Effect of supercritical fluid activation on the diameter of carbon fibres

•The reactivity of carbon fibres is greater towards SCW and SCCO2 than steam and ATCO2.•The gasification reaction with supercritical fluids is diffusion-controlled.•There is a critical porosity after which the fibre becomes fragmented.•A model of percolative fragmentation accounts for the different evolution of the porous structure.•The thinning of the fibre is due to a perimetral fragmentation brought about by external gasification.