[ RadSafe ] Re: Fallout "preserved" in concrete

welch at jlab.org welch at jlab.org
Tue Feb 24 23:31:20 CST 2009

Thank you all for the ideas and information regarding radioactivity in
concrete.  Because I did not give a lot of detail in asking the question,
several of you have mentioned things that we have of course thought about
and assessed.  We have done gamma analysis on core samples of the blocks,
and we have a fair idea of the inventory of gamma emitters.  What was a
little puzzling to us was that we detected tritium at considerably higher
levels than the gamma emitters (it seemed high even considering half-life
differences), and the tritium was in every block, even the ones that have
no detectable gamma emitters.  There also appears to be a lack of the
expected depth profile (the expectation is that the tritium concentration
should simply be proportional to the neutron flux at a given depth.  But,
we have not done an exhaustive study (number of samples is relatively
small), and there's some question as to the reliability of the analysis
technique.  Also, we should probably not expect an idealized distribution,
given variations in neutron fields and geometrical factors in the actual
exposure conditions.

There's also the possibility of an interfering agent (radiological or not)
in the material that is causing "false positive" tritium results.  That
was the impetus for my original post - the idea that there is interference
from fallout (since the blocks were made in the early sixties).  But,
since we're not seeing any Cs137 in the blocks, the possibility that other
fallout nuclides are causing a problem seems remote.  We're trying a few
things to evaluate other interferences (sampling concrete from the
building in areas where there should not have been any activation), and
looking at sending our samples out for alternative analysis techniques.  I
will keep you posted as to what we find out.  Again, thank you for your
ideas, suggestions and reference materials offered.  We will follow up on
all of them.

Best regards,

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