Fluorination of nuclear graphite IG-110 in molten 2LiF-BeF2 (FLiBe) salt at 700 ºC

Abstract

It is important for the development of advanced nuclear reactors to understand if the exposure to molten fluoride salts changes physical properties of graphite and its ability to absorb tritium. This paper demonstrated that the interaction between high purity nuclear grade graphite IG-110 and molten salt mixture 2LiF-BeF2 (FLiBe) at 700 °C for twelve hours in argon envirnoment resulted in graphite fluorination and changes in graphite microstructure. XPS C1s spectra analysis shows an increase in F functionalization (from 3.5% to 9.0–19.3%), an increase in the sp3/sp2 ratio (from 0.1496 to 0.1798), and F1s spectrum analysis confirms appearance of CF bonds. Raman spectra show an increase in ID/IG ratio from 0.35 to 0.69 and an appearance of peak shoulders characteristic of CF bond formation. GDMS depth analysis from the surface to 5 μm depth shows a higher depth penetration of F compared to that of Li and Be, indicative of surface salt reduction and bulk graphite fluorination. FLiBe reduction by graphite fluorination is not thermodynamically favorable, but certain carbon sites in the graphite are more reactive than the majority of carbon atoms in graphite. We argue that graphite fluorination occurred at these reactive sites, leading to the reduction of salt at the graphite surface. XRD and Raman confirmed microstructural changes. This indicates that the fluorination process may introduce new active sites as existing active sites are consumed, which could be advantageous for tritium chemisorption from FLiBe.