Technological process for treating radioactive TBP/OK spent solvents

Tributyl phosphate (TBP) and kerosene (OK) spent radioactive organic solvents will be produced during the nuclear fuel reprocessing process, which may cause equipment corrosion by direct incineration. Therefore, additives need to be added to prepare a uniformly stable suspension. The amount of each raw material added during the preparation process and the stirring operation can have a significant effect on the performance of the suspension. In this paper, the influence of the amount of each raw material and the stirring rate on the emulsion formation, stabilization time, and fluidity of suspension is studied through different experiments to optimize the suspension formulation of the pyrolysis/incineration processes. The experimental results show that when the volume content of TBP was 30%~60%, the prepared suspension had excellent performance and could be used as the basic solution for suspension formulation. In contrast, when the volume content of TBP was more than 80%, the prepared suspension became less stable, the emulsion phase broke and settled, and the performance of the suspension was poor. When the volume contents of TBP were 30% and 60%, respectively, the DBP content was less than 0.26% and 0.6%, respectively, the Ca-P ratio was 1.25~1.75. Water content was 8~12%. The prepared suspension had great emulsion formation and fluidity. The viscosity and stability time meet the requirements with little secondary waste. The suspension can be applied in TBP/OK spent radioactive organic solvents pyrolysis/incineration reprocessing project.


Introduction
During nuclear fuel cycle reprocessing, the Purex process is commonly used for the separation of uranium and plutonium.The commonly used extractant is tributyl phosphate (TBP), with kerosene (OK) as the diluent.The degradation of TBP and OK during the production process leads to a decrease in liquid extraction performance and eventually forming spent radioactive organic solvents [1] .With the development of the nuclear engineering industry, the amount of spent radioactive organic solvents generated has increased significantly, posing a huge challenge to the progress of the nuclear industry and environmental protection.Many advanced methods have been applied to treat these radioactive organic solvents, such as pyrolysis [2,3] , steam reforming [4] , supercritical water oxidation [5,6] , wet oxidation [7,8] , and electrochemical catalytic oxidation [9,10] .Among the methods of treating TBP/OK spent radioactive organic solvents, pyrolysis/incineration is one of the most commonly used methods in engineering applications because of its low secondary waste liquid generation, corrosion of equipment, and nuclide entrainment in the tail gas [11][12][13] .
However, the direct pyrolysis/incineration of TBP will produce phosphoric acid, which will cause great corrosion to the equipment, so it is necessary to add an alkaline neutralizer to the spent radioactive organic solvents to avoid damage to the equipment; a suitable proportion of surfactant, water, and additives should also be added to prepare a uniform and stable suspension mixture [14] .Since the suspensions are characterized by a large number of solid particles, relatively large viscosity, and the easiness for the emulsion to settle or break, the types and amounts of added raw materials, as well as the characteristics of the mixing process in the preparation process will have different degrees of influence on the characteristics of the final suspension; this will, in turn, affects the efficiency of the pyrolysis/incineration of the suspension [15] .
In summary, in the pyrolysis or incineration of TBP/OK spent radioactive organic solvents, the formulation of raw materials and the characteristics have a significant impact on the working performance of the suspension.The adoption of a mixing process that can maintain the best performance of the suspension is of great significance for its efficient transportation and pyrolysis treatment.At present, the optimal parameters for TBP/OK spent radioactive solvents have not been defined in China.The cost-effectiveness and efficiency of TBP/OK spent radioactive solvent treatment are inferior.Therefore, developing a technical process capable of treating radioactive TBP/OK spent solvent is essential to solve the dilemma of pyrolysis/incineration treatment of spent radioactive organic solvents in China.
In this study, several suspension formulations suitable for the pyrolysis/incineration system of the reprocessing process were developed through laboratory-scale tests.The optimal formulation was recommended based on the test results, which could be applied to the relevant process system of the spent fuel reprocessing plant.
The density of TBP used in this study is 0.96 g/cm 3 , with a relative molecular mass of 266.32; the kerosene used in the laboratory formulation study is hydrogenated kerosene with a density of 0.75 g/cm 3 , and the density of DBP used is 1.05 g/cm 3 , with a relative molecular mass of 210.21.According to the research of the reprocessing plant, the composition of the spent solvent in this study is shown in Table 1.
Table 1.Composition of waste TBP kerosene organic waste solvent in this study.

