WO2007123436A1

WO2007123436A1 - Method for recycling a still residue of liquid radioactive wastes

Info

Publication number: WO2007123436A1
Application number: PCT/RU2006/000571
Authority: WO
Inventors: Valentin Alexandrovich Avramenko; Vitaly Georgievich Dobrzhansky; Valentin Ivanovich Sergienko; Sergei Ivanovich Shmatko
Original assignee: Obschestvo S Ogranichennoi Otvetstvennostyu 'nauka - Tekhnologii - Proizvodstvo'
Priority date: 2006-04-21
Filing date: 2006-11-01
Publication date: 2007-11-01
Also published as: RU2297055C1

Abstract

The invention relates to nuclear energy engineering, in particular to recycling a still residue of liquid radioactive wastes, for example of nuclear power plants. The method for recycling the still residue of liquid radioactive wastes consists in passing said still residue through cesium-selective inorganic sorbent for removing cesium-137 radionuclides, wherein ferrocyanide sorbents can be used in the form of the cesium-selective inorganic sorbent, in carrying out oxidation associated with subsequent separation of a radioactive cobalt-60 hydroxide-containing sludge. The oxidation is carried out by means of ozone-free oxygen-containing oxidisers at a high temperature and a pressure which is greater than the pressure of liquid saturated steam at said temperature. The oxidiser in embodied in the form of oxygen, hydrogen peroxide, potassium permanganate and sodium or ammonium peroxosulphate. Said invention makes it possible to simplify the method, to reduce the cost thereof, simultaneously increase the safety of the method and to accelerate the recycling method by reducing the oxidation process time.

Description

METHOD FOR CUBE REMAINING OF LIQUID RADIOACTIVE WASTE

Technical field

The invention relates to the field of nuclear energy, in particular, to the processing of bottoms of liquid radioactive waste (LRW) from nuclear installations, for example, waste from nuclear power plants (NPPs).

One of the main problems determining the existence and further development of nuclear energy is the solution of the problem of collecting and conditioning radioactive waste.

State of the art

During operation of a nuclear power plant, a relatively large amount of liquid and solid radioactive waste is generated. Special water treatment systems available at existing NPPs continuously process low-salt LRW by evaporation with crystallization of partially soluble salts. Evaporation of LRW produces condensate and bottoms of LRW that accumulate in special LRW storage facilities. Significant material and financial resources are spent on maintaining the technical condition of existing storage facilities in accordance with the requirements of regulatory documents, as well as on their protection.

Processing of bottoms of LRW nuclear power plants is one of the most popular and large-scale tasks in the field of processing and storage of radioactive waste.

Liquid radioactive waste (LRW) of nuclear power plants in the form of bottoms is a salt solution of high concentration (the amount of salts reaches 500 g / dm ³ ) contaminated with fission products, radionuclides of corrosive origin, various substances used to maintain the water-chemical regime and decontamination of equipment. The traditional methods of processing bottoms are deep evaporation, cementing and bitumen. These methods make it possible to convert LRW into an inert form suitable for disposal, but do not give a significant reduction in the volume of the final radioactive product, and also require significant material costs.

An economically and environmentally sound solution to the problem is the treatment of bottoms of LRW from radionuclides and the processing of the latter into solid radioactive waste. Radioactive substances in solutions of bottoms of LRW are in the form of simple and complex ions, neutral molecules and colloidal particles. The main radionuclides in the bottoms are ^{134> W} Cs, ⁶⁰ Co, ⁵⁴ Mn. Cesium isotopes are characterized by their ionic form. Cobalt and manganese radionuclides in bottoms are in the form of complexes with compounds that are widely used for equipment decontamination. At nuclear power plants, sodium ethylene diamine acetate (EDTA) and oxalic acid are the most commonly used substances. The finding of cobalt and manganese in a complex form determines the need for the destruction of complexes to solve the problem of the separation of these radionuclides from solutions.

