With the aging of nuclear installations, the need to transport large volumes of waste draws the community to inquire after the criticality risk of materials containing low concentrations of solid fissile material. The main issue about it is to determine the ratio between the solid non-fissile mass and the solid fissile that ensures an infinite muliplication factor koo lower than 0.95. US calculations on 235U in SiO2 matrix establish that this ratio should be 2000. This paper presents the IRSN approach studying any solid non-fissile matrix and the different fissile isotopes considered in the IAEA Regulation for ther Safe transport of Radioactive Material which are 235U, 233U, 239Pu, 241Pu. In the plutonium case, moreover, the existence of a positive temprerature coefficient is brought to light.