Indicator Characteristics
Chemical composition (2) Neutralizer The exothermic phenomenon of using CaO formulated with water to form Ca(OH) 2 makes the formed lime milk prone to polymerization, leading to a decrease in the accuracy of the high-temperature gas filter.Therefore, this study uses Ca(OH) 2 instead of CaO as a neutralizing agent in the suspension.The Ca(OH) 2 for this study has true density of 2.24 g/mL, purity > 96%, granularity (>0.2 mm) ≤5%, and granularity (>0.09 mm) ≤15%.
(3) Emulsifier Since nonionic surfactants are highly stable, with good emulsification and acid-base resistance [16] , which do not affect the pyrolytic combustion process, nonionic surfactant was used in this study.The surfactant used in this study was produced in a plant in Jiangsu, China.It is characterized by its stable nature, the absence of phosphorus in the components, and the non-corrosive nature of stainless steel containers, as well as its low cost and easy availability.
(4) Water Distilled water was used for the formulation of spent TBP/OK suspensions in this study.

Experimental methodology (1) Preparation of suspensions
The test is intended to develop the formulation of water-containing emulsion suspension.The specific test method is: a certain amount of emulsifier was dissolved in distilled water, the appropriate amount of neutralizer was added and stirred for 10 min with a rotation speed of 1000~1200 r/min at room temperature, and the lime milk was obtained.After that, the lime milk was poured into the mixed oil phase of TBP, DBP, and OK.The rotation speed was increased from 200 r/min to 1500 r/min for 20 min to form an emulsion.The final suspension was prepared by stirring emulsion for 5 min. (

2) Suspension viscosity
The viscosity of the suspension was measured by the HAAKE RheoStress RS300 rheometer coaxial rotor cartridge system at room temperature.20 mL of emulsified suspension was placed into the cartridge.The shear rate (s -1 ) was increased uniformly from 0 to 1000 within 5 min.The change curve of apparent viscosity (mPaꞏs) was measured to obtain the final apparent viscosity, which was plotted against the shear rate.The viscosity curve was obtained.
(3) Oil-water interfacial tension The oil-water interfacial tension of the suspension was measured by a JYW-200A automatic interfacial tension meter.During the test, the specimen adjusted to 25°C is poured into the glass container to a height of 20-25 mm.The glass container is placed on the lifting platform and raised so that the platinum ring reaches 4-6 mm below the liquid surface.By turning the knob and moving the platform down, the display value will gradually increase to a maximum value, which is the measured surface tension value of the suspension.
Finally, the appropriate formulation was determined based on a comprehensive analysis of the emulsion formation time (<4 h), stabilization time (>10 h), and fluidity (apparent viscosity<50 mPaꞏs) of the aqueous formulation to obtain the best parameters for different test factors.

