多气泡对燃料颗粒应力分布的影响研究

利用有限元模拟方法建立了UO2弥散型燃料颗粒受内部多个气泡内压作用的模型,计算得到了燃料颗粒内部存在多个气泡时的应力分布结果。结果表明,当气泡沿x轴均匀排列时,y方向的最大正应力随气泡数量的增多而增大,且增大幅度逐渐减小,气泡对燃料颗粒内部最大正应力的影响存在极限;当存在多行气泡时,燃料颗粒内部x方向的最大正应力随气泡行数的增加而增大,y方向的最大正应力随气泡行数的增加而减小;气泡造成的燃料颗粒内部的应力集中效应随距径比的增大而减小。     

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。     

高燃耗下核燃料颗粒内部的应力状态及关键影响因素研究

高燃耗是先进反应堆堆芯的发展方向,高燃耗下核燃料内部微结构的精细化建模是燃料强度分析和性能评估的基础。本研究开发了高燃耗燃料颗粒微结构的自动化建模和力学计算程序,综合考虑燃料颗粒内气孔尺寸、位置的非均匀分布特征,系统分析了基体材料的力学性能、燃料颗粒间距、运行环境静水压力及裂变碎片损伤层对燃料颗粒开裂行为的影响规律。结果表明：燃料颗粒间距越大,燃料颗粒越不易开裂;裂变碎片损伤层的存在使得燃料颗粒开裂风险小幅增大;燃料颗粒内气孔尺寸、位置分布的非均匀性,会导致燃料颗粒从多处开裂,且颗粒在外层开裂的概率更大;开裂危险区普遍具有气孔尺寸较大且大气孔串联的特征;基体材料对燃料颗粒表面作用的约束压应力具有较大的波动性,但应力均值随燃料元件所受静水压力的增大而近乎线性增大;增大弥散燃料基体材料的弹性模量可在一定程度上抑制燃料颗粒的开裂行为;燃料元件所受静水压应力越大,燃料颗粒越不易开裂,而燃料颗粒间距缩小能削弱环境压应力的影响程度。本工作为高燃耗条件下弥散燃料安全评估及优化设计提供了分析方法及数值参考。     

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。     

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。     

钍基氟盐冷却高温堆TRISO包覆燃料颗粒结构优化分析

钍基氟盐冷却高温堆(Thorium-based Pebble Bed Fluoride Salt-cooled High-temperature Reactor,PBTFHR)作为第四代核反应堆的堆型之一,其燃料元件由TRISO(TRi-structural ISOtropic)包覆燃料颗粒组成,具有较好的中子性能和安全性。本工作采用SCALE 6.1程序开展PB-TFHR的临界和燃耗性能计算,结合PANAMA模型研究包覆燃料颗粒的破损率,分析了PB-TFHR中TRISO包覆燃料颗粒的kernel半径、包覆层的厚度和密度对堆芯中子学性能、裂变气体氪、氙和碘产量及包覆燃料颗粒破损率的影响,给出优化的包覆燃料颗粒结构,为其物理设计提供参考。研究发现:当保持包覆层的厚度和密度不变时,较大的kernel半径(≥0.01 cm)可使堆芯处于欠慢化区,且堆芯温度反应性系数均为负值;在相同的燃耗下,kernel半径越小,堆芯中裂变气体的生成量越少,且包覆颗粒的破损率越小;当保持包覆层密度不变,只改变包覆层的厚度时,疏松热解炭层和内致密热解炭层的厚度对keff有较大影响;而当保持包覆层厚度不变只改变包覆层的密度对keff影响较小。     

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。     

包覆燃料颗粒几何参数统计规律对破损率的影响

将包覆燃料颗粒的碳化硅包覆层看作模型中的承压壳,利用蒙特卡罗方法,将颗粒几何参数的统计分布应用到压力壳式模型,考虑颗粒包覆层几何参数的统计规律对破损率的影响。研究结果表明,包覆燃料颗粒核芯半径、疏松层厚度、内致密热解炭层厚度以及碳化硅层厚度等参数对破损率有较为明显的影响;其中,核芯半径和疏松层厚度是影响破损率相对关键的因素。     

