RU2183871C1 - Method for decontaminating spent cation- exchange resin of radioactive waste treatment plants at nuclear power station - Google Patents

Abstract

FIELD: nuclear power engineering; decontamination of radioactive wastes. SUBSTANCE: spent cation-exchange resin is placed by hydraulic charging into series- connected filters and decontaminating solution is passed through them to extract radionuclides from cation-exchange resin. For the purpose use may be made of either acid solution of salts of bivalent or trivalent metals or still bottoms of power-plant trap-water cleaning units cleaned of radionuclides and corrected with respect of concentration of hydrogen ions Washing solution may be doped with ferric oxide dissolution promoters. Dirty decontaminating solution is collected in sediment bowl with mixer wherein mixture of ferrocyanides and ferricyanides of light alkali metals are added to function as settling agents for settling down and sediment separating. EFFECT: enhanced effectiveness of radionuclide desorption and decontaminating solution cleaning from cesium radionuclides and corrosion group metals. 5 cl, 1 dwg

Description

 The invention relates to nuclear energy, in particular to a technology for the treatment of radioactive waste from a nuclear power plant (NPP).

 A known method of deactivating spent cation exchange resin of radioactive medium treatment facilities of a nuclear power plant, including cleaning the cation exchange resin from radionuclides by passing a solution of a strong mineral acid through it and purifying the resulting dirty solution from radionuclides on a ferrocyanide sorbent (1).

 The use of an acid solution with a low pH for the purification of cation exchange resin does not provide a high degree of purification of the acid desorbate, which does not sufficiently reduce the amount of radioactive waste to be disposed of.

 Closest to the proposed solution in terms of technical nature and the effect achieved is the prototype method for deactivating the spent cation exchange resin of radioactive medium treatment facilities of a nuclear power plant, which includes cleaning the cation exchange resin from radionuclides by passing a deactivation solution through it and cleaning the resulting dirty decontamination solution from radionuclides on a ferrocyanide sorbent (2 ) In this method, an acid solution of sodium salt is selected as the decontamination solution, which ensures the effective purification of cation exchange resin mainly from cesium radionuclides.

 However, the spent cation exchange resin of radioactive process media purification plants in addition to cesium radionuclides is contaminated with radionuclides of metals of the corrosion group (cobalt, manganese, etc.), the decontamination of which is not provided for in the known method, which is the main disadvantage of the latter.

 The objective of the invention is to reduce the amount of radioactive waste to be disposed of by increasing the efficiency of desorption of radionuclides from cation exchange resin and cleaning the dirty decontamination solution from radionuclides of cesium and metals of the corrosion group.

 To solve this problem, in a method involving the purification of cation exchange resin from radionuclides by passing a deactivating solution through it and purification of the obtained dirty decontamination solution from radionuclides on a ferrocyanide sorbent, an acid solution of salts of divalent and / or trivalent metals is selected as a decontamination solution, and before purification from radionuclides on a ferrocyanide sorbent of a dirty decontamination solution, it is collected in a precipitation tank, a mixture of ferro and ferricyanide is added as a precipitating agent s of light alkali metals and the precipitate is separated.

 In this case, as a decontaminating solution, you can choose a solution of salts of organic acids or purified from radionuclides and adjusted for the concentration of hydrogen ions, the bottom residue of the treatment facilities for floor drain water of a nuclear power plant.

 Activators of the dissolution of iron oxides can be added to the washing solution.

 The washing solution can be passed through the cation exchange resin in two filters connected in series, installed with the ability to turn off the overhead stream of the cleaned filter and simultaneously turn on the filter with a fresh charge of spent cation exchange resin at the stream outlet.

 As our studies have shown, the purification of spent cation exchange resin from cesium radionuclides of metals of the corrosion group is faster and much more effective when using acidic solutions or salts of divalent and / or trivalent metals, or a mixture of salts of mono- and trivalent metals in comparison with using an acidic solution of sodium salt a known method. In addition, the collection of a dirty decontamination solution in a sedimentation tank allows the cation exchanger to be cleaned under optimal conditions (regardless of the process of cleaning a dirty washing solution), and the use of a mixture of light and alkali metal ferro- and ferricyanides as a precipitating agent allows efficient simultaneous deposition of cesium radionuclides and , for example cobalt, in the form of one sparingly soluble compound, which makes it possible to increase the degree of concentration of radionuclides and reduce the volume m radioactive waste.

 Use as a decontamination solution, purified from radionuclides and adjusted according to the concentration of hydrogen ions, of the bottom residue of the floor drain treatment plants of a nuclear power plant, reduces the cost of preparing a decontamination solution and further reduces the volume of liquid radioactive waste (LRW) and, accordingly, environmental pollution of the nuclear power plant.

 During prolonged operation of ion exchangers, a phase of iron oxides with the inclusion of radionuclides can form, and the removal of the latter from the resin is very difficult. For this case, it is proposed to add activators of the dissolution of iron oxides into the decontamination solution: fluorine ions, chlorine ions, oxalic acid, etc. The presence of activators accelerates the process of dissolution of oxides and allows radionuclides included in the oxide phase to be transferred to the decontamination solution.

