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Folks --
Today Harry Anagnostopoulos provided
an excellent summary of the deposition and detection aspects of a melted
Ir-192 source in Louisiana. He noted that the melted Ir-192 tended to stick to
the surfaces it landed on and that detection with a pancake probe worked
best.
To compare the Louisiana experience with
the ORNL (Oak Ridge) experience, as I recall the ORNL particles were not
sticky. I assumed this was because they were not melted and fell mostly on the
concrete reactor bay floor. This may also have had something to do with their
composition, which escapes my memory at the moment. At any rate, they were
indeed somewhat air-dispersible in their fragmented form.
Also, these particles, fairly fresh from the
reactor, had a very high specific activity. So even small bits were very hot,
both radioactively and themally. At least one was located by the use of a heat
sensor (thermal detector) with a real-time imaging device. Others were
detected through a window high up in the reactor bay, using a collimated
detector. These methods saved dose by allowing remote detection to within inches
prior to "nailing down" the exact locations with regular detectors and
picking up the particles. One problem was that until the cask was removed, the
background in the bay was high and so the detection of the individual particles
was more difficult.
Janet Westbrook
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