Annotation of the results obtained in 2018
The composition, structure, and properties of the low-temperature magnesium-potassium phosphate (MPP) compound after radiation and thermal exposure have been studied. The initial samples of the compound MPP-HLW consisted of the main phase, which is an analogue of the natural mineral K-struvite, as well as some cesium enriched inclusions, wollastonite CaSiO3, and the KNO3 phase, which is formed when potassium is replaced with alkaline metals of the high-level waste simulator (HLW). The structure of the MPP compound changed after heat treatment at 450 ° C. Thus, the oxygen content in the sections of studied samples decreased, that indicated removal of bound water. The calcium silicate phase remained unchanged, and the KNO3 phase was not detected, probably due to its degradation. The absence of changes in the phase composition of the compound samples after irradiation with accelerated electrons (dose 1 MGy) is noted. Differential radionuclide leaching rate from the original samples of the MPP compound on the 90th day of contact with water according to GOST R 52126-2003 was: for 239Pu, 152Eu and 90Sr - .0∙10-9; 1.0∙10-8 и 9.6∙10-7 g/(cm2∙day), respectively. Despite the fact that the leaching rate of radionuclides increased by almost an order of magnitude after heat treatment at 450 ° C, it corresponds to the regulatory requirements for a glass compound for immobilization of HLW. Thus, MPP compound has thermal stability sufficient for practical solidification of HLW. It was established that the leaching mechanism of 239Pu and 152Eu from initial samples of MPP-HLW in the first 2 weeks was determined by washing them from the compound surface and diffusing radionuclides from the compound, respectively, and in the following 11 weeks of the test – by depletion of the surface. It was noted that the heat treatment of compound samples at 450 ° C did not significantly affect the leaching mechanism of radionuclides. The behavior of 90Sr during leaching of compound samples both before and after their heat treatment was clearly determined by the diffusion of the radionuclide from the inner layers of the compound. The stability of the compound to leach at 90 ° C was determined. The leaching rate of the compound components was, g/(m2∙day): Mg - 6.8 ∙ 10-7; P - 6.3 ∙ 10-4; K - 2.0 ∙ 10-3; Na –3.3 ∙ 10-3, Cs - 3.8 ∙ 10-6; Sr - 1.9 ∙ 10-8; Nd - 2.0 ∙ 10-9, U - 3.5 · 10-6. Thus, the leaching rates of Na, Cs, Sr from the MPP compound are comparable with the data for borosilicate glass. The compound was studied during leaching with various solutions - groundwater simulators at the sites of underground waste storage. It was shown that during contact of the compounds with mineralized solutions, the leaching rate of Mg, K, and P did not change significantly in comparison with the data of the standard procedure GOST R 52126-2003. At the same time, the degree of leaching of these elements from the blank MPP matrix to mineralized solutions decreases, that may be due to the strengthening of the surface layer due to the formation of additional phases on the compound surface with the participation of the solution components. The degree of Mg, K, P leaching from the sample MPP-HLW increased by ~ 3 times. On the other hand, the degree of leaching of the HLW components from the MPP-HLW sample during contact of the compound with bentonite water significantly decreased in comparison with leaching during contact with other solutions. That may be due to the substitution of metals in the structure of the matrix MgKPO4 ∙ 6H2O with components of the HLW simulator. To study the characteristics of the composition and structure of the phosphate compound, microtomography was used. It was shown that the compound consists of seven phases, however, due to the close medium uptake, these phases are difficult to separate. It was clearly noted that one of the phases clearly described the pores and after leaching the samples it increased by 6 vol. %, that is obviously connected with the leaching of readily soluble KNO3. The Cs-containing phase in the MPP compound sample after leaching diffused through the sample volume, while the La-containing phase remained stationary. The relative increase of the cesium phase content confirms its resistance to leaching. The resulting data is visualized using 3D models. The behavior of the low-temperature MPP compound during long-term storage was modeled. The optimal values of the parameters used for development of the behavior model of the MPP compound were obtained. It was shown almost complete compliance of the calculated and experimental data for K and P. Some discrepancy between the calculated and experimental data for Mg was explained by the absence of phosphate recrystallization in the model and imperfect parameterization of the model due to the insufficiency of the existing thermodynamic databases. It was noted that the content of K, P and Mg in solutions after leaching after 22 days of the MPP compound contact with water becomes constant and their content in solution for 365 days is 7.35∙10-2, 3.88∙10-2, 2.69∙10-3 mol/l, respectively. On the basis