碳化硅基新型包覆燃料颗粒的设计及制备

TRISO型包覆燃料颗粒可将核裂变产生的气体、固体裂变产物束缚在燃料颗粒内部,是高温气冷堆安全性的重要保障。为满足未来超高温气冷堆在更高温度及更高燃耗条件下对燃料元件的要求,需对传统TRISO颗粒进行优化和改进。基于包覆颗粒的破损机制,设计了两种SiC基新型包覆颗粒,一种采用疏松SiC层替代疏松热解炭层,包覆层由内而外依次为疏松SiC层、内致密热解炭层、致密SiC层、外致密热解炭层;另一种为全SiC包覆结构,包覆层由内而外依次为内层疏松SiC层、SiC过渡层、外层致密SiC层。根据结构设计,采用流化床化学气相沉积法实验探索了疏松SiC的形成机制及包覆工艺条件,并利用SEM、XRD等进行材料分析,最终成功实现了两种新型包覆颗粒的大规模制备。更进一步,提出了全SiC基燃料元件的概念,并制备了球形和柱形全SiC基模拟燃料元件。     

高温气冷堆燃料元件发展前景

第4代核能系统主要候选堆型之一的超高温气冷反应堆(very-high-temperature reactor,简称VHTR)氦气出口温度要求大于1 000℃;从经济性考虑,模块式高温气冷堆的单堆功率愈高愈好,燃料的燃耗深度也愈高愈好,这些对近代低富集度3层包覆颗粒(modern LEU TRISO particles)燃料元件提出了更高燃耗深度和耐更高温的要求.为满足上述要求,本文介绍了ZrC层代替包覆燃料颗粒的SiC层、UCO(UO2 UC2)核芯代替包覆燃料颗粒的UO2核芯和进一步降低现有低富集度3层包覆颗粒SiC层破损率的高温气冷堆燃料元件的研究和发展.     

10 MW高温气冷堆首次装料的包覆燃料颗粒研制

10 MW高温气冷堆采用全陶瓷TRISO型包覆颗粒燃料元件.TRISO型包覆燃料颗粒由燃料核芯、疏松热解炭层、内致密热解炭层、碳化硅层和外致密热解炭层组成.本工作研究用于生产包覆燃料颗料的具有多气体入口的新型喷动流化床和4层连续包覆工艺.采用化学气相沉积方法在150 mm直径流化床沉积炉中生产出10 MW高温气冷堆的包覆燃料颗粒.用扫描电镜观察研究了包覆燃料颗粒的微观结构.包覆燃料颗粒的制造破损率为3.4×10-6.包覆燃料颗粒的辐照考验结果(包覆燃料颗粒的裂变产物85Krm释放率为10-6)表明,包覆燃料颗粒的性能可以满足我国10 MW高温气冷堆的设计要求.     

10MW高温气冷堆包覆燃料颗粒的研制

我国10MW高温气冷堆采用全陶瓷型包覆颗粒球形燃料元件。TRISO型包覆燃料颗粒由燃料核芯、疏松热解碳层、内致密热解碳层、碳化硅层和外致密热解碳层组成。采用丙烯和乙炔混合气体制备致密热解碳层以及四层连续包覆的新工艺,开展生产工艺条件试验,系统地研究了生产工艺和性能之间的关系,摸索出最佳生产工艺条件。用化学气相沉积方法在150mm流化床沉积炉系统中批量生产出TRISO型包覆燃料颗粒。用扫描电镜观察分析了包覆燃料颗粒的微观结构,包覆燃料颗粒的制造破损率为3.4×10-6,冷态性能达到我国10MW高温气冷堆设计要求。包覆燃料颗粒辐照考验结果(放射性裂变产物释放率R/B为1×10-6左右)表明,包覆燃料颗粒的质量可以满足10MW高温气冷堆安全运行的要求。     

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。     

高温气冷堆燃料元件固有循环安全特性的研究

介绍了高温气冷堆TRISO型的包覆燃料颗粒及球形燃料元件的结构特点及其安全原理.高温气冷堆具有低功率密度特点和负温度反应性特点,其与球形燃料元件有安全循环关系,实现了高温气冷堆固有循环安全特性.     

气冷堆用棱柱型弥散微封装燃料性能分析及优化研究

棱柱型弥散微封装燃料是将三重各向同性包覆（TRISO）燃料颗粒弥散于金属或陶瓷基体形成的颗粒增强复合燃料,具有良好的结构稳定性、裂变产物包容能力和辐照稳定性,是高温气冷堆中较具发展前景的燃料形式之一。本文提出将TRISO燃料颗粒弥散于SiC基体的棱柱型弥散微封装燃料设计方案,并基于有限元分析软件COMSOL建立了该燃料元件三维热流固耦合分析模型,初步实现了该燃料元件性能分析和优化设计。结果表明,棱柱型弥散微封装燃料元件的温度最大值位于燃料元件外侧,应力峰值位于冷却剂通道壁面,边距比为0.76~0.84、孔距比为0.68~0.75时燃料元件热应力最小。本文建立的棱柱型弥散微封装燃料性能分析方法和研究结论,可为后续该型气冷堆燃料元件设计提供指导和参考。      

HTGR颗粒燃料包覆工艺研究

包覆颗粒燃料涂层工艺是高温气冷堆(HTGR)关键技术之一。在研究制备工艺参数对包覆层性能的影响的基础上,确定了制备包覆颗粒燃料的最佳工艺条件,并制备出达到冷态设计要求的 Triso 型包覆颗粒燃料。     

