Method for processing liquid radioactive waste

 

(57) Abstract:

Usage: in chemical technology, specifically in the nuclear environment and in the processing of liquid radioactive waste (LRW) generated by the operation of the various nuclear power plants. Summary of the invention in carrying out sequential processing of liquid radioactive waste containing radionuclides of cesium and strontium in the following sequence: first, LRW serves on stage pretreatment, which may include blocks of mechanical treatment, ultrafiltration and microfiltration unit, then put them through selective inorganic sorbent on the basis of ferrocyanides transition metals copper, Nickel, cobalt and a porous inorganic carrier, and then passed through the reverse osmosis module when the salt content of 1 g/l in one stage, and when the salt content of 1 g/l in two separate flow of concentrate and permeate, which is then subjected to purification by passing through a sorbent selected from the series: synthetic zeolite type a, chabazite hexagonal structure or natural zeolite monoclinic structure, which is used as the modified zeolite "Selex-KM". In another embodiment, through their selective relates to the field of chemical technology, specifically for nuclear ecology and can be used in the processing of liquid radioactive waste (LRW) generated by the operation of the various nuclear power plants (NPP) NPP, vehicles (nuclear-powered icebreakers, submarines, floating NPP).

In the process of nuclear energy cycle formed of various types of LRW, which must then be processed with a view to their subsequent disposal in the compact solid form. As a rule, all types generated LRW have a well-defined chemical and radionuclide compositions, determined by the specifics of the maintenance of the water-chemical mode of the first circuit of NPP and formulations decontamination equipment used for decontamination of equipment.

In the operation of the NPP are three main types of LRW belonging to the class of medium - and low-level, the composition of which is given in the table.

Based on the ecological requirements existing in the Russian Federation in NRB 76/87 [1] and IAEA recommendations, the processing of LRW should include cleaning up the total content of active radionuclides is less than 10-10Ku/L. As a rule, limiting this figure are the radionuclides cesium-oceny difficult to be cleaned, especially from salt solutions. In addition, due to sanitary requirements (SPORO-85), activity subject to final disposal in the form of cement blocks brines 150 - 240 g/l should not exceed 1 to 10-4Ku/L. This requirement for permissible levels of radiation exposure of maintenance personnel repositories.

The literature describes many methods of LRW treatment [2, 3].

As in solutions of LRW in addition to radionuclides, there are other harmful chemical impurities, as a rule, in the processing using complex methods, including various chemical operations.

The closest to describing the technical essence and the achieved result is a method for processing of liquid radioactive waste containing radionuclides of cesium and strontium, which includes stages pretreatment, purification and desalination using reverse osmosis, with the separation of the flow on the permeate and the concentrate followed by purification of the permeate on the sorbents [4].

In this way LRW sequentially subjected to the following stages of processing.

First, the solutions are sent to the stage pretreatment. Depending on the degree of contamination of suspended solids, oil and surfactant this article is the material, retention of organic matter and petroleum products (foam, highly porous organic sorbents type Porous-TM", active carbons);

Subsequent filtering LRW through the cartridge filter element with filtration 20 micron and ultrafiltration.

At low concentrations in the original LRW suspended solids and oil products from this stage eliminates the process of ultrafiltration. Then purified from impurities LRW received on the primary reverse osmosis purification unit, in which the removal of salts and radionuclides. This unit is also equipped with a cartridge filter elements with a fineness of filter 20 and 5 μm. LRW under the working pressure to 5.9 MPa micro-filtrated, and then through the membrane elements, which is the reverse osmosis separation to obtain filtrate (permeate and concentrate.

In the cleaning process on the reverse osmosis module desalted to content < 10 mg/l permeate allocated block for further purification, and the concentrate flows into the intermediate tank.

The amount of the produced concentrate, which is subject to further utilization is determined by the physico-chemical composition and concentration in the concentrate is at 60 65 g/DM3.

Typically, the resulting concentrates are then subjected to further processing (concentration) for disposal in solid form. We can loparite to the salt content of ~ 200 - 250 g/l and then translate into an insoluble state by a method of cementing or was evaporated to dryness with the room dry salt in special containers for long term storage. On the basis of radiochemical composition LRW all used methods of disposal should provide reliable isolation of solid blocks for ~ 300 years (10 half-lives of the most long-lived of the present radionuclides of cesium and strontium). The permeate is fed to the reverse osmosis unit on sorption purification using traditional organic resin is a strongly acidic cation exchange resin of the type KU-2-8 es and strong-base anion exchange resin of the type AB-17-8 Yak, where it is cleaned before the content < 10-10Ku/l

This method provides the basic task of processing - cleaning discharge water from all toxic impurities up to sanitary standards.

The disadvantage of this method is that the radionuclides of cesium, due to their chemical nature limit the treatment processes at the reverse osmosis membranes, forcing prcu and reduce its volume performance.

