UMo/Zr单片式燃料板起泡行为数值模拟

燃料板的鼓泡临界温度是制定研究试验堆安全系数的标准之一,通常由鼓泡退火实验确定。针对鼓泡退火实验,本研究发展了一种对UMo/Zr单片式燃料板宏观起泡行为进行计算模拟的方法,并计算分析了气腔内裂变气体原子数对鼓泡高度的影响,获得了包壳内的Mises应力和等效塑性应变随温度升高而演化的规律。研究发现,气腔周围包壳产生塑性变形是起泡的一个关键原因。此研究为燃料板起泡机理和关键影响因素分析打下了基础。     

中国先进研究堆燃料板性能试验研究和行为评述

文章介绍中国先进研究堆燃料板及其性能试验结果,并对燃料行为进行了评述.通过对铀密度为4.3 g/cm3的U3Si2-Al 弥散体燃料和包壳材料的热物性测量、包壳的腐蚀试验、燃料板的机械性能测量、热循环和辐照性能试验,可以确认,燃料板在最高温度不超过起泡温度(550 ℃)情况下一般不会破损,裂变产物不会从燃料板泄漏.燃料芯体能够经受温度高达400 ℃的多次热循环,芯体不开裂,不碎化,芯体与包壳之间的结合、U3Si2颗粒与基体铝结合良好.温度直到250 ℃,燃料板不会产生变形.在热流密度直到4.0 MW/m2、芯体最高温度为230 ℃、样品辐照燃耗达71.8%(原子百分数)条件下,燃料板无变形和损伤,燃料板肿胀不明显.     

UMo/Al弥散燃料板内裂变气体肿胀的静压效应研究

将核燃料的裂变气体肿胀与静水压力计算相耦合,并考虑重要的辐照蠕变,编制了定义其复杂力学本构关系的子程序。将定义各部分材料热-力学本构关系的用户子程序引入ABAQUS软件,获得了燃料板细观尺度下辐照-热-力耦合行为的计算模拟方法,并计算分析了核燃料裂变气体肿胀的静压效应。与不考虑裂变气体肿胀静压相关性的计算结果对比发现,在裂变气体肿胀计算中引入静压的影响,将使得核燃料颗粒内的辐照肿胀应变显著减小,引起板内最高温度降低,并减弱燃料颗粒和基体间的力学相互作用,减小燃料颗粒内的等效蠕变应变,致使基体内最大Mises应力和第一主应力减小。     

Cr涂层对板状燃料元件起泡特性影响数值模拟

鉴于Cr涂层能够有效地缓解棒状燃料元件包壳在失水事故时的鼓胀现象,本文提出将Cr涂层应用于板状燃料元件以抑制其起泡的方案。为研究Cr涂层对板状燃料元件起泡现象的抑制作用,本文采用有限元分析工具,分别添加Zr和Cr涂层的材料物性,并根据起泡现象的特征,考虑了Zr和Cr的热蠕变和受辐照硬化后的塑性行为。采用开发的工具对起泡退火实验进行了模拟,分析了温度和厚度对Cr涂层抑制作用的影响。研究结果表明：Cr涂层的屈服极限较低,在起泡现象发生时容易进入塑性阶段,从而不能有效地阻止起泡现象的产生;但其在大于700 K的高温下,较低的蠕变率能够降低起泡的高度,且温度越高降低的效果越明显。因此,Cr涂层可以抑制包壳的蠕变以降低起泡的高度。     

钠冷实验快堆(Ks—40)燃料棒气腔高度计算

本文叙述了实验快堆燃料棒气腔高度计算方法。其中,温度场计算把燃料轴向分成二十段径向分成未扰动区、等轴晶区、柱状晶区和中心孔区。利用包壳强度设计准则算得包壳最佳厚度及燃料棒最大许用内压。利用带有裂变气体平均温度的理想气体定律计算气腔高度。最后将计算结果与国外10座钠冷快堆作比较符合得比较理想。     

燃料破损在线系统计算机监控系统硬件的安装和调试

中国实验快堆燃料破损在线系统的计算机监控系统是快堆控制系统的组成部分，系统通过监测反应堆气腔内裂变产物比活度的变化，判断在反应堆活性区是否存在燃料棒包壳破损，并按给定的报警阈值在控制室发出声光报警信号。     

弥散型燃料板的辐照起泡机理分析

弥散型燃料在研究堆和动力堆中有着广泛的应用。起泡是弥散型燃料特有的失效模式,起泡的发生将导致堆芯传热性能恶化,威胁反应堆的运行安全。在分析总结国内外弥散型燃料板的辐照后起泡退火试验结果的基础上,从微观尺度到宏观尺度分析了起泡发生的机理,重点研究了弥散型燃料板的一种重要起泡模式——孔洞连通模式,剖析了孔洞连通发生的3个基本过程。同时应用孔洞连通机理,在估算裂变气体压力的前提下,通过力学计算给出了可引起起泡的孔洞连通的圆形区域尺度约为1.8mm,这与实验观察结果相符。本文分析表明,燃料板的孔洞连通起泡机理涉及到高燃耗效应、燃料相的肿胀和开裂、裂变碎片损伤和应力腐蚀开裂等过程,建立起泡模型需做弹塑性力学和断裂力学的数值计算。     