TRISO包覆燃料颗粒裂变产物扩散释放特性研究

为研究三结构各向同性（TRISO）燃料颗粒裂变产物扩散释放特性,建立了辐照-热-力耦合作用下TRISO包覆燃料颗粒裂变产物Fick扩散模型,并通过IAEA CRP-6基准题进行了验证;利用所建模型对高温气冷堆典型工况下TRISO包覆燃料颗粒的性能进行了分析,同时,考虑裂变产物的反冲效应和热扩散效应,对TRISO包覆燃料颗粒不同温度及颗粒功率下裂变产物释放特性进行了分析。研究结果表明,高温会使TRISO包覆燃料颗粒裂变产物包容能力丧失,功率的提升则对裂变产物的释放影响较小。     

包覆燃料颗粒制备的自动化控制系统设计与研制

针对高温气冷堆包覆燃料颗粒生产逐渐规模化大批量发展的趋势,原制备工艺的手动控制体系已不能适应,需发展现代化工业级别的包覆燃料颗粒制备自动化控制系统。针对TRISO型包覆燃料颗粒4层连续包覆工艺进行分析,将包覆炉系统划分为5个子系统,将整个包覆过程分解为9个操作状态,提出建立分布式控制系统（DCS）自动化控制系统的思路。根据对包覆颗粒制备工艺的严格要求提出了控制系统设计原则,包括连锁控制、安全可靠、集成规范、实用易用、开放和易更新原则,并在具体建设过程中实现了这些原则要求,建立起一套完整的包覆燃料颗粒制备工艺自动化控制系统。该系统在我国高温气冷堆示范工程项目辐照样品的生产中投入运行,经实践检验,证明该系统可较好地实现包覆燃料颗粒制备工艺控制,满足工厂规模的生产要求。     

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.     

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.     

小型模块化超级安全气冷堆中子学特性研究

为分析小型模块化超级安全气冷堆堆芯中子学特性，建立六棱柱燃料组件模型，利用蒙特卡罗程序和ORIGEN程序的耦合计算，研究TRISO颗粒致密度、燃料富集度、TRISO颗粒大小、栅距比、TRISO颗粒包层厚度和燃料棒直径等物理参数对寿期等特性的影响。研究结果表明，寿期长度随着燃料富集度、栅距比的增大而单调增大；燃料棒直径、TRISO颗粒致密度、TRISO颗粒尺寸大小对寿期长度也有一定的影响；TRISO颗粒包层厚度对寿期长度的影响很小。基于该结果，初步设计出小型模块化超级安全气冷堆的堆芯装载方案，其寿期满足20 a不换料的寿期长度要求。      

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。     

Performance modeling of Deep Burn TRISO fuel using ZrC as a load-bearing layer and an oxygen getter

The effects of design choices for the TRISO particle fuel were explored in order to determine their contribution to attaining high-burnup in Deep Burn modular helium reactor fuels containing transuranics from light water reactor spent fuel. The new design features were: (1) ZrC coating substituted for the SiC, allowing the fuel to survive higher accident temperatures; (2) pyrocarbon/SiC “alloy” substituted for the inner pyrocarbon coating to reduce layer failure and (3) pyrocarbon seal coat and thin ZrC oxygen getter coating on the kernel to eliminate CO. Fuel performance was evaluated using General Atomics Company’s PISA code. The only acceptable design has a 200-μm kernel diameter coupled with at least 150-μm thick, 50% porosity buffer, a 15-μm ZrC getter over a 10-μm pyrocarbon seal coat on the kernel, an alloy inner pyrocarbon, and ZrC substituted for SiC. The code predicted that during a 1600 °C postulated accident at 70% FIMA, the ZrC failure probability is <10^?4.     

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.     

Nondestructive measurement of the coating thickness for simulated TRISO-coated fuel particles by using phase contrast X-ray radiography

The TRISO-coated fuel particle for a HTGR (high temperature gas-cooled reactor) is composed of a nuclear fuel kernel and outer coating layers. The coating layers consist of a buffer PyC (pyrolytic carbon) layer, an inner PyC (I-PyC) layer, a SiC layer, and an outer PyC (O-PyC) layer. X-ray radiography is one of the nondestructive alternatives to measure a coating thickness without generating a radioactive waste. Phase contrast X-ray radiography technology is more powerful for acquiring a radiograph with clear boundaries, when compared with a conventional X-ray radiography. The contrast can be enhanced for weakly absorbing materials in a phase contrast X-ray radiograph by detecting an intensity variation due to the variation of a phase of the X-rays in the boundary between two objects. Phase contrast X-ray radiograph was acquired from a simulated TRISO-coated fuel particle with a micro-focus X-ray imaging system. The coating thickness was nondestructively measured from the phase contrast X-ray image for the fuel particle.     

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     

气冷微堆燃料设计的中子学特性影响研究

为分析气冷微堆燃料设计的中子学特性影响,基于方形燃料组件模型,利用蒙特卡罗程序RMC研究了TRISO颗粒、燃料芯块在燃料设计中的主要参量对组件中子学特性的影响。研究结果表明,燃料颗粒体积占比和包覆层厚度不变时,组件寿期随燃料核芯直径的增大先显著增大,而后趋于平稳;燃料颗粒体积占比和燃料核芯直径不变时,组件寿期随包覆层厚度的增大而减小;燃料装载量不变时,芯块直径增大,组件寿期显著增大,而芯块高度影响较小;无燃料区厚度的增加对组件中子学特性有明显的负面影响,基体材料密度、基体杂质硼当量对组件中子学特性的影响较小。研究结果将为后续气冷微堆包覆颗粒弥散燃料的设计优化提供指导。