The yellow cake was not discovered by US troops in 2003, the UN's IAEA already had it safeguarded
Senior Defense Official: As you know, the United States government has invited the IAEA to conduct a Nuclear Nonproliferation Treaty safeguards inspection at the Baghdad yellow-cake storage facility at Tuwaitha, more commonly known as Location C. This inspection will be under the protection and auspices of coalition forces. The safeguard inspection will begin on or about June 7th. And I'm sure the next person will speak to the actual logistics, but for planning purposes, we and the IAEA expect that this process will likely take a couple weeks. The purpose of the inspection is to inventory and assess the condition of the material that is under IAEA safeguards at the Baghdad yellow-cake storage facility. The material at this facility includes approximately 500 metric tons of safeguarded uranium and several non-fissile radioisotope sources that are not under IAEA safeguards. The uranium is mostly in the form of yellow cake, an isotopically natural form that is an impure oxide. There is a small quantity of low-enriched and depleted uranium. Typically, the IAEA would conduct an NPT safeguards inspection at this location annually. The last inspection was conducted in December of 2002. Given the changed circumstances, the United States has determined it would be helpful to have the IAEA reinventory this location. "
Also see: "U.N. inspectors documented and safeguarded the yellowcake, which had been stored in aging drums and containers since before the 1991 Gulf War. There was no evidence of any yellowcake dating from after 1991, the official said."