of the developed model, prognostic kinetic leaching curves for K, P and Mg were obtained during storage of the MPP compound for 100 years. It is established that the coefficient of thermal expansion of the MPP compound is (11.6±0.3)·10-6 1/°С, which corresponds to the regulatory requirements of NP-019-15 to the matrix for HLW solidification. At the same time, the thermal conductivity coefficient of the MPP compound averages 0.5 W/(m∙K), that is lower than this characteristic for aluminophosphate glass (changes within 0.7-1.6 W/(m∙K)). This indicates the feasibility of increasing the thermal conductivity of the MPP compound, for example, by introducing additives with a higher thermal conductivity. To develop practical recommendations for the used waste solidifier, the phase composition, particle morphology and particle size distribution of commercial MgO powders were studied. It has been established that for the preparation of a mineral-like MPP compound with high mechanical strength (15 MPa on average) that corresponds to the requirements for matrix materials for LRW solidification, it is necessary to use pre-heat treated (≥1300 ° C) magnesium oxide with a particle size of not more than 50 μm with a high their crystallinity (crystallite size not less than 40 nm). Also for the synthesis of the MPP compound samples, it is necessary to use potassium dihydrogen phosphate, crushed to a particle size of 0.15-0.25 mm. The samples should be prepared in the following ratio, g: MgO: H2O: KH2PO4 = 1: 2: 3. The excess of MgO relative to the stoichiometry of the reaction (10) is about 10 wt.%. To reduce the rate of the reaction (10) boric acid should be added to the initial mixture based on the calculation of its content in the compound 1.3 ± 0.1 wt. %. In addition, we can recommend the following: 1) You should use MPP matrix (as an alternative to cement compound) for solidification of liquid high-salt radioactive waste of low and medium activity levels with complex chemical and radionuclide composition, primarily with a high content of alpha-emitting nuclides and containing components that are impossible or prohibited to solidify in cement (ammonium, radiocarbon, silt deposits). 2) For immobilization of high-level waste, you should use the dehydrated MPP matrix to minimize the formation of radiolytic hydrogen, as well as with the addition of mineral fillers to increase hydrolytic stability and mechanical strength.

 

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Annotation of the results obtained in 2016
All research works according to the plan in 2016 have been done to the reporting year and the expected research results have been obtained, including: The critical analysis of published data on the subject of the project was made. It was noted that the development of the nuclear industry depends on solving the problem of liquid radioactive waste treatment. The prospects of using the low-temperature phosphate matrices for radioactive waste immobilization was shown. These cement like matrices are forming by the room temperature and have a crystal structure similar to ceramics. It was noted that the practical using of such matrices for the actinides-containing radioactive waste treatment doesn’t require unlike with vitrification to create a special costly high temperature furnaces, the elimination of which after the end of exploitation time is a big radioecology problem and doesn’t realize in our days. Thus, the relevance and the scientific importance of the project were shown and they consist from the conducting systematic studies on the phase composition, structure and physical and chemical properties of the low-temperature phosphate matrices, depending on the conditions of synthesis and storage and/or disposal. On basis of preliminary studies magnesium potassium phosphate (MPP) matrix on the basis of MgKPO4∙6H2O and the calcium phosphate (CP) matrix CaHPO4∙2H2O were shown that they to be more promising for the solidification of liquid radioactive waste. The optimal conditions for the preparation of matrices were chosen, including the preliminary preparation of hardener and phosphate binder has been noted to make possible to control blend setting speed and to lead to increase the compressive strength of the matrices. The efficiency of introducing of the mineral fillers (wollastonite, bentonite, clinoptilolite) in the matrix was established. It was shown that optimum filling matrices salts of up to 20 wt. % without free water, i.e. solution/binder ratio is about 0.74 L/kg, which is more in double high than the ratio for cement. The experimental samples of MPP and CP matrices were prepared with the weight to 50 g and density is to 2.0 and 1.7 g/cm3, respectively. The phase composition and structure of the prepared matrix samples were studied, and so was the elements distribution in their volume. It was shown that the main crystalline phase of MPP matrix is MgKPO4∙6H2O, and other phases is MgO (periclase) and also availability of KH2PO4 (<1 wt.