Near-surface and bulk behavior of Ag in SiC

The diffusive release of fission products, such as Ag, from TRISO particles at high temperatures has raised concerns regarding safe and economic operation of advanced nuclear reactors. Understanding the mechanisms of Ag diffusion is thus of crucial importance for effective retention of fission products. Two mechanisms, i.e., grain boundary diffusion and vapor or surface diffusion through macroscopic structures such as nano-pores or nano-cracks, remain in debate. In the present work, an integrated computational and experimental study of the near-surface and bulk behavior of Ag in silicon carbide (SiC) has been carried out. The ab initio calculations show that Ag prefers to adsorb on the SiC surface rather than in the bulk, and the mobility of Ag on the surface is high. The energy barrier for Ag desorption from the surface is calculated to be 0.85–1.68 eV, and Ag migration into bulk SiC through equilibrium diffusion process is not favorable. Experimentally, Ag ions are implanted into SiC to produce Ag profiles buried in the bulk and peaked at the surface. High-temperature annealing leads to Ag release from the surface region instead of diffusion into the interior of SiC. It is suggested that surface diffusion through mechanical structural imperfection, such as vapor transport through cracks in SiC coatings, may be a dominating mechanism accounting for Ag release from the SiC in the nuclear reactor.     

Ag diffusion in cubic silicon carbide

The diffusion of Ag impurities in bulk 3C-SiC is studied using ab initio methods based on density functional theory. This work is motivated by the desire to reduce transport of radioactive Ag isotopes through the SiC boundary layer in the Tristructural-Isotropic (TRISO) fuel pellet, which is a significant concern for the Very High Temperature Reactor (VHTR) nuclear reactor concept. The structure and stability of charged Ag and Ag-vacancy clusters in SiC are calculated. Relevant intrinsic SiC defect energies are also determined. The most stable state for the Ag impurity in SiC is found to be a Ag atom substituting on the Si sub-lattice and bound to a C vacancy. Bulk diffusion coefficients are estimated for different impurity states and values are all found to have very high activation energy. The impurity state with the lowest activation energy for diffusion is found to be the Ag interstitial, with an activation energy of approximately 7.9 eV. The high activation energies for Ag diffusion in bulk 3C-SiC cause Ag transport to be very slow in the bulk and suggests that observed Ag transport in this material is due to an alternative mechanism (e.g., grain boundary diffusion).     

Microstructure of TRISO coated particles from the AGR-1 experiment: SiC grain size and grain boundary character

Pre-irradiation SiC microstructures in tristructural-isotropic (TRISO) coated fuel particles from the Advanced Gas Reactor Fuel Development and Qualification program’s first irradiation experiment (AGR-1) were quantitatively characterized using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). From EBSD, it was determined that only the cubic polymorph of as-deposited SiC was present and the SiC had a high fraction of coincident site lattice (CSL) Σ3 grain boundaries. Additionally, the local area misorientation (LAM), which is a qualitative measurement of strain in the SiC lattice, was mapped for each sample fuel variant. The morphology of the SiC/IPyC interfaces were characterized by TEM following site-specific focused ion beam (FIB) specimen preparation. It was determined that the SiC layer had a heavily faulted microstructure typical of chemical vapor deposition (CVD) SiC and that the average grain diameter increased radially from the SiC/IPyC interface for the samples manufactured with similar CVD conditions, while the last sample showed a nearly constant grain size across the layer.     

UN核芯TRISO燃料颗粒破损概率模型研究

TRISO燃料颗粒由核芯和4层包覆层组成,具有良好的裂变产物包容能力。TRISO燃料颗粒破损概率是表征TRISO燃料事故安全特性的关键参数。本文基于修正的PANAMA破损概率计算方法,在考虑UN核芯裂变气体释放导致的气体内压以及内外致密热解炭层辐照蠕变和收缩作用的基础上,开发了UN核芯TRISO燃料颗粒压力壳式破损概率计算方法,并采用IAEA基准题6和基准题9对模型进行了验证;基于开发的UN核芯TRISO颗粒破损概率计算方法,采用随机抽样统计方法分析了事故工况下UN核芯和包覆层设计参数（包括包覆层尺寸及密度）对UN核芯TRISO燃料颗粒破损概率的影响。研究结果显示,疏松热解炭（Buffer）层设计参数是影响TRISO颗粒破损概率的关键因素,可通过降低Buffer层尺寸及密度分布设计标准偏差的方法降低UN核芯TRISO燃料颗粒的破损概率。     