压水堆燃料包壳破损条件下裂变气体非稳态释放数值模拟研究

压水堆燃料包壳破损后,芯块-包壳间隙内积累的裂变气将释放到冷却剂中,其内部的微观机理还尚不清楚。为了揭示裂变气体释放过程中冷却剂与气体的相互作用规律,基于三维计算流体力学（CFD）方法对该物理过程展开数值模拟,所利用的模型为VOF模型以及k-ε模型。模拟结果表明,包壳破损后冷却剂首先进入芯块-包壳间隙,在芯块-包壳间隙内蒸发,引起芯块-包壳间隙内压强上升,而后裂变气体释放到子通道;裂变气体从芯块-包壳间隙释放到子通道可分为2个阶段。第一阶段:芯块-包壳间隙与子通道间压差较大,气体射流进入子通道,该阶段持续时间较短,裂变气体释放率较大,且变化也较大。第二阶段:芯块-包壳间隙与子通道间压差较小且相对平稳,裂变气体通过破口内涡的对流传质进入子通道,该阶段持续时间较短,裂变气体释放率较小,且相对稳定。      

三维燃料棒精细化模拟软件FUPAC3D与FUPAC的对比验证研究

为验证基于三维有限元分析平台建立的三维燃料棒精细化模拟软件FUPAC3D在分析评价压水堆燃料棒辐照-热-力耦合行为方面的能力和精度,本文给出了三维FUPAC3D软件采用的热学模型、燃料棒力学模型、裂变气体释放模型以及腐蚀模型,以华龙一号典型燃料棒参数和运行工况作为输入参数,分别使用三维FUPAC3D软件和已工程化应用的1.5维FUPAC软件进行建模分析,并针对2种软件在芯块和包壳温度、包壳应力与应变、芯块与包壳间间隙宽度的计算结果进行对比研究。研究结果表明,FUPAC3D软件与FUPAC软件具有相当的精度,FUPAC3D软件具备压水堆燃料棒辐照-热-力耦合行为的精细化模拟能力。      

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。     

热蠕变对UMo/Zr单片式燃料板起泡行为的影响

For a UMo/Zr monolithic fuel plate with a gas space, a method is developed to simulate the macroscale blister behavior considering the thermal creep effects of the cladding, in which the calculation of cladding deformation is coupled with the gas space pressure. Based on the developed simulation method, the effects of thermal creep strain of cladding and the internal fission gas atom number on the blister behavior are analyzed. The research results indicate that with the thermal creep of cladding considered, if the fission gas atom number is 4.0×1017, the predicted blister threshold temperature will be 100℃ lower than the case without considering the thermal creep of cladding, with the blister threshold temperature set as the temperature at which the blister height reaches 0.1 mm, with the fission gas atom number increasing from 2.5×1017 to 4.0×1017, the blister threshold temperature might decrease by 40℃. The blister threshold temperature of the fuel plates could be improved by using a cladding material with low thermal creep rate.     

Enhancing the ABAQUS thermomechanics code to simulate multipellet steady and transient LWR fuel rod behavior

A powerful multidimensional fuels performance analysis capability, applicable to both steady and transient fuel behavior, is developed based on enhancements to the commercially available ABAQUS general-purpose thermomechanics code. Enhanced capabilities are described, including: UO₂ temperature and burnup dependent thermal properties, solid and gaseous fission product swelling, fuel densification, fission gas release, cladding thermal and irradiation creep, cladding irradiation growth, gap heat transfer, and gap/plenum gas behavior during irradiation. This new capability is demonstrated using a 2D axisymmetric analysis of the upper section of a simplified multipellet fuel rod, during both steady and transient operation. Comparisons are made between discrete and smeared-pellet simulations. Computational results demonstrate the importance of a multidimensional, multipellet, fully-coupled thermomechanical approach. Interestingly, many of the inherent deficiencies in existing fuel performance codes (e.g., 1D thermomechanics, loose thermomechanical coupling, separate steady and transient analysis, cumbersome pre- and post-processing) are, in fact, ABAQUS strengths.     

A model for analysis of fuel behaviour in transients

For many applications, analysis of fuel element behaviour must take non-linear thermal, and elasto-plastic effects into account. This is particularly true if the fuel undergoes large deformations and rapid temperature transients. To meet this need a multi-dimensional fuel model based on finite element stress and thermal analysis has been developed. The model is solved for the transient temperature distribution by a step-by-step time incremental procedure. The temperature is then introduced into the elasto-plastic analysis as a thermal load and stresses and deformations are calculated. A model for treatment of creep and a special element for the gap between fuel pellet and cladding is incorporated together with semi-empirical procedures for calculating fission gas release, fuel pellet to cladding heat transfer coefficients, etc.The fuel model has been compared with both analytical solutions and in-reactor experimental results. The observed and predicted results are in good agreement.     