TBP content
The content of TBP has a significant effect on the physical stability of the suspension.When the content of TBP is small, the suspension has excellent fluidity and low viscosity, resulting in poor overall stability.As the amount of TBP blended increases, the content of the P-fixation agent is also increased to avoid phosphoric acid generating, which increases the particle concentration and the viscosity of the suspension.Meanwhile, the increase in TBP content can further reduce the oil-water interfacial tension, which has an adverse effect on the performance of the suspension.Therefore, by changing the amount of TBP in the suspension (30, 60, 70, 80, 90 vt.%), this experiment studied the effect of TBP content on the oil-water interfacial tension and suspension performance, respectively.The experimental formulation is shown in Table 2, where the amount of water added is 10% of the total mass of solid raw materials.The results are shown in Figure 1 and Table 3.
Table 2. Experimental formulation of the effect of TBP content on the performance of suspensions.As can be seen from Figure 1, with the increase of TBP content from 30% to 90%, the equilibrium interfacial tension between the oil phase and water of the suspension was 11.058 mN/m, 8.943 mN/m, 8.847 mN/m, 8.466 mN/m, and 7.851 mN/m.This indicates that the oil-water interfacial tension of the mixed suspension gradually decreased as the TBP content increased, and the viscosity of the suspension gradually increased due to the incorporation of TBP.The results in Table 3 show that under the condition that the Ca/P molar ratio and water content remain unchanged, the suspension containing 30% TBP had rapid emulsion formation time, brief stabilization time, poor stability, and favorable fluidity.With the increase of TBP content to 60%, the stability of the suspension was enhanced, but the viscosity increased, and the fluidity became poor.This is because the solid particle concentration in the suspension increased, and the proportion of water decreased accordingly, which could not guarantee the fluidity of the suspension.Thus, the viscidity of the suspension increased.For the suspension containing 70vt.%TBP, it can meet the stability time of 10 h, with great fluidity and stable physical properties.For the suspension containing 80~90 vt.% TBP, its stability became poor, and the phenomenon of emulsion phase settling occurred.The water phase precipitated at the bottom after settling to a certain extent.Then, the emulsion broke, and the suspension performance was found to be poor.
From the above experimental results, it can be seen that when the contents of TBP were 30 vt.%~60 vt.%, under the conditions of Ca/P molar ratio of 1.25 and water content of 10%, the suspension had great emulsion formation.The fluidity, viscosity, and stability time meet the requirements of the nuclear fuel reprocessing process, so these two formulations are used as the base formulations of mixed suspensions.

Ca/P molar ratio
According to the pyrolysis reaction equation (e.g., Equation 1 and Equation 2), the molar ratio of Ca to P should be greater than 1 to ensure the phosphorus fixation rate in the pyrolysis/incineration process.The effect of Ca/P molar ratio on the emulsion formation and fluidity of suspensions was investigated.
The range of Ca/P molar ratios that could make different formulations emulsify was determined.The formulations with different Ca/P molar ratios of suspensions are shown in Table 4, and the experimental results are listed in Table 5 As can be seen from Table 5, when the volume content of TBP was 30%, the final apparent viscosity of the suspension increased with the increase of the Ca/P molar ratio.When the Ca/P molar ratios were 1.25, 1.5, and 1.75, the final apparent viscosities of the suspension were 50.14 mPaꞏs, 52.89 mPaꞏs, and 55.35 mPaꞏs, respectively.The emulsion formation time was extended from 5 min to 15 min.The increase of the concentration of solid particles in the suspension with the increase of the Ca(OH) 2 content resulted in the increase of the viscosity of the suspension.In addition, with the increase of TBP content to 60%, the final apparent viscosities of the suspensions were 78.96 mPaꞏs, 86.08 mPaꞏs, and 55.35 mPaꞏs when the Ca/P molar ratios were 1.25, 1.5 and 1.75, respectively, likewise an increasing trend.The emulsion time was greatly reduced, and the emulsion could be formed after only 1 minute of stirring.As shown in Figure 2, long-term standing tests of suspensions with different Ca/P molar ratios showed that all suspension samples precipitated a small amount of water or oil phase after 12-13 days of standing.The emulsified suspensions could be recovered by simple stirring, indicating that when the Ca/P molar ratio was 1.25-1.75, the prepared suspensions were significantly recoverable and stable.Among them, when the Ca/P molar ratio was 1.25, the amount of secondary waste generated by the pyrolysis/incineration process was the least, which was conducive to waste minimization.

