U3Si2—Al弥散型燃料的辐照肿胀研究

介绍了U3Si2-Al弥散型燃料的辐照肿胀机理。将弥散型燃料的芯体视为连续基体中的微型燃料元件,应用裂变气体的行为机理描述燃料相中的气泡形成过程。研究结果表明：燃料相的肿胀引起燃料颗粒和金属基体之间的力学相互作用,金属基体能抑制燃料颗粒的辐照肿胀。在一定辐照条件下,本模型对燃料元件辐照肿胀的预测值与测量值相符。     

U_3Si_2Al弥散型燃料的辐照肿胀研究

介绍了U3Si2 Al弥散型燃料的辐照肿胀机理。将弥散型燃料的芯体视为连续基体中的微型燃料元件 ,应用裂变气体的行为机理描述燃料相中的气泡形成过程。研究结果表明 :燃料相的肿胀引起燃料颗粒和金属基体之间的力学相互作用 ,金属基体能抑制燃料颗粒的辐照肿胀。在一定辐照条件下 ,本模型对燃料元件辐照肿胀的预测值与测量值相符     

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。     

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。     

烧结二氧化铀燃料裂变气体肿胀的数值模拟研究

建立低温条件下烧结二氧化铀燃料(简称UO2燃料)中裂变气体的肿胀计算模型,采用有限差分方法编写计算程序,定量计算不同燃耗和温度条件下UO2燃料中固溶态的裂变气体份额、裂变气体气泡的密度与平均半径以及它们对燃料肿胀的贡献.计算表明,该模型能用于预测低温条件下UO2燃料中裂变气体所导致的肿胀随燃耗的变化规律.     

溶液堆气泡及瞬态中子学模型

溶液堆的燃料（UO₂(NO₃)₂水溶液）具有液体形态，兼作核燃料和慢化剂，裂变碎片与水分子碰撞产生辐照裂解气体气泡，气泡在燃料中存在和运动使得溶液堆的瞬态中子学模拟十分困难。本研究首先基于稳态的物理热工耦合方法对溶液堆进行模拟，对模拟结果体现的气泡行为特征进行提炼，建立溶液堆气泡数值模型，再将该气泡模型应用于溶液堆瞬态中子学分析程序中，使用该程序对瞬态试验进行模拟并与测量结果进行比较，发现气泡行为特征与耦合方法的模拟结果一致。     

弥散燃料颗粒裂纹起源的有限元模拟分析

通过建立含多气泡的燃料颗粒模型,采用有限元方法分析了燃料颗粒在裂变气体气泡内压作用下的应力分布,统计了燃料颗粒内部气泡位置对气泡内壁处的最大拉应力的影响,并结合实验结果探寻了弥散燃料颗粒在辐照后退火时的裂纹起源。结果表明:当弥散燃料颗粒内部含有多个裂变气体气泡时,受气泡内压作用,气泡内壁径向应力为压应力,环向应力为拉应力;气泡位置距燃料颗粒心部越远,气泡内壁处的最大环向拉应力越大;表层气泡的最大环向拉应力远大于心部气泡的;燃料颗粒裂纹起源于表层气泡内壁。     

溶液堆物理热工耦合程序开发及堆芯气泡分布初步研究

溶液堆内辐照裂解气体的存在是影响溶液堆运行稳定性的重要因素。浮力、碰撞等复杂的相互作用造成这些气体在燃料溶液中形成气泡后的行为难以预测;现有的分析手段通常用关于坐标的函数来近似估计气泡的分布。使用计算流体力学(CFX)程序对溶液堆气-液两相试验工况进行数值模拟的结果表明,应用多尺寸组分模型(MUSIG)能够很好地反映堆内气泡的迁徙过程。FMCAHR_CFX是基于溶液堆燃料管理程序(FMCAHR)和计算流体力学程序(ANSYS CFX 10.0)的耦合程序;本文使用该程序分析简单溶液堆模型中的气泡分布。     

UO2燃料裂变气体渗流模型研究

为分析UO₂燃料晶界气泡连通导致裂变气体间歇性释放的动力学过程,从而解决目前扩散模型预测的沿芯块径向释放份额与实验测量不符的问题,采用二维渗流模型模拟UO₂燃料晶界气泡网络的演化及与燃料棒内自由空间连通的释放过程。研究结果表明,渗流模型预测沿芯块径向的裂变气体释放份额在芯块中间部分出现局部峰值,并随着时间向芯块外侧推进,与辐照试验观察到不同燃耗下径向裂变气体分布现象定性符合。因此,本研究建立的渗流模型能够从机理上解释此前扩散模型未能预测的UO₂燃料裂变气体释放份额沿径向非单调分布现象。      

