Creep Analysis and Material Properties Research of Rotary Calciner

Unavoidably, during the aqueous reprocessing of spent fuel, high level liquid waste is produced, which is typically vitrified. A rotary calciner is an essential component of a two-step cold crucible melter equipment system. Owing to the calciner’s long-term operation under high temperature condition, potential effects from metal creep are essential design factors. Stable temperature and stress fields of components of a rotary calciner have been calculated based on the finite element analysis software ANSYS. Furthermore, creep life of creep effects has been evaluated and comparisons among Inconel 690, Inconel625, and SUS 310s tubes’ mechanical property performance have been conducted. The results indicate that all three materials require the mechanical property requirements and SUS 310s exhibits the minimum creep displacement.


Introduction
For the reprocessing of spent fuel in commercial nuclear power plants, a more mature water treatment method in engineering is uranium-plutonium separation method, which uses tributyl phosphate to extract valuable nuclides in solution for reuse.This process will inevitably produce high-activity radioactive waste liquid containing strong acidity, fission products and residual actinides.Due to the risk of leakage in the transportation and storage of high-level waste liquid, high-level waste liquid is usually evaporated, concentrated and stored for decades, and then solidified with borosilicate glass for permanent disposal [1][2][3].
The glass solidification of high-level liquid waste enables long-lived fission nuclides to be stored stably and the final waste volume is minimized, and it can operate stably and continuously on an industrial scale [4].Glass solidification in electric melting furnaces has been applied successfully in a wide range of industrial applications.However, due to the inconnel electrode immersed directly in acidic high-level liquid waste, the maximum operating temperature of the electric melting furnace will not exceed 1150 °C.The heating coil of the two-step cold crucible is heated by a magnetic field and does not directly contact the high-level waste liquid, so that the operating temperature can reach 1300-1400 °C [5].Moreover, the refractory wall material around the electric melting furnace will also be corroded directly by high temperature glass, so its service life has been limited for several years.Due to the cold shell protection, the cold crucible avoids wall corrosion and has a service life of more than ten years.In addition, unlike the electric melting furnace as a whole, the cold crucible components have the advantages of flexible disassembly, easy maintenance and replacement [6][7][8][9][10].The first step of the two-step cold crucible glass solidification is the rotary calciner, which dehydrates and denitrifies the high-level liquid waste through a high-temperature furnace tube to become a metal oxide [11,12].Calcination treatment can greatly improve the processing capacity of the cold crucible.The burning furnace tube rotates and heats the high-level liquid waste at room temperature.The metal material is used in high temperature environment for a long time, and creep damage will inevitably occur.Creep refers to the trend of slow and permanent movement or deformation of solid materials under stress, which can be regarded as a time-related plastic deformation [13][14][15].Under the stress of 40 % higher than the melting point temperature of the material and far lower than the yield strength of the metal, the metal is prone to high temperature creep, and the deformation of the material increases with time.The design life of rotary calciner is 5 to 10 years, and the working temperature of the furnace tube needs to reach more than 800 °C.It is a long-running working part in a typical high temperature environment.The influence of creep on the stability of equipment operation should be considered in the design process.Therefore, this paper selects Inconel 690, Inconel 625 and SUS 310s three materials for creep comparison.All three materials have high strength and corrosion resistance.Inconel 690 alloy has excellent resistance to intergranular corrosion and intergranular stress corrosion cracking.It is mainly used as heat transfer tube material for steam generator of PWR nuclear power plant.Inconel 625 alloy is a solid solution strengthening nickel-based wrought superalloy with molybdenum and niobium as the main strengthening elements.It has excellent corrosion resistance and oxidation resistance.It has good tensile properties and fatigue properties from low temperature to 980 °C, and is resistant to stress corrosion in salt spray atmosphere.SUS 310s is a kind of high nickel austenitic stainless steel, which has excellent corrosion resistance and oxidation resistance, especially high strength and high heat resistance at high temperature.It is widely used in furnaces, heat treatment baskets, burners, combustion chambers and heat exchangers.The stable operation of the rotary calciner furnace tube has high requirements for deformation.It is necessary to consider the maximum deformation of the material during long-term operation at high temperature, high acid corrosion and low speed, and select the furnace tube material with the best stress and creep performance under high acid and high temperature conditions.

Model Analysis
This paper mainly evaluates the fatigue creep life of rotary furnace.According to the bearing capacity of the rotary furnace during the start-up process, considering the gravity, heating process, motor acceleration process and working process, based on the finite element software ANSYS, the hexahedral high-order grid is mainly used.Firstly, the steady-state analysis of the temperature field and stress field is carried out to obtain the temperature field and stress field distribution of the furnace tube.Then, based on this, the creep life of the second stage creep effect is considered, and the mechanical properties of the furnace tube when using three different materials of Inconel 690, Inconel 625 and SUS 310 s are analyzed.

Geometric Model Processing
The structure of the rotary calciner mainly includes drive mechanism, jamb, scraper, furnace, heating insulation module, furnace tail, holder and furnace tube bracket.Due to the complexity of the model, the model is simplified in the calculation process.The simplified structure is shown in figure 1.

