RU2090944C1 - Method for decontaminating highly radioactive water from radionuclides - Google Patents

Abstract

FIELD: sorption procedure for decontaminating highly radioactive water. SUBSTANCE: method involves filtering water through combination charge of cation-exchange inorganic sorbents of zirconium phosphate and/or titanium phosphate in hydrogen and salt forms. Volume proportion of hydrogen and salt forms of sorbents in combination charge is 1:2-2:1. Lithium, sodium, or potassium forms are used, as a rule, as salt forms of sorbent. Sorbents are usually arranged in combination charge in layers; first layer along decontaminated water path has sorbent in hydrogen form and second layer has sorbent in salt form. EFFECT: provision for maintaining steady and neutral pH value of medium at sorption column outlet; facilitated procedure of water decontamination. 7 cl, 2 tbl

Description

The invention relates to a sorption technology for water purification and can be used for the purification of high- and intermediate-level liquid wastes, in particular, the water of spent fuel pools of nuclear power plants in the recirculation mode or of the aqueous coolant of nuclear reactors in emergency situations in the direct filtration mode with effluent discharge. It is assumed that the invention is used for decontamination from long-lived biotoxic nuclides of the type ¹³⁷ Cs, ¹³⁴ Cs, ⁹⁰ Sr, ⁶⁰ Co, primarily from cesium radionuclides, the share of which in the total activity of liquid waste 0.16 Ci / l reaches 98% according to accident data at the American Three Mile Island NPP [1]
In practice, water purification of the coolant of nuclear reactors, as well as decontamination of liquid wastes of low and medium levels of activity, methods of purification using organic ion-exchange ion exchangers are known [2-5]. Thus, in the purification of purge reactor waters, strongly acidic cation exchange resin and strongly basic anion exchange resin are used in H ⁺ and OH ^- forms in the form of mixed loading or use separate filters. According to [2, p. 267] an ion-exchange filter loaded with a head layer of KU-2-8 cation exchanger and a mixed layer of KU-2-8 and anion exchanger AB-17-8 in H ⁺ and OH ^- forms, respectively, is used to purify the water of a pool-type reactor from activity . According to [3, p.131], for the deactivation of the aqueous coolant of nuclear power plants in the United States, an installation consisting of two filters is used, respectively comprising cation exchange resin in the H ⁺ form and a mixed charge of cation exchange resins in the same form and anion exchange resin in the OH ^- form in the ratio 1 :1. It is known [5, p. 338] that a column loaded with Amberlite IR-120 cation exchanger in an amount of 0.4 m ³ is used to purify ¹³⁷ Cs of radionuclide from the recirculated water of the exposure pool of the irradiated fuel.

A known installation and method of removing radioactive strontium and / or cesium from an aqueous solution containing chemical rigidity [6] For this, the solution is passed through a first column containing an organic cation exchange resin in a Na ⁺ form, and then through a second column loaded with synthetic zeolite (type A, erionite, chabazite, philipsite) or a mixture of these zeolites.

The disadvantage of sorption methods for the deactivation of medium- and highly active solutions using organic ion-exchange resins is their low radiation resistance [2-4], as a result of which the base is degraded, the exchange capacity decreases and the kinetics of exchange deteriorates [2, p.23-25; 3, p. 73] For example, sulfo and carboxyl cation exchangers noticeably worsen sorption capacity at absorbed radiation doses of 10 ⁶ Gy (10 ⁸ rad), and at doses of 10 ⁷ Gy these resins can no longer be used [2, p.24] At the same time, in the process ion-exchange excretion of such radiotoxic long-lived fission products as ¹³⁷ Cs, ⁹⁰ Sr, of this dose level are obtained when an ion exchanger absorbs activity of the order of 1-5 Ci / l already during the exposure time of 5-7 days [2, p.18 and 270] Another disadvantage of organic resins is their low selectivity, in particular to cesium radionuclides, such as ¹³⁷ Cs and ¹³⁴ Cs.

 The decontamination method has similar disadvantages, using activated carbon based sorbents for cleaning the aqueous coolant of a nuclear reactor, the surface of which is modified with oxine or its derivatives [7] Another disadvantage of this method is that activated carbon based sorbents have low cation exchange capacity.