α-U中裂变气体和合金元素相互作用的第一性原理研究

<正>金属燃料在使用过程中经历着剧烈的演变过程,制约其高燃耗的关键问题有:裂变气体释放和燃料的肿胀;燃料成分的重布,主要是指合金元素重分布;裂变产物的迁移及其对包壳的腐蚀。随着燃耗的升高,产生大量的裂变气体会影响合金元素的重布及其析出行为。本工作运用基于密度泛函理论的第一性原理计算程序VASP研究了α-U中裂变气体氙(Xe)和合金元素锆(Zr)和     

Modeling of fission gas swelling in LWR UO2 fuel under irradiation

An understanding of the behavior of fission gas in uranium dioxide (UO₂) fuel is necessary for the prediction of the performance of fuel rods under irradiation. A mechanistic model for matrix swelling by the fission gas in LWR UO₂ fuel is presented. The model takes into account intragranular and intergranular fission gas bubbles behavior as a function of irradiation time, temperature, fission rate and burn-up. The intragranular bubbles are assumed to be nucleated along the track of fission fragments, which play the dual role of creator and destroyer of intragranular bubbles. The intergranular bubble nuclei is produced until such time that a gas atom is more likely to be captured by an existing nucleus than to meet another gas atom and form a new nucleus. The capability of this model was validated by a comparison with the measured data of fission gas behavior such as intragranular bubble size, bubble density and total fuel swelling. It was found that the calculated intragranular bubble size and density are in reasonable agreement with the measured results in a broad range of average fuel burn-ups 6–83 GW d/tU. Especially, the model correctly predicts the fuel swelling up to a burn-up of about 70 GW d/tU.     

Computer modeling of steady state fission gas behavior in carbide fuels

A model for the simulation of long-term, steady-state fission gas behavior in carbide fuels is formulated. It is assumed that fission gas release occurs entirely through gas atom diffusion to grain boundaries and cracks. Fission gas bubbles are assumed to remain stationary and to grow as the net result of gas atom precipitation into the bubbles from the matrix solid and gas atom re-solution from the bubbles into the matrix. Furthermore, assuming that local gas atom redistribution process in the immediate neighborhood of a bubble is very rapid, the bubble size is assumed to correspond to the equilibrium size that maintains exact balance between the rate of gas atom re-solution and that of gas atom precipitation.The model also treats the effect of attachment between bubbles and second-phase precipitates; the experimentally observed faster growth rate of precipitate bubbles is simulated using a reduced re-solution parameter for precipitate bubbles. With the grain matrix assumed to be spherical, the model allows the computation of the radial distribution of the intragranular bubbles and the gas atom concentration in the matrix.The flux of gas atoms arriving at the grain boundary is computed. The continual growth of grain boundary bubbles, resulting from the accumulation of gas atoms on the grain boundary, leads to grain boundary interlinkage and all gas atoms that subsequently reach the grain boundary are assumed to be released. Similarly, all gas atoms generated following the interlinkage of intragranular bubbles are also assumed to be immediately released.Application of the model indicates that fission gas swelling is largely due to intragranular bubbles. Grain boundary bubbles, although very large in size, contribute little to fission gas swelling and the contribution from gas atoms in solid solution in the matrix is even less significant.Physical parameters entering the model were assigned numerical values that closely represent the physical characteristics of the irradiation samples. Careful comparisons between the results of sensitivity studies and the experimental data readily identify the re-solution parameter to have the strongest influence on the results predicted by the code and that the grain size, and not the temperature, is the dominant factor affecting gas release.When allowance is made for the uncertainties of the experimental data, the predicted fission gas swelling also correlates well with experiment. The spread in the fuel swelling data, however, indicates that fuel cracking, and not fission gas swelling alone, very often contributes significantly to the fuel external dimensional changes. The linear fission gas swelling rate prediceted by the model exhibits almost a linear variation with temperature. This result correlates well with the linear swelling rate obtained from experimental swelling data if immersion density data alone are used, in order to eliminate the sources of uncertainties associated with fuel cracking.     

Radiation re-solution of fission gas in uranium dioxide and carbide

Calculations have been performed to estimate the removal rate of fission gas atoms from bubbles due to collisions with energetic fission fragments and recoil cascades. The efficiency of this process was found to be higher than estimated earlier, but is still too low to be responsible for the experimental observations of fission gas bubble destruction during irradiation of oxide fuel. An irradiation experiment to investigate the interaction of fission spikes with free surfaces has enabled a simple theory to be developed which can explain the shrinkage of bubbles and pores by the surface relaxation of a shock wave produced by the passage of a fission fragment. This mechanism occurs in oxides but not carbides because of the faster dispersion of the fission fragment energy and provides the major reason for the difference in gas bubble distributions in oxide and carbide fuel. This process, however, does not remove gas atoms from the bubbles. Since high levels of apparently diffusive fission gas release are observed in oxides, the “effective solubility” of the fission gases required for this release must be sought in phenomena other than the fission spike.     