%) is likely. It was shown that the main phase of the samples MgKPO4∙6H2O is retained and the phase of KNO3 (Niter) is formed at the immobilization of aqueous solutions of nitrates and solution-simulator of radioactive waste. The presence of KNO3 indicates on replacement of the potassium by metals cations and ammonium and is confirmed by the presence in the samples of the various element orthophosphate phases, the structure of which is similar to MgCsPO4∙6H2O, Sr3(PO4)2, MgNaPO4∙6H2O, Na3PO4 (Olympite), Mg(NH4) PO4∙6H2O (Struvite). It was established that the synthesized СP matrix samples consist of the phase with the structure of CaHPO4∙2H2O (Brushite), SiO2 (Stishovite), partially unreacted CaSiO3 as well as obviously Сa5(SiO4)2(OH)2 (Reinhardbraunsite), СaPO3(OH) (Monetite) and Ca4O(PO4)2 (Thomas phosphate). It was shown that the MPP matrix is homogeneous and mainly consists from particle size of 2-10 micron. It was found that the calculated average elemental composition of the matrix is MgKPO4∙6H2O ((K) - Struvite). Moreover, the particles with different atomic ratio by structure-forming elements Mg, K and P were detected, also Mg1.16K0.68PO4·4.5H2O (or mixture MgKPO4·6.0H2O and Mg3(PO4)2·2.6H2O in a molal ratio of 4.3/1) and also Mg0.62KH0.76PO4·3.8H2O (probably a mixture MgKPO4·6.0H2O and KH2PO4 partially unreacted in the molar ratio 1.6/1). It was found that the potassium in the MPP matrix samples obtained in the immobilization of nitrate solutions of cesium, strontium and lanthanum, is preferably in a single phase of potassium nitrate that connects with substitution of potassium by elements of radioactive waste. It was shown that the cesium is localized in the specific inclusions, such as composition Cs2.69K0.31PO4, containing cesium up to 23.4 % at., and also the phases are similar to phosphate minerals namely Struvite and Newbury composition Mg0.97K0.29Cs0.75PO4·6.3H2O and Mg0.55K0.38Cs0.77H0.75PO4·2.8H2O, respectively. The distribution uniformity of lanthanum and strontium corresponding to the distribution of phosphorus and magnesium was established that indicates the binding of elements into phosphate compounds, including the composition Mg0.6K0.23Sr0.69PO3.91·6.1H2O and Mg0.60K0.68La0.36PO4·6.3H2O, are similar to (K) -Struvite and also Sr3(PO4)2 and Mg0.55K0.70La0.44PO4.06. Thus, it was found that the immobilization of cesium, strontium, lanthanum as simulator of transplutonium elements and ammonium also in MPP matrix occurs as a result of their chemical bonding in the composition of slow-soluble phosphates. Compressive strength, radiation and thermal resistance of MPP matrices were determined. It was established that the compressive strength of MPP matrix samples is 12-18 MPa that corresponds to requirements of NP-019-15 (> 5 MPa). It was shown that the introduction of wollastonite, bentonite and clinoptilolite leads to increase the mechanical stability of the matrix: compressive strength of the sample containing 38 wt% of wollastonite is 22 MPa. It was found that the strength of optimum samples MPP matrices after immobilization of simulator of radioactive waste, later on long-term contact with water (92 days), and after the radiation electron exploration (1 MGy) and the thermal influence (at 450°C for 4 hours) is a value above 5 MPa, which also corresponds to regulatory requirements. It was established that crystallization water during heating MPP matrices is removed from the structure MgKPO4∙6H2O, peak of the endothermic effect corresponds to 118°C. It was shown that water disposal from the matrix takes place during the isothermal holding at that temperature for 24 hours. Thus, it was shown that the immobilization of the actinides-containing radioactive waste of radiochemical plants can include the step of MPP matrix dehydration during thermal influence that will ensure the impossibility the formation of a gas phase because of the radiolysis of bounded water molecules under the influence of alpha particles during the storage/disposal of radioactive waste. In studies of hydrolytic stability of matrices according to GOST R 52126-2003 the obtained solutions have been shown to have pH about 8.5. At the same time the increase of pH to 9.5-10.5 was observed in solution after leaching from MPP the matrix samples obtained by solidification of radioactive waste simulated that may be related with hydrolysis of Na3PO4. The difference of leaching mechanism of matrix-forming elements of MPP and CP matrices was established. It was shown that the amount of phosphorus and potassium leached from MPP matrix exceeds the amount of magnesium that may be related with dissolving of partially unreacted KH2PO4. It was also observed that the mechanism of depletion of the surface layer corresponds to the leaching of potassium and phosphorus, and diffusion mechanism corresponds to the leaching of magnesium for 10 days after contact MPP matrix with water. In the case of MPP matrix it was shown that the silicon yield obviously is controlled by the dissolution mechanism of highly soluble phase SiO2∙4H2O. At the same time this difference between the behavior of calcium and phosphorus can be caused by the presence in the composition of the CP matrix the compounds of these elements with different solubility, including perhaps, of