In radiochemical practice and at nuclear power plants, methods are known for cleaning long-lived radionuclides ( ¹³⁷ Cs, ⁹⁰ Sr) of water from storage pools and spent fuel storage using inorganic sorbents that have higher radiation resistance than organic resins and coals - at least 10 ⁷ Gy [ 2, p.22 and 28] Of these, the most widely used or proposed to use synthetic sorbents are zeolites, hexacyanoferrates (ferrocyanides) and zirconium phosphate.

A known method of decontamination from ¹³⁷ Cs in the mode of recirculation of neutral demineralized water from the fuel holding pool at the Bradwell nuclear power plant [5, p. 343] The purified water is first passed through a Decalsoi zeolite column, and then through an ion-exchange demineralizer. The disadvantage of this method is the low chemical stability of the zeolite used, which leads to the introduction of an additional ion-exchange purification step. In another technical solution [8], 16 thousand column volumes of refrigerant were passed through granular synthetic mordenite from the pool to store spent fuel in recirculation mode; as a result, more than 95% of the radionuclides ¹³⁴ Cs, ¹³⁷ Cs and ¹¹⁰ Ag were removed. The disadvantage of this method is the low selectivity of the sorbent, as indicated by a low coefficient of purification by radiocaesium (20) with a rather large filter cycle. Another disadvantage is the low chemical resistance of the sorbent and zeolites in general, as a result of which impurities of the base silicon, aluminum, sodium, and others enter the effluent.

Known methods for deactivation using hexacyanoferrates of the aqueous coolant of the reactor from radiocaesium [9] and the pool water of spent fuel from nuclear plants from ¹³⁷ Cs and ⁹⁰ Sr (90). In the first method [9] through a glass column loaded with 1 cm ^{3 of} titanium hexacyanoferrate impregnated into a cation exchange resin in an amount of 23%, a simulated water coolant of the first circuit of the WWER-440 reactor (0.065 mol / L H ₃ BO ₃ , 0.025 mol / l KOH, 0.002 mol / l NH ₄ OH; temperature -50 ^o C; pH 6.5) at a speed of 10 m / h (the sorbent was obtained according to the Polish patent N p-2255191, 1985 [9]). At the end of the filtration cycle, in 25 thousand column volumes (co), the purification coefficient for ¹³⁷ Cs was 100, and the concentration of hexacyanoferrate ions was 2 mg / L. According to the second method [10], the water of the pools for spent fuel from radioactive impurities in the recirculation mode is treated using a mixed-action filter consisting of domestic KU-26 cation exchange resin in the H ⁺ form and AV-17 anion exchange resin in the OH ^- form in the ratio 1: 1, 10-20% of the amount of which of the total load of 300 l is preliminarily modified with nickel hexacyanoferrate by the impregnation method. The purification coefficient of ¹³⁷ Cs and ⁹⁰ Sr, equal to 10, is achieved when passing 5000 and 10 k.o. water having a salt content of 400 mg / l and a total beta activity of 1 • 10 ⁷ Bq / dm ³ (0.27 Ci / l).

The main disadvantage of both methods [9, 10] is the use of an inorganic sorbent based on a radiation-unstable organic base, which, with a high specific activity of water (n • 10 ⁷ Bq / l and higher), will significantly reduce the life of the filter, and especially with a cyclic mode of operation, when the absorbed activity of the sorbent is in the columns for a long time (weeks and months). Another drawback, as can be seen from [10], is the low resource of the loading operation over the long-lived ⁹⁰ Sr, which is associated with the low selectivity of the sorbent modifier.

