Update: I have redesigned this containter. You can find it here: http://www.thingiverse.com/thing:1558097
I needed a place to keep my Trinitite collection, along with a couple tiny samples of uranium ore and uranium glass. The little plastic jar that I had was getting a bit too cramped. So, I decided to design something much bigger and fancier, with UV reactive ABS and an obnoxiously large trefoil on the top.
I'm sure you could use if for non-radioactive things as well. :)
If you are printing this to hold mildly radioactive samples, print both parts at 100% infill. I didn't use supports on the top, and the trefoil printed just fine without it. I used Makerbot Nuclear Green ABS(for obvious reasons), but the extra density of PLA may help with shielding a bit.
Without going into all of the details around radiation shielding, 100% infill will block all alpha particles and a good percentage of beta particles. Plastic actually works better than metal when blocking beta particles, because beta particles can induce temporary radioactivity in certain metals. But, it all depends on how energetic the beta particles are, so it likely won't block all of them. You can always add some lead foil shielding to make it better. But, this is not exactly intended for laboratory conditions.
This will do absolutely nothing to block gamma or X-rays! If you think otherwise, maybe you shouldn't be playing with radioactive things in the first place :)
If you're printing this just as a container, 15-20% infill should be fine. The top and sides are particularly thick, so reduced infill will help to keep the plastic use down quite a bit. If you use Makerbot Nuclear Green and hit it with a UV lamp, it will light up like a christmas tree (as seen in the first photo). I painted the trefoil black, but it looks great unpainted as well.