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。      

LOCA事故下碳化硅复合包壳失效概率计算

碳化硅（SiC）复合包壳是未来轻水堆耐事故燃料（Accident Tolerant Fuel,ATF）包壳候选方案之一。本文主要研究了在失水事故（Loss Of Coolant Accident,LOCA）下双层SiC结构包壳的失效概率问题。基于SiC复合材料包壳失效概率计算方法,采用准稳态方法模拟计算了瞬态工况下的SiC复合材料包壳失效概率。通过分析各种应力在事故工况下的占比,对Weibull分布的两个特征参数进行敏感性分析;研究了不同燃耗对于失效概率的影响,模拟了在不同层厚比下包壳的失效概率。结果表明：复合层所占比例的变化、Weibull参数改变、在不同燃耗下发生LOCA事故,对于SiC复合材料包壳的瞬态失效概率都有着较为明显的影响。本文的研究有助于耐事故燃料包壳的开发与设计,为SiC复合材料包壳失效概率的进一步研究提供参考。     

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。      

基于流固热耦合的燃料元件包壳结构完整性分析

反应堆系统发生瞬态工况时,冷却剂温度的瞬间大幅度变化会对燃料元件包壳结构完整性造成冲击,危及反应堆安全。本文以某压水堆3×3燃料组件为对象,采用流固热耦合方法对冷水事故下燃料组件的流动换热特性和燃料元件包壳温度、变形及应力进行了三维精细化模拟。结果表明:定位格架能够增强燃料棒表面的对流换热强度;包壳变形时向与刚凸接触的一侧折弯,向与弹簧接触的一侧凸起;包壳与定位格架接触部位的温度和最大等效应力随事故时间不断增大,且最大等效应力超过了包壳材料的屈服强度,将发生强度失效,影响其结构完整性。本文研究可为反应堆燃料元件包壳瞬态工况下的完整性评价提供借鉴。      

聚变堆第一壁C SiC涂层微观结构研究

用光学显微镜(OM)、扫描电镜(SEM)、X射线衍射仪(XRD)、X射线波谱仪(WDS)和能谱仪(EDAX),观察研究了C_3H_6-CH_3SiCl_3-Ar混合气体在石墨基体上化学气相沉积(CVD)的C SiC涂层的显微组织、晶体结构、SiC含量及分布与制备工艺的关系,以及氢离子(H~ )化学溅射和高能电子束热冲击对C SiC涂层微观结构和性能的影响。研究结果表明,在1600℃下制备的C SiC涂层,具有良好的结合强度、抗化学溅射和抗热冲击性能。     

锆合金包壳水侧SiC涂层研究

研究了聚碳硅烷（PCS）粉末的高温裂解特性及PCS粉末与锆粉间的化学反应机理,并在900 ℃制备了SiC涂层。研究发现,900 ℃开始,PCS裂解产物由无定形态SiC向结晶态转变。不同温度下,PCS粉末与锆粉的混合物发生一系列化学反应,产物为ZrC、Zr₂Si、Si₃Zr₅,通过调节反应温度,可控制该化学反应的程度,进而实现对涂层成分的调节。采用先驱体转化法（PIP）在锆合金包壳表面制备了SiC涂层,经PCS溶液浸涂-裂解3次循环可得到SiC陶瓷层,厚度为4 μm,涂层成分为SiC,ZrC为过渡层。划痕法测试得到涂层附着力等级为1～2级。     

金属基弥散微封装燃料中TRISO燃料颗粒的尺寸优化设计

弥散微封装燃料是将包覆燃料颗粒弥散在基体中形成燃料芯块或者燃料棒,是目前耐事故燃料（ATF）中最具发展潜力的燃料之一。包覆燃料颗粒为三结构同向型（TRISO）或者两结构同向型（BISO）包覆燃料颗粒,基体可以是金属也可以是陶瓷。本文用有限元分析软件ABAQUS对金属基弥散微封装燃料进行了分析计算。通过分析TRISO燃料颗粒各包覆层厚度对燃料性能的影响,提出优化改进的建议。研究结果表明,疏松热解碳层（Buffer）厚度越大,燃料颗粒发生破损失效的燃耗越高,因此设计时应考虑增加其厚度;内部致密热解碳层（IPyC）厚度越大,其自身的最大环向拉应力越大,因此设计时应降低其厚度;碳化硅（SiC）层厚度越大,其自身环向压应力越小,因此设计时应降低其厚度。本文的研究结果可为金属基弥散微封装燃料的优化设计提供指导。      

