RU2399974C1 - Cleaning method of process water medium of nuclear productions from radionuclides - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: method involves water filtration through loading of granulated ferrocyanide-containing sorbent; sorbent includes 0.2-2 wt % of hydrazine, 35-48 wt % of water and 20-35 wt % of nickel ferrocyanide; composition Me(I)_4-2x[Ni_xFe(CN)₆], where Me(I) - Li⁺, Na⁺, K⁺, NH₄ ⁺ or their mixture, x=1.2-1.8, the rest is zirconium hydroxide. Size of composite sorbent granules is 0.05-1.5 mm.

EFFECT: increasing cleaning coefficient and operational life of sorption loading.

Description

The invention relates to a sorption technology for the purification of radionuclides from technological aqueous media of nuclear production, in particular liquid radioactive waste, water from the spent fuel storage and storage pools, unbalanced water, shower and laundry water, or an aqueous reactor coolant in emergency situations. Presumably, the invention is used for decontamination from long-lived radionuclides ¹³⁷ Cs, ¹³⁴ Cs, ⁹⁰ Sr, ⁶⁰ Co, primarily from cesium radionuclides, the share of which in the total activity is 90% or more.

In the practice of cleaning radionuclides from aqueous reactors of nuclear reactors, zeolites, sparingly soluble compounds of multivalent metals, such as zirconium and titanium phosphates, and also ferrocyanides (hexacyanoferrates) of transition metals (Ni, Zn, Cu, Co, Cd), are used as selective inorganic sorbents (Amphlet Ch. Inorganic ionites. M: Mir, 1966. 188 pp. Egorov EV, Makarova SB Ion exchange in radiochemistry. - M .: Atomizdat, 1971, 408 pp.) The highest selectivity to cesium radionuclides, especially from saline solutions possess transitional ferrocyanides metals. To use them in column mode, ferrocyanide components are applied to a granular carrier.

Composite ferrocyanide sorbents on an inorganic carrier possess high radiation resistance. A known method and device for cleaning solutions of radionuclides of strontium and cesium (RF Patent No. 2118856, class G21F 9/12), in which composite ferrocyanide sorbents on inorganic carriers are used for selective extraction of cesium-137: NZHA, FEZHEL, ferrocyanide on zeolite and others). The main disadvantage of the known inorganic ferrocyanide composite sorbents is associated with their poor chemical stability in alkaline media, especially at pH> 9, due to the nature of the inorganic matrix.

The closest in technical essence and the achieved result is a method of purification of cesium radionuclides from water technological environments of nuclear production, including passing a stream of water through a ferrocyanide-containing sorbent at a speed of not less than 1 · 10 ^-4 m / s and a temperature of 30-90 ° C (Patent RF No. 2113025, class G21F 9/12, publ. 06/10/1998). For the implementation of the method used a composite sorbent on an inorganic basis - nickel ferrocyanide on an aluminosilicate matrix (sorbent brand Fenix). When dynamically cleaning alkaline borate liquid radioactive waste of a WWER nuclear power plant with a total salt content of 312.5 g / l on a ferrocyanide sorbent with a total salt content of 312.5 g / l, a resource of 180-200 column volumes was achieved, which is inefficient when processing liquid radioactive solutions. The low life of the sorbent is associated with insufficient chemical stability and low kinetic characteristics of the aluminosilicate matrix.

The aim of the invention is to increase the cleaning efficiency and increase the service life of an inorganic sorbent.

The goal is achieved by the proposed method of purification from cesium radionuclides of water technological environments of nuclear production, in accordance with which the water is filtered through a granular charge of a ferrocyanide-containing sorbent. As a composite sorbent, granular zirconium hydroxide is used containing 20-35 wt.% Mixed nickel ferrocyanide, 0.2-2 wt.% Hydrazine and water 35-48 wt.%. Zirconium hydroxide used as a carrier of a composite sorbent is completely insoluble in alkaline solutions, which ensures high chemical stability of the sorbent. The presence of 20-35 wt.% Cesium selective mixed nickel ferrocyanide in the sorbent, which has the highest chemical resistance in alkaline media compared to other transition metal ferrocyanides (Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ^{2 +} , Cd ²⁺ , Zn ²⁺ ), allows you to get high resources of the sorption load. To achieve the content of the active component in the sorbent above 35 wt.% Fails using the sol-gel method for its preparation (Patent RU No. 2113024, CL G21F 9/12, publ. BI No. 16, 06/10/1998).