In practice, the processing of liquid waste of high and medium levels of activity uses sorption methods for the isolation or separation of long-lived radionuclides by inorganic sorbents based on insoluble compounds of zirconium and titanium, in particular zirconium and titanium phosphates [11, 12] Sorbents obtained by the gel method are highly selective for cesium and high radiation resistance, maintaining without changing the sorption properties of the dose of gamma radiation of the order of (1-3) • 10 ⁷ Gy [12, 13] According to the method [11, example 13], cesium is isolated from 0.01 mol / L NH ₄ Cl solution on a 0.2 cm ² x 12.5 cm column loaded with zirconium phosphate in partially Na $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ form, after which it is eluted with a saturated solution of NH ₄ Cl. According to example 18 [11], sorption of cesium and barium ions is carried out on zirconium phosphate in the H ⁺ form on a 0.2 cm ² x 3.1 cm column. The disadvantage of the method [11, example 13] using a sorbent in Na $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ -form, it consists in a significant reduction in the selectivity of zirconium phosphate to cesium and, accordingly, the working resource of the load in the filtration mode until breakthrough. When filtering neutral solutions containing cationic impurities through a sorbent in hydrogen form according to the method of [11, Example 18], a significant acidification of the effluent occurs as a result of the release of hydrogen ions during the exchange of H ⁺ -M ^{+ n} , which will adversely affect the corrosion resistance used in deactivation equipment, pipelines and fittings and will additionally cause pollution of the cleaned environment by corrosion products.

The closest technical solution is a method for purifying a radionuclide and corrosion products from an aqueous coolant of a nuclear reactor by filtering it through a two-layer charge, including granulated zirconium phosphate Zr (HPO ₄ ) ₂ • nH ₂ O in the H ⁺ form and granular zirconium hydroxide in the direction of water in the OH ^- form [14, 15] Sorbents are obtained by the gel method in the form of granules of irregular shape [14] or by the sol-gel method in spherical granulation [15]
According to the method [14], through two connected columns loaded respectively with phosphate and zirconium hydroxide at a weight ratio of 1: 5, water with a temperature of 100 ^° C. containing 0.1 mmol / L Fe ³⁺ , 0 is passed at a speed of 10 m / h , 1 mol / L Ni ²⁺ and 5 mmol / L sodium acetate, designed to maintain a pH of 6-8. As a result, 1 mmol of iron and 2.3 mmol of nickel were absorbed on zirconium phosphate (calculated per 1 g of phosphate groups). In the effluent effluent after the second column, designed to capture phosphate ions that enter the solution in small quantities as a result of hydrolysis of zirconium phosphate, the concentration of phosphates was 0.2 mg / L.

 The proposed method [14, 15] is complicated because it involves the introduction of a sodium acetate chemical reagent to stabilize the environment at a neutral level, the pH value of which must be maintained for holding pools at domestic nuclear power plants at 5.5–8 [16, p. 292] Another disadvantage is in complicating and increasing the cost of the method due to the introduction of an additional anion-exchange charge from zirconium hydroxide, which is ineffective against cationic radioactive impurities (cesium, strontium, rare-earth elements) and the amount of which is many times greater AET weight basic zirconium phosphate sorbent.

 The aim of the invention is to simplify the sorption method of purification from radionuclides of cooling water in the exposure pool or spent fuel storage while maintaining a stable and neutral pH value at the outlet of the column. The goal is achieved in that the cooling water is filtered through an inorganic cation exchange sorbent in hydrogen and salt forms, which are selected as zirconium and / or titanium phosphate.

The method is as follows. A granular inorganic cation exchanger in hydrogen and salt forms is loaded into the column. Moreover, it can be used both in layers and in the form of a mechanical mixture. In the case of a two-layer arrangement of the load from the mentioned sorbents, it is recommended to use hydrogen as the first layer along the cooling water filtration, and the salt form of the cation exchanger as the second layer. The proposed method, in addition to using a two-layer loading in one column, also allows the use of a sorbent in the form of two separate, “split” loads (layers) of hydrogen and salt forms, placed in two successive columns, respectively. Then, in this case, the sorbent in hydrogen is loaded into the first column in the course of water filtration, and in the second column in salt form. As the salt form, the sodium form is preferred, although other forms, for example lithium, can be used. The optimal ratio of the volumes of sorbents loaded into the column in salt and hydrogen form depends on various factors (radioisotope and macroelement composition of the coolant, pH of the medium) and is specified in practice experimentally. When decontaminating water with a low salinity and neutral pH, which is usually characteristic of a cooling pool cooler and spent fuel storage, this volume ratio is largely determined by the ratio of long-lived nuclides ¹³⁷ Cs and ⁹⁰ Sr and can be selected from 2: 1 to 1: 2 s Taking into account the fact that selective sorption of cesium is better on the H ⁺ form of cation exchange resin, and strontium is better absorbed by the Na ⁺ form of the sorbent. In accordance with the proposed method, it is recommended to use zirconium phosphate, titanium phosphate or a mixture of them as a granular inorganic sorbent for loading into the column, since these sorbents have good sorption-selective properties with respect to long-lived radionuclides (Cs, Sr, REE) and uranium [12, 15, 19] and methods for their synthesis are also known [12, 17, 18, 20]. When implementing the proposed method in a production environment, water is filtered through a charge using a pump of the corresponding capacity, p By putting the solution to be cleaned into top-down or bottom-up mode. In the process of passing the solution through the load, the radioactive substances are extracted by the sorbent from the solution, and the purified effluent, depending on the purpose, is sent to the discharge or is returned back (recirculation mode) to the container. After the selective capacity is exhausted, spent sorbents are sent for disposal as highly active waste.

