RU2702096C1 - Complex for immobilisation of radionuclides from liquid hlw - Google Patents

Abstract

FIELD: processing and recycling wastes; technological processes.

SUBSTANCE: group of inventions relates to a complex for immobilisation of radionuclides from liquid HLW. Complex for immobilisation of radionuclides from liquid HLW includes serially arranged plant for synthesis of non-selective sorbent, facilities of sorbent supply, sorption unit, system of high-active sediment separation from low- or very low-active solution, unit of precipitation compaction. Compacting unit consists of a drying unit, a mechanical press and a sintering furnace and converting the residue into a stable ceramic. Sorbent synthesis plant comprises three reactor vessels, two of which have fluoroplastic lining, it is also equipped with heating and thermal regulation systems with at least one mixing device and drain mechanism. Furnace for sintering and transfer of precipitate into stable ceramics is equipped with gas catching system and device for blowing with gas. Sorption unit and compacting unit are arranged in "hot chamber". There is also a method for processing wastes containing technetium on a complex for immobilisation of radionuclides from liquid HLW.

EFFECT: group of inventions enables single-step immobilisation of wide nomenclature radionuclides.

3 cl, 1 dwg, 6 ex

Description

The invention relates to nuclear energy, and in particular to the disposal of liquid high-level waste, and can be implemented when disposing of radioactive waste by curing in a stable form matrices obtained by sorption of HLW on a non-selective universal sorbent.

The combination of a sorbent synthesis process, HLW sorption on this sorbent and subsequent solidification with the simultaneous compaction of high-level waste into a single complex allows the processing of HLW on an industrial scale, which is very important for the accumulated amounts of radioactive waste.

Known methods for concentrating and curing liquid HLW are associated, as a rule, with the high selectivity of the proposed matrices (Aloi A.S., Trofimenko A.V., Iskhakova O.A., Kolycheva T.I. “Development of the matrix composition for vitrification of Sr and Cs from VAO "," Radiochemistry ", 1997, v. 39, No. 6, pp. 562-568; Lifanov F.A., Savkin A.E., Slastennikov Yu.T." Purification of high-salt liquid radioactive waste by selective sorption ". In the collection" Radioactive waste. Storage, transportation, processing. Impact on humans and the environment. Abstracts "M materials of the international conference October 14-18, 1996 St. Petersburg, Central Research Institute of CM "Prometheus"). The disadvantage of the selective inclusion of radionuclides in the curing matrix is the high cost of such fractionation of the processed radwaste and a significant increase in waste volumes.

The technology of complex immobilization of HLW into SINROK type ceramic ceramics is very expensive, since it requires complex expensive equipment and the use of high temperatures (Ringwood A.E., Oversby VM, Kesson SE "Immobilization of high-level nuclear reactor wastes in SYNROC". " Nuclear and Chemical Waste Management "1981. V. 2, p. 287-305).

Patent RU No. 2432631 describes the complex immobilization of radionuclides in phosphate ceramics, the main matrix of which is sodium zirconium phosphate with a natural cosnarite structure. Elements such as Na, Cs, Sr, Ln, An, Fe can be included in this ceramic. But the possibility of including Mo, Tc, and other elements has not been analyzed. In addition, complete fixation of cesium requires information on the composition of the waste in order to adjust the cesium content in cosnarite.

Existing laboratory and industrial plants for the processing of HLW are sets of equipment for vitrification of HLW. The vitrification technology, as a rule, includes the stages of preliminary fractionation and evaporation, which complicates and enlarges the design of the installation (Sat. “90 years to the Radium Institute named after VG Khlopin”, 2013, ANO “IZAO”). In addition, during vitrification, not all radionuclides can be immobilized in glass in one stage. The existing installations for the immobilization of HLW into ceramics and glass ceramics are very dimensional, energy-intensive, and the matrices used are selective with respect to the included radionuclides (Sat. "90 years of the VG Khlopin Radium Institute", 2013, ANO "ICAO").

The proposed complex for the immobilization of radionuclides from liquid HLW uses a sorbent - layered hydrazinium titanate, the properties of which are described in patent RU 2560407, as well as previously described in several publications (Patent WO 2011/116788 A1; St. Petersburg University journal, No. 6 (3864 ) dated April 16, 2013). In this material, obtained in the form of a nanosized powder, a large surface area, ion-exchange, and redox properties are very successfully combined for sorption of various elements. The possibility of sorption of radionuclides by this sorbent, as well as many other elements, has been tested and described in the above publications. However, the authors of the application are not currently aware of compact, easy to manufacture and operate single complexes that use a non-selective sorbent and allow the process of immobilization of HLW on an industrial scale.

