Material-dependent radiation shielding

Material-dependent radiation shielding

Radiation is attenuated exponentially by passing through shielding
material.  Radiation resistance values are assigned to all bulk-material
nodes, and the radiation damage ABM traces the path of each radiation ray
to count up the shielding.  The relative radiation resistance values are
essentially real, but the effectiveness of all shielding is scaled down
by a factor of about 70 for game purposes.  Strength of the existing
radiation sources is increased by varying amounts to compensate for
shielding.  Uranium block and ore, both usable as shielding, are made
slightly radioactive, the latter only very slightly.