 Passing the decontamination solution through two series-connected filters with cation exchange resin placed in them allows you to effectively clean the cation exchange resin from radionuclides and at the same time use the decontamination solution economically.

 The drawing shows a diagram of the method.

 The proposed method for the decontamination of spent cation exchange resin of radioactive media treatment facilities of a nuclear power plant is as follows.

 The spent cation exchange resin is placed hydraulically in sequentially connected filters 1 and 2. From the tank 3 for the preparation of the decontamination solution, the latter is first passed through filter 1 and then through filter 2. In the filters, cesium radionuclides and metals of the corrosion group are transferred to the decontamination solution. A dirty decontamination solution is collected in a settling tank 4 with a stirrer.

 The amount of decontamination solution passed through the filters corresponds to the amount of active substance, for example, aluminum salts, a larger exchange capacity of the cation exchange resin of one filter, but a smaller exchange capacity of the cation exchange resin of two filters.

 After washing the resin in the filter 1, it is turned off, the cation exchange resin is discharged and sent for reuse in the installation of the processing of radioactive environments of nuclear power plants. The supply of decontamination solution to the filters is continued, but now it is first passed through filter 2 with partially washed resin, and then through filter 1 with a fresh charge of spent cation exchange resin, etc.

 A mixture of light alkali metal ferro- and ferricyanides is added to the tank 4 as a precipitating agent, mixed with a dirty decontamination solution, sedimentation and separation of the precipitate are carried out, which can subsequently be thickened using a microfilter or centrifuge and homogenized by cementing or other suitable method.

 Almost all the radionuclides of metals of the corrosion group and part of the cesium radionuclides are deposited from the decontamination solution.

 Post-treatment of decantate of capacity 4 from cesium radionuclides is carried out in a filter container 5 loaded with a selective sorbent based on nickel ferrocyanide and silica gel.

 The purified decontamination solution is collected in a container 3 for the preparation of the decontamination solution, it is analyzed, the necessary chemical agents are added and reused to deactivate the spent cation exchange resin. The filter container 5 after exhaustion of the resource is turned off, sealed and sent for storage and / or disposal.

 The whole process is carried out under the control of the radioactivity of the media in accordance with the "Radioactive Safety Standards (NRB-99)" using gamma-ray spectral analysis.

 Example 1

Spent KU-2-8 cation exchanger with specific radioactivity: ¹³⁷ Cs-1.32 • 10 ^-5 Ku / l; ¹³⁴ Cs-7.07 • 10 ^-6 Ku / l and ⁶⁰ Co-4.55 • 10 ^-8 Ku / l were loaded into two filters 1 and 2 with a volume of one liter each. A deactivating 0.7 N calcium nitrate solution with an addition of nitric acid to pH 1 was passed sequentially through filter 1 and then filter 2. After passing 10 l of the solution, the specific radioactivity of cation exchange resin in filter 1 was: ¹³⁷ Cs-6.3 • 10 ^-9 Ku / l; ¹³⁴ Cs-5.9 • 10 ^-10 Ku / l and ⁶⁰ Co-8.0 • 10 ^-9 Ku / l, which, according to the NRB-99 standards, allowed cationite to be considered deactivated and suitable for storage (burial) as non-radioactive waste.

 The deactivated cation exchange resin was discharged from filter 1, which was then filled with a new portion of spent cation exchange resin with the specific radioactivity indicated above.

The specific radioactivity of cation exchanger in filter 2 was: ¹³⁷ Cs-0.6 • 10 ^-5 Ku / l; ¹³⁴ Cs-3.3 • 10 ^-6 Ku / l and ⁶⁰ Co-3.9-10 ^-8 Ku / l.

The washing solution was continued, but now the solution first passed to filter 2, and then to filter 1. After passing 6 l of the solution, the specific radioactivity of the cation exchange resin in filter 2 was: ¹³⁷ Cs-6.3 • 10 ^-9 Ku / l: ¹³⁴ Cs- 5.9 • 10 ^-10 Ku / l and ⁶⁰ Co-8.0 • 10 ^-9 Ku / l, deactivated cation exchange resin was discharged from filter 2, and it was filled with a new portion of spent cation exchange resin and the process was repeated as described above.

 Throughout the process, the spent decontamination solution was collected in sedimentation tank 4, a solution of potassium ferro- and ferricyanide was added in a mass ratio of 1: 1 at the rate of 20 mg / l dry salts.

The solution was stirred and after settling, the specific radioactivity of the decantate was ¹³⁷ Cs-6.3 • 10 ^-9 Ku / l; ¹³⁴ Cs-5.9 • 10 ^-10 Ku / l and ⁶⁰ Co-8.0 • 10 ^-9 Ku / l.

 The decantate was separated from the precipitate and passed through a filter container 5 with a composite sorbent nickel iron-hydrogen sulfide on silica gel (NSS).

The specific radioactivity of the solution in tank 4 was ¹³⁷ Cs-6.3 • 10 ^-9 Ku / l and ⁶⁰ Co-8.0 • 10 ^-9 Ku / l.

