RU2391727C1 - Procedure for neutralisation of low mineralised low-activity waste under field conditions - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: at neutralisation low mineralised low-activity liquid wastes are purified on mechanical and ultra-filters, are desalinated on reverse-osmosis filters and after-purified on ionite filters. Concentrates of reverse-osmosis filters are additionally stripped till saturation with salts at atmospheric pressure and temperature less, than 100°C in a tank designed for successive cementing and waste disposal; saturated salt concentrates are successively included into Portland cement. Condensate of evaporator is returned to ultra-filters, while concentrate of ultra-filters is returned to mechanical filters. Additionally wastes are stripped from radio-nuclides on filters with selective sorbents with protection from ionising radiation; also radio-active wastes of specific activity for beta-emitting nuclides over 0.1 MBk/kg and/or radioactive concentrates before stripping at their specific activity over 0.1 MBk/kg are directed to these filters.

EFFECT: facilitating simplification of assembly for after-stripping process; increased efficiency of liquid radioactive wastes (LRW) stripping under conditions of contamination with surface active substances (SAS) and oil products and decreased radiation dose for maintenance personnel.

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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.), which are distinguished by salinity and radioactivity, large volumes of low-mineralized natural waters are contaminated with radionuclides to concentrations exceeding permissible values by 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), which is maintained in LRW to convert iodine radionuclides into 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 the 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 the solutions (400-500 g / l), the degree of radionuclide purification 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 hence the volume of landfill waste. Moreover, cementing NPP concentrates while increasing the 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 an installation that includes cleaning on mechanical and ultrafilters, desalination on reverse osmosis filters and post-treatment on ion exchange filters with curing of the resulting radioactive concentrates by inclusion in slag Portland cement [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 salt content of 200 g / l in the concentrate, the pressure in the reverse osmosis apparatus should be increased to values of more than 20 MPa, which is almost impossible due to the insufficient strength of the reverse osmosis membranes. Moreover, with an increase in the concentration of LRW, the degree of their 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 3–4 times higher 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 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 it 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 with the direction of the condensate of the evaporator to reverse osmosis filters provided for by this method, high molecular weight contaminants (primarily oil products) will interfere with the operation of reverse osmosis elements.

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

To achieve the specified 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 radion wedges on filters with selective sorbents with protection against ionizing radiation, additionally concentrating reverse osmosis filters at atmospheric pressure in a container designed for subsequent cementing and disposal of waste, the condensate of the evaporator is returned to ultrafilters, the ultrafiltered concentrate is returned to mechanical filters, and to filters with selective sorbents direct the initial radioactive waste when its specific activity for beta-emitting nuclides is more than 0.1 MBq / kg and / or radio active concentrates before evaporation when their specific activity is more than 0.1 MBq / kg.

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 reduces the dose load for staff [Sivosh K., Lovash D., Litpak L. et al. Evaporation of liquid radioactive waste at a temperature below the boiling point. - In the book: Research in the field of neutralization of liquid, solid and gaseous radioactive waste 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 / kg, the formation of medium-level waste (more than 1 MBq / kg) is possible, which avoids their preliminary purification on filters of selective sorbents. The salinity of the reverse osmosis concentrates, as a rule, does not exceed 50 g / l and with their specific activity for beta-emitting nuclides of more than 0.1 MBq / kg during the evaporation to saturation with salts (depending on the salt composition up to 400-500 g / l), the formation of medium-level waste (more than 1 MBq / kg ) that pos It makes it possible to avoid 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 protection for disposal, 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 drawing shows a diagram of the implementation of the method, where the positions indicated:

1 - filter with selective sorbents,

2 - mechanical filter, 3 - ultrafilter,

4 - reverse osmosis filter, 5 - ion exchange filter.

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 waste is directed at a specific activity for beta-emitting nuclides of more than 0.1 MBq / kg, first to filters 1 with selective for radionuclides with sorbents with protection against ionizing radiation, and with a specific activity of less than 0.1 MBq / kg, directly to mechanical 2 and ultrafilters 3 for cleaning from suspensions and oil products. Then the waste is fed for desalination to reverse osmosis filters 4 and for post-treatment to ion exchange filters 5 (the salt content of the filtrate is less than 10 mg / l and the specific activity is not more than 5 Bq / kg) The concentrate formed during reverse osmosis with a salt content of not more than 50 g / l is sent, when the specific activity for beta-emitting nuclides is more than 0.1 MBq / kg, first for filters 1 with sorbents selective for radionuclides with protection against ionizing radiation, and for specific activity less than 0.1 MBq / kg, directly for additional evaporation at a pressure and temperature of less than 100 ° C in a container intended 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 grade of which is selected depending on the salt composition of the condensate from evaporation returned to the ultrafilters 3, and the ultrafiltered concentrate returned to the mechanical filters 2. This method reduces the amount of LRW contaminated with surfactants and petroleum products by at least 400 times, even with the initial specific activity, reaching 1 MBq / kg. At the same time, at all stages of the cleaning process for personnel, the maintenance conditions for equipment for low-level waste are maintained and the degree of purification of solutions from radionuclides by at least 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) As a low-saline low-level liquid waste, a solution of bicarbonate chloride-sulfate natural water (salinity ~ 450 mg / l) containing 25 mg / l surfactant and 25 mg / l oil products (pH 8.5) was used. 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 into a rotor-film vacuum concentrator to a salinity (dry residue) of 500 g / l (when alkalizing 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 for the curing of an extrapaired 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-shaped container 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 previously purified from radionuclides on filters with a selective sorbent based on ion-exchange resins, modified by sequential treatment with solutions of potassium ferrocyanide and cobalt salts [RF Patent No. 2267176. 2005, Bull. No. 36.] (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 spent selective sorbents are disposed of directly in filters with concrete protection against ionizing radiation (filter containers) as conditioned solid radioactive waste, since the radionuclides on the sorbents are so firmly fixed that they are not washed out when they enter water.

The proposed method facilitates the operation of equipment for the disposal of low-saline low-level waste, as it allows additional evaporation directly in containers intended for cementing and disposal of waste, and even with a relatively high specific activity of the waste, it requires protection from ionizing radiation only on filter containers with selective sorbents, therefore the method is industrially applicable. In the case of contamination of surfactant waste and oil products with this method, even without the use of filters with selective sorbents, an orderly more effective cleaning of radionuclides is provided.

Claims (1)

A method of neutralizing 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 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 it additionally purifies the waste from radionuclides on filters with selective sorbents with protection from ionizing radiation, the evaporation of concentrates of reverse osmosis filters at atmospheric pressure in a container designed for subsequent cementing and disposal of waste, and the condensate from the additional evaporation is returned to ultrafilters, the ultrafiltered concentrate is returned to mechanical filters, and the original radioactive waste is sent to filters with selective sorbents at their specific activity of beta-emitting nuclides of more than 0.1 MBq / kg and / or radioactive concentrates before evaporation with their specific ac ivnosti more than 0.1 MBq / kg.

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