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。     

气冷堆用棱柱型弥散微封装燃料性能分析及优化研究

棱柱型弥散微封装燃料是将三重各向同性包覆（TRISO）燃料颗粒弥散于金属或陶瓷基体形成的颗粒增强复合燃料,具有良好的结构稳定性、裂变产物包容能力和辐照稳定性,是高温气冷堆中较具发展前景的燃料形式之一。本文提出将TRISO燃料颗粒弥散于SiC基体的棱柱型弥散微封装燃料设计方案,并基于有限元分析软件COMSOL建立了该燃料元件三维热流固耦合分析模型,初步实现了该燃料元件性能分析和优化设计。结果表明,棱柱型弥散微封装燃料元件的温度最大值位于燃料元件外侧,应力峰值位于冷却剂通道壁面,边距比为0.76~0.84、孔距比为0.68~0.75时燃料元件热应力最小。本文建立的棱柱型弥散微封装燃料性能分析方法和研究结论,可为后续该型气冷堆燃料元件设计提供指导和参考。      

弥散核燃料等效辐照肿胀计算模拟

本文将弥散核燃料芯体看作一种特殊的颗粒复合材料,利用细观计算力学的方法,假设燃料颗粒在芯体中周期性分布,建立了对芯体等效辐照肿胀进行计算模拟的有限元模型。考虑颗粒的辐照肿胀和基体材料的辐照硬化效应,分别建立了燃料颗粒和基体材料的应力更新算法,编制了用户材料子程序,在Abaqus软件中实现了芯体等效辐照肿胀的有限元模拟。计算分析了颗粒大小和体积含量对芯体等效辐照肿胀的影响,并得到了等效辐照肿胀的拟合公式。研究结果表明,影响芯体等效辐照肿胀的主要因素是颗粒的辐照肿胀和体积含量。     

研究堆降低铀浓缩度计划（RERTR）燃料研究开发进展

20世纪90年代中期开始的UMo弥散型燃料辐照试验结果表明，UMo燃料的辐照性能良好；但2003年发现，在高功率和高温时出现枕状肿胀。随后的深入研究表明，肿胀不是由于UMo燃料颗粒本身引起的，而是由于辐照使铝与燃料反应所形成的反应层产生的大气泡引起的。要继续成功开发很高铀密度的低浓铀燃料元件需要有详细的研究计划，这包括：燃料制造工艺开发、堆外性能研究、辐照、辐照后检查、燃料性能评价和模型以及程序的开发。可以通过几个途径克服肿胀问题：稍为改变燃料和基体材料的化学成分；用另一种材料取代铝基体，甚至取消铝基体，开发单片式燃料板。目前6个国家（阿根廷、加拿大、法国、韩国、俄罗斯、美国）正在合作进行上述工作。本文总结了RERTR．6辐照试验及相关实验的最新试验结果，讨论了到2010年开发和鉴定出很高铀密度的低浓铀燃料元件的途径。     

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。     

低富集U-Mo弥散型燃料小板RERTR-6辐照试验结果——Si对U-Mo／Al弥散燃料辐照行为影响的初步分析

辐照时Al基体中弥散的U-Mo燃料颗粒表面形成包覆的反应物。在一些辐照试验中,反应物和舢基体交界面处有气孔产生。受辐照条件的影响,气孔可能会长大并彼此连接形成连通的大气孔,严重时形成连续网络结构使燃料板出现不可接受的枕形肿胀。在美国和其他国家的辐照实验中都观察到了这种现象。冶金学和热动力学分析表明,Al基体中加入Si以及U-Mo燃料中添加Zr或Ti都可以提高U-Mo／Al弥散型燃料反应产物的稳定性。本文介绍了添加Si辐照试验的初步结果,即将适量的Si添加到Al基体中能有效降低U-Mo／Al燃料扩散反应和消除气孔形成。     

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。     

U_3Si_2-AI 弥散型燃料元件

U_3Si_2-Al 弥散型板状燃料元件是新发展的高铀密度的研究堆燃料.辐照和全堆芯试用表明:元件外形尺寸稳定;无裂变产物泄漏;直到燃耗达98%时,U_3Si_2颗粒肿胀都是裂变密度的线性函数;U_3Si_2与铝基体和铝包壳在制造和辐照中都是相容的。芯体铀密度最高达4.8g(U)/cm~3的 U_3Si_2-Al 燃料都是适合研究堆使用的低浓铀燃料.     