Young’s modulus measurements of SiC coatings on spherical particles by using nanoindentation

Spherical silicon carbide coatings are deposited by fluidised bed chemical vapour deposition for the application to Tristructural Isotropic (TRISO) coated fuel particles. The silicon carbide exhibits columnar structure and grows along the radial direction during deposition. In this work, two measurements are made with nanoindentation, one is measured vertically to the grain growth direction, which gives a Young’s modulus of 391.1 ± 12.9 GPa, and the other is measured along the grain growth direction which gives a Young’s modulus of 442.5 ± 13.3 GPa. Finite element analysis and a theoretical effort are introduced to estimate the bending contribution when the indentation is carried out on the external surface of SiC. The relationship between grain orientation of SiC and its Young’s modulus has been examined.     

HTGR包覆燃料颗粒碳化硅层细晶化研究

高温气冷堆(HTGR)是能适应未来能源市场的第四代先进核反应堆堆型之一,其固有安全性的第一道保障是使用的TRISO型包覆燃料颗粒。在TRISO型燃料颗粒4层包覆结构中,SiC包覆层是承受包覆燃料颗粒内压和阻挡裂变产物释放的关键层,制备高质量SiC包覆层是核燃料领域中的重大问题和关键技术之一。本文介绍高温气冷堆燃料颗粒的基本结构,详述制备SiC包覆层的流化床-化学气相沉积过程,提出SiC层细晶化这一研究方向,并系统阐述包覆燃料颗粒SiC包覆层细晶化的优势。在细晶化SiC材料制备方法方面,系统分析SiC粉体、陶瓷、薄膜和厚膜材料的研究现状,并结合本实验室前期研究成果提出制备细晶SiC包覆层的可行制备策略。     

包覆燃料颗粒尺寸及其标准偏差对失效概率的影响

三结构同向性型(Tristructural isotropic,TRISO)包覆燃料颗粒是目前高温气冷堆和固态燃料熔盐堆采用的燃料元件。TRISO包覆燃料颗粒破损会导致裂变产物不可接受的释放,由此影响反应堆的安全运行。基于TRISO包覆燃料颗粒压力壳式破损模型,分析了TRISO包覆燃料颗粒核芯和各包覆层的尺寸对失效概率的影响,研究了TRISO包覆燃料颗粒核芯半径、疏松热解碳(Buffer)层厚度和碳化硅(Si C)层厚度的合理设计范围。同时,利用随机抽样统计的方法分析了TRISO包覆燃料颗粒核芯半径分布和各包覆层厚度分布对颗粒失效概率的影响。研究发现,降低Buffer层厚度分布的标准差至16μm可以使TRISO包覆燃料颗粒的失效概率降低一个数量级。     

Kinetic Monte Carlo (KMC) simulation of fission product silver transport through TRISO fuel particle

A mesoscale kinetic Monte Carlo (KMC) model developed to investigate the diffusion of silver through the pyrolytic carbon and silicon carbide containment layers of a TRISO fuel particle is described. The release of radioactive silver from TRISO particles has been studied for nearly three decades, yet the mechanisms governing silver transport are not fully understood. This model atomically resolves Ag, but provides a mesoscale medium of carbon and silicon carbide, which can include a variety of defects including grain boundaries, reflective interfaces, cracks, and radiation-induced cavities that can either accelerate silver diffusion or slow diffusion by acting as traps for silver. The key input parameters to the model (diffusion coefficients, trap binding energies, interface characteristics) are determined from available experimental data, or parametrically varied, until more precise values become available from lower length scale modeling or experiment. The predicted results, in terms of the time/temperature dependence of silver release during post-irradiation annealing and the variability of silver release from particle to particle have been compared to available experimental data from the German HTR Fuel Program (Gontard and Nabielek [1]) and Minato and co-workers (Minato et al. [2]).     

