After DESERT STORM The UN had the Tuwaitha stockpile pretty much under its observation, if not outright control.
The controversy with Joe Wilson and Niger was over whether Iraq was planning an additional future yellowcake purchase -
this did not occur but plenty of good reporting indicated the prospects were at least explored by Iraq
which would have been necessary to establish a secret fuel materiel stockpile not under UN observation, possibly not even in Iraq but possibly in a country cooperating with Iraq (Libya?)
What kind of control did the UN have after UNSCOM inspectors were removed from Iraq in 1998?