residue CaSiO3 or composition phase Ca5(SiO4)2(OH)2. It was found that the hydrolytic stability of MPP matrix is slightly reduced during solidification of radioactive waste simulator. It was found that the stability of CP matrix to leaching of matrix-forming elements generally exceeds stability of MPP matrix, for example, the integral phosphorus leaching rate of phosphorus from the MPP and CP matrices after 90 days of contact with water is 1.0∙10-3 and 3.0∙10-5 g/(cm2·day), respectively. The samples of MPP matrices containing uranium and lanthanum (to 10 % wt.) were synthesized at the solidification of their nitrate solutions and so were samples at the immobilization of radioactive waste simulator containing actinides with Eu-152 (specific activity 4.0 kBq/g) as the transplutonium elements simulator). The influence of uranium and lanthanides on the physical and chemical properties and hydrolytic stability of the matrices was studied. It was shown that the immobilization of uranium and lanthanum doesn’t affect the density (about 1.8 g/cm3) and the compressive strength (10-15 MPa) of obtained samples of MPP matrices. It was established that uranium and lanthanides have no significant effect on the hydrolytic stability of MPP matrix to the leaching of matrix-forming elements, according even to the PCT test at 90°C. It was shown that the incorporation of the wollastonite into the composition of MPP matrix samples leads to decrease of the rate and extent of leaching Eu-152 in 5 times. The characteristics of hydrolytic stability of MPP matrix have been established at the 28th day of contact with water according to GOST R 52126-2003: differential and integral Eu-152 leaching rates – 6.5∙10-5 and 2.6∙10-4 g/(cm2∙day), respectively, and 152Eu leaching degree is not more than 1.0 g/(cm2∙day), respectively. These data are significantly lower than the permissible limit of NP-019-15 for cement (137Cs leaching rate is less than 1∙10-3 g/(cm2∙day), extent of leaching 152Eu is no more than 1.0 % wt.). It was shown that the Eu-152 leaching index from MPP matrix according to the ANS 16.1 is 11.5, which corresponds to the requirements of the US Department of Energy (> 6).

 

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Annotation of the results obtained in 2017
The research on the development of low-temperature mineral-like phosphate matrices, primarily the magnesium potassium phosphate (MPP) is being carry out at the Vernadsky Institute. The MPP matrix is synthetic analogue of the natural mineral K-struvite and the main feature of such matrix is its formation at room temperature as a result of the reaction between potassium dihydrogen phosphate and magnesium oxide in an aqueous solution. The use of this matrix, which has high hydrolytic, physical and chemical stability, is the base of the new technology developed for the immobilization of liquid radioactive waste (LRW). The phosphate compound based on the MPP matrix has a number of significant advantages over the cement-like compound, primarily the large volume of solidified LRW per unit mass of binding agents, the higher the density of the connection and the degree of its filling in the salts. Finally, all these advantages reduce the volume solidified forms of waste in comparison with cementing twice. Considering the huge volume of accumulated LRW that have to be solidify, the use of the technology developed on the basis of the MPP matrix will significantly reduce costs for these purposes in industry. In 2017 all planned work was carried out and the following main results were obtained. The phase composition of the MPP compound samples prepared at solidification of nitric acid solution-simulators of LRW was specified. The МРР compound samples with the solidification of ammonium nitrate solution were prepared. It was shown that the main phase of the obtained compound is a synthetic analogue of the natural mineral struvite MgNH4PO4·6H2O. However, the atomic ratio of potassium and ammonium in individual particles reaches the value 0.26, which corresponds to the composition of the compound MgK0.21(NH4)0.79PO4·6H2O. The mechanical and radiation stability of the obtained compound samples was determined. The microhardness of the MPP matrix was 26.2 ± 0.7 GPa, which considerably exceeds the microhardness of the magnesian cement and similarly for high-temperature ceramic materials. Compressive strength of the MPP compound containing 6.7 wt% of lanthanum corresponds to the requirements for cement-like compound (at least 5 MPa). The constancy of the phase composition of the MPP matrix after its contact with water for 90 days was shown. The optimal content of wollastonite (23-28 wt%) and zeolite (17-23 wt%) introduced into the compound was determined, which allows to synthesize samples with compressive strength (up to 50 MPa). Radiation stability of the compound was evaluated by studying the structure and properties of the compound before and after irradiation with an electron beam (dose 1 MGy), and also after γ-irradiation with a Cs-137 source (dose 0.12 MGy). It was established that the average compressive strength of the MPP matrix after irradiation