Properties and microstructures of a matrix graphite for fuel elements of pebble-bed reactors after high temperature purification at different temperatures
-
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非常重要和必要。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: The matrix graphite (MG) of pebble fuel elements for a High Temperature Gas-cooled Reactor (HTGR), composed of 71wt% natural graphite, 18 wt% artificial graphite and 11 wt% phenolic resin-derived carbon, was purified by high temperature treatment (HTT), and its properties and microstructure were analyzed to investigate the effect of different HTT temperatures and optimize the purification temperature. Results showed that with increasing HTT temperature, its density and thermal conductivity gradually increased, but pore size and d002 gradually decreased. The rate of erosion caused by friction as the fuel pebbles move in the reactor also decreased. The ash content decreased significantly from to 18.2 to 12.3×10−6 after HTT at 1 600 ℃, but changed little when the HTT temperature was further increased to 1 900 ℃, especially for catalytic metals such as Fe, Ni and Ca that are related to its corrosion rate. The microstructure improvement and ash content reduction at high temperatures jointly contributed to the increase in the anti-corrosion performance of MG. Based on properties such as crushing strength, erosion resistance, and corrosion rate, a HTT of 1 600 ℃ is adequate although the MG gradually became more ordered with a further increase of HTT temperature from 1 600 to 1 900 ℃. This determination of an appropriate HTT temperature for the production of MG for the fuel elements of an HTGR should improve the production efficiency and reduce the mass production cost of this material for a commercial HTGR.
-
Table 1. The ash contents and EBCs of raw materials for A3-3 MG (μg g−1).
Natural flake graphite powder Artificial graphite powder Phenolic resin Ash contents 10.0 11.5 43.0 EBC 0.223 0.087 0.330 Table 2. Specimen 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 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 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 ratea (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 conductivityb (W·m−1·K−1) 31.73±0.56 35.93±0.78 36.78±0.66 37.40±0.82 37.86±0.98 ≥25.0 TR Thermal conductivityb (W·m−1·K−1) 36.78±0.76 38.90±0.64 39.33±0.83 39.65±0.77 39.88±0.75 Ash content (1×10−6) 18.20±1.00 12.30±0.50 11.00±1.00 10.00 ±1.50 10.50±1.00 ≤300 Note: a 1000 ℃, 10 h, atmosphere was He+1 vol% H2O. b The value of thermal conductivity at 1000 ℃. Table 5. The ash contents and typical impurity elements of MG pebbles (1×10−6).
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 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 Table 7. Porosities 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 -
[1] Zhang Z Y, Dong Y J, Li F, et al. The Shandong Shidao Bay 200 MWe high-temperature gas-cooled reactor pebble-bed module (HTR-PM) demonstration power plant: An engineering and technical innovation[J]. Engineering,2016,2(1):112-118. doi: 10.1016/J.ENG.2016.01.020 [2] Zhou X W, Yang Y, Song J, et al. Carbon materials in a high temperature gas-cooled reactor pebble-bed module[J]. New Carbon Materials,2018,33(2):97-108. doi: 10.1016/S1872-5805(18)60328-2 [3] Zhou X W, Lu Z M, Zhang J, et al. Preparation of pebble fuel elements for HTR-PM in INET[J]. Nuclear Engineering and Design,2013,263:456-461. doi: 10.1016/j.nucengdes.2013.07.001 [4] Zhou X W, Lu Z M, Zhang J, et al. Study on the comprehensive properties and microstructures of A3-3 matrix graphite related to the high temperature purification treatment [J]. Science and Technology of Nuclear Installations, vol. 2018, Article ID 6084747, 10 pages, 2018. [5] Zhou X W, Yang Y, Ma J T, et al. Effects of purification on the properties and microstructures of natural flake and artificial graphite powders[J]. Nuclear Engineering and Design,2020,360:110527. doi: 10.1016/j.nucengdes.2020.110527 [6] ASTM Standard test method for determination of pore volume and pore volume distribution of soil and rock by mercury intrusion porosimetry[S]. Tech Rep ASTM D4404-10, ASTM International, West Conshohocken, PA, USA, 2010, https://www.astm.org/. [7] Pappano P J, Burchell T D, Hunn J D, et al. A novel approach to fabricating fuel compacts for the next generation nuclear plant (NGNP)[J]. Journal of Nuclear Materials,2008,381:25-38. doi: 10.1016/j.jnucmat.2008.07.032 [8] Windes W, Burchell T, Carroll M. Graphite technology development plan[R]. Technical report 23747, Idaho National Laboratory, Idaho, USA, 2010. [9] Lee J J, Ghosh T K, Loyalka S K. Comparison of NBG-18, NBG-17, IG-110 and IG-11 oxidation kinetics in air[J]. Journal of Nuclear Materials,2018,500:64-71. doi: 10.1016/j.jnucmat.2017.11.053 [10] Walker P L, Rusinko F, Austin L G. Gas reactions of carbon[J]. Advances in Catalysis,1959,11:133-221. [11] Michio I. Materials Science and Engineering of Carbon: Characterization[M]. USA: Elsevier Science, 2016. [12] Zheng G Q, Xu P, Sridharan K, et al. Characterization of structural defects in nuclear graphite IG-110 and NBG-18[J]. Journal of Nuclear Materials,2014,446:193-199. doi: 10.1016/j.jnucmat.2013.12.013