状态方程靶的纳米胶连复合技术研究

为了模拟和解释材料在激光冲击产生的高应变率和压力下的实验现象，物理实验对状态方程实验靶多层材料之间的胶层厚度提出了亚微米级的需求。针对现有复合方法的不足，本文采用引发式化学气相沉积方法制备固体原胶薄膜，并采用液相活化方法对原胶薄膜进行活化和固化，实现了状态方程实验靶的亚微米级无损胶连复合。采用多种表征技术对纳米胶连复合样品的胶层厚度及形貌进行表征，胶层厚度为亚微米级，最薄时仅300 nm。本文所报道的纳米胶连复合方法能为激光加载压缩物理实验的精度提高和实验结果的准确解释提供制靶技术保障。     

UMo/Zr单片式燃料板堆内热力耦合行为研究

UMo/Zr单片式燃料板在堆内辐照环境下会经历复杂的多场耦合及多尺度关联的行为。针对均匀辐照的堆内工况条件,建立了对UMo/Zr单片式燃料板的堆内行为进行多尺度模拟的方法,并计算分析了元件的温度场、变形和主要应力场随燃耗演化的规律,获得了芯体与包壳界面层间应力的分布与演化规律。研究结果表明,芯体的最高温度会随着辐照时间持续增长;芯体厚度随着辐照时间而增大,在靠近芯体的边界附近厚度增长较多,与辐照后相关检测结果相符;芯体的Mises应力要远小于包壳中的Mises应力;芯体和包壳界面正应力最大值位于靠近芯体角部的位置,界面角部区域较大的界面拉应力易导致此处首先产生界面破坏。     

国产不锈钢包壳管与裂变产物的化学相互作用──堆外模拟试验

在两种氧势下（分别以Ｃｒ／Ｃｒ２Ｏ３和Ｎｉ／ＮｉＯ模拟），对国内３个单位研制的不锈钢包壳管进行ＦＣＣＩ（燃料包壳化学相互作用）堆外模拟试验。低氧势下，包壳管内壁受裂变产物侵蚀不明显，受侵深度小于包壳管腐蚀设计裕量值。高氧势下，出现了基体腐蚀、晶界侵蚀和剥蚀３种类型兼有的腐蚀形貌，其侵蚀深度超过了腐蚀设计裕量值。化学相互作用深度和晶界渗透深度随氧势增高而加大，晶界渗透深度随着Ｃｓ：Ｔｅ比增大而减小。     

3D-Cf/SiC复合材料的弯曲强度及热膨胀性能分析

采用料浆浸渗和CVI工艺制备了含有ZrB₂陶瓷颗粒的3D-C_f/SiC复合材料,对其进行弯曲强度和线热膨胀系数测试,通过扫描电镜观察复合材料的表面及断口形貌。结果表明,3D-C_f/SiC复合材料的弯曲强度为107.99 MPa,满足一般热防护材料的使用要求;其线热膨胀系数随温度变化的规律是由于碳纤维和SiC陶瓷基体之间线热膨胀系数的不匹配及热残余应力造成的。     

Fabrication of SiC–SiC composites for fuel cladding in advanced reactor designs

Both advanced fission reactor concepts and fusion energy systems demand materials that can survive extremely harsh operating environments having persistent high temperature and high neutron flux conditions. Silicon carbide fiber/silicon carbide matrix (SiC–SiC) composites have shown promise for these applications, which include fuel cladding and reactor structural components. However, the composite fabrication process is time consuming and the fabrication of complicated geometries can be difficult.In this work, SiC–SiC and carbon fiber–SiC composite samples were fabricated using chemical vapor infiltration (CVI), and the mechanical and thermal properties of samples with a range of densities and total infiltration times were characterized and compared. Both sample density and the reinforcing fiber material were found to have a very significant influence on the composite mechanical and thermal material properties. In particular, internal porosity is found to have a significant effect on the mechanical response, as can be observed in the crack propagation in low density samples. In order to better understand the densification of the composites, a computer model is being developed to simulate the diffusion of reactants through the fiber preform, and SiC deposition on the fiber surfaces. Preliminary modeling has been correlated with experimental results and shows promising results.     

