Real-time imaging of cobalt mobility in graphite at elevated temperatures for advanced nuclear applications

Radionuclide retention within the core at elevated temperatures is essential for the safe operation of Generation IV graphite-moderated reactors. Here, we report real-time in situ Scanning Electron Microscopy (SEM) imaging of cobalt migration in graphite at high temperatures. Cobalt nanoparticles nucleate near 400 °C and undergo a distinctive spiral fragmentation at ~520 °C, coinciding with the cobalt hexagonal close-packed (hcp)–face-centred cubic (fcc) phase transition temperature, before re-growing above 800 °C. Minimal surface diffusion was observed at ~750 °C with movement confined within 1 µm over four hours. Transmission Electron Microscopy (TEM) confirms subsurface clustering and demonstrates the role of graphite microstructure and defect density in retaining cobalt. Mass spectrometry after heating between 550 and 1100 °C validates these observations, indicating that substantial cobalt release occurs only near 1100 °C, above normal operating conditions. These findings demonstrate the low mobility of cobalt at reactor-relevant temperatures and establish a framework for probing high-temperature ion dynamics in advanced materials.