The objective of the invention is to increase process efficiency by reducing energy consumption and increasing performance; improve environmental safety of the process LRW processing by reducing cycle disposal of radioactive cesium.

The task is achieved by the described method of processing of liquid radioactive waste containing radionuclides of cesium and strontium, which includes stages pretreatment, purification and desalination using reverse osmosis, with the separation of the flow on the permeate and concentrate, which further LRW subjected to purification by passing through the selective inorganic sorbent on the basis of the ferrocyanides of transition metals (copper, Nickel, cobalt) and a porous inorganic carrier or after stage pretreatment or after reverse osmosis, single stage when the salinity LRW < 1 g/l or two when salinity LRW > 1 g/l, on the concentrate stream with subsequent purification of the permeate on the sorbents selected row: synthetic zeolite type "A", chabazite hexagonal structure or natural zeolite monoclinic structure, which is used as the modified zeolite brand "Selex-KM".

Hallmark speed selective inorganic sorbent.

Another difference is that, as a selective inorganic sorbent use composite ferrocyanide sorbent on the basis of the ferrocyanides of transition metals (copper, Nickel, cobalt) and a porous inorganic carrier.

The difference method is that the cleaning are liquid radioactive waste after stage pretreatment or after reverse osmosis in the concentrate stream.

Other differences of the method lies in the fact that at the stage of purification of the permeate is passed through inorganic sorbent is selected from the series: synthetic zeolite type "A", chabazite hexagonal structure or natural zeolite monoclinic structure, which is used as the modified zeolite brand "Selex-KM, and in that the processing solutions having a salt content of < 1 g/l using a single-stage reverse osmosis processing, and the processing solutions with a salt content of > 1 g/l - two.

The invention due to the following patterns.

Despite the selective properties of reverse osmosis membranes, they are not sufficient to ensure the required performance of cleaning them is thew absoluet LRW, what you need to spend a lot of energy, cesium still gets to permeate and for his final cleaning requires the use of ion-exchange resin. When used on stage pretreatment selective to the radionuclides cesium ferrocyanide sorbent capable of removing cesium 99.5% at any salinity LRW) almost all of the cesium is absorbed by this sorbent to the stage reverse osmosis. In this case, the reverse osmosis module can operate in mode a smaller desalination, which means better performance. Ultimately, the process of sorption purification is also simplified, since the required number of sorbents for purification of reduced 5 to 10 times. Thus eventually decreasing and subject to disposal of solid waste. The process of cleaning concentrate after stage reverse osmosis also leads to problem solving as a unique selective properties ferrocyanide sorbents allow to extract cesium from selenaselena solutions with almost the same degree that also allows you to improve the performance of reverse osmosis plants and reduce the amount of solid waste. Extraction for ferrocyanide sorbents and when used after purification will extract only the radionuclides cesium (i.e., to show the same properties as in the sorption of brines). When cleaning LRW type I or type II is preferable to use ferrocyanide sorbents before reverse osmosis system, because in this case they will be extracted in addition to the radionuclides cesium and strontium, cerium and other isotopes.

The effectiveness of the described method is illustrated by examples.

Example 1. Are LRW purification by the method prototype. LRW type III, containing 200 mg/l of petroleum contaminated with suspended solids and petroleum (NP) serves to block prefilter. First they passed through a filter with floating foam boot, then filtered through a cartridge filter element with filtration 20 micron.

When this is cleaned from NP 97% of suspended solids ~95%. Filtering is carried out at an operating pressure of 0.2 - 0.3 MPa. Next LRW filtered under pressure of 0.04 MPa through the ultrafiltration module, where there is a complete purification from suspended solids, NP and surfactants. Throughout these stages is achieved by the removal of radionuclides with a factor 3 to 4.

Then LRW at a temperature of 15oC serves on the reverse osmosis unit, equipped with two roll-up of reverse osmosis elements of the CSO bandwidth LRW through two elements.

Delaying the ability of the used membranes trademark "FT-30 Filmtec" company Dow chemical company (USA) ions of sodium, cesium, chlorine is at least 99.3%, and ions of calcium, magnesium, strontium, heavy metals, surfactants is at least 99.9%.

After the second module desalted solutions arrive at a final purification on ion-exchange resin KU-2-8 es and AV-17-8 es in the amount of 0.3 m3each.

Concentrates with a salinity of 80 - 100 g/l are collected in a special tank for further processing. Purified solutions to salinity < 1 mg/l and total activity of < 10-10Ku/l (coefficient of cleaning < 105can be discharged into open waters.

The cleaning process is performed during the automatic control using spectrometers. This achieves the following major process indicators: the performance of the reverse osmosis modules - 0.2 m3/h; purification > 105; the final salt content of < 1 mg/l; volume treated in a continuous mode LRW - 200 m3.

