Just getting into the sun isnt enough. The sun's outer layers are not very dense. A blob of uranium might survive long enough to come out the other side, at least on the initial few orbits.
I don't think we have to be very precise. It doesn't really matter if a blob of uranium orbits the sun for a few thousand years.
In that case, why get it into the sun at all? Just put it into an orbit that doesn't intersect Earth.