External Load
The power of the driving motor of the furnace tube is 2000 W, the rated working condition is reached within 3 seconds, and the rated speed is 30 rpm (standard working condition).Accordingly: The rotary inertia of the furnace tube without scrap is 200 kg• m 2 , and the torque generated during the 3 second acceleration process: In summary, considering the most dangerous working conditions, the gear disc torque is loaded at 636.7 N• m when the motor is fully loaded.The radius of the gear disc is 0.3275 m, and the tangential force is: The rolling wheel is considered as a sliding contact, and the thermocouple and scraper are gap fit between the two sides and the sleeve, which is considered as a sliding contact.The bolt connection surface and the interference contact surface are considered as binding constraints.The thermocouple and the scraper are clearance fit between the two sides and the sleeve, which is considered as sliding contact.

Material Model
The material configuration of each part is shown in the figure, and the materials of each part are shown in table 1.The creep model can be expressed by Arrhenius equation: Accordingly, the strain rate can be obtained: In the formula:  is strain rate, A is constant,  is material constant, n is stress exponent, Q is activation energy of hot deformation, R is gas constant (8.314J/mol), T is absolute temperature.The creep parameter of Incolnel690, Inconel625 and SUS 310s are shown in table 2.

Calculation Results of Typical Material Inconel 690
The furnace barrel is divided into five sections to give the convective heat transfer coefficient (interpolation according to coordinates).The convective heat transfer coefficient is applied to the outer surface of the furnace body according to the following diagram.The temperature distribution of the furnace tube can be obtained by heat transfer calculation.The specific results are shown in figures 2-4.
In the axial sampling path on the surface of the furnace tube, the path temperature and stress distribution are shown in figure 5 (the attached figure is the path schematic).It can be seen that the maximum temperature on the surface of the cylinder is 1153 K, and the specific position is concentrated on the left side of the furnace tube.The equivalent stress distribution of the furnace tube (material Inconel 690) under the action of temperature field and centrifugal force (working speed 30 rpm) is shown in figures 4, 5 and 6.It can be seen that there are three obvious peaks in the axial stress distribution, while the radial distribution does not fluctuate much.Among them, the maximum Mises stress of the furnace tube is 40.8MPa, which occurs at the connection between the shaft sleeve (gear) and the furnace tube (axial position C); the stress at the left end of the furnace tube (axial position A) is 28 MPa.Although the stress value at the connection position between the sleeve and the furnace tube is not the largest, it needs to be investigated.Although the peak stress at the axial position B is less than 25 MPa, it can be seen from the temperature distribution cloud map that it is in the highest temperature area and needs to be investigated.The axial stress distribution trend of the furnace tube of Inconel 690, Inconel 625 and SUS 310s is shown in figure 8. From the diagram, it can be seen that the stress distribution is affected by the temperature distribution.Compared with Inconel 625 and SUS 310s, the peak stress of Inconel 690 is shifted to the right as a whole, and the peak stress is also different.The material peak stress of Incolnel690, Inconel625 and SUS 310s are shown in table 3.

Figure 2 .
Figure 2. Distribution of convective heat transfer coefficient of furnace tube.

Figure 5 .
Figure 5.The change curve of the degree and stress along the axial path (the following diagram is the path diagram).

Figure 6 .
Figure 6.The radial stress distribution map at the axial position 1 and 2.
Three Materials Static Analysis Results (Temperature, Stress, Displacement) Curve Diagram After investigating the temperature stress distribution of the Inconel 690 material furnace tube, the temperature and stress distribution of the furnace tube under the three materials of Inconel 690, Inconel 625 and SUS 310s were compared.The axial distribution trend of furnace tube temperature of Inconel 690, Inconel 625 and SUS 310s is shown in figure 7. It can be seen from the figure that the temperature distribution under Inconel 625 and SUS 310s materials is basically the same.Compared with Inconel 690, the temperature distribution shows an overall left shift trend, which is the same as the difference trend of heat conduction properties of materials.

Figure 7 .
Figure 7.The temperature variation curves of Inconel 690, Inconel 625 and SUS 310s along the axial path.

3. 3 .
Comparison of Creep Results of Three MaterialsThe distribution trend of axial creep displacement of Inconel 690, Inconel 625 and SUS 310s furnace tubes under continuous loading for 9000 hours is shown in figure9.It can be seen from the figure that the overall creep displacement distribution trends of the three materials are basically the same.The creep displacement is the largest at the left end of the furnace tube (position A), because the stress here is large and the temperature is high (about 1050K), and the creep displacement of the middle position of the furnace tube (position B) is also obvious, because the stress here is small (20-30MPa), but here is the highest temperature area of the furnace tube (1153K), while the right end of the furnace tube (position C) has the highest stress, but the temperature is low, so the creep displacement is small.At the same time, the overall trend of Inconel 690 material has a certain right shift compared with the other two materials, which is consistent with the difference of temperature and stress distribution.

Figure 9 .
Figure 9. Creep and stress-time curves of typical axial position of furnace tube.

Figure 10 .
Figure 10.Curves of creep displacement with time at typical axial positions A and B of furnace tube.

Table 1 .
Table of main components of rotary furnace.