A known method of processing bottoms of liquid radioactive waste of nuclear power plants, which consists in the fact that the bottom residue is treated with carbon monoxide and / or carbon dioxide to convert the salts contained in the bottom residue into sparingly soluble forms. Carbon oxide and / or carbon dioxide are introduced in stoichiometric amounts with respect to the converted salt components. Then, the formed crystalline phase is separated, and the remaining liquid phase is treated with ozone at 20-60 ^° C in the presence of an oxidation catalyst and / or a collector for extracting radionuclides from the liquid phase. During ozone treatment, the oxidation of organic compounds and the destruction of complex compounds formed by organic liands and radionuclides occur, resulting in the formation of a radioactive sludge that is separated from the solution. The solution is sent for additional evaporation. Polyvalent ions, mainly iron and zirconium ions, are used as an oxidation catalyst. As the collector, preferably iron and cobalt compounds are used. The treatment of the solution with carbon oxides is preferably carried out in two stages with a partial pressure of 0.01-0.99 MPa and an overpressure of 0.05-1.15 MPa with separation of the crystalline salt component after each stage. In this case, the first stage of processing is carried out at 20-70 ⁰ C, and the second at 0-5 ⁰ C. The method allows to reduce the volume of stored bottoms, to obtain a radioactive sludge suitable for disposal by known methods, as well as to isolate solid crystalline salts with normative permissible for open storage with the content of radionuclides and water suitable for reuse in the water circulation of nuclear power plants (see R.U2066493, publ. 09/10/1996) The disadvantage of this method is its complexity, caused, in particular, by processing in several stages, if necessary, maintaining different values of pressure, temperature and concentration of reagents at different stages, as well as the high cost of processing associated with the need to use ozone and catalysts.

Closest to the claimed is a method of processing bottoms LRW, which includes ozonation of bottoms at a solution pH of from 12 to 13.5. The resulting radioactive sludge containing iron hydroxide (the main part) and hydroxides of other transition metals, as well as radionuclides of the corrosion group Co; ⁵⁹ Fe ⁶³ Ni ⁵⁴ Mn and others are separated. After separation of the radioactive sludge, the filtrate is passed through a filter container with a cesium selective inorganic sorbent, then the spent filter container is sent for storage or disposal. The filtrate contains ¹³⁷ Cs 1.2 ^* 10 ^"10 Ci / L, and ⁶⁰ Co 0.5" 10 ^" ⁹ Ci / L. It is concentrated by deep evaporation to obtain a crystalline hydrate monolith (salt melt), the specific activity of which is 1.6 10 ^"9 Ki / l. An advantage of the invention is the provision of deep purification of liquid radioactive waste with high salinity from radionuclides (see RU2226726, publ. 10.04.2004).

A significant drawback of this method is its high cost, due to the consumption of electricity and reagents used (50-100 kg of ozone for the oxidation of 1 m ^{3 of} cubic residue), the need to install an ozonizer station (for an oxidation plant with a capacity of 0.5 M ³ MaC, a station with a capacity of up to 1 MW), which is not economically viable and leads to an explosive process. In addition, ozonation of the bottom residue may require the use of an appropriate amount of alkalizing reagent (NaOH), as The pH of the bottom residue is 8-13, and the optimum oxidation mode is pH 12-13.5, otherwise the ozone consumption increases greatly due to its destruction in an acidic environment. Before passing through the sorbing filter container, the filtrate obtained by ozonation is acidified with nitric acid. This complicates the process. Another significant disadvantage of this method is that the oxidation of the bottom residue is carried out before the sorption extraction of cesium. This leads to the need to work with large volumes of highly radioactive sediment and a solution containing cesium, which increases the radiation hazard of the process, because cesium radionuclides in LRW are among the most active, long-lived and hard to remove. The amount of radioactive sludge is 4.0-4.2 g for each liter of bottoms of LRW.

Disclosure of invention

The objective of the invention is to develop a method for processing bottoms of LRW, eliminating the use of ozone and allowing to separate cesium at the initial stage of processing.

The technical result of the method is to simplify the method, reduce its cost while increasing the safety of the method, as well as accelerating the processing process by accelerating the oxidation process.

