添加Al2O3和SiO2的大晶粒UO2芯块制备研究

研究了Al2O3和SiO2添加剂对UO2芯块晶粒尺寸的影响.结果表明:加入少量的Al2O3和SiO2,可有效促进烧结过程中UO2芯块的晶粒度长大,过量加入则会阻碍烧结过程中UO2芯块的致密化;在添加量一定的情况下,添加不同比例的Al2O3和SiO2,对芯块晶粒尺寸有较大影响,只添加SiO2,对芯块晶粒尺寸影响不大,Al2O3添加量增加,芯块晶粒尺寸随之增加;添加Al2O3和SiO2促进UO2芯块晶粒长大的机制是在烧结期间发生了液相烧结.     

添加微量元素制备大晶粒UO_2芯块

介绍了添加微量元素制备大晶粒UO2芯块的研究情况。添加的三种微量元素是:普通级Al2O3／SiO2、纳米级Al2O3／SiO2和Cr2O3。得出了阶段性的结论:添加这三种微量元素均促进UO2芯块晶粒的增加。控制适当的添加量,其性能完全满足现有的技术条件。     

UO2?-Er2O3燃料芯块制备技术初步研究

对Er₂O₃质量分数为4.32%的UO₂-Er₂O₃可燃毒物燃料芯块的制备技术进行了初步研究。通过对比不同工艺条件（混料、成型、烧结）下,芯块的外观完整度、密度、晶粒度等性能,初步得到了UO₂-Er₂O₃燃料芯块的制备技术。试验表明:干法球磨混合6?h,添加5‰的聚乙烯醇（PVA）,300~350?MPa压力下冷压成型,1700~1750℃、H₂气氛中烧结2~3?h,可得到外观完整、密度大于等于95%理论密度（T.D.）、晶粒尺寸大于8?μm的UO₂?-Er₂O₃燃料芯块。      

大晶粒UO2燃料芯块和试验燃料组件的设计与制造

本工作涉及大晶粒UO2燃料芯块的研究、试验燃料组件的设计与制造。所谓大晶粒是在UO2粉末中分别添加Al2O3／SiO2、Cr2O3粉末，烧结后形成了大晶粒UO2燃料芯块，它能有效降低裂变气体释放、减少燃料棒内压、减少芯块和包壳的PCI作用，     

大晶粒UO2燃料芯块堆内试验组件的研制

为了开发高性能的压水堆燃料,研制了大晶粒燃料芯块。试验燃料芯块具有高的235U富集度、小直径和大晶粒尺寸的特点。通过堆内辐照试验可以对不同制造工艺的燃料芯块进行评价和筛选,以便确定燃料制造工艺。为了在中国原子能科学研究院池式研究堆中随堆考验,设计了一种试验组件,包含四根双包壳的燃料棒。双包壳燃料棒是在外包壳内装入两根单包壳燃料棒。试验组件直接由反应堆一次循环水冷却,不设专门的冷却回路。试验组件上安装了多种堆芯测量传感器,包括燃料中心温度热电偶、自给能中子探测器和冷却剂出、入口温度热电偶,可以在线监测燃料试验参数。描述了大晶粒UO2燃料芯块的研制、试验燃料组件的研制和检验。     

UO_2-Er_2O_3燃料芯块的烧结工艺研究

对UO_2-Er_2O_3可燃毒物燃料芯块的烧结工艺进行研究。试验表明:烧结过程中选择生坯密度在55%~60%理论密度的生坯芯块,在1700~1750℃,H2气氛中烧结2~3 h,可得到完整度、密度、晶粒尺寸等性能满足要求的燃料芯块。     

Gd2O3-UO2芯块晶粒尺寸的影响因素研究

用光学显微镜和图象分析仪研究了影响Gd2O3-UO2芯块晶粒尺寸的因素。对烧结气氛、混料方式、芯块在烧结炉中的位置、以及在UO2芯块中添加U3O8、草酸铵和助烧剂（Al、Ti、V的氧化物）等给Gd2O3-UO2芯块晶粒尺寸带来的影响进行了研究。结果表明,球磨工艺、添加助烧剂和微氧化气氛烧结等都有利于芯块的晶粒生长,晶粒大小分布均匀;添加U3O8和草酸铵对芯块的晶粒生长无明显的影响。     

