Kravchuk Dmytro V, Forbes Tori Z
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States.
ACS Mater Au. 2021 Aug 26;2(1):33-44. doi: 10.1021/acsmaterialsau.1c00033. eCollection 2022 Jan 12.
Understanding the alteration mechanisms of UO-based nuclear fuel has a range of practical implications for both short- and long-term storage of spent fuel rods and environmental ramifications for the mobility of radioactive material at the Chernobyl and Fukushima sites. The major identified alteration phases on the surface of nuclear waste are analogues of schoepite UO·2HO, studtite UO(O)·4HO, rutherfordine UOCO, and čejkaite NaUO(CO). While α-radiolysis has been shown to cause the ingrowth of uranyl peroxide alteration phases, the prevalence of uranyl carbonate phases on solid waste forms has not been mechanistically explained to date, especially since the alteration chemistry is largely affected by the high temperatures of the spent nuclear material. Herein, we demonstrate the first mechanistic link between the formation of the uranyl superoxide () phase, its reactivity at temperature ranges relevant to the spent nuclear fuel (40-350 °C), and its thermodynamic transformation into a potassium uranyl carbonate mineral phase, agricolaite K[UO(CO)], using thermogravimetric analysis, calorimetry, vibrational spectroscopy, and powder X-ray diffraction techniques. The thermodynamics data reveal the metastability of the uranyl superoxide phase through decomposition of the hydrogen peroxide within the solid-state lattice. Increasing the temperature does not result in the breakdown of the superoxide anion bound to the uranyl cation but instead enhances its reactivity in the presence of CO gas, resulting in potassium carbonate phases at intermediate temperatures (150 °C) and in uranyl carbonate phases at higher temperatures (350 °C).
了解铀基核燃料的蚀变机制对于乏燃料棒的短期和长期储存以及切尔诺贝利和福岛核事故现场放射性物质迁移对环境的影响具有一系列实际意义。已确定的核废料表面主要蚀变相类似于柱铀矿UO·2HO、水铀矿UO(O)·4HO、碳酸铀酰钍矿UOCO和切尔卡ite矿NaUO(CO)。虽然α辐射分解已被证明会导致过氧化铀酰蚀变相的生长，但迄今为止，尚未从机理上解释碳酸铀酰相在固体废物形式中的普遍存在，特别是因为蚀变化学在很大程度上受乏核材料高温的影响。在此，我们利用热重分析、量热法、振动光谱和粉末X射线衍射技术，首次证明了超氧化铀酰()相的形成、其在与乏核燃料相关的温度范围(40 - 350°C)内的反应性以及其向碳酸铀酰钾矿物相——水碳铀矿K[UO(CO)]的热力学转变之间的机理联系。热力学数据通过固态晶格中过氧化氢的分解揭示了超氧化铀酰相的亚稳定性。升高温度不会导致与铀酰阳离子结合的超氧阴离子分解，反而会增强其在CO气体存在下的反应性，在中等温度(150°C)下生成碳酸钾相，在较高温度(350°C)下生成碳酸铀酰相。