Water content
The water content in the suspension has a direct influence on the fluidity and viscosity of the suspension.Because of the high heat of vaporization of water, the increase in water content will increase the heat consumption of the pyrolysis reaction and reduce the processing capacity of the system.However, the low water content will lead to an increase in viscosity and poor fluidity of the suspension.While ensuring the good fluidity of the suspension, the heat consumption required for subsequent treatment should be reduced as much as possible.This experiment explores the effect of water content on the viscosity and heat consumption of the suspension when the TBP dosing is 30 vt.% and the Ca/P molar ratio is 1.25.The optimal water content is obtained by evaluating the different performances of the suspension.The suspension formulations are shown in Table 6, and the experimental results are listed in Table 7.As can be seen from Table 7, when the water content was 5%, the suspension became emulsion after 60 minutes of stirring.The final apparent viscosity of suspension was the highest among all schemes, reaching 67.6 mPaꞏs with the immediate occurrence of shear precipitation of oil.The stability time was only 4 hours, with the overall performance of the suspension extremely poor.As the water content increased to 8%, the emulsion time was reduced to 30 minutes, and the viscosity of the suspension was reduced to 58.0 mPaꞏs, with no shear oil precipitation.The overall performance was improved to some extent, but the stabilization time and fluidity were not increased.The apparent viscosity still exceeded the suitable range of suspension for pyrolysis/incineration.
With the further increase of water content, TBP/OK, nonionic surfactant, and water formed an oil-in-water system with great stability, and the viscosity of the suspension was reduced substantially.The apparent viscosities of suspensions were 50.14 mPaꞏs, 47.12 mPaꞏs, and 43.16 mPaꞏs at 10%, 12%, and 15% of water content, respectively, which met the pyrolysis/incineration process requirements on the apparent viscosity of suspension.The emulsion stirring time was reduced to 15 min, and the stability time of suspension increased to more than 10 hours, meeting the performance requirements of suspension.The fluidity of the suspensions prepared with different water contents is shown in Figure 3.As can be seen from Figure 3, with the increase in water content, the suspension gradually changed from a paste-like morphology to a slurry with stable and homogeneous properties.This significantly improved the fluidity and apparent viscosity of the suspension.According to the composition of the raw materials of each suspension, the heat consumption when heating up to 400℃ for each kg of suspension and the resulting theoretical maximum treatment capacity of TBP were calculated.The effect of water content on the treatment efficiency of suspension can be evaluated.The results are shown in Figure 4.
From Figure 4, it can be seen that with the increase in water content, the heat consumption per unit of suspension gradually increased, and the theoretical maximum treatment capacity of TBP then decreased.At 5% and 8% water content, the heat consumption per kg of suspension was 2490.3 kJ/h and 2873.4 kJ/h, respectively.The theoretical maximum treatment capacities were 18.3 kg/h and 16.7 kg/h.The overall heat consumption was small, and the theoretical maximum treatment capacity of TBP was large in these formulations.From the fluidity test results, it can be seen that the suspension made of this water content became emulsion, but the suspension is almost paste-like, with the fluidity extremely poor.When the water contents were 10% and 12%, the heat consumption per unit of suspension was 3131.7 kJ/h and 3310.4 kJ/h, respectively, and the theoretical maximum treatment capacities of TBP were 13.2 kg/h and 10.7 kg/h.It can be seen that as the water content increased, the heat consumption per unit increased to a certain extent, and the maximum treatment capacity of TBP gradually decreased.Combined with the experimental results of Figure 3, the fluidity of this suspension was improved with great stability, and heat consumption and TBP treatment capacity were moderate.As the water content increased to 15%, the heat consumption increased significantly to 3858.2 kJ/h, and the theoretical maximum treatment capacity of TBP decreased to 7.8 kg/h.Although the the prepared suspension in this formulation had good fluidity, the significant increase in heat consumption reduced the treatment capacity of TBP greatly, therefore increasing the cost of pyrolysis/incineration of the suspension and reducing the efficiency of the project.In summary, from the results of this experiment, it can be seen that when the water content is 10-12%, the comprehensive results of the fluidity, stability, treatment cost, and efficiency of the prepared suspensions are favorable.