Modeling spent TRISO fuel for geological disposal: corrosion and failure under oxidizing conditions in the presence of water

High temperature gas reactors (HTGRs) are being considered for near term deployment in the United States under the GNEP program and farther term deployment under the Gen IV reactor design (U.S. DOE Nuclear Energy Research Advisory Committee, 2002). A common factor among current HTGR (prismatic or pebble) designs is the use of TRISO coated particle fuel. TRISO refers to the three types of coating layers (pyrolytic carbon, porous carbon, and silicon carbide) around the fuel kernel, which is both protected and contained by the layers. While there have been a number of reactors operated with coated particle fuel, and extensive amount of research has gone into designing new HTGRs, little work has been done on modeling and analysing the degradation rates of spent TRISO fuel for permanent geological disposal. An integral part of developing a spent fuel degradation modeling was to analyze the waste form without taking any consideration for engineering barriers. A basic model was developed to simulate the time to failure of spent TRISO fuel in a repository environment. Preliminary verification of the model was performed with comparison to output from a proprietary model called GARGOYLE that was also used to model degradation rates of TRISO fuel. A sensitivity study was performed to determine which fuel and repository parameters had the most significant effect on the predicted time to fuel particle failure. Results of the analysis indicate corrosion rates and thicknesses of the outer pyrolytic carbon and silicon carbide layers, along with the time dependent temperature of the spent fuel in the repository environment, have a significant effect on the time to particle failure. The thicknesses of the kernel, buffer, and IPyC layers along with the strength of the SiC layer and the pressure in the TRISO particle did not significantly alter the results from the model. It can be concluded that a better understanding of the corrosion rates of the OPyC and SiC layers, along with increasing the quality control of the OPyC and SiC layer thicknesses, can significantly reduce uncertainty in estimates of the time to failure of spent TRISO fuel in a repository environment.     

Effect of oxygen on ion-beam induced synthesis of SiC in silicon

The properties of Si-structures with a buried silicon carbide (SiC) layer created by high-dose carbon implantation into Cz–Si or Fz–Si wafers followed by high-temperature annealing were studied by Raman and infrared spectroscopy. The effect of additional oxygen implantation on the peculiarities of SiC layer formation was also studied. It was shown that under the same implantation and post-implantation annealing conditions the buried SiC layer is more effectively formed in Cz–Si or in Si (Cz-or Fz-) subjected to additional oxygen implantation. So we can conclude that oxygen in silicon promotes the SiC layer formation due to SiO_x precipitate creation and accommodation of the crystal volume in the region where SiC phase is formed. Carbon segregation and amorphous carbon film formation on SiC grain boundaries were revealed.     

UN核芯TRISO包覆燃料颗粒性能分析

为分析致密热解碳层、内压等因素对TRISO包覆燃料颗粒热-力学性能的影响,基于多物理场耦合软件COMSOL建立了以UN为核芯的TRISO包覆燃料颗粒三维热-力学耦合模型,并通过IAEA CRP-6基准题进行了验证。利用本文模型对稳态运行及反应性引入事故（RIA）工况下典型TRISO包覆燃料颗粒的性能进行了分析,结果表明,正常运行工况下SiC层能维持结构完整性,但IPyC层存在失效风险,需进一步优化TRISO包覆燃料颗粒的设计方案,而RIA工况下热膨胀是造成TRISO包覆燃料颗粒发生结构失效的主要原因。该模型能对轻水堆运行环境下的TRISO包覆燃料颗粒进行复杂的多物理场耦合性能分析,为进一步优化FCM燃料元件设计打下基础。     