Re-solution effects and fission gas swelling in UO2

A fission gas swelling model is proposed which enables one to calculate swelling in the vicinity of grain boundary networks and in imperfection-free regions. The grain boundary swelling requires a knowledge of the gas accumulation and the reaction-rate at the boundary. The gas accumulation was calculated by deriving a modified form of Fick's second law wherein it was assumed that because of re-solution effects the in-pile diffusion coefficient can be described as a function of the gas concentration but is independent of the actual operating time. Reaction-rates for bubbles at grain boundaries were derived in the manner discussed by de Jong and Koehler in their treatment of vacancy clustering. The results indicate that there is a grain size of about 10⁻⁴ cm for which the swelling is a maximum, which increases somewhat with irradiation temperature and with depletion at a constant temperature. The results enable one to predict the swelling and the mean radii of both intergranular and intragranular bubbles. Mean bubble radii predicted using the re-solution swelling model are in reasonable agreement with radii obtained from electron micrographs of irradiated UO₂ fuel samples. It is argued that gas bubble migration is the predominant means by which gas atoms arrive at grain boundaries at irradiation temperatures above about 900°C.     

The migration of intragranular fission gas bubbles in irradiated uranium dioxide

The mobility of intragranular fission gas bubbles in uranium dioxide, irradiated at 1600–1800°C, has been studied following isothermal annealing at temperatures below 1600°C. The intragranular fission gas bubbles, average diameter approximately 2 nm, are virtually immobile at temperatures below 1500°C. The bubbles have clean surfaces with no solid fission product contamination and are faceted to the highest observed irradiation temperature of 1800°C. This bubble faceting is believed to be a major cause of bubble immobility. In fuel operating below 1500°C the predominant mechanism allowing the growth of intergranular bubbles and the subsequent gas release must be the diffusion of dissolved gas atoms rather than the movement of entire intragranular bubbles.     

The fission gas bubble distribution in a mixed oxide fast reactor fuel pin

The fission gas bubble distribution has been studied in a mixed oxide fast reactor fuel pin irradiated in DIDO MTR to 2.8% burn-up at centre and surface temperatures of 2000 and 1000°C. The intragranular fission gas bubbles are very small (<6 nm diameter) and this is a consequence of the high re-solution rate at fast reactor ratings. The bubbles nucleate heterogeneously and linear arrays of bubbles, due to nucleation on fission tracks, are observed up to irradiation temperatures of 1900°C. At 1980°C ~4% of the fission gas produced is present in intragranular bubbles. There is no definite evidence for gas bubble mobility or coalescence. Apart from any effects of columnar grain growth fission gas release in fast reactor fuel pins seems to occur predominantly by the diffusion of single gas atoms, at least up to irradiation temperatures of 2000°C.     

The growth of gas-bubbles by coalescence in solids during continuous gas generation

In order to get some insight into the mode of gas bubble growth during the high temperature irradiation of solids, we have calculated the distributions of bubbles grown by coalescence. The calculations are based on Gruber's coalescence model for annealing which we have extended to the case of an irradiation induced gas generation. The dominant assumption is that the continuously generated inert gas atoms go into new bubbles distributed around small radii. The results show that the bubble growth rate is markedly increased by the irradiation induced gas generation and that the calculated bubble distribution profile exhibits a double peak as often observed experimentally.     

Investigations on swelling and fission gas behaviour in uranium dioxide

UO₂ irradiated at temperatures between 1000 and 2100 K was investigated with respect to fission gas behaviour and swelling. The amount of fission gas was measured in three steps as released fission gas, fission gas retained in bubbles and pores, and fission gas in the fuel matrix. The retained fission gas reaches concentrations up to $\text{1.6 × 10}{}^{\mathrm{?2}}$ gas atoms per uranium atom at temperatures below 1250 K and decreases with increasing temperature. The swelling was evaluated by measuring the volume changes and by immersion density measurements. The maximum fission gas swelling without extensive bubble migration is about 20% at 2000 K. It diminishes to about 5% at 1250 K.     

Temperature and dose dependence of fission-gas-bubble swelling in U3Si2

Large fission gas bubbles were observed during metallographic examination of an irradiated U₃Si₂ dispersion fuel plate (U0R040) in the Advanced Test Reactor (ATR). The fuel temperature of this plate was higher than for most of the previous silicide-fuel tests where much smaller bubble growth was observed. The apparent conditions for the large bubble growth are high fission density (6.1 × 10²¹ f/cm³) and high fuel temperature (life-average 160 °C). After analysis of PIE results of U0R040 and previous ANL test plates, a modification to the existing athermal bubble growth model appears to be necessary for high temperature application (above 130 °C). A detailed analysis was performed using a model for the irradiation-induced viscosity of binary alloys to explain the effect of the increased fuel temperature. Threshold curves are proposed in terms of fuel temperature and fission density above which formation and interconnection of bubbles larger than 5 μ are possible.