The problem is solved by the method of processing the bottoms of LRW by its oxidation with an oxygen-containing oxidizing agent, followed by separation of the radioactive sludge containing cobalt-60 hydroxide, and purification of cesium-137 radionuclides by passing through an inorganic sorbent selective for cesium, in which cesium-137 radionuclides are purified before oxidation and oxidation is carried out with oxygen-containing oxidizing agents that do not contain ozone at a temperature not lower than the boiling point of the bottom residue and at a pressure above the pressure Nia liquid saturated steam for this temperature.

Preferably, the oxidation is carried out using hydrogen peroxide, potassium permanganate, sodium or ammonium peroxosulphate or oxygen as an oxidizing agent.

Ferrocyanide sorbents can be used as cesium selective inorganic sorbent.

The invention consists in the following.

The bottom residue of LRW obtained after evaporation of the LRW of a nuclear power plant is passed through an inorganic sorbent selective for cesium to extract cesium-137.

A cesium selective inorganic sorbent can be, for example, nickel ferrocyanide modified aluminosilicate (EDKA), nickel ferrocyanide modified silica gel (NSA), copper ferrocyanide modified silica gel (MHS), nickel ferrocyanide modified with zirconium hydroxide 35 and other sorbents known from the prior art.

Cesium is in the bottom residue in ionic form, which allows sorption with high efficiency before the oxidation process. It is known that during the sorption process, a partial decrease in the process rate occurs due to the possible coating of sorbent particles with organic substances insoluble in the bottom residue. However, the degree of such a decrease is very insignificant, since the real rates of processing the bottom residue on inorganic sorbents are much (5-10 times) lower than the limiting flow rates at which the effect of the reduction in speed will affect. However, an increase in the processing rate on inorganic sorbents is not of practical importance, since the process is “inhibited” by the oxidation stage, both in the claimed method and in the known method with ozonation. It should be noted that there is no decrease in the capacity of inorganic sorbents for cesium radionuclide (which can lead to a decrease in the quality of purification) due to the presence of organic substances, because cesium ion practically does not form complexes with organic ligands.

Extraction of cesium-137 before oxidation avoids contamination of the entire further processing scheme (solutions, precipitation, and equipment), i.e. to increase the safety of the method due to a decrease in the activity of the bottom residue at the initial stage of processing by removing the ¹³⁷ Cs radionuclide, whose activity is 10 times higher than the activity of ⁶⁰ Co.

It is known that cobalt (IH) complexes, in particular with EDTA, have a stability constant that is several orders of magnitude higher than the stability constants of complexes with EDTA of ions of other transition metals contained in the solution, such as iron, nickel, cobalt, nickel, copper, etc.

As a result, the destruction of cobalt complexes due to the oxidative destruction of organic compounds is a difficult technical problem.

For oxidation, the solution purified from cesium is mixed with an oxidizing agent, and the oxidation is carried out with oxygen-containing oxidizing agents that do not contain ozone at a temperature not lower than the boiling point of the bottom residue and at a pressure above the saturated vapor pressure of the liquid for this temperature.

It is completely obvious that the boiling point of the bottom residue is determined by the salt composition of the bottom residue of LRW and, as a rule, is not lower than 130 ⁰ C, and the pressure exceeding the saturated vapor pressure of the liquid for this temperature (the saturated vapor pressure of the liquid is the pressure at which the liquid begins to boil or evaporate), can easily be determined by a specialist. Under pressure equal to or lower than the pressure of saturated vapor of the liquid, the process is inefficient and slow, because most of the liquid goes into the vapor phase.

The required amount of oxidizing agent is preliminarily determined in a known manner in each particular case, depending on the qualitative and quantitative composition of the bottom residue.

As the oxidizing agent, you can use any oxygen-containing oxidizing agents that do not contain ozone, in particular, hydrogen peroxide, potassium permanganate, sodium or ammonium peroxosulfate in the form of solutions, as well as oxygen. The listed oxidizing agents are more affordable in cost and in the way they are obtained.

