高燃耗陶瓷燃料颗粒的参数化建模及强度分析

弥散燃料芯体中的陶瓷燃料颗粒在辐照条件下会形成裂变气孔,燃料颗粒内部气孔间的相互干涉作用及气孔内压的增长致使局部拉应力超过材料强度极限,进而导致燃料颗粒开裂。本文考虑高燃耗燃料颗粒内气孔尺寸和位置分布的非均匀性,实现了颗粒内部的细观结构参数化建模。运用有限元方法计算并分析了气孔尺寸、基体约束压应力、温度和气孔分布方式对颗粒内部最大拉应力的影响,研究了颗粒内开裂危险区的分布规律。结果表明,陶瓷燃料颗粒最大拉应力随气孔尺寸和温度的增加而增大,随基体约束压应力的增加而减小;燃料相的断裂强度减小,开裂危险区面积增大;燃料颗粒从内部多处开裂破坏,而表层处开裂的概率更大。本文为弥散燃料失效研究及优化设计提供了分析方法及数值参考。     

基于弥散燃料颗粒开裂的金属基体裂纹特征模型

金属基弥散燃料元件在特殊工况下会发生表面起泡失效。燃料颗粒开裂是金属基体开裂的前提条件,只有当金属基体开裂后元件才会发生表面起泡。燃料颗粒开裂后,裂纹宽度和塑性区长度等裂纹特征决定了金属基体开裂行为。基于弹塑性断裂力学和应力平衡条件,建立了基于弥散燃料颗粒开裂的金属基体裂纹特征模型。计算结果表明:裂纹张开位移随退火温度和燃耗深度的升高而增加;裂纹尖端塑性区长度主要与退火温度相关。裂纹张开位移和塑性区长度的计算结果与实验数据均符合较好,验证了金属基体裂纹特征模型的有效性。     

基于弥散燃料颗粒开裂的金属基体裂纹特征模型

金属基弥散燃料元件在特殊工况下会发生表面起泡失效。燃料颗粒开裂是金属基体开裂的前提条件，只有当金属基体开裂后元件才会发生表面起泡。燃料颗粒开裂后，裂纹宽度和塑性区长度等裂纹特征决定了金属基体开裂行为。基于弹塑性断裂力学和应力平衡条件，建立了基于弥散燃料颗粒开裂的金属基体裂纹特征模型。计算结果表明：裂纹张开位移随退火温度和燃耗深度的升高而增加；裂纹尖端塑性区长度主要与退火温度相关。裂纹张开位移和塑性区长度的计算结果与实验数据均符合较好，验证了金属基体裂纹特征模型的有效性。     

弥散型燃料元件等效热传导系数的有限元模拟

弥散型燃料元件的热传导与燃料颗粒的数量、形状、大小及其分布,以及元件的几何形状和堆芯内热工条件等密切相关.采用细观计算力学的方法,按照燃料颗粒不同的排布方式从整个元件中取出单胞和代表体积单元,运用有限元法计算了弥散型燃料元件在不同温度、燃耗和颗粒体积含量下的等效热传导系数,并和理论公式进行了比较.结果表明,计算值和MaxweU模型的理论值最为接近.     

裂变气体气泡尺寸对弥散燃料颗粒内部特征的影响规律

在前期均匀裂变气体气泡尺寸弥散燃料颗粒开裂模型基础上,基于不同尺寸气泡压力作用于燃料相的米塞斯(Mises)应力相等这一假设条件,建立了非均匀气泡尺寸的燃料颗粒开裂模型,并通过模型计算了裂变气体气泡尺寸对燃料相等效层厚度、气泡中气体原子数、气泡压力、燃料相最大张应力等内部特征的影响规律。计算结果表明:当气泡半径较大时,燃料相等效层厚度与气泡半径近似呈线性关系,当气泡尺寸较小时,等效层厚度与气泡半径之比随气泡半径减小急剧增加;随着气泡半径减小,气体原子数浓度增加;在升温过程中气泡内壁最大张应力的增大速率明显高于开裂阻力,气泡半径越小,燃料颗粒开裂温度越低。     

改进弦长抽样方法开发及在弥散燃料蒙特卡罗模拟的应用

弥散型燃料广泛应用于高温气冷堆、事故容忍燃料、实验研究堆及核动力舰船等，是重要的燃料类型之一。弦长抽样(CLS)方法可简化弥散燃料几何建模，提高计算效率，然而传统CLS方法只能描述单种颗粒的填充，同时在高体积填充率时误差较大。针对CLS方法的两大问题，本文在自主化堆用蒙特卡罗程序RMC中开发了改进CLS方法，并应用于全陶瓷微胶囊封装燃料棒算例及含毒物颗粒的高温堆燃料球算例。计算结果表明，改进CLS方法可解决多种颗粒混合填充的问题，并且可保证体积填充率的准确性，为弥散燃料的临界及燃耗计算提供了高效、精确的方法。     