Nitrogen and helium permeation tests of SiCf/SiC composites

The permeance of inert gases (N₂ and He) through coated SiC based ceramic composites both in form of tubes and discs has been measured, in particular SiC-SiC_f/SiC functionally graded material tubes made of SiC-SiC_f with a SiC polycrystalline sealing coating and chemically vapour infiltrated (CVI) SiC_f/SiC composite discs have been tested.The permeation tests have been aimed at verifying the effectiveness of coatings obtained by chemical vapour deposition (CVD) and used for closing the porosity of the composite materials and assuring their gas sealing. The measurements have been carried out at room temperature with operating pressure in the range 130-500 kPa. The SiC-SiC_f/SiC functionally graded material has shown very low permeability values. The CVI SiC_f/SiC composite tested has shown rather low permeance values, although the material is not gas tight: its reduced permeance was attributed to both the high fibre percentage and the dense SiC coating.     

工作压强对SiC空心微球结构及性能的影响

采用化学气相沉积（CVD）-高温热解法,在不同工作压强条件下,制备了惯性约束聚变靶用SiC空心微球。利用X射线光电子能谱仪、扫描电子显微镜、白光干涉仪、X射线照相机对SiC空心微球的成分、表面形貌、表面粗糙度、球形度以及壁厚均匀性进行了测试与分析。研究结果表明：随工作压强的增加,SiC空心微球的表面均方根粗糙度先减小后增加,当工作压强为15 Pa时,表面均方根粗糙度达到最小值98 nm;随工作压强的增加,SiC空心微球的球形度未发生明显变化,且均优于97%;而壁厚均匀性则随工作压强的增加先增加后减小,当压强为15 Pa时,壁厚均匀性可达95%。     

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.     

Evaluation of Thermocouple Fin Effect in Cladding Surface Temperature Measurement during Film Boiling

Thermocouple fin effect on surface temperature measurement of a fuel rod has been studied at elevated wall temperatures under film boiling condition in a reactivity initiated accident (RIA) situation. This paper presents an analytical equation to evaluate temperature drops caused by the thermocouple wires attached to cladding surface. The equation yielded the local temperature drop at measuring point depending on thermocouple diameter, cladding temperature, coolant flow condition and vapor film thickness. The temperature drops by the evaluating equation were shown in cases of free and forced convection conditions. The analytical results were compared with the measured data for various thermocouple sizes, and also with the estimated maximum cladding temperature based on the oxidation layer thickness in the cladding outer surface.

It was concluded that the temperature drops at above 1,000°C in cladding temperature were around 120 and 150°C for 0.2 and 0.3 mm diameter Pt-Pt-Rh thermocouples, respectively, under a stagnant coolant condition. The fin effect increases with the decrease of vapor film thickness such as under forced flow cooling or at near the quenching point.     

SiC辐照损伤对耐高温氧化性能的影响

碳化硅（SiC）材料在高温氧化时会生成SiO2保护膜,但经辐照后高温氧化,材料结构和氧化速率会发生变化,影响材料性能。为研究其晶格损伤与氧化规律之间的关系,利用6H-SiC单晶片和烧结SiC多晶片,开展了在室温下经过能量为5 MeV、注量为5×1014 cm-2的Xe20+离子辐照后再进行1 300 ℃氧化1 h的实验。利用X射线衍射、拉曼光谱、傅里叶变换红外光谱、扫描电镜、透射电镜进行表征,对比了不同晶型的SiC氧化前后辐照与未辐照区域。结果显示：辐照破坏了晶格有序度,造成了晶格损伤,这些损伤在氧化过程中促进了多种SiO2基团的形成;生成的SiO2形成氧化层,由于与SiC基体热膨胀系数的差异,以及重结晶作用,导致碳化硅产生内应力,使氧化膜破裂;截面TEM图像显示,辐照引起的层错致使氧化程度加深,这是导致氧化速度加快的重要原因。     

Development of the tailored SiC/SiC composites by the combined fabrication process of ICVI and NITE methods

In order to improve the thermo-mechanical performances of SiC/SiC composite, process improvement and modification by the combination of nano-infiltration and transient eutectic-phase (NITE) method and chemical vapor infiltration (CVI) method were studied. Multilayered PyC/SiC fiber coating and matrix infiltration within fiber-tows were prepared with isothermal/isobaric CVI (ICVI) method and full-densification of SiC matrix was examined with NITE methods using four kinds of processing options. Applied pressure was useful for nearly-full matrix densification due to the promoting infiltration driving force of SiC nano-powder intra-fiber-tows, but simultaneously caused the sever degradation of fibers and interphase with fracture, resulting in lower strength. Increase of additives amount and additional polymer were effective ways for matrix densification by SiC nano-power infiltration intra-fiber bundles without pressure. Thermal conductivity was greatly improved with the decrease of matrix porosity. The tailoring of thermo-mechanical properties might be easily controlled by the SiC matrix porosity without process-induced fibers and interphases degradations.