It was found that the treatment of the bottom residue purified from cesium by oxidizing agents such as hydrogen peroxide, potassium permanganate, sodium or ammonium peroxosulfate or oxygen at elevated temperatures and pressures above the saturated vapor pressure of the liquid at this temperature can significantly intensify the oxidation of organic compounds, including complexing agents such as EDTA, oxalic and citric acid, etc. The process conditions allow to accelerate the oxidation process by 2-3 times in comparison with the closest analogue.

Under these conditions, oxidation of complexes with transition metals (iron, cobalt, manganese, nickel, etc.) is ensured, and at the same time the solution is destabilized with respect to the ions of these metals held in solution by organic ligands, with the formation and precipitation of sparingly soluble hydroxides of these metals.

Thus, the treatment of the bottoms of LRW, purified from cesium, with an oxidizing agent in a flow reactor under the indicated conditions provides the maximum depth of the processes of destruction of transition metal complexes with organic ligands. The high oxidation rate achieved at the same time allows for a given productivity to reduce the volume of liquid in the duct and, accordingly, to make both the high-temperature oxidation process and the entire processing as a whole safer.

In addition, due to the oxidation by an oxidizer that does not contain ozone and, consequently, the absence of a plant for producing ozone, the explosion hazard of the process is eliminated and its cost is reduced. Brief Description of the Drawings

Fig.l - diagram of the method

The implementation of the invention

The cubic residue of RZhO NPP placed in the tank 1 with the help of pump 2 is fed to column 3 with an inorganic sorbent selective for cesium. When passing through the column, ^ι Cs is extracted from the bottom residue. The spent filter is sent for storage or disposal as solid radioactive waste by known methods, and the solution purified from radioactive cesium is sent to the oxidation (oxidative destruction) of organic substances that make up its composition.

Next, the solution purified from radioactive cesium is mixed with an oxidizing agent, which is supplied from the tank 4 by means of a metering pump 5 to the supply line 6. The mixture of the solution with the oxidizing agent from the main 6 is fed into a heated flow reactor 7 equipped with a throttle (not shown), by means of a high pressure pump 8. Heating of the high-pressure flow reactor is carried out using a special electric heater 9. The pumping rate of the bottom residue mixture with the oxidizing agent through the flow reactor 7 with a throttling device is determined It is the time of the oxidation reaction and depends on the volume of the vessel and the pump supply. In each case, the pumping speed is determined experimentally.

If a gaseous oxidizing agent, oxygen, is used for oxidation, then it is supplied under pressure directly to a high-pressure flow reactor (not shown). The oxidation by oxygen is carried out at a partial pressure of 8-12 MPa.

The mixture processed in the high-pressure flow reactor 7 is cooled in the heat exchanger 10 and throttled to atmospheric pressure.

The resulting radioactive sludge, consisting mainly of transition metal hydroxides, including cobalt-60 radioactive hydroxides (main part) and manganese-54, is filtered off using filter 11 and sent in the form of concentrated pulp through outlet 12 using one of the known methods.

The filtrate, purified from radionuclides to the standards established by the requirements of radiation safety, through outlet 13 is transferred to storage as industrial waste for possible subsequent processing in order to extract useful components.

The possibility of carrying out the invention is confirmed by the following examples.

In all the examples, the bottoms of LRW obtained after evaporation of LRW of nuclear power plants and used as a highly concentrated solution with pH 9-12, with a total salt content of up to 400-450 g / dm, ¹³⁷ Cs activity of about 10-15 Bq and ⁶⁰ Co activity of about 10 were used for processing. -15 Bq, characterized by the following macro-salt composition: anions, g / l: BO ^{3 "s-} 150-160; NO ₃ ^" - 155-165; SO ₄ ^" - 6-15; CO ₂ ^2" - 50-55; cations, g / l: Na ⁺ -140-150; K ⁺ -35-40; NB ^ ₄ - 0, 1-03.

The activity of the bottom residue of LRW for cesium after sorption purification using ferrocyanide sorbents was 150 Bq / L.

