Re: Field measurement of hard-to-detect radionuclides

[Quang Le <quangle@SLAC.Stanford.EDU>]

Grant,

At my previous job, contamination control was a big part of our program. We
dealt with curie amounts of "loose" H-3, C-14, P-32, P-33, S-35, Cr-51,
iodines... The best way we found that would give us confidence to release
objects and people is to swipe check them and run the swipes through an
"open window" liquid scint. counter. I realize this may not be a good
option for large facilities. Thin window GM detectors should be able to see
C-14, so you may want to do a GM check in conjunction with your
scintillation-probe check. I am not so sure about Ni-63 though. If you have
the luxury of time and knowledge of the nuclides involved AND these
nuclides have considerably different half-lives, you may be able to take
measurements at half-life intervals and set up a system of equations to
solve for the contributing activities. Just some random thoughts. Hope they
can help.

<<<We (Bechtel Hanford RadCon) are searching for any information on how other
facilities, primarily DOE or DOE contractors, have handled the issue of
significant proportions of the radionuclide mix in a given area are hard to
detect (HTD) using most instruments.  In our specific case, the HTD nuclides
are Ni-63 and C-14, in proportions that typically are 25% or less.  However,
we have seen one sample that shows the proportion to be as high as 77% of
the total activity.

At Hanford, and in particular at the shutdown reactors, we've seen the more
easily measured constituents (e.g., Co-60, Cs-137, Sr-90) decay at their
shorter half lives while the longer lived HTD are becoming an increasing
contributor to the total activity.  Obviously, their relative contribution
will continue to increase.  

Our stock scintillation-probe based instrument has an efficiency of less
than 5% for these nuclides.  The level of contamination present does not
pose a significant external or inhalation dose hazard to the workers, but
renders release of objects and people difficult.

Our current strategy is to ratio the detectable nuclides to the total
activity present.  However, as the potential error is large, we are seeking
alternative strategies.  Possible strategies include finding a more suitable
instrument (would allow C-14 detection), seeking specific radionuclide
exemption/modification to the surface release requirements of DOE Order
5400.5.

Any information or suggestions would be welcome.  Please post either back to
RADSAFE, or reply directly to GMCeffal@bhi-erc.com.>>>>




_____________________________________________________________________ 
Quang Le 
SLAC/OHP 
(650) 926-2610 
<quangle@slac.stanford.edu> 
Note: The above is my own opinion only!  
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