锆基弥散微封装燃料等效传热系数数值模拟研究

为研究锆基弥散微封装燃料(M3燃料)的等效传热系数,假定TRISO(三层各向同性)颗粒球在锆基体内呈现体心立方排布,通过ABAQUS软件,基于均匀化理论,建立了M3燃料等效传热系数的模拟方法。依据建立的模拟方法,对不同相体积M3燃料的等效传热系数进行了研究分析。模拟结果显示:等效传热系数会随着温度的升高而升高、随燃耗和相体积的增加而降低。     

基于弥散燃料颗粒开裂的裂变气体释放模型

根据弥散燃料颗粒开裂后裂变气体的3种释放途径，分别建立了裂纹连通释放模型、气泡连通释放模型以及原子扩散释放模型，综合得到了基于弥散燃料颗粒开裂的裂变气体释放模型，并采用该模型对裂变气体释放量进行了计算。结果表明:裂变气体释放量主要由裂纹连通释放途径贡献；燃耗深度越高，裂变气体释放量的增加速率会越大；随着退火温度的增加，裂变气体释放量迅速增加，而退火时间越长，裂变气体释放量的增加速率越低。通过裂变气体释放量模型计算得到的裂纹宽度与实验观察到的裂纹宽度符合较好，对比结果验证了基于弥散燃料颗粒开裂的裂变气体释放模型的合理性。      

弥散型核燃料热导率计算模型研究

弥散型燃料热导率是反应堆安全分析和燃料元件性能评估中的重要参数。本文基于多孔物体热导率理论，考虑基体中弥散颗粒分布相关性的影响，建立弥散型燃料热导率的计算模型，并初步验证了模型的合理性，在此基础上研究了孔隙率、燃料相体积分数以及燃料相/基体热导率比对弥散型燃料热导率的影响。结果表明:弥散型燃料热导率随燃料相体积分数和孔隙率的增加而降低；燃料相与基体热导率之比越大，燃料相体积分数对热导率的影响越小。      

基于有限元方法的惰性基弥散燃料传热性能研究

惰性基弥散燃料芯块(Inert Matrix Dispersion Pellet,IMDP)以高温气冷堆燃料技术为基础,采用惰性材料作为三重各向同性型(Tristructural Isotropic,TRISO)燃料颗粒的弥散基体,相比传统的UO2燃料,其最典型特征是具备高热导率。采用通用有限元软件ABAQUS,结合其二次开发功能,建立有限元计算模型,研究了温度、燃耗以及燃料颗粒与惰性基体间热阻对IMDP燃料有效热导率影响规律,并与UO2陶瓷燃料进行对比。结果表明:IMDP有效热导率随燃耗及温度的增加而减小,且在不同寿期及不同温度下,IMDP有效热导率均明显高于UO2热导率;反应堆正常运行工况下,相比UO2芯块,IMDP较高的热导率会使芯块中心温度显著降低;此外,燃料颗粒与惰性基体间热阻在0~4×10~(-4) m~2·oC·W~(-1)范围内对IMDP的有效热导率影响程度最为敏感。     

VVER-1000应用PC级燃料的燃料管理

核电厂应用后处理制造的燃料组件,便可完成核燃料的"闭式循环",可以增加燃料的利用率和缓解乏燃料的储存难题。PC级燃料是利用后处理产品铀进行浓缩及加工所获得的一种核燃料,俄罗斯的VVER-1000机组中使用PC级燃料已有20年的历史。田湾核电厂1号机组已确定从第10燃料循环开始使用PC级燃料。使用KASKAD程序包,对VVER-1000使用PC级浓缩铀制造的TVS-2M组件展开研究设计,分析其应用的可行性,给出优化的燃料管理方案。     

核燃料

在法国核然料工业组织中,法杰马公司主要销售燃料组件。法比燃料公司(FBFC)的3个从属工厂都负责燃料组件的制造,该公司每年生产装铀量为1500t 的燃料组件。自1985年以来,法杰马公司又销售先进燃料组件(AFA)。该 AFA 的主要特点是使用了锆合金定位格架和可拆式上、下管嘴。大亚湾核电站要用的燃料组件正是该种与一般组件不同的先进燃料组件。法杰马公司采用钆作可燃毒物,以保证燃料组件的良好特性。近来该公司又推出混合氧化物燃料组件(MOX)。由于法杰马公司在设计和制造的各阶段都严格遵守了质量保证和质量控制制度,所以其产品质量优良、可靠性好。展望未来,法杰马公司将与法国核燃料工业中的其它集团一起,努力为用户提供尽可能好的产品。     

Fuel R & D to Improve Fuel Reliability

Light water reactor fuel is operating in an increasingly challenging environment. Fuel burnup extension and cycle length increase both can increase the local duty. Reactor water chemistry modifications for the purpose of protection the plant system materials have the potential of increasing fuel surface deposition and cladding corrosion and hydriding. The status of fuel performance in US reactors is summarized and an update of the “Fuel Reliability Program” established by the utility industry to ensure reliability is provided.     