阿贡国家实验室单片式燃料研制

最初的U-Mo弥散型燃料元件已经最示出铀装载量方面的优越性，可以用它来成功地把部分研究堆燃料转换成低浓铀。另外，辐照实验显示出在高温下U-Mo弥散型燃料元件的芯体与铝基发生了相互反应。解决这些现象的潜在方法是使用燃料合金片代替铝基弥散型燃料。这种单片式燃料提供了比弥散型燃料更低的燃料一基体反应层和更高的铀密度。由于弥散型燃料基体的不足，单片式燃料元件生产需要研究新的制造方法。经过阿贡国家实验室研究人员的努力，已经得到一种可行的单片式燃料板生产方法，本文将介绍这种方法。     

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。     

Simulation of irradiation hardening of Zircaloy within plate-type dispersion nuclear fuel elements

Within plate-type dispersion nuclear fuel elements, the metal matrix and cladding attacked continuously by fast neutrons undergo irradiation hardening, which might have remarkable effects upon the mechanical behaviors within fuel elements. In this paper, with the irradiation hardening effect of metal materials mainly considered together with irradiation growth effect of the cladding, the three-dimensional large-deformation constitutive relations for the metal matrix and cladding are developed. The method of virtual temperature increase in the previous studies is further developed to model the irradiation swelling of fuel particles; the method of anisotropic thermal expansion is introduced to model irradiation growth of the cladding; and a method of multi-step-temperature loading is proposed to simulate the coupling features of irradiation-induced swelling of the fuel particles together with irradiation growth of the cladding. Above all, based on the developed relationship between irradiation growth at certain burnup and the loaded virtual temperatures, with considering that certain burnup corresponds to certain fast neutron fluence, the time-dependent constitutive relation due to irradiation hardening effect is replaced by the virtual-temperature-dependent one which is introduced into the commercial software to simulate the irradiation hardening effects of the matrix and cladding. Numerical simulations of the irradiation-induced mechanical behaviors are implemented with the finite element method in consideration of the micro-structure of the fuel meat. The obtained results indicate that when the irradiation hardening effects are introduced into the constitutive relations of the metal matrix and cladding: (1) higher maximum Mises stresses for certain burnup at the matrix exist with the equivalent plastic strains remaining almost the same at lower burnups; (2) the maximum Mises stresses for certain burnup at the cladding are enhanced while the maximum equivalent plastic strains are reduced; and (3) the maximum first principal stresses for certain burnup at the matrix or the cladding are lower than the ones without the hardening effect, and the differences are found to increase with burnup; and the variation rules of the interfacial stresses are similar.     

研究堆燃料的发展现状与前景

在过去33年中,国际降低研究和试验堆铀浓度计划已成功开发和应用了U₃Si₂-Al弥散型燃料。但由于U₃Si₂的抗辐照性能限制了它可能承受的运行温度与裂变密度,所以该燃料只适用于低功率密度的研究堆。U7Mo-Al弥散型燃料中的UMo颗粒与Al基体发生广泛的化学反应,将引起严重的肿胀与起泡问题。近年来,给U7Mo颗粒表面涂敷ZrN隔离层,获得防止反应的显著效果,使U7Mo-Al弥散型燃料有望应用于实践。U10Mo单片型燃料的芯体铀密度可达16g/cm³,辐照性能良好,但制造方法需进一步完善;应用中国核动力研究设计院改进的框架结构与轧制方法,能够控制UMo芯体与Al包壳具有相近的延伸率,从而可成功地轧制出合格的U10Mo合金单片型燃料板。     

