合金元素对氢化锆制备及其原位氧化行为影响研究

Zirconium hydride is one of the ideal neutron moderators for the space reactor power. However, the hydrogen loss always exists due to the higher operating temperature of the moderator parts. Fortunately, synthesizing oxide film on the surface of such parts is an effective way to slow down the speed of hydrogen loss. In-situ oxidation process has been commonly used to synthesize the hydrogen layer on ZrHx substrate because of its low preparation temperature(﹤600℃) and its suitability for the special-shaped pieces. We find in the previous reseach that the oxide film on ZrHx substrate synthesized by in-situ oxidation process has a certain hydrogen resistance performance, but such resistance performance can not meet the requirements of the special usage due to the permeability of the film and the micro-crack defects. It is found after analysis that the black dense tetragonal phase in the inner layer and white loose monoclinic phase in the outer layer coexist in the oxide film and it is the formerplaying a major role for hydrogen resistance. The tetragonal phase is a metastable phase at low temperature and will be inevitably transformed into the monoclinic phase during the oxidation which lead to the generation of micro-crack defects in the oxide films. This project intends to study the effect on the phase transition by adding the alloying elements such as Y and Ce during the hydrogenation process of zirconium alloy by establishing the thermodynamic model of the Zr-M(Nb、Y、Ce)-H-O system andapplying relevant software and research the influence of oxide film synthesized through in-situ oxidation process by adding the above alloying elements by adopting the first-principles calculation combined with experimental study to determine the type and addition amount of the alloying elements and to lay the foundation for the application of Zirconium hydride as moderator.

氢化锆是空间反应堆电源理想的中子慢化材料，但慢化剂部件工作温度较高而存在氢损失，在其表面制备氧化膜是降低失氢速度的有效途径。原位氧化工艺因其制备温度低（﹤600℃）、适合异型件而成为ZrHx阻氢层的常用制备方法。前期研究发现，ZrHx表面原位氧化获得的氧化膜有一定阻氢性能，但膜层不致密，存在裂纹缺陷，阻氢性能无法满足使用要求。经分析，发现氧化膜中四方相与单斜相共存，外层以白色疏松的单斜相为主，内层以黑色致密的四方相为主，起阻氢作用的主要为黑色致密四方相。四方相在低温时为亚稳相，随氧化进行必然会向单斜相转变，而使氧化膜出现裂纹。本项目拟通过建立Zr-M(Nb、Y、Ce)-H-O热力学模型，应用相关软件，研究添加Y、Ce等合金元素对锆合金氢化过程中相转变规律的影响，并采用第一性原理计算结合实验研究合金元素对原位氧化制备氧化膜的影响，确定合金元素的种类和添加量，为氢化锆作为慢化剂应用奠定基础。

氢化锆由于其较高的热稳定性、高的氢密度、低的中子俘获截面、良好的导热性能和负的温度反应系数成为微型核反应堆的理想慢化材料，但在使用温度下，由于ZrH-H2平衡会导致氢损失，从而导致其慢化性能降低和服役寿命缩短。为解决使用过程中氢的损失问题，本项目开展合金元素对氢化锆制备及其原位氧化行为影响研究。采用第一性原理、声子谱计算和Debye模型相结合，根据相图和热力学实验数据进行参数优化计算，收集、评估并修正Zr-Y和Zr-H二元热力学数据库，结合优化得到的Y-H体系数据库，外推建立Zr-Y-H三元系热力学数据库。设计制备出无杂质污染、合金成分准确均匀锆钇合金；通过高温等温氢化-相变区等压氢化-终点氢化三段氢化工艺，制备无裂纹的含Y氢化锆块体。Y合金元素具有晶粒细化作用，抑制了ε-ZrHx孪晶组织的形成，缓解相变应力。采用CO2原位氧化法在含10wt%Y氢化锆表面制备出致密完整的膜层。研究含钇氢化锆在CO2氧化气氛中的失氢行为。Y在合金中对相结构的影响在于层中存在Y2O3抑制t-ZrO2向m-ZrO2的相转变。Y的添加既提高氢化锆的抵抗形变破坏和裂纹扩展的能力，利于抑制裂纹的形成；又同时稳定氢化锆基体和表面氧化层，抑制氢化锆的分解失氢，且同时阻止氢向外扩散，抑制氢化锆在工作温度下的氢损失。本项目的完成为制备慢化性能好、机械性能强、可靠性高、寿命长的无裂纹氢化锆慢化剂部件奠定了基础，对发展微型核反应堆电源系统具有重要的实际价值。