Hi all, I have a question. In fast reactors, it is generally considered unsafe to have too high an amount of americium (mainly am-241) in the core, because it diminishes all the safety parameters of the core: - it has too high an absorption cross section, so that it "shadows" the U-238 resonances which give the Doppler effect - it has a much lower retarded neutron fraction, hence diminishing the safety margin to prompt criticality - it has a rising fission cross section which overshadows that of U-238, hence increasing fission when the spectrum hardens, which is bad for the void factor. At least, that's what I understand. But now, my question is: when I look at the Pu-240 cross sections, they seem to have similar problems. So why isn't Pu-240 then such a problem ? cheers, Patrick.