Liquid radioactive waste is saline solutions in which the diffusion mobility of cesium radionuclides is reduced. To improve the kinetic characteristics and, consequently, increase the degree of purification, the pore size of the carrier is increased, in this case, humidity (water content), which is selected within 35-48 wt.%. This provides in dynamic mode high coefficients of purification from cesium radionuclides. An increase in humidity of more than 48% is irrational, because the mechanical strength of the sorbent is significantly reduced. A decrease in humidity of less than 35% by weight impairs the kinetic characteristics of the material. In addition, liquid radioactive waste contains dissolved oxygen and other oxidizing agents, which are able to oxidize ferrocyanide to ferricyanide in accordance with the equation:

[Fe (CN) ₆ ] ^4- + e ^- → [Fe (CN) ₆ ] ^3-

Mixed nickel ferricyanide Me (I) _4-2x [Ni _x Fe (CN) ₆ ] has much lower chemical resistance and selectivity to cesium radionuclides. To prevent oxidation and increase the service life and cleaning efficiency, 0.2-2.0 wt.% Hydrazine is introduced into the sorbent composition. Hydrazine is an effective reducing agent and, first of all, enters into a chemical reaction with oxidizing agents that enter the filter with a sorbent with the solution. For example, hydrazine interacts with oxygen in accordance with the equation:

N ₂ H ₄ + O ₂ = N ₂ + 2H ₂ O

Hydrazine actively interacts with ozone, hydrogen peroxide and other oxidizing agents. Thus, hydrazine prevents the oxidation of ferrocyanide to ferricyanide and thereby contributes to an increase in the purification efficiency and an increase in the service life of the sorbent.

The upper limit of the hydrazine content in the sorbent is due to the maximum sorption capacity of hydrazine on nickel ferrocyanide, and the lower one is due to insufficient efficiency when the hydrazine content is less than 0.2 wt.%.

Composite sorbent has a granule size of 0.05-1.5 mm, which is optimal for use in dynamic mode in bulk and alluvial layers (filters). The active component, mixed nickel ferrocyanide, has the composition:

Me (I) _4-2x [Ni _x Fe (CN) ₆ ],

where Me (I) - Li ⁺ , Na ⁺ , K ⁺ , NH ₄ ⁺ or a mixture thereof,

x = 1.2-1.8.

Before passing through a ferrocyanide-containing sorbent, aqueous media can be oxidized, for example, with ozone to destroy organic complex compounds; purification from solid particles and suspensions using, for example, ultrafiltration, as well as purification from petroleum products. The purification of aqueous media from cesium radionuclides can also be carried out on at least two series-connected or parallel connected filters.

Example 1. The most optimal variant of the implementation of the cleaning method. A granular ferrocyanide-containing sorbent - nickel ferrocyanide on zirconium hydroxide of a certain moisture content was obtained using the sol-gel method (Patent RU No. 2113024, class G21F / 12, publ. BI No. 16, 06/10/1998). Then the granules were treated with a solution of hydrazine of a given concentration. When sorbent granules are treated with a hydrazine solution, hydrazine is adsorbed predominantly on nickel ferrocyanide. After equilibrium was established, the solution was separated from the sorbent, and the equilibrium concentration of hydrazine was determined in the solution by the volume permanganometric method. The content of hydrazine in the sorbent (S) (wt.%) Was determined by the formula

where Co, Cp is the initial and equilibrium concentration of hydrazine; V is the volume of the solution, m is the mass of the sorbent.

Obtained by the described method, the sorbent brand "Thermoxide-35" had the following characteristics:

1. The chemical composition, wt.%:

- zirconium hydroxide thirty - Nickel ferrocyanide, Me _1,6 (I) [Ni _1,2 (CN) ₆ ] 27 - hydrazine, N ₂ H ₄ 1,1 - water 41.9 - salt form of nickel ferrocyanide mixed (Na ⁺ , NH ₄ ⁺ )

2. The size of the granules of a spherical shape, mm, 0.4-1.0.

3. The distribution coefficient of cesium from simulated more than 10 ⁵

borate solution with a salinity of 300 g / l and pH = 11.5.

The sorbent was loaded into a column through which boron-containing liquid radioactive waste from WWER nuclear power plants with a total salt content of 450 g / l and a concentration of B ₂ O ₃ 56 was passed preliminarily oxidized by ozone in a top-down direction with a bulk velocity of 3 column volumes per hour (h ^-1 ). 5 g / l and pH 10.8. The initial activity of the ¹³⁷ Cs solution was 5 · 10 ⁷ Bq / L. Periodically, ¹³⁷ Cs activity was determined in the filtrate. When 940 column volumes were passed through, the activity of ¹³⁷ Cs in the filtrate was lower than the level of intervention, i.e. less than 110 Bq / l. With a further increase in the number of missed column volumes, a breakthrough of cesium radionuclides occurred, and the activity of the filtrate exceeded the level of intervention. Thus, the service life of the sorbent amounted to 940 column volumes, while the cleaning coefficient in this period was more than 4.5 · 10 ⁵ . In the prototype (RF Patent No. 2113025, class G21F 9/12, publ. June 10, 1998) under similar experimental conditions using a Fenix brand sorbent, the maximum resource was 200 column volumes. Using the proposed method allows to increase the resource of the download more than 4 times.

