Airborne high specific activity

[mcnaught@lanl.gov (Mike McNaughton)]

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)