was about 9.0 ± 0.4 MPa, and the constancy of the phase composition, structure and hydrolytic stability of the samples was shown. The samples of the MPP compound containing uranium (6.2 wt%) were synthesized. The obtained MPP compound contains the hydrated phase of the potassium uranyl orthophosphate, an analogue of the natural metaankoleite K(UO2)PO4∙3H2O, the phase of the MPP matrix is MgKPO4∙6H2O (K-struvite), and the MgO phase (periclase). The phase of the uranyl nitrate in the MPP compound was not detected. The average composition of the uranium enriched compound particles corresponds to the general formula for the compound Mg0.33K(UO2)0.67PO4∙2.3H2O (uranium content about 45 wt%). The main matrix phase of the compound contains not more than 3 wt% of uranium, and its average composition corresponds to the formula Mg1.05K0.90PO4∙1.1H2O. Nitrate ions in the MPP compound are in the form of potassium nitrate; the impurities of magnesium, phosphorus and uranium in this phase do not exceed 0.6, 1.4 and 1.3 wt%, respectively. The hydrolytic stability of the MPP compound containing uranium and lanthanum was studied. The pH of the solutions after the contact of the uranium-containing MPP compound with water is 8.0-8.4. The leaching rate of uranium from the compound decreases depending on the contact time of the studied compound samples with water. In accordance with GOST R 52126-2003, the differential, integral leaching rates and leaching degree were established (1.7∙10-6, 2.7∙10-6 g/(cm2∙day) and 0.014% respectively). It was shown that during the first 7 days uranium leaching occurs due to dissolution of the surface layer of the compound, and in the next 21 days the leaching of uranium is uniquely determined by the diffusion mechanism from the inner layers of the compound. The behavior of lanthanum during leaching is almost the same except that leaching of lanthanum occurs by elution of compound surface layer in the interval of 14-28 days. The MPP compound samples containing about 0.1 wt% Np-237, Pu-239 and Am-241 were synthesized. The uniformity of the distribution of actinides in the compound was established. It was shown that the behavior of Np-237, Pu-239 and Am-241 during leaching from the MPP compound is similar. The leaching rate of actinides from the MPP compound depending on the contact time of the studied compound samples with water decreases significantly. In accordance with GOST R 52126-2003 the differential leaching rate of Np-237, Pu -239 and Am-241 (9.4∙10-7, 3.9∙10-7 and 1.8∙10-7 g/(cm2∙day), respectively) and the leaching degree of studied radionuclides (0.5, 0.4 and 0.03%, respectively) were established. High leaching indexes of Np-237, Pu-239 and Am-241 were established in accordance with ANS 16.1 - 12.8, 13.4 and 14.5, respectively. It was shown that the introduction of wollastonite or zeolite improves the hydrolytic stability of the compound to 5 times average. It was shown that at 28th day the integral rate of Pu-239 leaching decreases from 3.6∙10-6 g/(cm2∙day) to 2.1∙10-6 and to 8.6∙10-7 g/(cm2∙day) under addition of wollastonite and zeolite in optimal amounts (28 and 23 wt.%, respectively). It was shown that the leaching of actinides from the MPP compound samples is controlled by similar mechanisms. At the beginning of the ANS 16.1 test, the linear equations of the logarithmic dependence of Np-237, Pu-239 and Am-241 yields on the contact time with water of the MPP compound have slopes about 0.3-0.35 that corresponds to the mechanism of leaching of actinides from the sample surface. This is followed by reducing of slope to a value -0.74 ±0.05 which corresponds to the depletion of surface layer. In the subsequent contact time of the compound with water the leaching of Np-237, Pu-239 and Am-241 occurs by the diffusion mechanism from the inner layers of the compound (slope 0.54±0.03), which is similar to the behavior of uranium and lanthanum. The leaching rate of the structure-forming components of the compound, taking into account the significant specific surface area of the sample powders of the MPP compound (up to 40 m2/g) is, g/(m2∙day): Mg - (0.6 ... 2.1)∙10-6; K - (0.22 ... 1.5)∙10-2; P - (0.34 ... 7.9)∙10-3; Na - no more than 5.3∙10-2. The low leaching rate of metal as simulators of the behavior of the most hazardous LRW radionuclides, g/(m2∙day): Cs - 1.4∙10-5; Sr - 1.1∙10-6; La - 2.3∙10-7 was established. The fixed values of the leaching rate of the MPP compound components are lower than the literature data for aluminum- or iron- phosphate glass. Thus, the obtained data on the high physical, chemical and hydrolytic stability of the MPP compound, including at elevated temperature, allow us to state the prospects of practical use of the compound for solidifying LRW, including actinide-containing and highly active waste. To substantiate the effectiveness of the use of the MPP compound, it is necessary to conduct systematic studies of its radiation resistance, as well as stability in contact with aqueous solutions - groundwater simulators, which are the main objectives of the project in 2018.

 

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