Measuring U XAFS for UO2 in fluorescence is likely to not work well no matter how much you dilute it.   There will greatly suppressed amplitudes of the XAFS amplitudes.  For XANES, the fluorescence intensity will also be reduced quite a bit, giving and apparent edge shift to lower energy. 

A sample with the element exceeding 30 wt % more or less has to be measured in transmission or electronic yield.  This is pretty well-documented in the literature (the effect is called "over absorption" or sometimes "self absorption") and pretty much any tutorial or overview you can find.     It's been discussed on this forum a number of times.