In all experiments, the oxidation process was carried out at temperatures in the flow vessel with a throttling device at 200 ⁰ C, 250 ⁰ C and 300 ⁰ C and the rates for pumping the bottom residue through the high pressure flow reactor 100 cm ³ / h, 250 cm ³ / h and 500 cm ³ / hr and radioactivity was determined by cobalt-60 in the bottom residue.

Example 1

The bottoms of LRW are passed through a column with a ferrocyanide sorbent NJA with a capacity of 400 ml at a rate of 2 column volumes per hour. The bottom residue that has passed through the sorbent using a high-pressure pump (up to 150 kg / cm) is introduced into a flow vessel equipped with a 100 cm ³ choke, heated with a spiral electric heater, and oxygen is simultaneously supplied to the vessel under a pressure of 10 MPa from an oxygen cylinder. The process is carried out at 200 ⁰ C. The saturated vapor pressure of the still bottom solution at this temperature is 1.2 MPa. The pressure in the vessel exceeds the saturated vapor pressure by 5 times. After leaving the vessel, the bottoms of LRW are cooled in a heat exchanger and throttled to atmospheric pressure. The precipitate of the resulting hydroxides is filtered on a cermet ultrafiltration membrane and separated in the form of a concentrated pulp.

Example 2

The bottom volume of LRW, passed through a column with a ferrocyanide sorbent, NLF with a transmission speed of 5 column volumes per hour, is mixed with 30% hydrogen peroxide supplied in an amount of 200 ml per 1 liter of bottom residue with using a proportional dosing pump directly to the supply line. The resulting mixture using a high pressure pump is introduced into the flow vessel and subjected to oxidation under the conditions indicated in the table. Further processing of the oxidized bottoms of LRW and the resulting precipitate is carried out analogously to example 1.

Example h.

The example is carried out analogously to example 2, except for the following conditions. To clean the bottom residue of LRW from cesium-137 radionuclides, the ferrocyanide sorbent Tepmoksid-35 is used; the rate of transmission of the bottom residue through the sorbent 10 column volumes per hour. As an oxidizing agent, a 5% potassium permanganate solution is used, which is taken in an amount of 200 ml per 1 liter of bottoms. Oxidation in a leak-proof flow vessel and further processing of the purified bottoms of LRW and the precipitate formed as in Example 1.

Example 4

The example is carried out analogously to example 2, except for the following conditions. To clean the bottom residue of LRW from cesium-13 7 radionuclides, a ferrocyanide sorbent is used, the rate of transmission of the bottom residue through the sorbent is 5 column volumes per hour. As an oxidizing agent, a 40% solution of ammonium peroxosulfate is used, which is taken in an amount of 100 ml per 1 liter of bottoms. The oxidation and further processing of the purified bottoms and the resulting precipitate as in example 1.

The oxidation conditions and experimental results are shown in the table.

The experimental data show that when processing the bottom residue of LRW by the proposed method, the specific activity of the bottom residue for cesium-137 decreases by about 7,000 times, and the specific activity for cobalt-60 decreases by 20-125 times, which confirms the effectiveness of the developed method.

The amount of radioactive sludge is also reduced by 1.5-2.0 times, which leads to lower costs for its further processing.

Claims

Claim

1. The method of processing the bottom residue of liquid radioactive waste by oxidizing it, followed by separation of the radioactive sludge containing cobalt-60 hydroxide, and purification of cesium-137 radionuclides by passing through an inorganic sorbent selective for cesium, characterized in that the cesium-137 radionuclides are purified before oxidation and oxidation is carried out with oxygen-containing oxidizing agents that do not contain ozone at a temperature not lower than the boiling point of the bottom residue and at a pressure above the saturated vapor pressure of the liquid ty for this temperature.

2. The method according to claim 1, wherein the oxidation is carried out using hydrogen peroxide, potassium permanganate, sodium or ammonium peroxosulfate or oxygen as an oxidizing agent.

3. The method according to claims 1 or 2, characterized in that ferrocyanide sorbents are used as cesium selective inorganic sorbent.

SUBSTITUTE SHEET (RULE 26)