氦离子辐照烧结碳化硅损伤效应研究

利用中国科学院近代物理研究所320 kV高压平台提供的氦离子辐照烧结碳化硅,辐照温度从室温到1 000 ℃,辐照注量为1015~1017 cm-2。辐照完成后,进行退火处理,然后开展透射电子显微镜、拉曼光谱、纳米硬度和热导率测试。研究发现,烧结碳化硅中氦泡形核阈值注量低于单晶碳化硅。同时,氦泡形貌和尺寸与辐照温度、退火温度有关。另外,对辐照产生的晶格缺陷、元素偏析进行了研究。结果表明,辐照产生了大量的缺陷团簇,同时氦泡生长也会发射间隙子,在氦泡周围形成间隙型位错环。在晶界处,容易发生碳原子聚集。辐照导致材料先发生硬化而后发生软化,且热导率降低。     

Verification of Fission Product Release Model from High Temperature Engineering Test Reactor Fuel

A fuel assembly of the High Temperature Engineering Test Reactor (HTTR) is composed of fuel rods and a hexagonal graphite block. A fuel rod is composed of the fuel compacts and a graphite sleeve. The coated fuel particles are incorporated into a graphite matrix to form a fuel compact. The fuel consists of microspheres of low-enriched U02 with a TRISO coating. The TRISO coatings consist of a porous pyrolytic carbon (PyC) buffer layer followed by an isotropic PyC layer, a SiC layer and a final (outer) PyC layer.

In order to evaluate amounts of fission products released from the HTTR fuel rods during normal operation, analytical models have been developed. Fractional releases of noble gases and iodine are calculated based on release data of ⁸⁸Kr which are obtained by irradiation tests with failed coated fuel particles. The transport of the metallic fission products through the kernel, coatings, fuel compact and graphite sleeve is modeled as a diffusion process. These analytical models have been verified by comparison with measured fractional releases in in-reactor tests and have been concluded to be applicable to the HTTR fuel condition.     

Analysis of the anisotropy,stoichiometry and polytypes in pyrolytic carbon and silicon carbide coatings

Silicon carbide (SiC) and pyrolytic carbon (PyC) coatings in tristructural isotropic (TRISO) coated fuel particles were characterised by a combination of 2-modulator generalised ellipsometry microscopy (2-MGEM), Raman spectroscopy and transmission electron microscopy. We compared the values of anisotropy obtained from 2-MGEM and Raman spectroscopy to investigate the effect of sampling area and microstructure. No linear correlation in anisotropy was found between these two techniques despite both sampling areas of 2–5 μm. The largest deviations were observed for highly anisotropic samples with optical anisotropy factors (OPTAFs) above 1.1. For medium and low anisotropy samples (OPTAF < 1.1) the relationship is close to linear. The limited use of only the average value of diattenuation does not give an accurate representation of the characteristics of the coatings as significant areas can exist with considerably higher diattenuations that could increase the probability of failure under neutron irradiation. We also observe a change in the diattenuation of SiC due to the presence of stacking faults as confirmed by Raman spectroscopy. Raman spectroscopy was also used to perform semiquantitative analysis of the Si and carbon excess in SiC in four TRISO particles.     

TRISO coated fuel particles with enhanced SiC properties

The silicon carbide (SiC) layer used for the formation of TRISO coated fuel particles is normally produced at 1500-1650 °C via fluidized bed chemical vapor deposition from methyltrichlorosilane in a hydrogen environment. In this work, we show the deposition of SiC coatings with uniform grain size throughout the coating thickness, as opposed to standard coatings which have larger grain sizes in the outer sections of the coating. Furthermore, the use of argon as the fluidizing gas and propylene as a carbon precursor, in addition to hydrogen and methyltrichlorosilane, allowed the deposition of stoichiometric SiC coatings with refined microstructure at 1400 and 1300 °C. The deposition of SiC at lower deposition temperatures was also advantageous since the reduced heat treatment was not detrimental to the properties of the inner pyrolytic carbon which generally occurs when SiC is deposited at 1500 °C. The use of a chemical vapor deposition coater with four spouts allowed the deposition of uniform and spherical coatings.     