球形燃料孔隙内汽泡生长与脱离

根据汽泡动力学原理及可视化试验结果,分别构建了球形燃料孔隙内的汽泡滑移和脱离模型,并对汽泡受力及传热进行分析,通过计算得到了汽泡滑移临界曲率半径、汽泡脱离半径的变化规律。汽泡滑移时的临界半径主要受颗粒大小影响,颗粒越大,汽泡的相对临界半径r_cr/R_p越小。颗粒半径对汽泡脱离半径的影响最大,颗粒半径越大,汽泡的相对脱离半径r_d/R_p越小。流速、压力、壁面过热度对汽泡脱离半径也有影响,其他条件相同时,流速越大,汽泡脱离半径越小;压力越大,汽泡脱离半径越大;壁面过热度越大,汽泡脱离半径越大。     

包覆燃料颗粒应力的有限元分析

高温气冷堆的燃料元件由包覆燃料颗粒弥散在石墨基体中组成。在反应堆运行过程中,辐照及各复杂的物理化学反应产生的应力会使包覆燃料颗粒发生破损,对包覆燃料颗粒进行应力分析是评价燃料元件和反应堆运行安全性能的主要内容之一。本文基于压力壳模式,主要考虑内压作用下的球形壳层应力及包覆燃料颗粒的非球形因素,用有限元法对应力进行了分析。     

Transmission electron microscopy investigation of irradiated U-7 wt%Mo dispersion fuel

The microstructural evolution of atomised U-7 wt%Mo alloy fuel under irradiation was investigated by transmission electron microscopy on material from the experimental fuel plates used in the FUTURE irradiation. The interaction layer that forms between the U(Mo) particles and the Al matrix is assumed to become amorphous under irradiation and as such cannot retain the fission gas in stable bubbles. As a consequence, gas filled voids are generated between the interaction layer and the matrix, causing the fuel plate to pillow and finally fail. The present analysis confirms the assumption that the U(Mo)-Al interaction layer is completely amorphous after irradiation. The Al matrix and the individual U(Mo) particles, with their cellular substructure, have retained their crystallinity. It was furthermore observed that the fission gas generated in the U(Mo) particles has formed a bubble superlattice, which is coherent with the U(Mo) lattice. Bubbles of roughly 1-2 nm size have formed a 3-dimensional lattice with a lattice spacing of 6-7 nm.     

钨中氦泡融合条件的分子动力学模拟研究

在托卡马克聚变装置中,钨偏滤器会受到低能高束流的氦等离子体冲刷,导致材料表面形成绒毛状纳米结构或针孔状表面损伤,使钨材料使用性能发生退化,影响等离子体的稳态运行。目前普遍认为,氦致表面损伤的形成与钨表面下氦泡的生长密切相关。钨受到氦等离子体辐照后会在材料的近表层形成高密度的小氦泡,它们可通过融合的方式长大,氦泡的融合是近表层大氦泡形成的关键环节。为了解氦泡的相对位置、温度、氦空位比(He/V)、氦泡初始间距对氦泡融合的影响,本文采用分子动力学方法模拟氦泡在金属钨中的融合过程。结果表明：氦泡的相对位置、温度、He/V、氦泡初始间距都会影响氦泡的融合,但影响的机理并不相同。其中,氦泡的相对位置是影响氦泡融合的关键因素,当氦泡沿〈100〉方向排列时,氦泡易发生融合,而沿 〈111〉方向排列则不易发生融合,其原因是氦泡附近存在各向异性的应力场。温度升高有利于氦泡体积得到更快、更充分的弛豫,进而促进氦泡发生融合。高He/V的氦泡具有较高的压力,更易发生融合。当温度为1 500 K时,2个He/V为3、半径为1 nm的氦泡之间的相互作用距离可达1.28 nm甚至更远,但它们发生融合的最大初始距离为0.96 nm。本研究可促进对钨中氦泡融合机理的理解,为钨中大氦泡的形成提供可能的解释。此外,本研究结果可为大尺度模拟（如动力学蒙特卡罗、团簇动力学）提供相关输入参数用于研究高密度氦泡的长时间演化。     

文丘里式气泡发生器气泡碎化特性研究

熔盐堆在运行过程中须不断地去除氙等气体裂变产物。熔盐堆除气系统中气泡发生器的作用是通过向回路中注入一定量的直径为0.5 mm的小气泡,在扩散作用下吸收熔盐中的氙,最终气泡被分离出来,达到除氙的目的。在橡树岭国家实验室设计的基础上,本文为钍基熔盐研究堆设计气泡发生器,并在专门建造的水回路中对其工作特性进行了可视化研究。利用高速摄像系统跟踪气泡的运动和碎化过程,分析气液相流速对碎化后气泡尺寸的影响。结果表明：在实验条件下,当气体流量一定时,气泡尺寸随液体流量的增大而减小;当液体流量一定时,气泡尺寸随气体流量的增加而增大。     

