Well, which are not? Uranium 238 is not. It fisses at a very significant amount under influence of fast neutrons; yet the fission cross-section even for fast (fission spectrum) neutrons is low enough that infinite multiplication factor is below unity even in bare metal, unmoderated U-238. Yet it is big! When uranium 238 is irradiated with fast neutrons, like from fusion, the convergent fission chain reaction produces much more energy than the energy of the initial neutrons. Thorium 232 is another actinide isotope known to have small fission cross-section even at high energies. But are there any others? Just because a nucleus will not fiss under influence of a slow neutron does not mean that its fission under influence of fast neutrons cannot sustain chain reaction. Example: plutonium 238. It will not fiss due to slow neutrons. Yet its cross-section for fission under fast neutrons is larger than the cross-section of plutonium 239 - so much so that the critical mass of plutonium 238 is smaller than that of plutonium 239! Plutonium 240 and 242 are less liable to fission - yet both are fissile! The critical mass of plutonium 240 is 33 kg - significantly less than that of uranium 235. And even plutonium 242 has a critical mass of under 90 kg. So how about the other even isotopes - which do not fiss due to slow/moderated neutrons but can fiss due to fast/fission spectrum neutrons? What is the infinite multiplication factor of plutonium 244? And how about the other long-lived even isotopes of uranium - uranium 236, 234 and 232?