Tina, UrAc is thought to be sensitive to light - especially UV type of light found in fluorescent light and sunlight. Causes breakdown of the stain and possible precipitation. Very little radioactivity is present - but some is there, so be careful. I am not sure why one wouldn't wrap UAc containers in Al foil, but it has nothing to do with radiation. Be is the best mechanically sound metal for avoiding brehmsstrahlung, but has other problems.
Plastic is another good material for avoiding it.