球形燃料元件温度分布对包覆燃料颗粒失效概率的影响

固态熔盐堆采用TRISO(Tristructural isotropic)包覆颗粒球形燃料元件。在运行工况下,燃料元件内部存在一定的温度分布,填充在燃料元件内部不同位置的TRISO颗粒的失效概率会因此受到影响。利用体积微元的方法分析了温度分布对包覆颗粒失效概率的影响,并进一步研究了球形燃料元件尺寸对TRISO颗粒平均失效概率的影响。结果表明,在一定的功率密度下,如果利用球心温度或者平均温度计算燃料元件内部TRISO颗粒的平均失效概率,结果相比实际值会有至少一个数量级的差别;在相同功率密度和相同燃耗条件下,燃料元件直径每减小1 cm,其包覆颗粒平均失效概率降低两个数量级左右。     

PANAMA程序及其在10MW高温气冷实验堆安全分析中的应用

ＰＡＮＡＭＡ程序是德国在高温气冷堆安全研究中开发的一个实用程序，可以用来计算ＴＲＩＳＯ－包覆燃料颗粒在事故条件下的破损率，本文简介ＰＡＮＡＭＡ模型，着重开发了ＰＡＮＡＭＡ程序中ＳｉＣ压力容器失效模式，并利用１０ＭＷ高温气冷实验堆（ＨＴＲ－１）包覆燃料颗粒的设计参数，计算了燃耗，温度，核芯直径以及各包覆层厚度对颗粒破损率的影响，结果分析表明破损率阻燃耗，温度和核志直径的增大面而增长较快，对缓冲层和Ｓ     

Meltdown of palladium nanoparticles due to prolonged hydrogen ion bombardment

Nanometric crystalline particles of palladium are known to form on polycrystalline palladium under the bombardment of low-energy hydrogen ions. Mostly, the Pd particles disperse on an amorphous medium or in a matrix protruding from the target surface. Here evidence is presented that Pd nanoparticles originally present on the matrix surface are liquefied by bombarding hydrogen ions. The macroscopic target temperature for this to occur is around one-third of the melting point of Pd. This phenomenon is too unfamiliar to be described in terms of existing scientific models and an as-yet unknown process might underlie its occurrence.     

泡沫不锈钢层TRISO颗粒的堆内行为模拟

三向同性燃料（TRISO）颗粒中疏松热解碳层堆内辐照收缩产生间隙后,会导致TRISO颗粒热导恶化。为解决该问题,本文采用泡沫不锈钢替代TRISO颗粒中的疏松热解碳层。对泡沫不锈钢TRISO颗粒的堆内行为模拟结果表明,采用泡沫不锈钢可以避免疏松层堆内密实化,提高疏松层的传热效率,有效降低核芯运行温度;不论采用泡沫不锈钢还是疏松热解碳作为疏松层,内层致密热解碳层（IPyC层）和外层致密热解碳层（OPyC层）的应力均会超过包覆层强度;碳化硅（SiC）层的环向应力随泡沫不锈钢层弹性模量的减小而减小,通过降低泡沫不锈钢弹性模量可以有效控制SiC层应力,保证其结构完整性。因此,应选取气孔率高、弹性模量低的泡沫不锈钢作为TRISO颗粒的疏松层,可在改善热导恶化问题的同时保证SiC层的结构完整性。该研究为TRISO颗粒在工程应用中的优化设计提供了指导。      

XBIC/μ-XRF/μ-XAS analysis of metals precipitation in block-cast solar silicon

The results of the investigations of the interaction between the different impurities in intentionally contaminated block-cast multi-crystalline silicon by means of synchrotron-based microprobe techniques XBIC (X-ray beam induced current), μ-XRF (X-ray fluorescence microscopy) and μ-XAS (X-ray absorption microspectroscopy) recently implemented at beamlines ID-21 and ID-22 of ESRF, Grenoble, are presented. It was found that Si₃N₄/SiC particles frequently observed in the upper part of multi-crystalline Si blocks represent effective sinks for Fe and Cu impurities. The amount of precipitated iron was the same order magnitude both at nitride and carbide particles. The amount of Cu precipitated at the SiC inclusions was significantly larger than that at Si₃N₄ rods. Chemical state of the copper precipitates was identified as copper-rich silicide Cu₃Si. The anneal at 950 °C that is known to enhance oxygen precipitation in silicon was found to accompany with the enhanced formation of nanoscale iron disilicide precipitates both inside the grains and at grain boundaries.