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。     

弥散型燃料的裂变气体行为研究

探讨了弥散型燃料中对辐照肿胀有重要影响的裂变气体的行为机理。裂变气体原子聚集成气泡引起燃料相肿胀,气泡的尺寸分布是影响辐照肿胀的重要因素。决定气泡生长的裂变气体的行为机理主要有：裂变气体原子的产生和热扩散迁移,气泡的成核和聚合长大,气泡内气体原子的重溶,燃料相的辐照亚晶化等过程。燃料中各种尺寸的气泡浓度随时间的变化率可用气泡生长的动力学速率方程组来描述。当裂变密度较高时,辐照产生的缺陷引起燃料相的     

Reliability analysis of dispersion nuclear fuel elements

Taking a dispersion fuel element as a special particle composite, the representative volume element is chosen to act as the research object. The fuel swelling is simulated through temperature increase. The large strain elastoplastic analysis is carried out for the mechanical behaviors using FEM. The results indicate that the fission swelling is simulated successfully; the thickness increments grow linearly with burnup; with increasing of burnup: (1) the first principal stresses at fuel particles change from tensile ones to compression ones, (2) the maximum Mises stresses at the particles transfer from the centers of fuel particles to the location close to the interfaces between the matrix and the particles, their values increase with burnup; the maximum Mises stresses at the matrix exist in the middle location between the two particles near the mid-plane along the length (or width) direction, and the maximum plastic strains are also at the above region.     

Modeling of the heat transfer performance of plate-type dispersion nuclear fuel elements

Considering the mutual actions between fuel particles and the metal matrix, the three-dimensional finite element models are developed to simulate the heat transfer behaviors of dispersion nuclear fuel plates. The research results indicate that the temperatures of the fuel plate might rise more distinctly with considering the particle swelling and the degraded surface heat transfer coefficients with increasing burnup; the local heating phenomenon within the particles appears when their thermal conductivities are too low. With rise of the surface heat transfer coefficients, the temperatures within the fuel plate decrease; the temperatures of the fuel plate are sensitive to the variations of the heat transfer coefficients whose values are lower, but their effects are weakened and slight when the heat transfer coefficients increase and reach a certain extent. Increasing the heat generation rate leads to elevating the internal temperatures. The temperatures and the maximum temperature differences within the plate increase along with the particle volume fractions. The surface thermal flux goes up along with particle volume fractions and heat generation rates, but the effects of surface heat transfer coefficients are not evident.     

金属基弥散燃料元件失稳肿胀的静态弹塑性模型

针对金属基弥散燃料元件金属基体开裂导致的失稳肿胀,在不考虑粘塑性变形情况下建立了裂纹面的静态弹塑性模型,采用有限元模拟对静态弹塑性模型进行了验证。当金属基体发生全屈服后,其主要变形方式从弹性变形转变为塑性变形;根据金属基体的主要变形方式,分别建立金属基弥散燃料裂纹面的弹性变形模型和塑性变形模型;结合内应力与弯矩的平衡条件,获得了裂纹面弹塑性变形的临界转变条件。弹性变形模型和塑性变形模型的计算结果与有限元模拟结果符合较好,验证了金属基弥散燃料失稳肿胀的静态弹塑性模型的有效性。      

Amorphization of the interaction products in U–Mo/Al dispersion fuel during irradiation

The microstructures of the product resulting from interaction between U–Mo fuel particles and the Al matrix in U–Mo/Al dispersion fuel are discussed. We analyzed the available characterization results for the Al matrix dispersion fuels from both the out-of-pile and in-pile tests and examined the difference between these results. The morphology of pores that form in the interaction products during irradiation is similar to the porosity previously observed in irradiation-induced amorphized uranium compounds. The available diffraction studies for the interaction products formed in both the out-of-pile and in-pile tests are analyzed. We have concluded that the interaction products in the U–Mo/Al dispersion fuel are formed as an amorphous state or become amorphous during irradiation, depending on the irradiation conditions.