Stirring time
Stirring can make the solid particles in the suspension evenly dispersed and ensure the good homogeneity of the suspension.Hence, the stirring time has an important role in ensuring the good performance of the suspension.In this experiment, the suspensions were prepared with 30 vt.% and 60 vt.% of TBP.The effect of the stirring process on the apparent viscosity of the suspensions prepared by different formulations was studied.The experimental results are shown in Figure 5.
The experimental results showed that when the TBP content was 30%, the apparent viscosity of the suspension gradually increased from 83.38 mPaꞏs in the initial state to 96.07 mPaꞏs with the increase of stirring time to 60 min.This indicates that the extension of stirring time could improve the overall performance of the suspension.For the 60% suspension, the apparent viscosity gradually increased from 145.56 mPaꞏs initially to 154.06 mPaꞏs as the stirring time increased from 0 to 30 min.When the stirring time was further increased to 60 min, the apparent viscosity of this suspension slightly decreased and reached 151.70 mPaꞏs, indicating that for the suspension with 60% TBP, the apparent viscosity reached its maximum value when the stirring time was 30 min.The suspension performance reached the best point at this time.

DBP content
DBP is the main radiation degradation product in the TBP/OK spent radioactive organic solvents.In actual engineering practice, the general TBP/OK solvent will contain a small amount of DBP and other impurities, and the content of DBP has a very significant effect on the performance of prepared suspensions.In general, the lower the DBP content is, the more stable the performance of suspensions is.In this experiment, the effect of different DBP content on the performance of suspensions prepared with 30% and 60% TBP was investigated, which is shown in Figure 6.It shows that the interfacial tension between TBP and water decreased significantly with the increase of DBP content, indicating that the polarity of DBP reduced the interaction force and surface tension difference between the atomic molecules of substances in the suspension and gradually reduced the interfacial energy of the suspension.These reactions resulted in the great reduction of interfacial tension in the suspension system, which seriously affected the formation of emulsion and the performance of the suspension.
The effect of DBP content on emulsion formation and fluidity of 30% TBP suspension are shown in Table 8, and the results of long-term standing tests are shown in Figure 7.With the increase of DBP content, the apparent viscosity of suspension gradually decreased, and the formation of emulsion gradually deteriorated.When there is no DPB in the suspension, its apparent viscosity is 155.97 mPaꞏs, the emulsion formation time of the suspension increased from 10 minutes to 30 minutes, and the stability time reached more than 10 hours, with moderate fluidity.When the DPB content reached 0.26%, the suspension became emulsion after stirring for 45 minutes, and the apparent viscosity reduced to 113.85 mPaꞏs, with no significant difference in stabilization time and fluidity.When the DPB content was 0.6%, the 30% TBP suspension became emulsion after 45 minutes of stirring, the apparent viscosity was significantly reduced to 78.07 mPaꞏs, and the fluidity was significantly improved.When the DPB content reached 1%, the suspension failed to become emulsion, and the oil phase was precipitated during the apparent viscosity test, and the stability decreased accordingly.From the above results, it can be seen that when the DPB content was no more than 0.6%, 30% TBP suspension could become emulsion and maintain good performance.The effect of DBP content on the emulsion formation and fluidity of 60% TBP suspension are shown in Table 9.It can be seen that similar to the experimental trend of 30% TBP suspension, DBP content has a great influence on the stable formation of emulsion.When there was no DPB in the suspension, its apparent viscosity was 141.47 mPaꞏs, and the suspension formed an emulsion after 10 minutes of stirring.The stability time was more than 10 hours, with moderate fluidity.As DBP content reached 0.26%, the emulsion formation time of the suspension increased from 10 minutes to 15 minutes.The apparent viscosity decreased with the fluidity enhanced, and the suspension maintained good stability.Unlike 30% TBP suspension, when the DBP content reached 0.60%, its apparent viscosity significantly reduced to 40.74 mPaꞏs, and the suspension of 60% TBP became emulsion after stirring only for 5 minutes.After the formation of the emulsion, it is very easy for the emulsion to precipitate out of the aqueous phase.The emulsion settled and broke after 7 minutes with poor stability.When the DBP content reached 1.00%, the suspension failed to form an emulsion after stirring and precipitated the oil phase during the test, which may be related to the interfacial adsorption of DBP.In summary, when the DBP content was less than 0.26%, the suspension of 60% suspension became emulsion rapidly, with great stability and fluidity.The long-term standing test results of 60% suspension are shown in Figure 8.