Examples 2-6. On the ferrocyanide-containing sorbent - mixed nickel ferrocyanide on zirconium hydroxide, kinetic curves of sorption of ¹³⁷ Cs radionuclide depending on the moisture content of the sorbent were obtained. Were synthesized, as in example 1, samples of the composite sorbent with a moisture content of 19, 36, 39, 45 and 48 wt.%. Kinetic curves were taken from a simulated solution of liquid radioactive waste (LRW) of the following chemical composition:

- NaNO ₃ 100 g / l - KNO ₃ 100 g / l - H ₃ BO ₃ 40 g / l - NaOH 27.4 g / l - Na ₂ CO ₃ 10 g / l - pH 10.9

A non-bearing ¹³⁷ Cs activity of ~ 10 ⁵ Bq / L was introduced into the solution. The results are presented in the drawing, where Kd is the distribution coefficient of ¹³⁷ Cs. The distribution coefficient was calculated from the expression:

,

,

where C _o and C _W - ion concentration in the initial and equilibrium solution, respectively; With _t - ion concentration in the sorbent; V is the volume of the solution; m is the mass of the sorbent.

As can be seen from the data presented (see the drawing), the kinetics of sorption of cesium radionuclides from a simulated solution of LRW is very dependent on the moisture content of the ferrocene-containing sorbent. The higher the moisture content of the composite sorbent, the better its kinetic characteristics. An increase in humidity of more than 48% is irrational, because the mechanical strength of the composite sorbent is significantly reduced. A decrease in humidity of less than 35% by weight impairs the kinetic characteristics of the material.

Examples 7-10. Under the same conditions, comparative tests were conducted on various grades of composite sorbents on different media for the purification of oxidized LRW of the Kalinin NPP. The following sorbents were tested: sorbent of the Thermoxide-35 brand (nickel ferrocyanide, carrier — zirconium hydroxide, composition of the sorbent, as in Example 1); HCF brand sorbent (manufactured in Finland); Fenix sorbent (nickel ferrocyanide, carrier - aluminosilicate); RCoFeCN-PAN sorbent (cobalt ferrocyanide, carrier - organic polymer, Czech Republic). The initial bottom residue had the following chemical and radiochemical composition:

- pH 11,4 - Total salinity, g / l 484 - H ₃ BO ₃ , g / l 70.8 - Chemical absorption of oxygen, g / l 7.4 - Volumetric activity of the solution, Ki / l ¹³⁷ Cs - 5.3 · 10 ^-4 ; ⁶⁰ Co - 7.0 · 10 ^-6

The bottoms were pre-oxidized with potassium permanganate, which reduced the volumetric activity of ⁶⁰ Co to a value of 5.4 · 10 ^-8 Ci / L. The tests were carried out in dynamic conditions with a bulk velocity of 2.5 h ^-1 . The following characteristics of sorbents were compared: purification coefficient ¹³⁷ Cs under the same filtration conditions and service life (number of missed column volumes, bp). The results are presented in the table.

Table The results of sorption of ¹³⁷ Cs by various composite sorbents Thermoxide-35 HCF Fenix KCoFeCN-PAN Skipping. volume, k.o. Cleaning ratio Skipping. volume, k.o. Cleaning ratio Skipping. volume, k.o. Cleaning ratio Skipping. volume, k.o. Cleaning ratio 25 4.0 · 10 ⁵ 47 1.5 · 10 ⁴ 12 8.7 · 10 ⁴ 10 1,110 ⁴ 68 1.9 · 10 ⁴ 62 1.2 · 10 ⁴ 56 6.810 ³ fifty 1,0 · 10 ³ 112 9.310 ⁵ 101 6.510 ³ 73 5.210 ³ 70 310 129 6.7 · 10 ⁴ 118 3.0 · 10 ³ 116 9.3 · 10 ⁴ 114 277 171 1,0 · 10 ⁵ 165 1.9 · 10 ³ 134 6.210 ³ 130 325 198 1.410 ⁵ 183 1.9 · 10 ³ 175 8.7 · 10 ³ 233 9.910 ⁴ 227 1.3 · 10 ³ 188 7.2 · 10 ³ 249 2.210 ⁵ 242 1,2 · 10 ³ 199 1,0 · 10 ⁴ 289 8.6 · 10 ⁴ 256 1,110 ³ 245 5.910 ³ 301 5.0 · 10 ⁴ 297 542 278 4.110 ³ 343 1,0 · 10 ⁵ 315 619 295 2.7 · 10 ³ 371 8.6 · 10 ⁴ 371 520 339 591 386 1.810 ⁵ 356 371 428 1.5 · 10 ⁵ 444 5.8 · 10 ⁵

The ferrocyanide-containing sorbent, mixed nickel ferrocyanide on zirconium hydroxide (Thermoxide-35 sorbent), which has a purification coefficient two orders of magnitude higher and a significantly longer working life, which was far from being used up by the end of the experiment, has the best characteristics. The experiment was terminated at the request of radiation safety.

Claims (2)

1. The method of purification from radionuclides of the aqueous technological environment of nuclear production, including filtering water through a granular charge of a ferrocyanide-containing sorbent, characterized in that they use a sorbent containing 0.2-2 wt.% Hydrazine, 35-48 wt.% Water and 20-35 wt.% nickel ferrocyanide composition Me (I) _4-2x [Ni _x Fe (CN) ₆ ], where Me (I) - Li ⁺ , Na ⁺ , K ⁺ , NH ₄ ⁺ or a mixture thereof, x = 1.2 -1.8, the rest is zirconium hydroxide. 2. The method according to claim 1, characterized in that the sorbent has a granule size of 0.05-1.5 mm

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