The objective of the invention is the development of an autonomous complex for the immobilization of radionuclides from liquid HLW with their subsequent transfer to cured ceramic.

A working complex can allow one-stage immobilization of alpha, beta, gamma-radionuclides of a wide range, on an industrial scale, both in continuous operation and with interruption of the process at any stage. The inventive complex provides the maximum reduction in the initial volume of HLW: the volume of highly active solidified waste is reduced ~ 100 times, and liquid waste is transferred to the category of low-level or very low-level; It has low cost and low power consumption; the simplicity of the design of all elements of the complex provides the possibility of compact dismantling of contaminated nodes of the complex and the disposal of each node separately.

The technical result is achieved by using the proposed complex for the immobilization of radionuclides from liquid HLW, containing a sequentially located installation for the synthesis of a non-selective sorbent, a sorbent supply means, a sorption unit, a system for separating a highly active sludge from a low-active solution, a sludge compacting unit including a drying unit, a press and a sintering furnace and converting the precipitate into stable ceramics. The installation for the production of sorbent contains three reactor vessels, two of which have a fluoroplastic lining, heating and thermoregulation systems, at least one mixing device and a drain mechanism. The furnace for sintering and converting sludge into stable ceramics is equipped with a gas trapping system, as well as a device for gas purging, if it is necessary to create a reducing medium, for example, when sintering tablets containing technetium. In addition, the sorption unit and the compacting unit are arranged to be located in a “hot chamber”.

When processing wastes containing technetium at the proposed complex for the immobilization of radionuclides from liquid HLW, sorption of technetium is carried out without preliminary reduction of dissolved technetates, and the resulting tablets with technetium are sintered with simultaneous blowing with dry argon.

When processing wastes containing technetium on the proposed complex for the immobilization of radionuclides from liquid HLW, the obtained tablets with technetium are sintered at a temperature of 1100 ° C.

After the immobilization of the waste on the described complex, the radionuclide sediments are transferred to compact chemically stable ceramics, and the solutions are converted into low-level or very low-level waste.

A block diagram of a semi-industrial complex for immobilizing liquid HLW is shown in the figure (Fig. 1). The numbers 1 and 2 indicate the reaction vessels with solutions of hydrazine and fluorotitanic acid, and the temperature of the vessel 2 is maintained at 90 ° C; 3 - a reactor for the synthesis of sorbent (which is carried out at 110 ° C and with constant stirring); vessel 2 and reactor 3 have an internal PTFE lining. The solution after the synthesis of the sorbent using a special device is poured from the reactor into the vessel 4, and the sorbent is separated into block 5. In block 6, by adding water, the sorbent is brought to a pulp state. Under the conditions of a “hot chamber”, the required amount of sorbent pulp is fed into a container with liquid HLW 7. After passing through the sorption block, liquid waste transferred to the NAO or VLLW discharge is removed to tank 8, and highly active waste solidified on the sorbent is collected in a receiving block 9. Then they enter the compaction unit 10, which consists of a drying unit, a press, and a furnace mounted in the “hot chamber”, which is equipped with a gas collection system. If necessary, gas purging can be carried out in the furnace. The equipment of block 10 can be installed in boxes connected to each other in a chain. High-level waste is recovered from the furnace in the form of compact, chemically strong tablets.

The following examples illustrate the use of the proposed complex for the extraction of various radionuclide elements contained in liquid HLW and their conversion to stable ceramics. For the experiments, a semi-industrial complex for the immobilization of radionuclides from liquid HLW was mounted. The productivity of the synthesis reactor is 2 kg of sorbent per day. By scaling up the process, the productivity of sorbent production and continuous immobilization can be increased. To verify the operation of all nodes of the manufactured complex, model and real liquid HLW was cured (by the composition of radionuclides close to the waste of nuclear power plants, including nuclear power plants in Fukushima) by deposition on the proposed sorbent and transfer to ceramic. The control of the activity of the initial liquid HLW, final cured products and the mother liquor showed that the residual total activity of the mother liquor was 0.08 - 3% of the initial, for Cs - 10 ^-3 -10 ^-4 %, for Pu, Am, Np - 10 ^-5 -10 ^-6% .

