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Jeff
Cope wrote:
-----Original Message-----
From: Rosita Cope [mailto:jcope@DDMINC.NET] Sent: Friday, September 13, 2002 12:15 PM To: radsafe@list.vanderbilt.edu Subject: Pu-238 Radsafers,
I am a radiological technician at Savannah River
Site. We work in some areas that are contaminated with Pu
oxide. I hear quite frequently that Pu-238 has characteristics quite
unlike other types of nuclides when in the forms of surface
and airborne contamination. One I hear a lot is that the
contamination can "move upwind". I ask why this happens and they say "the
generation of heat makes it move". Others say because the particles are so
light, they move in ways unlike other nuclides. Can anyone give me any
information on this, as to whether the movement characteristics are unique and
if so, why?
thank you,
Jeff Cope
====================
I responded to a
similar question on RADSAFE a few years ago as follows:
It is
a piece of folklore on the Hanford reservation that
if the cap is left off of a small canister of Pu dioxide, within a few minutes it will have spread over all the inside surfaces of the glove box. This
phenomenon seems to be real, with its physical basis being "aggregate
alpha-recoil transfer," by which is meant that a stochastic event of some number of alpha-decay events occuring along a grain boundary or other crystal-stucture weakness can cause the ejection (or spalling) of an atom cluster from the surface of the alpha-emitting material. Quoting from _Observations of the Distribution and Nature of Alpha-Active
Particulate Material in a HEPA Filter Used for Plutonium Containing Dust, M.T. Ryan and W.J. McDowell, ORNL/TM-5765, February 1977: "Aggregate recoil transfer is a phenomenon specific to
surfaces
of alpha-emitting radioactive materials where, due to kinetic energy made available by alpha decay, clusters of atoms are ejected into the surrounding medium. Such atom clusters, known as aggregate recoil particles, may contain up to [one million] atoms. Particles of alpha-emitting material, which are collected in normal HEPA filter operation with near 100% efficiency, may be sources of aggregate recoil particles. Aggregate recoil particles produced from a larger collected particle may undergo re-entrainment in the moving airstream and subsequent re-deposition downstream in the filter. If an alpha decay event occurs within this particle again, re-entrainment and re-deposition may occur. This process leads to a net transfer of radioactive material in the downstream direction." This
process would, clearly, be more significant in high specific activity
alpha emitting materials than in low specific activity materials, since the chance conjuction of enough decays along a crystal defect would be more likely in high activity materials. This
phenomenon is a potential explanation for the "Mysterious
Contaminant" described by Joel Antkowiak (in his 8/7/96 message to RADSAFE), although it doesn't seem to be consistent with all of the symptoms he describes. It may also explain the startling efficiency of alpha-emitting contaminants in penetrating protective clothing, glovebox gloves, etc.. Best
regards.
Jim
Dukelow
Pacific Northwest National Laboratory Richland, WA These
comments are mine and have not been reviewed and/or approved by my management or
by the U.S. Department of Energy.
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