The specific radioactivity of the purified decantate for all radionuclides turned out to be lower than 1 • 10 ^-10 Ku / l, which allowed according to the NRB-99 standards after replenishing this solution and adjusting the pH to reuse it.

 Example 2

The same as in example 1, but as a deactivating solution used 0.7 N solution of aluminum nitrate. After passing this solution, the activity of cation exchanger was ¹³⁷ Cs-6.3 • 10 ^-9 Ku / l; ¹³⁴ Cs-5.9 • 10 ^-10 Ku / l and ⁶⁰ Co-8.0 • 10 ^-9 Ku / l.

The specific radioactivity of the decontamination solution purified by precipitation and sorption turned out to be less than 1 • 10 ^-10 Ku / l.

 Example 3

The same as in example 1. but as a decontaminant used a mixture of 0.4 N sodium nitrate solution and 0.3 N aluminum nitrate solution with a pH value of 0.9. The average specific radioactivity of purified cation exchanger was ¹³⁷ Cs-6.3 • 10 ^-9 Ku / l and ⁶⁰ Co-8.0 • 10 ^-9 Ku / l.

The specific radioactivity of the purified decontamination solution did not exceed 1 • 10 ^-10 Ku / l.

 Example 4

 The same as in example 1, but as a deactivating used 0.7 N solution of sodium oxalate.

The average specific radioactivity of purified cation exchanger was ¹³⁷ Cs-6.3 • 10 ^-9 Ku / l and ⁶⁰ Co-8.0 • 10 ^-9 Ku / l.

The specific radioactivity of the purified decontamination solution did not exceed 1 • 10 ^-10 Ku / l.

 Example 5

Same as in example 1, but cation exchange resin with a specific activity of ¹³⁷ Cs-3.97 • 10 ^-5 Ku / l was purified; ¹³⁴ Co-1.64 • 10 ^-5 Ku / l and ⁶⁰ Co-3.96 • 10 ^-6 Ku / l and with visible finely dispersed black magnetite inclusions.

 A mixture of 0.5 N sodium nitrate solution and 0.2 N aluminum nitrate solution with the addition of 1 g / l sodium chloride was used as a deactivating agent.

The average specific activity of purified cation exchanger was ¹³⁷ Cs-1.5 • 10 ^-9 Ku / l and ⁶⁰ Co-1.05 • 10 ^-8 Ku / l.

The specific activity of the purified decontamination solution did not exceed 1 • 10 ^-10 Ku / l.

 Example 6

 The same as in example 1, but as the decontaminating solution used previously purified from radionuclides and pH-adjusted bottoms of the installation of treatment of drain water. Purification of the bottom residue with a salinity of 380 g / L was carried out by oxidative treatment, followed by filtration on a sorbent of non-ferrous metal. In the purified solution, the pH was adjusted to a value of 1.

The average specific activity of purified cation exchanger was ¹³⁷ Cs of not more than 1 • 10 ^-9 Ku / l and ⁶⁰ Co-5.3 • 10 ^-9 Ku / l.

The specific activity of the purified solution did not exceed 1.0 • 10 ^-10 Ku / l.

Comparative example
The same as in example 1, but as a decontaminating solution used 2 N sodium nitrate solution. Cleaning was carried out in one filter.

The average specific radioactivity of purified cation exchanger was: ¹³⁷ Cs-6.3 • 10 ^-6 Ku / l and ⁶⁰ Co-9.83 • 10 ^-8 Ku / l.

The specific radioactivity of the purified decontamination solution was ¹³⁷ Cs-5.6 • 10 ^-10 Ku / l and ⁶⁰ Co-4.1 • 10 ^-10 Ku / l.

Sources of information
1. British patent 1533955, CL G 21 F 9/12, publ. 1976.

 2. Patent of the Russian Federation 1762666, G 21 F 9/12, publ. 1996 - a prototype.

Claims (5)

 1. A method of deactivating spent cation exchange resin of a nuclear power plant’s treatment facilities, including cleaning the cation exchange resin from radionuclides by passing a decontamination solution through it and cleaning the resulting dirty decontamination solution from radionuclides on a ferrocyanide sorbent, characterized in that either an acid solution of two or two salts is selected as the decontamination solution and / or trivalent metals, or a mixture of salts of mono- and trivalent metals, and before cleaning from radionuclides to ferrocyanide ohm dirty deactivating sorbent solution thereof was collected in a precipitation vessel, was added as a precipitating agent a mixture of ferromagnetic and ferricyanides light alkali metals and separating the precipitate.  2. The method according to p. 1, characterized in that as a decontaminating solution, a solution of salts of organic acids is selected.  3. The method according to p. 1, characterized in that as a decontaminating solution, the bottoms purified from the radionuclides and adjusted according to the concentration of hydrogen ions are selected for the treatment of floor drains of a nuclear power plant.  4. The method according to p. 1, characterized in that activators of dissolution of iron oxides are added to the decontamination solution.  5. The method according to p. 1, characterized in that the deactivating solution is passed through the cation exchange resin in two filters connected in series, which are installed with the ability to turn off the head of the purified filter along the stream and simultaneously turn on the filter with a fresh charge of spent cation exchange resin at the stream outlet.

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