Smaller Is Not Always Better for Radiation Resistance

The Science

In nuclear reactors, energetic particles cause damage. In the materials, the boundaries between grains, regions with different crystalline orientation of the atoms, are important to both mechanical properties and how damage accumulates in the structures. Controlling grain size and distribution is a tool to enhance radiation stability in materials. In this research on silicon carbide ceramics of use in advanced reactors, scientists experimentally found that grain boundaries have both beneficial and detrimental roles in radiation damage. Scientists determined that grain boundaries beneficially annihilate defects caused by extra atoms stuck into the crystal structure. However, this defect elimination causes an accumulation of other types of defects (called vacancies; missing atoms in the crystal structure). Accumulation of vacancies led to a loss of crystallinity. This loss degraded the mechanical strength. Lowering the strength can lead to swelling, an increase in the volume of the atomic structure.

The Impact

This study provides new insights into the complexity of radiation-induced damage and healing as a function of grain size. These insights will allow the effective design of new, radiation-tolerant materials. These materials could aid in the design of advanced nuclear reactors.

Summary

It was believed that the nanocrystalline materials with more grain boundaries would always have enhanced radiation resistance. These grain boundaries can act like “defect sinks,” allowing the annihilation of the defects and the material to heal, and this is mostly what happens in metals. However, increased grain boundaries did not always lead to enhanced radiation resistance. In particular, for the ceramic material silicon carbide, which is a structural material and potential cladding for next-generation nuclear reactors, researchers predicted competing mechanisms that affect the evolution of radiation-induced damage and healing. A team led by researchers from the University of Wisconsin-Madison have experimentally validated a competing mechanism where under certain conditions more grain boundaries can decrease radiation resistance. The team used high-resolution electron microscopy to monitor the evolution of radiation damage in silicon carbide. Comparing the areas near the grain boundaries and within the grains revealed that a specific type of defect (called an interstitial, where an atom occupies a site in the crystal structure at which there is usually not an atom) preferentially was annihilated and therefore healed at the grain boundaries. However, the preferential annihilation of interstitial defects left behind another type of defect (called vacancies). The accumulation of vacancies near grain boundaries led to local amorphization (loss of crystal structure). These competing mechanisms have to be taken into account when optimizing grain size for radiation resistance in nanocrystalline materials similar to structural ceramics of interest for nuclear reactors.

Funding

Research supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences; the DOE Office of Nuclear Energy, Intermediate Voltage Electron Microscope (IVEM)-Tandem Facility, a DOE Office of Nuclear Energy user facility; and the National Science Foundation (Extreme Science and Engineering Discovery Environment).

Publications

X. Wang, L. Jamison, K. Sridharan, D. Morgan, P.M. Voyles, and I. Szlufarska, “Evidence for cascade overlap and grain boundary enhanced amorphization in silicon carbide irradiated with Kr ions.” Acta Materialia 99, 7 (2015). [DOI: 10.1016/j.actamat.2015.07.070]

M.J. Zheng, N. Swaminathan, D. Morgan, and I. Szlufarska, "Energy barriers for point-defect reactions in 3C-SiC." Physical Review B 88, 054105 (2013). [DOI: 10.1103/PhysRevB.88.054105]

N. Swaminathan, D. Morgan, and I. Szlufarska, "Role of recombination kinetics and grain size in radiation-induced amorphization." Physical Review B 86, 214110 (2012). [DOI: 10.1103/PhysRevB.86.214110]