The changes in both the crystallographic coefficient of thermal expansion (CCTE) and the coefficient of thermal expansion (CTE) of FLiNaK-infiltrated nuclear graphite were studied using in situ-heating X-ray diffraction analysis and horizontal push-rod dilatometry. It was found that, at temperatures lower than the melting point of FLiNaK, the CCTE of the d(002) spacing of graphite decreases with an increase in the weight of the graphite sample because of FLiNaK infiltration. On the other hand, the CTE of bulk graphite increases after molten salt infiltration. The CCTEs of the as-prepared FLiNaK salt, and the salt in the graphite pores were compared. The decrease in the CCTE of the FLiNaK salt after it had infiltrated into the graphite pores confirmed that interactions occur between the graphite and the salt. These interactions are probably induced by the difference in the CTEs of graphite and the solidified salt. Further, it is likely that the crystallization pressure also plays an important role here. Thus, both the causes need to be considered when using nuclear graphite in molten salt reactors.

Infiltration of molten FLiNaK salt into degassed nuclear graphite samples under inert gas pressure was studied. The weight gain of different grades (2020, 2114, IG-110, NBG-8, G1 and G2) of nuclear graphite during infiltration with different pressures was measured. Molten salt infiltration was compared with mercury intrusion porosimetry where it was found that mercury infiltration was a useful predictor of the threshold pressure and infiltration volume per gram graphite for molten salt infiltration. The distribution and morphology of salt in the graphite were observed by scanning electron microscopy, with very little difference between the molten salt content at the center and edge of samples for samples infiltrated at pressure higher than the threshold pressure. Increased molten salt infiltration with increased pressure resulted from the occupation of smaller pores and full occupation of the larger irregular pores. The similarity of weight gain between molten salt infiltration equilibrated at 20 and 100 h showed 20 h was adequate to obtain equilibrium.