Koroglu Batikan, Dai Zurong, Finko Mikhail, Armstrong Michael R, Crowhurst Jonathan C, Curreli Davide, Weisz David G, Radousky Harry B, Knight Kim B, Rose Timothy P
Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
Nuclear Plasma and Radiological Engineering, College of Engineering, University of Illinois at Urbana-Champagne, Urbana, Illinois 61801, United States.
Anal Chem. 2020 May 5;92(9):6437-6445. doi: 10.1021/acs.analchem.9b05562. Epub 2020 Apr 13.
The predictive models that describe the fate and transport of radioactive materials in the atmosphere following a nuclear incident (explosion or reactor accident) assume that uranium-bearing particulates would attain chemical equilibrium during vapor condensation. In this study, we show that kinetically driven processes in a system of rapidly decreasing temperature can result in substantial deviations from chemical equilibrium. This can cause uranium to condense out in oxidation states (e.g., UO vs UO) that have different vapor pressures, significantly affecting uranium transport. To demonstrate this, we synthesized uranium oxide nanoparticles using a flow reactor under controlled conditions of temperature, pressure, and oxygen concentration. The atomized chemical reactants passing through an inductively coupled plasma cool from ∼5000 to 1000 K within milliseconds and form nanoparticles inside a flow reactor. The analysis of particulates by transmission electron microscopy revealed 2-10 nm crystallites of fcc-UO or α-UO depending on the amount of oxygen in the system. α-UO is the least thermodynamically preferred polymorph of UO. The absence of stable uranium oxides with intermediate stoichiometries (e.g., UO) and sensitivity of the uranium oxidation states to local redox conditions highlight the importance of measurements at high temperatures. Therefore, we developed a laser-based diagnostic to detect uranium oxide particles as they are formed inside the flow reactor. Our measurements allowed us to quantify the changes in the number densities of the uranium oxide nanoparticles (e.g., UO) as a function of oxygen gas concentration. Our results indicate that uranium can prefer to be in metastable crystal forms (i.e., α-UO) that have higher vapor pressures than the refractory form (i.e., UO) depending on the oxygen abundance in the surrounding environment. This demonstrates that the equilibrium processes may not dominate during rapid condensation processes, and thus kinetic models are required to fully describe uranium transport subsequent to nuclear incidents.
描述核事故（爆炸或反应堆事故）后放射性物质在大气中的归宿和迁移的预测模型假定，含铀颗粒物在蒸汽冷凝过程中会达到化学平衡。在本研究中，我们表明，在温度快速下降的系统中，动力学驱动过程会导致与化学平衡产生显著偏差。这会使铀以具有不同蒸气压的氧化态（例如UO与UO）冷凝出来，从而显著影响铀的迁移。为了证明这一点，我们在温度、压力和氧气浓度可控的条件下，使用流动反应器合成了氧化铀纳米颗粒。通过电感耦合等离子体的雾化化学反应物在几毫秒内从约5000K冷却至1000K，并在流动反应器内形成纳米颗粒。通过透射电子显微镜对颗粒物进行分析发现，根据系统中的氧气量，会形成2 - 10nm的面心立方UO或α - UO微晶。α - UO是UO热力学上最不稳定的多晶型物。缺乏具有中间化学计量比的稳定氧化铀（例如UO）以及铀氧化态对局部氧化还原条件的敏感性凸显了高温测量的重要性。因此，我们开发了一种基于激光的诊断方法，用于在流动反应器内形成氧化铀颗粒时对其进行检测。我们的测量使我们能够量化氧化铀纳米颗粒（例如UO）的数密度随氧气浓度的变化。我们的结果表明，根据周围环境中的氧气丰度，铀可能更倾向于以具有比难熔形式（即UO）更高蒸气压的亚稳晶体形式（即α - UO）存在。这表明在快速冷凝过程中，平衡过程可能并不占主导，因此需要动力学模型来全面描述核事故后的铀迁移。
