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RE: Pu-238 -- and alpha-recoil spallation



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
jcope@ddminc.net
 
====================
 
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
jim.dukelow@pnl.gov
 
These comments are mine and have not been reviewed and/or approved by my management or by the U.S. Department of Energy.