In accordance with the proposed method in practice, you can use four combinations of downloads of zirconium phosphate (FC) and titanium phosphate (FT) in the direction of the purified water:
FC (H ⁺ form) FC (Na ⁺ form);
FT (H ⁺ form) FT (Na ⁺ form);
FC (H ⁺ form) FT (Na ⁺ form);
FT (H ⁺ form) FC (Na ⁺ form).

 The choice of a particular combination of downloads depends on the selectivity of the release of radionuclides by the sorbent from the purified water, its chemical composition, as well as the kinetics of metabolism, which depends on the type and working form of the sorbent.

 It should be noted that according to the claimed method, the above-mentioned sorbents should be used in the form of spherical granular material. This is due to the fact that practically decontamination of radioactive liquids is carried out in a dynamic mode using a column type apparatus (or columns), and the spherical shape of the grain provides good hydraulics of the process. Therefore, it is recommended that the sorbents according to the proposed technical solution be synthesized using a sol-gel method, for example [15, 17, 18, 20], resulting in spherical granules with good strength (fracture limit of more than 1 MPa) and operational (non-creep material during filtration, high rate of ion exchange, etc.) properties. Ultimately, this ensures the process of decontamination of highly active liquids in column mode for a long time and with high productivity.

 A comparative analysis with the prototype [14, 15] and analogs [2-12, 16, 19] allows us to conclude that such combined loadings of the same ion-exchange material in two working forms were not used for decontamination of liquids among both inorganic and organic cation exchangers. Thus, the claimed method differs from the known type (composition) of the combined load and the order of movement of the cleaned medium from hydrogen to salt form, which meets the criterion of "novelty."

Examples 1-3. Obtaining zirconium phosphate in hydrogen and salt forms
An aqueous 1.2 mol / L zirconium chloride solution is poured into a single-chamber (diaphragmless) electrolyzer with a capacity of 5 l square section made of titanium with a wall thickness of 1 mm. The cathode is the material of the cell, and the anode is platinum. The electrolysis is carried out at a cathodic current density of 310 A / m ² until the atomic ratio of chlorine to zirconium in the electrolyte reaches 0.65, resulting in a sol that is stable over time.

The synthesized anion-deficient sol is drip dispersed through a glass capillary with an inner diameter of 0.4 mm into a concentrated ammonia solution to obtain spherical gel-like particles of zirconium hydroxide. After washing the particles with deionized water, they were treated with a 1.1 mol / L solution of phosphoric acid with a solid: liquid volume ratio of 1: 5 and periodic stirring. After exposure for a day, the particles are washed with deionized water from excess reaction products and dried in air to obtain strong white spherical granules of zirconium phosphate in an H ⁺ form 0.4-1 mm in size.

To obtain the sorbent in sodium form, part of the granules of the obtained material is transferred into a beaker and poured with a 20-fold volume of 0.1 mol / L NaCl solution. Then, the hydrogen ions released during the exchange are neutralized by careful addition of 0.2 mol / L NaOH solution with continuous stirring until a constant pH value of 7.5 is established over time. The granules are separated from the solution, washed with water and dried in air, obtaining FC in the Na ⁺ form with the same fractional composition as the starting material.

