核燃料芯块裂变气体释放行为模拟研究

基于ABAQUS对UO_2核燃料芯块裂变气体行为做了研究,对两个经典模型:ANS-5.4单阶段模型和两阶段Forsberg–Massih释放模型作了详细描述,重点研究了这两个模型在ABAQUS中的实现及模型与ABAQUS结合时的技术难点。将研究模拟结果与该模型在相应成熟软件中的结果作比较,吻合度很好,验证了基于ABAQUS二次开发方式模拟裂变气体行为方法的可行性。     

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。      

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。      

贝叶斯推断法标定核燃料裂变气体释放模型

针对模型参数不确定性影响裂变气体释放(FGR)机理模型预测精度的问题,构建了一套贝叶斯标定方法。利用FGR实验测量数据标定晶内气体扩散系数等模型参数并推断其后验概率分布,采用Kriging模型和主成分分析法提高贝叶斯推断效率。分析结果表明,标定后模型FGR计算结果的准确性显著提高,总体均方根误差降低约70%;5个模型参数后验分布标准差相比先验分布均有所减小,进而降低了FGR预测值的不确定度。     

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     

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.     

重水堆燃料元件裂变气体测量技术研究

根据重水堆燃料元件空腔体积小的特点,设计重水堆燃料元件裂变气体测量装置,开展裂变气体释放测量工艺、刺孔技术、裂变气体测量技术和裂变气体加压取样技术研究,确定工艺流程和参数,通过保压实验和准确度测量实验验证系统密封性和体积测量,建立了重水堆燃料元件裂变气体测量技术,实现重水堆燃料元件裂变气体测量。     

压水堆燃料棒辐照行为模型研究

针对压水堆燃料棒辐照行为,建立分析模型,包括热学模型、力学模型、裂变气体释放模型、包壳辐照生长和腐蚀模型。通过与试验数据的对比,验证了模型的有效性。该模型可用于评价燃料棒的结构完整性、开发性能分析软件。     

燃料元件破损监测裂变气体法的理论计算

及时发现燃料元件包壳破损，与核安全有着密切的关系。结合裂变气体分离沉降装置和片型静电沉降器进行理论公式的推导和计算，并将理论计算与试验结果进行的比较。试验是在４９－２反应堆元件破损监测小回路上进行的。由于裂变气体子代产物β衰变的计数率不但与试验小元件、反应堆运行参数、探测仪表的特性有关，而且与试验回路的特性和运行参数有着密切的关系。因此，该计算对建立新的试验回路，对设计新型的静电沉降器和对从事于反应堆元件破损监测的科技人员都有一定的参考价值。     

高燃耗下裂变气体释放行为研究与程序校验

采用燃料棒性能分析程序COPERNIC,针对哈尔登（Halden）测试燃料组件 （IFA）519.9 DK 辐照试验燃料棒辐照试验进行了计算分析,研究了高燃耗下裂变气体释放行为,并与试验数据进行了对比验证。结果表明,在燃耗达到约100 GW?d/t(U)的辐照过程中,该程序对裂变气体释放率的预测值与试验测量结果符合较好;程序未精确预测芯块孔隙率在高燃耗“边缘结构”内的演化过程,但不影响其对燃料棒辐照综合性能分析的准确性和合理性。      

Fission gas release from UO2 fuel during low-temperature irradiation

A new mathematical interpretation is presented of fission gas release from UO₂ fuel during low-temperature irradiation in terms of a defect trap model and the knock-out process. In the present model it is assumed that gas in an intermediate state exists side by side with the dissolved fission gas and that trapped in bubbles. The present model gives a satisfactory interpretation of the relative proportion of isotopes in the steady state fission-gas release. The dependence of the fission-gas release rate on the fission rate is also interpreted; regimes either proportional to the square of fission rate or proportional to fission rate are predicted, depending on the fission rate interval considered.     

Fission product release in high-burn-up UO2 oxidized to U3O8

Results of oxidation experiments on high-burn-up UO₂ are presented where fission-product vaporisation and release rates have been measured by on-line mass spectrometry as a function of time/temperature during thermal annealing treatments in a Knudsen cell under controlled oxygen atmosphere. Fractional release curves of fission gas and other less volatile fission products in the temperature range 800-2000 K were obtained from BWR fuel samples of 65 GWd t⁻¹ burn-up and oxidized to U₃O₈ at low temperature. The diffusion enthalpy of gaseous fission products and helium in different structures of U₃O₈ was determined.     

