Yellorite = uranium. By default most yellorite ore (but not all for some reason) is replaced by IC2 uranium ore. Don't worry though; if you process the ores with mekanism machines (assuming you have mekanism, which you probably do if you also have big reactors) the output will still be yellorite ingots.
EDIT: Just realised monster doesn't have mekanism. There's probably another way to convert between yellorite and uranium.