A vacuum-pressure pulsing process was developed to gas impregnate graphite with carbon. An inductively heated graphite substrate is cycled between vacuum and 20-psig butadiene for 2 to 30 h at 750 to 950°C. The vacuum and pressure pulse periods were, respectively, 0.5 to 1 sec and 7.5 to 60 sec. Weight increases up to 8% were achieved in graphite of original density 1.86 g/cm3, and helium permeabilities were reduced from>10−2to<10−8cm2/sec, as required for molten-salt breeder reactor application.