TRISO燃料颗粒等效导热系数理论模型研究

三层各向同性碳包覆（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，其等效导热系数是计算弥散微封装燃料等效导热系数的重要基础。本文首先从球坐标下基本导热方程出发，基于多相固体宏观等效导热理论，建立了TRISO燃料颗粒等效导热系数理论计算模型；然后，结合固-固二元复合材料等效导热系数Chiew-Glandt模型分析了锆基微封装燃料（M3）芯体等效导热系数。结果表明，本文开发的模型可有效模拟TRISO燃料等效导热系数。基于开发的TRISO等效导热系数模型计算获得了全陶瓷微封装燃料（FCM）的等效导热系数。      

FCM燃料堆内行为模拟及结构设计研究

本文采用二维特征模型模拟不同无燃料区厚度全陶瓷微封装弥散（FCM）燃料的热力学行为，在保证堆芯装载要求的条件下，研究不同结构FCM燃料SiC基体和包覆燃料颗粒SiC层的应力状态。通过优化无燃料区厚度，调整TRISO颗粒间的间距，保证无燃料区和SiC层同时具有较低的应力水平。分析了无燃料区厚度为100 ~ 500 μm时基体SiC、无燃料区以及SiC层的应力分布，结果表明，基体SiC和SiC层最大应力随无燃料区厚度增大而增大，而无燃料区的最大应力则随其厚度增大而降低。当无燃料区厚度为400 μm时，无燃料区和SiC层均处于较低的应力状态，无燃料区SiC基体应力约为400 MPa，而SiC层的最大环向应力约为200 MPa，其失效概率约为2.5×10-4。因此，当无燃料区厚度为400 μm时，FCM燃料既能维持芯块结构完整，又能保证SiC层具有较低的失效概率。结构优化为FCM燃料的应用提供了基础。      

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。     

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。      

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。      

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。     

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。     

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。     

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。     

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.     

FCM燃料堆内行为模拟及结构设计研究

The thermal mechanical performance of the fully ceramics microencapsulated fuel (FCM) with different non-fuel part size was simulated using two-dimensional characteristic unit. When the fissile loading meet the requirements of the reactor core, the stress condition of SiC matrix and SiC layers were investigated for FCM pellets with different structures. Non-fuel parts and SiC layers suffered relative lower stress by optimizing FCM pellet structure and adjusting distance between different TRISO particles. The stress distribution of matrix, non-fuel part and SiC layer was discussed for the FCM pellets with non-fuel part size from 100 μm to 500 μm. The results indicate that, the maximum hoop stress of the matrix and SiC layer increased with the increasing of non-fuel part size, while the non-fuel parts exhibited crosscurrent. Non-fuel parts and SiC layer possessed lower stress when the non-fuel part was 400 μm. The stress of non-fuel part was about 400 MPa, and the maximum hoop stress of the SiC layers were about 200 MPa. The failure probability was 2.5×10-4. The structure integrity was maintained for the pellets with 400 μm non-fuel part, at the same time the failure probability SiC layer was low. Structural optimization is the basis for the application of FCM pellet.     

锆基弥散微封装燃料在稳态运行条件下的失效机理研究

为实现锆基弥散微封装燃料（M3燃料）的优化设计，进一步提升其在轻水堆（LWR）运行环境下的可靠性，需对其在稳态运行条件下的失效机理进行研究。本研究借助于ABAQUS有限元软件，通过二次开发建立了M3燃料的辐照-热-力耦合性能三维数值模拟分析方法，并基于此分析方法对M3燃料在稳态运行条件下的失效机理进行了研究。研究结果表明，稳态运行期间M3燃料的失效主要以辐照初期内致密热解碳层（IPyC层）的失效、辐照中后期疏松热解碳层（Buffer层）与IPyC层分开再接触后导致的碳化硅层失效为主。该研究结果可为后续M3燃料的优化设计提供指导。      

Hoop tensile strength testing of small diameter ceramic particles

A method to measure hoop tensile strength of 1-mm-diameter brittle ceramic spheres was demonstrated through the use of a ‘C-sphere’ flexure strength specimen. This innovative specimen geometry was chosen because a simple, monotonically increasing uniaxial compressive force produces a hoop tensile stress at the C-sphere’s outer surface that ultimately initiates fracture. This enables strength quantification and strength-limiting-flaw identification of the sphere itself. Such strength information is relevant to design optimization and durability assessments of ceramic fuel particles and breeder/multiplier pebbles for fusion when particle surfaces are subjected to tensile stresses during their manufacturing or service.     

TRISO燃料颗粒等效导热系数理论模型研究

TRISO (tri-structural isotropic) fuel particle consists of a fuel kernel in the center coated with four layers, with good fission product retention capability. The effective thermal conductivity of TRISO fuel particle is an important basis for calculating the effective thermal conductivity of dispersed fuels. In the present work, the theoretical model of the effective thermal conductivity of TRISO particle is built based on the theory of the effective thermal conductivity in multiphase solids in the framework of spherical coordinate and then the effective thermal conductivity of metal matrix microencapsulated fuel (M3) is analyzed combined with the Chiew-Glandt model which is the effective thermal conductivity model for solid-solid binary composite. The results show that the present model provides an excellent prediction of the thermal conductivity of TRISO particle. Finally the effective thermal conductivity of fully encapsulated fuel (FCM) is presented.     

Oxide thickness-dependent transient cladding hoop stress

A few thrice-burned Zry-4 fuel assemblies which were loaded in one of the PWRs operating in Korea were found to be failed due to PCI during a power ramp following a rector trip, while thrice-burned Zr–Nb fuel assemblies and twice-burned Zry-4 ones were intact. To investigate the effect of fuel rod oxide thickness on power ramp-induced cladding hoop stress, three intact fuel rods were selected, which include an intact twice-burned Zry-4 fuel rod, an intact thrice-burned Zr-4 fuel rod and an intact thrice-burned Zr–Nb fuel rod. With the use of a fuel performance analysis code, burnup-dependent steady-state cladding stress and ramp power-dependent cladding stresses at the power-ramped burnup were predicted for the three intact fuel rods. It was found that the cladding oxide thickness has a considerable effect on the ramp power-dependent cladding hoop stresses. In addition, the cladding maximum stress of the thrice-burned Zry-4 fuel rod with 125 μm oxide thickness exceeded an ultimate cladding tensile strength of the Zry-4 cladding when the pellet–clad friction coefficient-dependent cladding stress concentration ratio was considered. However, the thrice-burned Zr–Nb fuel rod with 50 μm oxide thickness was evaluated to have a considerable margin against the power ramp-induced PCI failure.