Researchers at the Pacific Northwest National Laboratory (PNNL) are uncovering new information about the way in which uranium dioxide (UO2) interacts with water. These new findings will improve our understanding of how spent nuclear fuel will degrade in deep geological repository environments.
    UO2 is the primary form of nuclear fuel used in commercial nuclear power reactors. During nuclear fission in a reactor, a variety of radionuclides are created in the fuel. Researchers want to acquire more information about UO2. They are especially interested in the dissolution mechanisms that come into play when the ceramic material’s surface contain water. These mechanisms control the release of the majority of the radionuclides. This could have important implications for the environment.
     Many laboratory instruments currently in use do not have the sensitivity, resolution and radiological controls necessary to effectively investigate UO2 surfaces. However, the PNNL has a one-of-a-kind instrumentation suite which recently enabled a multi-institute research team to take a closer look at surface areas. The team represented the University of Cambridge, the European Commission’s Joint Research Center, and PNNL. It has uncovered some key revelations for our understanding of nuclear energy.
    Deep geological repository research around the globe is focused on the saturation zone where the water is chemically reducing which can lead to a loss of oxygen. UO2 is thermodynamically stable in this zone. The major remaining challenge is to develop an approach to examine UO2 with enough chemical resolution and fidelity to predict how it may behave in these environments.  PNNL materials scientist Edgar Buck explains that “We’re just now developing the tools we need to answer longstanding questions about nuclear materials.”
     In the study, researchers from the University of Cambridge collaborated with PNNL scientists to investigate UO2 samples exposed to anoxic (oxygen free) corrosion utilizing PNNL’s flagship instrumentation in the Radiochemical Processing Laboratory’s Radiological Microscopy Suite. This is also called the “quite suite.” The belowground room is home to the JEOL GrandARM 300F scanning transmission electron microscope (STEM). Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy (EELS), the research team explored the progression of atomistic structures and defects. The PNNL team had previously shown that EELS is able to map non-equilibrium pathways for oxidation in UO2 that are difficult to probe with other methods and equipment.
     PNNL materials scientist Steven Spurgeon explains that “Our approach provides direct information at the atomic scale to improve our models for dissolution.” Better models can help with the development of more accurate, long-term predictions regarding the fate of spent nuclear fuel under anoxic disposal conditions.
     In their study, the researchers found that dissolution initiates at the material surface grain boundaries and film cracks. Significantly, they observed that there was no amorphous surface layer formation (no loss of its crystalline structure) during the dissolution process. This means that there is a different process for oxygen substitution in which oxygen substitution appears to create an oxidized passivating layer, which is responsible for the observed reduction in uranium release as a function of leaching time.
      Co-author Professor Ian Farnan of Cambridge said, “The collaboration with PNNL provided us with unique tools to uncover a behavior that would be inaccessible by other means. Through our shared expertise, we were able to show how subtle changes in the surface chemistry of used nuclear fuel can control its dissolution and the release of radioactive elements to the environment—a fundamental requirement for safe disposal.” The findings from the study are reported in the team’s paper “An Atomic-Scale Understanding of UO2 Surface Evolution During Anoxic Dissolution,” published in ACS Applied Materials & Interfaces.