Effect of non-equilibrium fission gas and fuel creep on swelling and release in irradiated carbide fuels

The code UCSWELL was developed to simulate fission gas behavior in carbide fuels. In the present work, one of the limiting assumptions in UCSWELL - that matrix gas bubbles are in equilibrium with gas atom concentration - is removed and non-equilibrium matrix fission gas bubbles are allowed, but with relaxation to equilibrium by means of vacancy diffusion and thermal and radiation-induced creep of the fuel. For a given grain size, the difference in swelling between equilibrium and non-equilibrium with relaxation bubble fission gas treatment increases with decreasing irradiation temperature. At a given temperature, the non-equilibrium effect is more pronounced for larger grain fuel. This is to be expected because the creep rate (and hence the rate at which bubbles grow to an equilibrium size) decreases as temperature decreases and/or as grain size increases. At temperatures, where the creep rate is grain size insensitive, grain size remains important to the equilibrium process in so far as the grain boundary is a source of vacancies to the non-equilibrium bubbles. While the difference in these quantities is at the most on the order of 20% for the steady operating conditions considered, it is anticipated that the non-equilibrium effects become more pronounced during reactor overpower and undercooling transients.     

Multiphysics coupled modeling of light water reactor fuel performance

A fuel performance code for light water reactors called CityU Advanced Multiphysics Nuclear Fuels Performance with User-defined Simulations (CAMPUS) was developed. The CAMPUS code considers heat generation and conduction, oxygen diffusion, thermal expansion, elastic strain, densification, fission product swelling, grain growth, fission gas production and release, gap heat transfer, mechanical contact, gap/plenum pressure with plenum volume, fuel thermal and irradiation creep, cladding thermal and irradiation creep and oxidation. All the equations are implemented into the COMSOL Multiphysics finite-element platform with a 2D axisymmetric geometry of a fuel pellet with cladding. Comparisons of critical fuel performance parameters for UO₂ fuel using CAMPUS are similar to those obtained from BISON, ABAQUS and FRAPCON. Additional comparisons of beryllium doped fuel (UO₂-10%volBeO) with silicon carbide, instead of Zircaloy as cladding, also indicate good agreement. The capabilities of the CAMPUS code were further demonstrated by simulating the performance of oxide (UO₂), composite (UO₂-10%volBeO), silicide (U₃Si₂) and mixed oxide ((Th_0.9,U_0.1)O₂) fuel types under normal operation conditions. Compared to UO₂, it was found that the UO₂-10%volBeO fuel experiences lower temperatures and fission gas release while producing similar cladding strain. The U₃Si₂ fuel has the earliest gap closure and induces the highest cladding hoop stress. Finally, the (Th_0.9,U_0.1)O₂ fuel is predicted to produce the lowest fission gas release and a lower fuel centerline temperature when compared with the UO₂ fuel. These tests demonstrate that CAMPUS (using the COMSOL platform) is a practical tool for modeling LWR fuel performance.     

PECITIS-II, a computer program to predict the performance of collapsible clad UO2 fuel elements

The codes devised and used in India for the design of fuel for their Pressurized Heavy Water Reactor (PHWR) programme are described. The scheme includes the use of collapsible fuel cladding for improved neutron economy.This code is made with reference to collapsible clad UO₂ fuel elements. This evaluates sheath strain and fission gas pressure. The fuel expansion is calculated by a two zone model which assumes that above a certain temperature the UO₂ deforms plastically and below that temperature it cracks radially and behaves as an elastic solid; the plastic core is under compression. The pellet clad gap conductance is calculated by using a modified Ross and Stoute model considering the effects of fuel and clad thermal expansion, fission gas release, dilution of filler gas and irradiation swelling. Stress relaxation of the sheath and its effect on fuel sheath contact pressure is also considered for arriving at the end result.     

Fuel swelling due to retained fission gas and thermal effects during high temperature transients

Fuel swelling of previously irradiated pressurized-water-reactor-type fuel rods tested under power-cooling-mismatch conditions is due to retained fission gas and thermal effects within the film boiling region. In this paper empirical correlations for fuel swelling are presented, and mechanisms contributing to the observed swelling and the applicability of an analytical fission gas behavior computer code (GRASS-SST) to fuel swelling are evaluated. Major contributors to fuel swelling are fuel melting and expansion, expansion of solid fuel, fission gas bubble coalescence, fission gas diffusion to grain boundaries, and change in surface tension of fuel upon melting. The contributions to fuel swelling of solid fission products and the effects of cladding contraction and wall thinning on rod swelling are also included. The overall empirically-calculated fuel swelling values and the GRASS-SST code calculated values are compared with measured values. The agreement between measured and empirically calculated fuel swelling is generally close. Fuel swelling due to retained fission gas during the film boiling transient, as calculated by the GRASS-SST code, was found to be in good agreement with experimental results.