掺杂Cr2O3对UO2芯块烧结行为的影响

提高燃料燃耗的一个有效手段是通过增大UO₂晶粒尺寸来减少元件内部气体压力,在大晶粒UO₂芯块中,裂变气体到达晶界表面的距离增加,因而裂变气体的释放速率降低,元件内部气体压力的增高缓慢。本文研究了添加Cr₂O₃对UO₂晶粒尺寸的影响。对纯UO₂、添加0.5% Cr₂O₃及5% Cr₂O₃ 3种配方的芯块进行了试验,在5%H₂Ar保护下,以10 ℃/min和5 ℃/min的升温速率升温至1 700 ℃,然后烧结2 h或4 h,对比纯UO₂芯块与添加Cr₂O₃的芯块发现,添加Cr₂O₃可有效增大晶粒尺寸;较长的烧结时间可促进晶粒长大;较低的升温速率也使晶粒长大。烧结过程产生液相烧结,液相浸润晶粒边界,促进晶粒长大。     

Gd2O3-UO2芯块中添加U3O8研究

Gd2O3-UO2可燃毒物燃料是近年来核电站采用较为普遍的可燃毒物之一。传统观点认为在芯块制造过程中添加U3O8粉末会降低芯块的烧结密度．本文研究了在Gd2O3-UO2芯块基体密度-94％T．D的基础上．添加不同品比例由AUC煅烧得到的U3O8粉末,经成型,H2/H2O气氛中1750℃烧结后表明．随着U3O8加入量的增加,芯块的密度也随之增加,U3O8添加量大于40wt％时芯块的密度达到-97％T.D．,U3O8的加入相当于起到了助烧剂的作用．这一现象和传统的U3O8降低芯块密度的观点正好相反．而芯块的平均晶粒尺寸-8μm。     

Al2O3覆盖层制备工艺对YSZ涂层耐熔盐腐蚀性能的影响

高密度石墨(HDG)由于其良好的抗热震性和高温强度,被认为是极具应用前景的坩埚候选材料之一。然而,熔盐电解法处理乏燃料过程中,高密度石墨坩埚会与高温氯化物熔盐、乏燃料等具有腐蚀性的介质直接接触,从而对坩埚造成腐蚀,影响坩埚的稳定性和使用寿命。坩埚表面等离子喷涂的氧化钇稳定的氧化锆(YSZ)涂层被证实具有优异的性能,但是涂层本身所存在的孔洞、裂纹以及液滴等缺陷严重制约了YSZ涂层在高温下的性能。因此,通过密封处理方法来提高涂层的致密度显得尤为迫切。本文利用磁控溅射(PVD)法和金属有机分解(MOD)法在等离子喷涂的氧化钇稳定的氧化锆(YSZ)涂层表面制备了Al₂O₃覆盖层,研究了这两种方法制备的Al₂O₃覆盖层对YSZ涂层在NaCl-KCl熔盐中的抗腐蚀性能的影响,并利用SEM、EDS、XRD等对腐蚀前后涂层的形貌以及微观结构进行了详细表征。结果表明：PVD法制备的Al₂O₃覆盖层主要分布于YSZ涂层表面,使其表面致密度得到一定程度提升,但涂层内部的致密度以...     

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.     

Phase studies in the UO2-Gd2O3 system

The incorporation of gadolinium directly into nuclear fuel is important regarding reactivity compensation, which enables longer fuel cycles. The incorporation of Gd₂O₃ powder directly into the UO₂ powder by dry mechanical blending is the most attractive process, because of its simplicity. Nevertheless, processing by this method leads to difficulties while obtaining sintered pellets with the minimum required density. This is due to the bad sintering behavior of the UO₂-Gd₂O₃ mixed fuel, which shows a blockage in the sintering process that hinder the densification process. Minimal information exists regarding the possible mechanisms for this blockage and this is restricted to the hypothesis based on the formation of a low diffusivity Gd rich (U,Gd)O₂ phase. The objective of this investigation was to study the phase formation in this system, thus contributing to clarifying the causes of the blockage. Experimental evidence indicated the existence of phases in the (U,Gd)O₂ system that revealed structures different from the fluorite-type UO₂ structure. These phases appear to be isostructural to the phases observed in the rare earth-oxygen system.     

Al2O3样品制备技术及^26Al的AMS测量

报道了用于加速器质谱计^26Al分析的Al2O3的制备流程及^26Al的测量过程。制备的空白样品^26Al/^27Al比值＜10^－13,显示出同量异位素^26Mg干扰小。制备流程是成功的。     

Laser-joined Al2O3 and ZrO2 ceramics for high-temperature applications

A laser process is presented that has been specially developed for joining oxide ceramics such as zirconium oxide (ZrO₂) and aluminium oxide (Al₂O₃). It details, by way of example, the design of the laser process applied for to producing both Al₂O₃-Al₂O₃ and ZrO₂-ZrO₂ joints using siliceous glasses as fillers.The heat source used was a continuous wave diode laser with a wavelength range of 808-1010 nm. Glasses of the SiO₂-Al₂O₃-B₂O₃-MeO system were developed as high-temperature resistant brazing fillers whose expansion coefficients, in particular, were optimally adapted to those of the ceramics to be joined. Specially designed measuring devices help to determine both the temperature-dependent emission coefficients and the synchronously determined proportions of reflection and transmission.The glass-ceramic joints produced are free from gas inclusions and macroscopic defects and exhibit a homogenous structure. The average strength values achieved were 158 MPa for the Al₂O₃ system and 190 MPa for the ZrO₂ system, respectively.     