Reduction of UO2 by H2

The reactivity of H₂ towards UO₂²⁺ has been studied experimentally using a PEEK coated autoclave where the UO₂²⁺ concentration in aqueous solution containing 2 mM carbonate was measured as a function of time at p_H2∼40 bar. The experiments were performed in the temperature interval 74-100 °C. In addition, the suggested catalytic activity of UO₂ on the reduction of UO₂²⁺ by H₂ was investigated. The results clearly show that H₂ is capable of reducing UO₂²⁺ to UO₂ without the presence of a catalyst. The reaction is of first order with respect to UO₂²⁺. The activation energy for the process is 130 ± 24 kJ mol⁻¹ and the rate constant is k_298K=3.6×10⁻⁹ l mol⁻¹ s⁻¹. The activation enthalpy and entropy for the process was determined to 126 kJ mol⁻¹ and 16.5 J mol⁻¹ K⁻¹, respectively. Traces of oxygen were shown to inhibit the reduction process. Hence, the suggested catalytic activity of freshly precipitated UO₂ on the reduction of UO₂²⁺ by H₂ could not be confirmed.     

Kinetics of initial stage of sintering of UO2 and UO2 with Nd2O3 addition

The kinetics of initial stage sintering of UO₂ powder were reinvestigated, using Ar-10% H₂ atmosphere. The effect of the addition of neodynium oxide was studied. The results revealed that surface and grain boundary diffusion mechanisms act simultaneously. The values of activation energies were found to be $\text{48.48 ± 3.51 kcal/mole}$ in the temperature range 870–942°C and $\text{89.88 ± 9.87 kcal/mole}$ in the temperature range of 942–1030°C for UO₂, and $\text{115.61 ± 7.77 kcal/mole}$ in the temperature range 1030–1150°C for UO₂ + Nd₂O₃. An important decrease in the calculated diffusion coefficient occurs by the addition of Nd₂O₃.     

Optical properties of UO2 and PuO2

We perform first-principles calculations of electronic structure and optical properties for UO₂ and PuO₂ based on the density functional theory using the generalized gradient approximation (GGA) + U scheme. The main features in orbital-resolved partial density of states for occupied f and p orbitals, unoccupied d orbitals, and related gaps are well reproduced compared to experimental observations. Based on the satisfactory ground-state electronic structure calculations, the dynamical dielectric function and related optical spectra, i.e., the reflectivity, adsorption coefficient, energy-loss, and refractive index spectrum, are obtained. These results are consistent with the available experiments.     

Sintering diagrams of UO2

Ashby's method of constructing sintering diagrams has been modified to obtain relative contribution diagrams directly from the computer. It is endeavoured here to study the interplay of sintering variables and mechanisms and determine the factors that affect the participation of mechanisms in UO₂. By studying the physical properties, it emerges that the order of inaccuracies is small in most cases and do not affect the diagrams. On the other hand, even a 10% error in activation energies, which is quite plausible, would make a significant difference to the diagram. The main criticism of Ashby's approach is that the numerous properties and equations used, communicate their inaccuracies to the diagrams and make them unreliable. Our study has considerably reduced the number of factors that need to be refined to make the sintering diagrams more meaningful.     

Fission-induced densification of UO2

A new technique has been developed to study fission-induced densification and hot-pressing of UO₂ at very low temperatures without complications from fracturing or other concurrent thermal effects. Thin disks of UO₂ of 0.22 to 20% enrichment and differing microstructural stability, were irradiated in the core of the CP — reactor at temperatures below 200°C and at pressures from 1 atm to 20.7 MPa. Results indicate that the pressurizing medium, NaK, had penetrated the open porosity at high pressure and impeded densification. To rationalize this effect, the previously proposed models for radiationinduced densification are critically reviewed. Modifications to models involving thermal sintering and hot-pressing, and pore resolution appear the most tenable. The former mechanism leads to predictions that fission-induced hot-pressing can occur, and ex-reactor sintering and hot-pressing should correlate with in-reactor densification. The proximity of pores and grain boundaries is also stressed. The effect of NaK logging is rationalized by changes in pore surface energy and by stabilization of small pores aginst complete resolution.