Reactivity Effect of Non-Uniformly Distributed Fuel in Fuel Solution Systems

A numerical method to determine the optimal fuel distribution for minimum critical mass, or maximum k-effective, is developed using the Maximum Principle in order to evaluate the maximum effect of non-uniformly distributed fuel on reactivity. This algorithm maximizes the Hamiltonian directly by an iterative method under a certain constraint—the maintenance of criticality or total fuel mass. It ultimately reaches the same optimal state of a flattened fuel importance distribution as another algorithm by Dam based on perturbation theory.

This method was applied to two kinds of spherical cores with water reflector in the simulating reprocessing facility. In the slightly-enriched uranyl nitrate solution core, the minimum critical mass decreased by less than 1% at the optimal moderation state. In the plutonium nitrate solution core, the k-effective increment amounted up to 4.3Δk within the range of present study.     

Characterization of High Burnup Fuel for Safety Related Fuel Testing

Fuel Assemblies designed and fabricated by Westinghouse Electric Sweden (WSE) to reach high burnup have been operated in the Leibstadt nuclear power plant (KKL) for seven cycles attaining an assembly average burnup above 60 MWd/kgU. The irradiation conditions in KKL featured linear heat generation rates ranging from 250 W/cm early in life down to 100W/cm in the last cycle and normal water chemistry with zinc injection. Selected rods have been extracted at both intermediate and final irradiation stages and hot cell examinations have been carried out. The results show that the fuel is well suited for high burnup applications and rod segments have been provided to the OECD Halden Reactor Program, the OECD Studsvik Cladding Integrity Program and the Japanese ALPS program for dedicated high burnup tests with regard to fission as release and cladding lift-off as well as behavior under power transient, RIA and LOCA.     

Fuel transient deformation

Cladding strains resulting from fuel-cladding mechanical interaction in a transient tested fresh fuel pin are assessed against laboratory measurements of high-temperature creep and hot pressing of mixed-oxide fuel pellets.A fuel pin containing nine different fuel sections with varying fuel pellet geometry and density was transiently tested in a MARK IIIA flowing sodium loop under conditions typical of a 1$/s overpower transient. Post-test cladding strain measurements indicated that the largest strains were generated by solid pellets with small gaps while large gap annular pellets generated the smallest strains.High-temperature creep and hot pressing tests on mixed-oxide fuel have been performed at temperatures up to 2600°C. The results indicate that at temperatures above 2300°C, an additional component of creep is operative; while the densification due to hot pressing was considerably less than expected by extrapolating the low-temperature behavior.Both the in- and out-of-reactor data suggest that fuel creep into the center void or hole of a fuel pin is a more effective means of reducing fuel-cladding stress than densification by hot pressing into fuel pellet porosity.     

Fuel Temperature Coefficients of Reactivity for Heavy-Water-Moderated,Cluster-Type Fuel Lattices

Temperature coefficients of reactivity have been measured up to 600°C on cluster-type UO₂ fuel for three kinds of ²³⁵U enrichment and on a hollow cluster of sus-cladding tubes by using a hot He gas loop in a heavy-water-moderated, pressure-tube-type critical assembly. A new experimental method has been developed which accurately eliminates the reactivity disturbance caused by heat leakage in the measurement of an extremely small change in reactivity. The fuel (fuel pellet, cladding land pressure-tube) temperature coefficients of reactivity obtained for the temperature range below 300°C are +1.00±0.04, ?3.48±0.13 and - 6.36±0.25 in the unit of l0^-5% Δk/k.°C for 0.2%, 0.7% and 1.5%²³⁵U enrichment, respectively. In the higher temperature region above 300°C, each coefficient shifts to positive side by about 2x10^-5 Δk/k.°C. Temperature coefficient of reactivity for the hollow cluster of sus-cladding tubes (cladding and pressure-tube) has a large constant value with positive sign, + (6.42±0.26) x 10^-5 Δk/k.°C, all through the temperature range. A calculational model to analyze a hot-loop-type measurement of temperature coefficients with use of WIMS-D code was proposed and could be successfully applied to the present measurement.     

157组燃料组件组成的堆芯燃料管理研究

本文应用SCIENCE程序包对157组燃料组件组成的压水堆堆芯进行换料优化燃料管理研究,给出了3个年换料和2个18个月换料共5个设计方案,每个设计方案给出了从首循环到第8循环共8个循环的主要计算结果,并进行了分析比较。综合来看,OUT-IN装载的设计方案功率峰值偏低,IN-OUT装载的设计方案功率峰值偏高,但均在设计限值以内;1/4堆芯换料设计方案的平均卸料燃耗最深,表明其组件燃耗得最充分,经济性较好。