Zirconium phosphate in the Li ⁺ form is prepared similarly to the previous FC (Na ⁺ ) sample, except that the neutralization of hydrogen ions by lithium hydroxide is carried out to a pH value of 4.8.

 Examples 4 and 5. Obtaining titanium phosphate in hydrogen and sodium form.

In a diaphragmless electrolyzer with a capacity of 3 l of square section, made of titanium with a wall thickness of 0.8 mm, pour a 1.1 mol / l solution of titanium tetrachloride with the addition of 1 at. zirconium in the form of chloride in relation to titanium. The cathode is a zirconium plate, the graphite anode. Hydrochloric acid is added to the electrolyte in the amount necessary to obtain a concentration of 0.1 mol / L, and the electrolysis is carried out in two stages: at the first with a cathodic current density of 30 A / dm ² until the atomic ratio of chlorine to metal in the electrolyte reaches 1.15 , the second at a cathodic current density of 12 A / dm ² until the atomic ratio of chlorine to metal is 0.16, resulting in a stable anion-deficient sol.

Then, the synthesized sol is processed as in Example 1, and the final product is dried in a thermostat at 105 ^° C for 6 hours. As a result, durable white (milky) spherical granules of titanium phosphate in an H ⁺ form of size 0.63-1 are obtained mm

Titanium phosphate in the Na ⁺ form is obtained by treating the starting FT (Na ⁺ ), as in Example 2, until a constant pH of 7.3 is established. As a result, durable FT spherical granules (Na ⁺ ) with a grain size of 0.63-1 mm are obtained.

Examples 6-9. Comparison of the effectiveness of water purification from ¹³⁷ Cs radionuclide in dynamic mode for various combined downloads of zirconium phosphate.

Evaluation of the cleaning efficiency of the proposed and known methods is carried out using a column of glass with an inner diameter of 16 mm, into which two layers of sorbent are loaded according to examples 1-3. The volume of each layer is 10 cm ³ . 20 l of an aqueous solution of a simulated spent fuel pool simulated with ¹³⁷ Cs nuclide are passed through a column at a constant speed of 20 kp / h. Composition of imitate:
NaHCO ₃ 47.0 mg / L
CaCl ₂ 0.29 meq / L
MgCl ₂ 0.04 meq / L
A ( ¹³⁷ Cs) 1.5 • 10 ^-5 Ci / L
pH 8.01 (adjusted to 0.1 mol / L NaOH)
The gamma activity of the effluent is determined using a scintillation detector and a single-channel amplitude analyzer. In the first and last fractions (100 ml each), the pH of the medium is determined on the I-120.1 ionomer and the average value for the entire effluent is calculated. The cleaning efficiency of the known method is evaluated under the same conditions on the same load from zirconium phosphate synthesized according to examples 1 and 2, only in the hydrogen or sodium form, examples 8 and 9, respectively.

The test results are shown in table 1. It obviously follows from them that the proposed processing method according to examples 6 and 7 provides, in contrast to the known (example 8), stable pH maintenance in the effluent following the column at a neutral level while maintaining a cleaning efficiency comparable to the known method. At the same time, the use of zirconium phosphate for decontamination only in the Na ⁺ form (Example 9), to maintain a neutral pH of the effluent, is apparently economically impractical, since it greatly reduces (almost twice) the cleaning efficiency.

Examples 10 and 11. Evaluation of the effectiveness of the purification of simulated swimming pool water from the ¹³⁷ Cs radionuclide in dynamic mode on a mixed charge of zirconium phosphate in H ⁺ and Na ⁺ forms.

In a glass column with an inner diameter of 8 mm, 10 ml of a mechanical mixture consisting of 7 ml of FC in the H ⁺ form synthesized according to examples 1 and 2 are loaded, for comparison, 20 ml of FC in the H ⁺ form obtained according to example are loaded into another column 1 (example 11). Through the column at a speed of 40 K. about. / h, an aqueous imitate is passed, as in examples 6-9, into which instead of a ¹³⁷ Cs nuclide a ⁹⁰ Sr radionuclide (without a carrier) is introduced in an amount of 3.2 • 10 ^-5 Ci / l. The specific activity of purified water is determined on a Ge (Li) detector of the DGDK-40B type using the AI-4096 multichannel analyzer using a 514 keV gamma line.