On the rate determining step in fission gas release from high burn-up water reactor fuel during power transients

The radial distribution of grain boundary gas in a PWR and a BWR fuel is reported. The measurements were made using a new approach involving X-ray fluorescence analysis and electron probe microanalysis. In both fuels the concentration of grain boundary gas was much higher than hitherto suspected. The gas was mainly contained in the bubble/pore structure. The factors that determined the fraction of gas released from the grains and the level of gas retention on the grain boundaries are identified and discussed. The variables involved are the local fuel stoichiometry, the amount of open porosity, the magnitude of the local compressive hydrostatic stress and the interaction of metallic precipitates with gas bubbles on the grain faces. It is concluded that under transient conditions the interlinkage of gas bubbles on the grain faces and the subsequent formation of grain edge tunnels is the rate determining step for gas release; at least when high burn-up fuel is involved.     

The effect of radiation induced gas on the stress rupture properties of an aluminium-lithium alloy

The changes in stress-rupture properties of an aluminium-lithium alloy induced by the presence of inert gas bubbles after neutron irradiation have been investigated in the temperature range of 175 °C to 300 °C. A bubble distribution with a mean diameter of 47 Å at an average spacing of 1400 Å produces a decrease in the minimum creep rate of two orders of magnitude below that for the bubble-free material under the same test conditions. For very fine bubble distributions and distributions which contain a narrow size range of large bubbles, a breakaway to a very high stress-sensitivity at low stresses occurs. This is absent in alloys which contain stronger bubble obstacle arrays which include a wide range of bubble sizes. All inert gas concentrations reduce creep ductility of aluminium-lithium alloy but the severity of embrittlement increases markedly with gas concentration and testing temperature.     

The growth of sub-critical bubbles on grain boundaries

A coalescence model is developed for the production of creep cavity nuclei. Small pre-existing bubbles situated in the grain boundary are swept by the dislocations responsible for grain-boundary sliding. The bubble motion induced by the dislocations results in bubbles continuously impinging and coalescing. The distribution of bubble sizes is calculated and the nucleus spacing is found for that part of the distribution with sizes above the critical radius. The spacing of nuclei is related to the high-temperature creep ductility of irradiated metals.     

Simulation of non-equilibrium fission gas behaviour during fast thermal transients

A code for predicting the behavior of non-equilibrium fission gas in oxide fuel elements undergoing fast thermal transients is developed. A new variable, the equilibrium variable (EV), is introduced which, together with bubble radius r, completely specifies a fission gas bubble with respect to its size and equilibrium condition. The code is used to simulate the measurements in two TREAT transients with peak temperatures of 2477 and 2000 K. The computations are in fair agreement with the observations for bubbles smaller than 964 Å in diameter, but not for the larger bubbles. In all simulations, bubbles that grew during the heat-up phase of the transient were found to be “frozen” at a larger than equilibrium size during the cooldown phase of the transient. This phenomenon can significantly affect posttransient swelling and gas release. It is also found that the assumption of equilibrium can introduce considerable error in the computed bubble distribution, swelling and gas release at the end of as well as at post fast thermal transients; for example, the non-equilibrium model releases more gas. The code is also used to simulate the H3 TREAT transient as analyzed by Stahl and Patrician (initial temperature equal to 785 K with a maximum of 2393 K attained in 4.2 seconds, maximum thermal gradient of 10 000 K/cm and grain diameter of 4 to 10 μm) using the ideal gas as well as the Van der Waal's equations of states. The gas inventory at the start of the transient is assumed to be at equilibrium in the smallest radius group (6.2 Å), and the initial bubble concentration is assumed to be 1.2 × 10¹⁹/cc. Release rate is found to be strongly dependent upon grain size and initial bubble concentration; a 4 micron diameter grain releases about 95% of the gas retained at the start of the transient, while 6 and 10 micron grains release 68% and 20% respectively. When the initial bubble concentration is reduced by a factor of 16 for the 10 micron grain, fractional release increases to 62%. Gas release is found to result primarily from small bubbles ( ).