Low-temperature diffusion in inert-gas bombarded Al2O3

Previous work on diffusion in inert-gas bombarded Al₂O₃ has revealed the presence of four diffusion processes, of which two take place well below the temperatures for self-diffussion, one agrees with self-diffusion, and one occurs at temperatures well above those for self-diffusion. The present work serves to explore in greater detail the two low-temperature processes. It is shown that the first, which is found in -Al₂O₃, Al(OH)₃, and γ-Al₂O₃beginning at about 100° C, is consistent with a range of ΔH's of 28 to 50 kcal/mole. The mechanism of the process is hinted at by the fact that it overlaps the temperatures both for Al(OH)₃decomposition and for point-defect motion in -Al₂O₃; the correlation with point defects is believed, however, to be the more significant. The second process, which is found only in -Al₂O₃ beginning at 500–650° C, implies an essentially single ΔH lying between 69 and 79 kcal/mole. It was suggested previously by Matzke and Whitton on the basis of electron diffraction that the process could be attributed to the amorphous-crystalline transition of -Al₂O₃. Further aspects of low-temperature diffusion in Al₂O₃ were revealed by comparing autoradiographs of specimens of -A₂O₃which were bombarded to various doses and then either heated to 850° C or immersed in unheated 12N NaOH. Thus regions exposed to a high dose and which would be expected to be amorphous, had an increased sticking factor, a greater tendency to lose gas during heating, and an enhanced chemical reactivity.     

The effect of Y2O3 on the dynamics of oxidative dissolution of UO2

In this work, we have studied the impact of Y₂O₃ on the kinetics of oxidative dissolution of UO₂ and the consumption of H₂O₂. The second order kinetics of catalytic consumption of H₂O₂ on Y₂O₃ was investigated in aqueous Y₂O₃ powder suspensions by varying the solid surface area to solution volume ratio. The resulting second order rate constant is 10⁻⁸ m s⁻¹, which is of the same magnitude as for the reaction between H₂O₂ and UO₂. Powder experiments with mixtures of UO₂ and Y₂O₃ show that Y₂O₃ has no effect on the oxidative dissolution of UO₂, whereas the consumption of H₂O₂ seems to be slightly slower in the presence of Y₂O₃ and H₂ respectively. UO₂ pellets with solid inclusions of Y₂O₃ show a decrease in oxidative dissolution by a factor of 3.3 and 5.3 under inert and hydrogen atmosphere, respectively. The rate of H₂O₂ consumption is similar for all cases and is well in line with kinetic data from powder experiments. The effects of H₂ and Y₂O₃ on the oxidative dissolution of UO₂ under gamma irradiation are similar to those found in experiments with H₂O₂. No significant difference in dissolution between inert and reducing atmosphere can be observed for pure UO₂.     

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.     

The creep of UO2 fuel doped with Nb2O5

The creep of UO₂ containing small additions of Nb₂O₅ has been investigated in the stress range 0.5–90 MN/m² at temperatures between 1422 and 1573 K. The functional dependence of the creep rate of five dopant concentrations up to 0.8 mol% Nb₂O₅ has been examined and it was established that in all the materials the secondary creep rate could be represented by the equation /.εkT = Aσⁿexp(?Q/RT), where /.ε is the steady state creep rate per hour, Q the activation energy and A and n are constants for each material. It was observed that Nb₂O₅ additions can cause a dramatic increase in the steady state creep rate as long as the niobium ion is maintained in the Nb⁵⁺ valence state. Material containing 0.4 mol% Nb₂O₅ creeps three orders of magnitude faster than the pure material.Analysis of the results in terms of grain size compensated viscosity suggest that, like “pure” UO₂, the creep rate of Nb₂O₅ doped fuel is diffusion-controlled and proportional to the reciprocal square of the grain size. A model is developed which suggests that the increase in creep rate results from suppression of the U⁵⁺ ion concentration by the addition of Mb⁵⁺ ions, which modifies the crystal defect structure and hence the uranium ion diffusion coefficient.