Re: How far would fallout travel from a N. Korean above-ground nuclear test?

According to my handy dandy copy of The Effects of Nuclear Weapons, the
answer is messy.  So, I'll go over the example in the book to give you an
idea what would happen.  

First off, we assume a ground shot similar to the Trinity test with a yield
around 20 kilotons.  We will also assume that they are not completely
insane and do this when it is dry to minimize rain out on their own soil. 
Given what the prevailing winds are in that region, we will also assume
that the shot is done near the east coast of the Korean peninsula.  With a
15mph wind, the primary fall out will be in an area ~16 miles wide and ~150
miles long.  At this contour, the dose rate is about 1 R/hr shortly after
the shot.  The debris that makes it to the upper atmosphere will spread
over many hours/days for quite some distance.  The problem here is that
there are so many variables with the weather that exact maps of radioactive
fallout hot spots are something that must be done after the fact.  

However, to answer your question, Japan is going to be really angry given
that the heart of their agricultural land is more or less directly in the
path of prevailing winds and will receive non-trivial fallout
contamination.  We aren't talking about massive amounts of cancer or severe
long-term effects, but I seriously doubt anyone would be willing to eat the
food for a while.  We will certainly be able to detect radioactive debris
(in incredibly tiny quantities) across all of the US.  By the time it gets
here, however, it won't be enough to cause a measurable health effect. 
After some time, the stuff that rises on the center of the plume will
spread across the entire northern hemisphere and take years to completely
drain out.  There is surprisingly little exchange between the northern and
southern hemispheres, so below the equator you won't really see anything
detectable.  

Hope for a below ground test.  

Scott Kniffin
Radiaiton Effects and Analysis Group
NASA Goddard Space Flight Center