In addition, to increase the efficiency of the complex, conditions were selected for the optimal quantitative deposition of radionuclides on a sorbent depending on the pH of the initial solution, nuclide nomenclature, aging time of sediments, and an assessment was made of the real reduction in the initial volumes of liquid HLW after transferring them to cured highly active ceramic.

Example 1

The proposed complex was used to immobilize the components of the model solution containing the corresponding elements in the same amounts in which they are present in real HLW (Ca - 7.0; Mg - 6.0; Sr - 4.5; K - 9.0; Zr - 5.5; Cs - 11.0, La - 12 g / l). For sorption, an aqueous suspension of the sorbent was prepared with a hydrazinium titanate content of ~ 80 g / L. The suspension was poured into a model solution - a simulator of LRW. After stirring, settling (1-30 hours) and filtering the precipitate, the mother liquor was analyzed for the content of control elements. The analysis results showed that sorption on the proposed complex using a selective sorbent was 94% for Ca, 97% for Mg, 96% for Sr, 96% for K, 98% for Zr, 92% for Cs, for La, 99.2%. Re-adding the sorbent to the solution after separation of the precipitate improves the purification of this solution to the content of each of the elements in an amount of not more than 0.8% of the original.

Example 2

The complex has been tested to immobilize real radionuclides. Solutions were prepared containing ¹³⁷ Cs, ⁹⁰ Sr, ²³⁹ Pu, ²³⁸ U, ²⁴³ Am, ²³⁷ Np in the form of nitrate and chloride salts in quantities from 80 to 140 MBq / L for ⁹⁰ Sr, ²⁴³ Am, ¹³⁷ Np and ¹³⁷ Cs and from 5 to 30 g / l for ²³⁹ Pu and ²³⁸ U. Very acidic stock solutions were made alkaline with ammonia to pH = 6-7. Then, the solution with the nuclides and the sorbent suspension were combined, as well as the sorption process was carried out as described in Example 1. If a precipitate formed in the solution after adding ammonia, sorption was performed without separation of the precipitate. It is proved that, having ion-exchange properties, layered hydrazinium titanate can sorb cations not only from solutions, but also from pulps with precipitates. A clear interface between the solution and the precipitate was formed in experiments on the sorption of cesium and strontium in 2-5 hours, and in experiments with uranium, americium and plutonium in 24-30 hours. The entire process of sorption and sedimentation of the sediment was carried out in the tank 7 of the complex. If necessary, more complete extraction of the waste components from the solution was added an additional amount of sorbent; It is also possible to conduct re-sorption from an already filtered solution. After separation of the highly active precipitate, the activity of the mother liquor decreased by a factor of 1000-10000. The filtered precipitate was dried, and tablets were pressed from the dry precipitate, which were calcined in an oven. A furnace specially developed for operation in a box or a hot chamber allows sintering of active samples according to a given program in the temperature range from room temperature to 1500 ° C. When the sorption of nuclides in this example, the sintering of the tablets was carried out in air at 1100 ° C. The content of nuclides in the solution after its separation from the precipitate was 0.04% for ⁹⁰ Sr, 0.82% for ¹³⁷ Cs, and 0.8-1.2% of the initial value for Am, Pu, Np, U.

Example 3

In the proposed complex, immobilization of solutions with plutonium with different valence composition was carried out. Solutions containing ²³⁸ Pu or ²³⁹ Pu in the tetravalent state and ²³⁹ Pu in the trivalent form were used. It turned out that the settling time before the appearance of a clear boundary between the solution and the formed precipitate is not the same for different valence forms of plutonium. Sorption from a solution of tetravalent plutonium takes place in 30-32 hours, while the maximum sorption of trivalent plutonium was carried out in 24-30 hours, as evidenced by the complete discoloration of the blue solution. It was proved that no radionuclides were released outside the complex into the external environment.

Example 4

The capabilities of the complex for immobilizing technetium contained in the initial solution in a heptavalent state were tested. All previous work on the immobilization of technetium in one or another strong matrix led to the need to convert technetium into tetravalent form, which required the use of reducing conditions and the complexity of technological operations. Using the proposed complex and the used sorbent, it became possible to sorb technetium without preliminary reduction of dissolved technetates. Technetium was best sorbed from solutions with pH = 6-8. The restoration of technetium during sorption is facilitated by the restoration properties of the interlayer hydrazine contained in the sorbent in combination with a very large specific surface area of the powder. Complete separation of the solution and the precipitate with adsorbed technetium takes 24-28 hours. During further processing, the sorbent tablets with technetium are sintered at 1100 ° C with simultaneous blowing with dry argon, after which they remain stable in the form of ceramics, consisting mainly of rutile-like phases (Ti, Tc) O ₂ . Dosimetric control of the working surfaces proved that when sintering highly active samples of volatilization of technetium and equipment contamination in the described manner

