperature of less than 100 ° C in a container designed for subsequent cementing and disposal of waste, until saturated with salts (up to 400-500 g / l) and cured by inclusion in Portland cement, the brand of which is selected depending on the salt composition. The condensation from the additional evaporation is returned to the ultrafilters, and the concentrate of ultrafilters is returned to the mechanical filters.

This method provides a reduction in the volume of LRW contaminated with surfactants and petroleum products, even with an initial specific activity of up to 1 MBq / kg, not less than 400 times, even if the personnel at all stages of the tinning conditions for the maintenance of equipment for low-level waste and the degree of purification of solutions from radionuclides at least than 2 · 10 ⁴ times.

Compared with the known methods of LRW neutralization, this method for waste contaminated with surfactants and oil products, even without the use of selective sorbents, provides higher cleaning coefficients on the same mechanical, ultra- and reverse osmosis filters, which does not follow explicitly from the prior art, i.e. . meets the criteria of inventive step.

Examples of specific performance

Example 1 (prototype). A solution of bicarbonate chloride-sulfate natural waters (salinity ~ 450 mg / l) containing 25 mg / l surfactant and 25 mg / l oil products (pH 8.5) was used as low-mineralized low-level liquid waste. The specific activity was 0.3 kBq / kg for cesium-137 and 0.6 kBq / kg for strontium-90 (according to radiation safety standards NRB-99 [Radiation Safety Standards (NRB-99). - M .: Ministry of Health, 1999] , the level of intervention in water HC _water is 11 Bq / kg for cesium-137 and 5 Bq / kg for strontium-90).

Neutralization was carried out by purification on mechanical and ultrafilters of radionuclides adsorbed on suspensions, colloids and emulsions, then desalination on reverse osmosis filters and after-treatment on ion exchange filters (KU-2 in H ⁺ form and AB-17 in OH ^- form) from radionuclides included in complexes and salts. Reverse osmosis filters were operated at a pressure of 7 MPa with LRW concentration of up to 50 g / L. Concentrates of ultra- and reverse osmosis filters were directed for additional evaporation at a temperature of 60 ° С and a vacuum of 0.2 from atmospheric to a rotor-film vacuum concentrator to a salinity (dry residue) of 500 g / l (when alkalized to pH ~ 11). The condensation from the additional evaporation was returned to reverse osmosis filters. Duplicated salt concentrate was included in Portland cement M-400 with a sorption addition of clay. The specific activity of the condensate and purified water was 0.3 Bq / kg for cesium-137 and 0.6 Bq / kg for strontium-90 (i.e., almost 10 times less than HC _water ), and the supplied paired salt concentrate - 0.3 MBq / kg for cesium-137 and 0.6 MBq / kg for strontium-90 (i.e., the concentrate belongs to low-level LRW).

Example 2 (prototype). It differs from Example 1 in that the specific activity of the initial solution was 0.3 MBq / kg cesium-137 and 0.6 MBq / kg strontium-90. The specific activity of the condensate and purified water was 0.3 kBq / kg for cesium-137 and 0.6 kBq / kg for strontium-90 (i.e. it exceeded HC _water for strontium-90 by more than 10 times), and curing of the double-salt salt concentrate - 0.3 GBq / kg cesium-137 and 0.6 GBq / kg strontium-90 (i.e. the concentrate is a medium-active LRW and, when it is cured, protection against ionizing radiation is required).

Example 3. It differs from example 1 in that the evaporation of the reverse osmosis filter concentrate was carried out at atmospheric pressure and a temperature of 60 ° C in a 200 liter barrel designed for subsequent cementing and disposal of waste. The condensation from the additional evaporation was returned to the ultrafilters, and the ultrafiltered concentrate was returned to the mechanical filters. The specific activity of the condensate was 0.003 Bq / kg for cesium-137 and 0.006 Bq / kg for strontium-90, purified water - 0.03 Bq / kg for cesium-137 and 0.06 Bq / kg for strontium-90 (i.e. less HC _{water (} almost 100 times)), and the supplied paired salt concentrate - 0.3 MBq / kg cesium-137 and 0.6 MBq / kg strontium-90 (i.e. the concentrate refers to low-level LRW) .

Example 4. It differs from example 3 in that the specific activity of the initial solution was 0.3 MBq / kg cesium-137 and 0.6 MBq / kg strontium-90. The initial solution and reverse osmosis concentrate were preliminarily purified from radionuclides on filters with Select-37 selective sorbent (Cs / Sr sorption efficiency is 99.9% -99.5%) with concrete protection against ionizing radiation (filter containers). The specific activity of the condensate was 0.003 mBq / kg for cesium-137 and 0.15 mBq / kg for strontium-90, purified water - 0.03 Bq / kg for cesium-137 and 0.3 Bq / kg for strontium-90 (t ie, less than HC _{water is} almost 20 times), and the supplied paired salt concentrate - 0.3 kBq / kg cesium-137 and 3 kBq / kg strontium-90 (i.e. the concentrate is classified as low-level LRW) .

It should be noted that the spent selective sorbents are disposed of directly in filters with concrete protection against ionizing radiation (filter containers) as conditioned solid radioactive waste, because radionuclides on sorbents are fixed so firmly that they are not washed out when they enter water.

The proposed method facilitates the operation of equipment for the disposal of low-mineralized low-level waste, as it allows additional evaporation directly in standard containers intended for cementing and disposal of waste (for example, 200-liter barrels), and even with a relatively high specific activity of the waste, it requires protection from ionizing radiation only on filter containers of industrially produced selective sorbents (for example, "Select-37"). Thus, the industrial implementation of the proposed method will not meet difficulties. In the case of contamination of surfactant waste and oil products with this method, even without the use of filters of selective sorbents, an orderly more effective cleaning of radionuclides is ensured.

Claims (1)

A method for the neutralization of low-mineralized low-level liquid wastes in the field, including cleaning on mechanical and ultrafilters, desalination on reverse osmosis filters and post-treatment on ion exchange filters with an additional evaporation of the resulting radioactive concentrates until they are saturated with salts at a temperature of less than 100 ° C and then include saturated salt concentrates in portland characterized in that they additionally purify the waste from radionuclides on the filters with selective sorbents with protection t of ionizing radiation, and the re-evaporation of concentrates of reverse osmosis filters is carried out at atmospheric pressure in a container designed for subsequent cementing and disposal of waste, and the condensate of the evaporator is returned to ultrafilters, the ultrafiltered concentrate is returned to mechanical filters, and the source radioactive waste is sent to selective sorbent filters when they are specific activity for beta-emitting nuclides of more than 0.1 MBq / kg and / or radioactive concentrates before evaporation when they are removed flax activity of more than 0.1 MBq / kg.

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