[hanslub42]

Uranium (both U238 and U235) is an alpha-emitter. Alpha particles travel at most a few centimeters in air.

[addandsubtract]

This is U8, though.

[snovv_crash]

(that's the name of the subway line for any non-Berliners who might be confused)

[gus_massa]

That's a good explanation!

I made a mistake and assumed it was Beta or Gamma radiation that travel easier in air.