The aim of the considerations below is to estimate the orders of magnitude for the production of RNBs at GANIL with possible evolutions of the present accelerator complex. This study was requested by the “Conseil Scientifique du GANIL”. We present and compare methods to produce ISOL beams by fission and/or low-energy spallation. Such reactions would complement the present SPIRAL beams which are limited to light elements if high intensities are required.