Properties and microstructures of A3-3 matrix graphite for pebble fuel elements after high temperature purification at different temperatures
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摘要: 为了研究高温纯化对高温气冷堆球形燃料元件用A3-3基体石墨的影响，对不同温度下纯化后A3-3基体石墨的综合性能和微观结构进行了对比分析和表征。结果表明，即使将纯化温度从1900 ℃降低到1600 ℃，高温纯化处理后基体石墨的综合性能均满足技术要求。 X射线衍射分析结果表明，纯化后基体石墨的微观结构得到了显著提升，并且随着高温纯化温度的升高，基体石墨的石墨化有序度逐渐提高，其微观组织结构也逐渐优化，这有利于基体石墨综合性能的提升。当纯化温度从1600 ℃继续升高时，纯化后基体石墨的灰分和杂质含量基本保持不变，在更高温度下纯化后基体石墨的微观结构优化对改善其抗氧化腐蚀性能起到了决定性作用。因此，高温纯化工艺在球形燃料元件的生产中是非常重要和必要的。该研究也为未来将球形燃料元件的高温纯化温度从1900 ℃降至1600 ℃提供了重要依据，有助于在将来的球形燃料元件商业化生产中提高生产效率和降低生产成本。Abstract: Matrix graphite (MG) was purified by high temperature purification (HTP), and their properties and microstructures were measured and analyzed to investigate the effect of HTP temperature on the property improvement of A3-3 MG as a pebble fuel element, and to optimize the purification temperature. Results showed that all the properties of MG specimens purified at temperatures from 1600 to 1900 ℃ met the technical requirements. X-ray diffraction analysis results showed that the microstructures of MG after HTP were significantly improved. With increasing the purification temperature from 1600 to 1900 ℃, MG gradually became ordered, the microstructures became better gradually for improving the comprehensive performance. The ash content decreased abruptly after HTP at 1600 ℃, but changed little when the purification temperature rose from 1600 to 1900 ℃. The microstructure improvement at high temperatures played a decisive role in increasing the oxidative corrosion resistance of MG. Therefore, HTP is very important and necessary, and cannot be canceled in the production of pebble fuel elements. This study provides an important reference to determine an optimal HTP temperature of pebble fuel elements for improving the production efficiency and reducing production cost in the commercial production of pebble fuel elements in the future.
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图 1 不同温度处理后酚醛树脂炭的X射线衍射结果
Figure 1. XRD patterns of PRC samples treated at different temperatures
图 2 压汞法得到的MG-800和MG-1600的孔分布信息
Figure 2. Pore information of MG-800 and MG-1600 characterized by mercury porosimetry
表 1 The ash contents and EBCs of raw materials for A3-3 MG (μg/g)
Table 1. The ash contents and EBCs of raw materials for A3-3 MG (μg/g)
Natural flake graphite powder Artificial graphite powder Phenol resin Ash contents 10.0 11.5 43.0 EBC 0.223 0.087 0.330 
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表 2 Specimens information for property and microstructure characterization of MG
Table 2. Specimens information for property and microstructure characterization of MG
Property Shape Dimension (mm) Orientation Amount Crush strength Pebble r=59.6-60.2 AX 5 TR 5 Thermal conductivity Cylinder Ø12.7×2 AX 3 TR 3 Erosion rate Pebble r=59.6−60.2 N/A 20 Corrosion rate Pebble r=59.6−60.2 N/A 3 Ash content Pebble r=59.6−60.2 N/A 2 Mercury intrusion
porosimetryCylinder Ø12.7×25 Random 1
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表 3 Weight and dimensional changes of MG pebbles through HTP
Table 3. Weight and dimensional changes of MG pebbles through HTP
Changes / 100% Heat treatment under different temperatures / ℃ 1600 1700 1800 1900 Weight 0.29 0.30 0.31 0.31 Axial dimension 0.17 0.23 0.28 0.31 Transverse dimension 0.14 0.20 0.25 0.29 Volume 0.45 0.63 0.78 0.89
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表 4 Comprehensive properties of MG treated with different heat treatment temperatures
Table 4. Comprehensive properties of MG treated with different heat treatment temperatures
Property Average±Deviation Specification MG-800 MG-1600 MG-1700 MG-1800 MG-1900 Density (g/cm3) 1.724±0.001 1.727±0.002 1.730±0.002 1.732±0.001 1.734±0.001 1.70-1.77 AX Crush strength (kN) 22.54±1.29 27.45±0.71 27.18±1.27 27.13±0.86 26.67±0.66 ≥18.0 TR Crush strength (kN) 17.43±1.20 19.37±0.68 18.99±0.41 19.47±0.34 19.57±0.47 Corrosion rate a (mg/cm2·h) 1.88±0.07 0.91±0.05 0.75±0.05 0.63±0.05 0.53±0.06 ≤1.3 Erosion rate （mg/h·Pebble） 7.94±0.48 3.07±0.11 2.03±0.13 1.81±0.16 1.48±0.09 ≤6.0 AX Thermal conductivity b (W/m·K) 31.73±0.56 35.93±0.78 36.78±0.66 37.40±0.82 37.86±0.98 ≥25.0 TR Thermal conductivity b (W/m·K) 36.78±0.76 38.90±0.64 39.33±0.83 39.65±0.77 39.88±0.75 Ash content (ppm) 18.2±1.0 12.3±0.5 11.0±1.0 10.0 ±1.5 10.5±1.0 ≤300 a 1000 ℃, 10h, atmosphere was He+1 vol% H2O.
b The value of thermal conductivity at 1000 ℃.
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表 5 The ash contents and typical impurity elements of MG pebbles (ppm)
Table 5. The ash contents and typical impurity elements of MG pebbles (ppm)
Element Apparatus MG-800 MG-1600 MG-1700 MG-1800 MG-1900 Ash content N/A 18.2 12.3 11.0 10.0 10.5 Al ICP-OES 0.302 0.222 0.256 0.128 0.257 Ca ICP-OES 1.936 0.849 0.799 0.667 0.711 Cr ICP-MS 0.185 0.156 0.133 0.113 0.119 Cu ICP-MS 1.861 0.014 0.011 0.010 0.010 Fe ICP-OES 4.473 3.151 1.917 1.451 1.387 Mn ICP-MS 0.128 0.065 0.051 0.027 0.021 Mo ICP-MS 0.104 0.020 0.027 0.016 0.010 Ni ICP-MS 1.313 0.523 0.324 0.207 0.194 Zn ICP-OES 0.260 0.015 0.020 0.014 0.012
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表 6 The d002 values of PRC samples treated at different temperatures by XRD
Table 6. The d002 values of PRC samples treated at different temperatures by XRD
Samples PRC-800 PRC-1600 PRC-1700 PRC-1800 PRC-1900 d002 / nm 0.3918 0.3694 0.3645 0.3587 0.3581
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表 7 Porosity of MG specimens measured by mercury porosimetry
Table 7. Porosity of MG specimens measured by mercury porosimetry
Specimens MG-800 MG-1600 MG-1700 MG-1800 MG-1900 Porosity (%) 15.1992 16.0932 16.4126 16.7677 17.0181 Skeletal density (g/cm3) 2.0674 2.0924 2.1082 2.1243 2.1376
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