Abstract: A heat transfer regulating mixture having a metallic component A with a melting point TA and a particulate ceramic component B which is non-wettable by the metallic component A, non-reactive therewith and which has a melting temperature TB which is higher than both the temperature TA and a desired operating temperature TD which is also higher than TA. The metallic component A and the particulate ceramic component B and the respective amounts will typically be selected to have a higher thermal resistivity below TA than above TA. The heat transfer regulating mixture may be incorporated in a thermal flux regulator having the mixture within an enclosure surrounding a heat generating reactor structure.