Yellowcake is uranium ore rich in U235, a fissionable isotope. Iraq is alleged to have attempted to buy it from Niger.
From what I've read here on FR, Iraq did get yellowcake - from Niger, and maybe another place or two - in the 80's. There were barrels of it sealed up by the UN. What is in ? is whether Saddamn had tried to get any recently. His head nuke dude went to Niger in 99 - either to get some uranium, cowpeas, or cattle (Niger's main exports).
CholeraJoe replies: "Yellowcake is uranium ore rich in U235..."
Yellowcake is neither "ore" nor "rich in U235". Yellowcake is the uranium oxide (U3O8) product that is the end result of milling and processing the ore. The uranium contains only 0.7% (1/140) of the U235 isotope. All isotopes of uranium found in the ore can fission, either spontaneously or by induced fission.
What makes the U235 isotope uniquely useful in atomic devices and reactors is that each fissioning U235 atom will produce a sufficient quantity of additional neutrons, with which it is possible (if enough of the isotope is in close proximity), to sustain a chain reaction all on its own.
--Boot Hill