Actually, the only environmental threat from warm water effluents of
nuclear power plants is warm water.  No radioactive material is
released from a nuclear power plant during normal operation (not to
minimize the occasional accidents that have occurred, sometimes with
release of radiation).  The cooling water doesn not come into contact
with radioactive material at any time.  Also, no chemicals are added
to the cooling water that passes through the plant.  The water coming
out of such a plant is essentially as clean as the water that went in.
 The one thing that is added to the cooling water is heat, and quite a
lot of it, and that can cause environmental problems.  An increase of
five degrees over a short period of time, in the annual average water
temperature of a river, can cause major metabolic and reprodictive
changes in cold blooded organisms.
Paul M.