John Savard, writing about space colony design in sci.space.history:
However, going over documents about space colonies, it is clear that because of cosmic rays, it is not possible, without using _very_ thick shielding, to reduce radiation to an *arbitrarily low level*. A little shielding cuts radiation exposure, but after a certain amount, secondary radiation from cosmic rays increases exposure.
In that area, it is *not* possible to achieve a level of shielding which reduces radiation exposure within the habitat to that found on Earth at sea level.
Of course, Denver, Colorado is not exactly overrun by mutants.