Dissolution of actinide dioxides, including neptunium dioxide (NpO₂(s)), is paramount for the prediction of the environmental fate of nuclear materials. Quantifying dissolution rates, as well as understanding qualitative dissolution mechanisms, informs performance assessment for geologic disposal of spent nuclear fuel and management of legacy radioactive waste. The aim of this research was to measure the dissolution rate of nanocrystalline NpO₂(s), produced through legacy nuclear waste processing, under oxidizing conditions, as well as to characterize surface alteration to the material. The solid phase was characterized using electron microscopy techniques (SEM/STEM) and X-ray photoelectron spectroscopy (XPS), indicating preferential dissolution of Np-hydroxide contained in the grain boundaries of NpO₂(s) and fragmentation of grains from the matrix. The oxidative dissolution was monitored over 40 weeks, yielding a two-step kinetic dissolution model involving hydration of NpO₂(s) and subsequent oxidation and dissolution of the hydroxide phase. The proposed dissolution models for nanocrystalline NpO₂(s) suggest that microstructural features such as grain boundaries are key factors affecting dissolution, including release of colloidal particles, and ultimately, environmental fate and transport of nuclear materials.

中文翻译：

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晶界促进了传统废物处理过程中纳米晶NpO2的溶解

包括二氧化n（NpO ₂（s））在内的act系元素二氧化物的溶解对于预测核材料的环境命运至关重要。量化溶出速率以及理解定性溶出机制，有助于对乏核燃料进行地质处置和遗留放射性废物进行管理的性能评估。这项研究的目的是测量纳米晶NpO ₂的溶解速率这些物质是在氧化条件下通过传统核废料处理过程产生的，并表征了材料的表面变化。使用电子显微镜技术（SEM / STEM）和X射线光电子能谱（XPS）对固相进行了表征，表明NpO ₂（s）晶界中所含的Np-氢氧化物优先溶解，并且晶粒从基体中碎裂。在40周内监测氧化溶解，得出两步动力学溶解模型，其中包括NpO _2的水合以及随后的氢氧化物相的氧化和溶解。拟议的纳米晶NpO ₂溶解模型（s）表明，诸如晶界之类的微观结构特征是影响溶解的关键因素，包括胶体颗粒的释放，以及最终的环境命运和核材料的运输。