[ RadSafe ] Re: Fallout "preserved" in concrete
Brennan, Mike (DOH)
Mike.Brennan at DOH.WA.GOV
Mon Feb 23 12:17:44 CST 2009
Benchmarking fallout could be very difficult for a couple of reasons.
The first is that the activity of fallout changes substantially, and in
a complex way. A thumb rule that I learned back when I was in the bomb
biz is that the half-life of fallout is about equal to the time since
the explosion. This is annoying to calculate for one source; it is
REALLY annoying if you are talking about dozens of sources spread out
over a decade or two.
The second is that fallout is really non-homogeneous geographically,
even from the same event and even in small areas. A presentation I
attended about Chernobyl said that 100 km downwind deposited activity
varied by 400% between a number of samples taken within a 10 m square.
That starts to even out as the short half-life isotopes decay out of the
picture, but then the differences increase again as biological and
p-chem factors start differentially concentrating different isotopes.
Tritium has long been a mystery to me, as it keeps showing up in
unlikely places. I am not convinced that most of it isn't an artifact
of counting, even though people whose lab coats I am barely fit to hold
tell me it isn't. It is on my list of questions to ask St. Peter in the
post exercise critique session.
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of welch at jlab.org
Sent: Sunday, February 22, 2009 10:24 AM
To: radsafe at radlab.nl
Subject: [ RadSafe ] Re: Fallout "preserved" in concrete
Thanks for the response, Mike.
We are indeed sampling and counting the concrete. Our early results
indicated the presence of tritium, which I did not expect to be
uniformly distributed in the material (this is a collection of large
shield blocks from an old cyclotron). We are scratching our heads a
little and trying to come up with possible explanations. One idea is
that the activity is really from Sr-90. But, we don't see any Cs137, so
it's a bit puzzling.
Other possible interference could be from naturally occuring activity
(as you mentioned), but we haven't put all the pieces together yet.
With respect to the fallout, I'm mainly wondering if there have been any
attempts to benchmark such a thing, similar as has been done with the
naturally occuring nuclides. If would be nice to have a reference to
help substantiate or refute the idea.
> Message: 2
> Date: Fri, 20 Feb 2009 14:38:43 -0800
> From: "Brennan, Mike (DOH)" <Mike.Brennan at DOH.WA.GOV>
> Subject: RE: [ RadSafe ] Fallout "preserved" in concrete
> To: <radsafe at radlab.nl>
> <37C41083D3480E4BBB478317773B845D0148EE62 at dohmxtum31.doh.wa.lcl>
> Content-Type: text/plain; charset="us-ascii"
> I would expect it to differ from location to location, due to
> different amounts of fallout. Any fallout that was cast in concrete
> in the 60s has decayed to a little bit of Cs-137, a little Sr-90, and
> so few plutonium atoms that you could name each one. I would expect
> that in almost all cases it would be insignificant in comparison to
> the naturally occurring radioactivity, which would also vary depending
> on where the materials came from. Without knowing the particulars of
> the gravel, sand, etc., I would guess that most of the activity comes
> from K-40 and isotopes in the U-238 and Th-232 decay chains (probably
> in secular equilibrium). For newer concrete, I suspect that the
> controlling factor would be how much fly ash was used in it, as I
> understand that radium and thorium is concentrated in the ash when the
> coal is burned (I have no direct experience with this, however).
> In the end, I suspect that if you want to know the activity from a
> given piece of concrete, you will just have to sample and count it.
> -----Original Message-----
> From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
> Behalf Of Keith Welch
> Sent: Friday, February 20, 2009 1:34 PM
> To: radsafe at radlab.nl
> Subject: [ RadSafe ] Fallout "preserved" in concrete
> Hello Radsafers,
> I'm looking for data regarding levels of radioactivity in concrete as
> a result of weapons fallout. I'm evaluating some concrete that was
> cast in the early to mid 1960s, and trying to determine if there might
> be elevated levels of Sr-90 or other nuclides in the concrete due to
> the prevailing levels in the environment at the time. I've searched
> the on-line literature pretty thoroughly, and have not come up with
> anything. Existing documentation that I can find only speaks to
> natural radioactivity in concrete (NCRP etc.).
> Thanks for anything you can suggest.
> Keith Welch
> Jefferson Lab
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