Example 5

The conditions for the most efficient deposition of radionuclides by the selected sorbent under the operating conditions of the proposed complex are determined. The precipitate after sorption consists of titanate-hydrated and hydrazine layers. In this case, metals with small ionic radii enter the titanate layers, and metal atoms with large sizes, as a rule, are deposited in the interlayer regions. Large ionic radii have atoms of alkali and alkaline earth metals. Atoms with smaller ionic radii that exist in different valence states, in the bonds of which with other atoms have a covalent component, are most likely to enter titanate layers. Such atoms include An, Ln, Mo, Zr, Tc. A difference in the deposition rate on the sorbent between the alkali and alkaline earth metal ions and the polyvalent, sometimes variable valence, metals was revealed. The former are precipitated at a faster rate than the latter. So, during cesium sorption, a clear boundary between the precipitate and the solution appeared after 2-3 hours, strontium - after 4-6 hours, and the precipitate containing actinides, lanthanides, technetium was completely formed in 24-38 hours. Since HLW solutions most often have a strongly acidic reaction, the optimal pH values of liquid wastes were determined before sorption. It turned out that the dependence of the deposition completeness on pH, as well as the deposition rate, are related to the type of adsorbed atoms. Ions of larger atoms with a valency of +1 - +2 are almost completely sorbed at a pH of 3-6, and for a more complete sorption of actinides, lanthanides, technetium, the solution should be pre-alkalized to a pH of 6-8.

Example 6

An assessment was made of the effective reduction in the volume of liquid HLW simulators after curing them in the proposed immobilization complex. Two liters of the prepared nitric acid model solution contained La - 12, Cs - 10, Mo - 8, Rb - 0.6, Zr - 4 g / l. With ammonia, the pH of the solution was adjusted to a value of 5, after which a loose precipitate formed. Without separating the precipitate, 1.8 L of the sorbent suspension was added to the container, in which the amount of the sorbent itself was 150 g. After settling for a day and a half, the transparent solution was separated from the precipitate. Analysis of this solution showed that a certain amount of lanthanum and zirconium did not completely precipitate, but remained in dissolved form. An additional introduction of 600 ml of the sorbent suspension into the solution led to the fact that after combining the two precipitates of the sorbents with the included model elements, practically no lanthanum and zirconium remained in the solution. A more complete precipitation of lanthanides was also promoted by an increase in the pH of the model solution before the introduction of the sorbent to a value of 6–7. The precipitate after gradual drying at temperatures of 80 → 380 ° C was pressed into tablets, which were then sintered at 1100 ° C with holding at this temperature for 1 hour. After cooling, the total volume of ceramic tablets was measured. It amounted to 20.5 cm ³ . Thus, the volume of the initial model solution - a simulator of liquid HLW - decreased by 97.6 times. Subject to the technological regulations for sorption in the proposed immobilization complex, the solution separated from the precipitate must be transferred to the category of low-level (NAO) or very low-level (VLLW) waste by the content of radionuclides in it.

Claims (3)

1. A complex for immobilizing radionuclides from liquid HLW, characterized in that it includes a sequentially located installation for the synthesis of a non-selective sorbent, a sorbent supply means, a sorption unit, a system for separating a highly active precipitate from a low or very low active solution, and a sediment compaction unit consisting of a drying unit mechanical press and furnace for sintering and transferring the precipitate into stable ceramics, while the installation for the synthesis of sorbent contains three reactor vessels, two of which have a fluoroplastic foot ings are also provided with heating and thermal regulation system, at least one stirring device and a discharge mechanism; the furnace for sintering and transferring the precipitate into stable ceramics is equipped with a gas trapping system, as well as a device for gas purging, and the sorption unit and the compacting unit are arranged to be located in a “hot chamber”. 2. A method of processing wastes containing technetium on a complex of immobilization of radionuclides from liquid HLW, characterized in that the sorption of technetium is carried out without preliminary recovery of dissolved technetates, and the obtained tablets with technetium are sintered with simultaneous blowing with dry argon. 3. The method according to p. 2, characterized in that the obtained tablets with technetium sinter at a temperature of 1100 ° C.

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