Discussion
In this study, several suspension formulations suitable for the pyrolysis/incineration system of the reprocessing process were developed through laboratory-scale tests.The optimal formulations were recommended based on the test results, which were applied to the processing system of the spent radioactive organic solvent reprocessing plant.At present, the pyrolysis/incineration system in China can only deal with low-level radioactive TBP/OK spent radioactive organic solvent, and its key equipment is introduced from abroad.Therefore, developing a technical process capable of treating TBP/OK spent radioactive organic solvent is essential to solve the dilemma of safe disposal of nuclear waste in China.
Due to the particularity of the nuclear industry, there are few published studies on reprocessing of TBP/OK spent organic solvent.Communication about the reprocessing process for spent radioactive solvent is relatively blocked among different countries.Therefore, this study is very innovative in the reprocessing process of nuclear spent organic solvent, which greatly enriches the research field of effective treatment of nuclear waste and can provide a useful data reference for the nuclear industry development.

Conclusion
In this study, we developed a suspension formulation suitable for reprocessing pyrolysis/incineration by changing the TBP/DBP content, emulsifier content, water content, and Ca/P molar ratio.We investigated the effect of the stirring process on the uniformity and stability of the suspension to obtain the optimal suspension formation with the highest efficiency.The following conclusions are drawn: When the TBP contents are 30% and 60%, and the water content is 10-12%, the Ca/P molar ratio is 1.25~1.75.The DPB contents are less than 0.26% and 0.6%, the overall homogeneity of the suspensions was improved.At the same time, the fluidity, viscosity, and stabilization time met the requirements of pyrolysis/incineration of spent radioactive organic solvents for the reprocessing process.The suspensions made by this formulation generate less secondary waste and heat consumption.They can be applied in the TBP/OK spent radioactive organic solvents pyrolysis/incineration reprocessing project, which provides an important reference for the safe and effective treatment of nuclear spent organic solvents and significantly reduces the cost-effectiveness of its harmless treatment.

Figure 1 .
Figure 1.Effect of TBP content variation on oil-water interfacial tension of suspensions.

Figure 3 .
Figure 3. Fluidity results from suspensions with different water contents.

Figure 4 .
Figure 4. Results of heat consumption/ treatment capacity of suspensions with different water contents.In summary, from the results of this experiment, it can be seen that when the water content is 10-12%, the comprehensive results of the fluidity, stability, treatment cost, and efficiency of the prepared suspensions are favorable.

Figure 5 .
Figure 5. Results of the effect of stirring time on the apparent viscosity of suspensions.

Figure 6 .
Figure 6.Effect of DBP content on the interfacial tension between 30 vt.% TBP and water.

Figure 7 .
Figure 7. Results of long-term standing test of 30% suspensions with different DPB contents.

Table 3 .
Experimental results of the effect of TBP content on the performance of suspensions.
≥ 10 h Easy to flow after tilting 80 20 min Settled after 20 min/precipitated oil Extremely easy to flow after tilting 90 10 min Rapid settled /emulsion broke Extremely easy to flow after tilting

Table 4 .
Formulations of the effect of Ca/P molar ratio on the performance of suspensions.

Table 5 .
Results of effect of Ca/P molar ratio on the performance of suspensions.

Table 6 .
Experimental formulations for water content variation.

Table 7 .
Effect of different water contents on the performance of suspensions.

Table 8 .
Effect of DBP content on the performance of 30% TBP suspensions.

Table 9 .
Effect of DBP content on the performance of 60% TBP suspensions.
Figure 8.Long-term standing test results of 60% suspensions with different DPB contents.