Example 2. Are LRW purification of type III in example 1, except that before the reverse osmosis unit LRW filtered through a sorption column filled composite ferronickel processing the permeate is filtered through 30 l of inorganic zeolite type A brand CIP.

The process of reverse osmosis treatment carried out at performance modules 0.3 m3/h and the final salt content in purified permeate is ~ 10 mg/l

The degree of purification from radionuclides > 105.

Volume treated in a continuous mode water is 400 m3.

Example 3. Carry out cleaning according to example 2, except that at the stage of sorption purification using zeolite IE-95, belonging to the group of zeolites Sabitova patterns.

However, the performance of the modules is 0.3 m3/h; the concentration of salts 10 mg/l; the degree of purification from radionuclides > 105volume treated in a continuous mode water is 420 m3.

Example 4. Conduct cleaning in example 1, except that on stage sorption purification using zeolite monoclinic structure - modified clinoptilolite in the Na-form of the brand "Selex-KM".

In this case, all the parameters of treatment corresponds to example 3.

Example 5. Carry out cleaning according to examples 1 to 4 in the following mode. After one cleaning cycle conduct flushing of the reverse osmosis module 50 - 70% alcohol solution, then 40% solution of formaldehyde. By what about the norms LRW reduced by up to 90 m3, i.e. more than twice. For processes with inclusion on stage pretreatment sorption on NA all indicators remained unchanged.

Example 6. Spend the purification of solutions of type I according to example 4, except that use single-stage reverse osmosis, and at the stage of the pretreatment stage ultrafiltration and sorption of petroleum products are excluded. As an FSC using the sorbent grade CIA on the basis of cobalt ferrocyanide.

The performance of the reverse osmosis module is 0.3 m3/h; the final salt content in purified permeate < 1 mg/l; the degree of purification from radionuclides > 105volume treated in a continuous mode water is 600 m3.

Example 7. Spend the purification of solutions of type II according to example 2, except that as an FSC using the sorbent grade MJA, on the basis of ferrocyanide of copper.

The performance of the process is 0.34 m3/h; the final salt content in purified permeate - 5 mg/l; the degree of purification from radionuclides > 105volume treated in a continuous mode water is 400 m3.

Example 8. Are LRW purification of type III according to example 2, except that in the FSC brand NA (Selex-C the Process of reverse osmosis purification is performed during performance of the modules of 0.25 m3/H. While the final content of radionuclides in cleared prior to the salt content of 6 mg/l solution will be < 10-10Ku/l

Volume continuously treated water in this example is 400 m3.

Thus, the proposed method allows a complex process LRW purification by reverse osmosis from all toxic contaminants, including radionuclides with lower power consumption and higher performance. In addition, it allows you to improve the environmental safety of the disposal process, LRW, as almost all radionuclides of cesium, which constitute about 70% of the specific activity of LRW are converted to a compact solid form.

The degree of concentration of these radionuclides in this way is ~ 5 - 8 thousand, which is 300 to 400 times higher than all other known methods of concentrating radionuclides.

The sources of information.

1. The norms of radiation safety NRB-76/87. - M.: Energoizdat 1987, 17 - 35.

2. Purification of low wastewater NPP radionuclide-selective inorganic sorbents. Proc. Dokl. All-Union seminar "Chemistry and technology of inorganic sorbents Dushanbe, 1986, T is 4. Nikiforov, A. S., Kulichenko centuries and Zhikharev M. I. Disposal of liquid radioactive waste. M: Atomizdat 1985, with 15 260 (prototype).

1. Method for processing liquid radioactive waste containing radionuclides of cesium and strontium, which includes stages pretreatment, purification and desalination using reverse osmosis, with the separation of the flow on the permeate and the concentrate followed by purification of the permeate on the sorbents, characterized in that the liquid radioactive waste is additionally subjected to purification by passing through the selective inorganic sorbent.

2. The method according to p. 1, characterized in that as selective inorganic sorbent use composite ferrocyanide sorbent on the basis of ferrocyanides transition metals copper, Nickel, cobalt and a porous inorganic carrier.

3. The method according to p. 1, characterized in that the additional treatment on inorganic sorbent is subjected to liquid radioactive waste after stage pretreatment or after reverse osmosis in the concentrate stream.

4. The method according to p. 1, characterized in that at the stage of purification of the permeate is passed through a sorbent selected from a number of synthetic zeolite type a, chabazite hexagonolepis fact, as natural zeolite monoclinic structure using modified zeolite brand "Selex-KM".

6. The method according to p. 1, characterized in that the processing solutions having a salt content of less than 1 g/l using a single-stage reverse osmosis processing, and the processing solutions with a salt content of more than 1 g/l of two step.

 

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