be densified is placed on a rotary mandrel or support 4 this support or mandrel made for example from graphite is placed in the lower part of the body of the reactor 6 which has a plug 5 provided with an opening through which can slide support 4 reactor 6 can be heated by means of an induction heating device 8 constituted by coils 10 in which can circulate a high frequency current supplied by a generator 9 the upper part of reactor 6 is sealed by a cover 12 provided with a pipe 14 used for introducing liquid hydrocarbon into the reactor and a pipe 16 used for introducing a neutral gas into the reactor and which is used for expelling the air contained in said reactor furthermore cover 12 is surmounted by a condenser 18 permitting the separation of the unconsumed hydrocarbon which is recovered and the neutral gas which is eliminated the condensation of the different vapours is ensured by a circulation of water in the condenser jacket 19 this device is also provided with a system making it possible to determine the temperature prevailing in reactor 6 said system being constituted e g by thermometer probes 20 located in the reactor body a description will now be given of a handling sequence with the device as described hereinbefore this handling or manipulation firstly consists of placing the structure to be densified 2 on its rotary support 4 and then sealing the upper part of reactor 6 by means of cover 12 surmounted by condenser 18 the reactor then being equipped with its heating system 8 when this has been done it is possible to fill.