Re: Airborne high specific activity

[js_dukelow@ccmail.pnl.gov]

On 6 August 1996, Mike McNaugton < mcnaught@lanl.gov > wrote:

> Subject: Airborne high specific activity 

> I have a question about airborne activity of U-238, U-235, Pu-239, etc.

> The conventional wisdom at Los Alamos is that high-specific-activity
> isotopes of uranium or plutonium become airborne more easily than
> low-specific-activity isotopes.  For example, there is almost never a
> problem with airborne activity of U-238, but a solid chunk of uncoated
> U-235 left undisturbed in a room (with an air sampler) will result in
> airborne activity, and Pu-238 is the biggest problem of all.
> 
> My questions are:
> 
> a. Is this a general rule, true of other types of materials?
> 
> b. Why?
> 
> Some possible explanations are as follows.
> 
> 1. The emission of the massive alpha particle causes microscopic particles
> to break loose.  I am skeptical of this explanation becasue this should be
> a function of the alpha particle energy and the total number of bequerels,
> not the specific activity.  Furthermore, calculations using conservation of
> momentum do not seem to support this explanation.
> 
> 2. High-specific-activity isotopes are easier to detect, so airborne
> microscopic  particles of low-specific-activity material go un-detected.
> 
> 3. Radiolytic catalysis causes high-specific-activity isotopes of uranium
> and plutonium to oxidize more rapidly.  Oxides such as UO3 are more powdery
> and so become airborne more easily.
> 
> In support of hypothesis number 3, I am told that a sample of U-235 left
> undisturbed for years in a dry room at Los Alamos formed a layer of yellow
> UO3. (This is in contrast to the usual black UO2).
> 
> I would appreciate any comments.  Thanks, mike
> 
> "Shlala gashle" (Zulu greeting, meaning "Stay safe")
> mike (mcnaught@LANL.GOV)
> 

No one seems to have responded to McNaughton's question, so ---

My vote is for explanation 1) above.

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

js_dukelow@pnl.gov
(509) 372-4074

These views are mine and have not been reviewed and/or approved by my 
management or by the U.S. Department of Energy.