According to the test data, after passing 50 l of water, the average purification coefficient for strontium by the proposed method is 432, and the pH of the effluent for the entire test life varies from 6.30 to 6.58. For the same conditions on the FC in the H ⁺ form, the purification coefficient from ⁸⁵ Sr is 105, and the effluent retains an acid reaction during the filtration of water (pH 3.63-3.81).

 The above results show a higher efficiency of decontamination of water from radiostrontium by the proposed method (about 4 times) while maintaining a neutral pH value of the solution being cleaned.

Examples 12 and 13. Evaluation of the efficiency of purification of simulated pool water of exposure from ¹³⁷ Cs on a combined load during filtration in recirculation mode.

 Evaluation of the cleaning efficiency of the combined charge according to the proposed method is carried out on a bench simulating the operation of the water purification system of the spent fuel exposure pool by filtering it through a combined charge from an inorganic sorbent with the effluent returning to the pool (recirculation mode). The stand includes a glass column with an inner diameter of 19 mm, loaded with 40 ml of sorbent in two layers of 20 ml each; a perfusion pump of the UNIROL-01 type, which provides a simulated water consumption (composition as in examples 6-9) of 1200 ml / h with a relative error of ± 1% polyethylene capacity per 50 l filled with simulated solution.

The radioactive imitate is continuously taken by the pump from the bottom and discharged after passing water through the column (filtering mode bottom-up) to the upper part of the polyethylene container. At the end of the filter cycle (2800 kb of water), the imitate in the tank is analyzed for pH and specific activity, and the average purification coefficient of the source water is calculated, as well as the degree of deactivation as the ratio of the sorbed activity to the initial water activity in the tank. In addition, the pH value and the column cleaning coefficient are estimated by analyzing the resulting imitate (effluent) after passing 1350 k.o. water and at the end of the filter cycle. The test results of two combined downloads (FT (H ⁺ ) and FT (Na ⁺ ) and FTs (H ⁺ ) and FT (Na ⁺ )) are given in Table 2. From the data it follows that the claimed method makes it possible to purify radioactive waters for a long time, including in the recirculation mode, from toxic nuclides.

 Thus, the proposed sorption method of water treatment allows high deactivation and long-term filtration to carry out decontamination of water flows from radiotoxic substances while maintaining neutral pH values in the cleaned environment. Therefore, with a high level of specific activity of the stream, it can be cleaned in recirculation mode with subsequent discharge (when maximum permissible activity levels are reached) of the treated water into the sewer. Given the low toxicity and high selectivity of the used sorbents, the method can also be useful in the treatment of drinking water in water treatment and food industry.

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Claims (7)

 1. A method of purification of radionuclides of water of a high level of activity, including filtering water through a combined granular charge from inorganic sorbents, characterized in that cation-exchange sorbents zirconium phosphate and / or titanium phosphate in hydrogen and salt forms are used as inorganic sorbents, and the volume ratio is hydrogen and the salt form of cation exchange sorbents in the combined load is 1 2 2 1.  2. The method according to claim 1, characterized in that the cooling water is exposed to the cooling pool or spent nuclear fuel storage, unbalanced water of the nuclear power plant, highly active water in the event of a severe accident at the nuclear power plant or liquid radioactive waste.  3. The method according to claim 1 or 2, characterized in that in the combined loading the sorbents are arranged in layers, the first layer along the water being purified contains the sorbent in hydrogen form, and the second layer contains the sorbent in salt form.  4. The method according to claim 1 or 2, characterized in that the combined load is a mechanical mixture of sorbents in hydrogen and salt forms.  5. The method according to PP. 1 3 or 4, characterized in that the lithium, sodium or potassium forms are used as the salt form.  6. The method according to claims 1 to 4 or 5, characterized in that they use a spherogranulated sorbent obtained by the sol-gel method.  7. The method according to PP.1 to 5 or 6, characterized in that the water is cleaned in recirculation mode.

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