Re: UF6

[Milton McLain <mem122a@Lanl.GOV>]

At 02:54 PM 4/8/96 -0500, Mike McNaughton wrote:

>I am editing a course on uranium safety.  The draft includes a statement
>about UF6 that I would like to clarify.  I hope some Radsafers can help.
>
>The draft says "Uranium can create widespread low-level gamma radiation
>fields less than 5 mrem/hr.  ... Larger sources of gamma radiation may exist
>from specific processes, including unflushed UF6 cylinders [in which case]
>gamma radiation from residual materials can result in radiation fields of
>several hundred mrem/hr.  This problem can be controlled by flushing empty
>cylinders to remove residual material."
>
>I assume the sentence means that "residual materials" other than UF6 cause
>the "several hundred mrem/hr". If it is UF6 emitting the gammas, I am
>puzzled how gammas from UF6 could be greater than from other forms of U.  
>
>I would be grateful for any clarification.  Thanks, mike
>
>


Mike,

The problem of gamma dose rate from UF6 shipping containers is that Th-234,
Pa-234m, and Pa-234, the immediate progeny of U-238 decay, are energetic
beta-gamma emitters. When the UF6 is removed by heating the cylinders,
vaporizing the UF6, the progeny remain behind as solid residues.  Their
fluorides are not volatile.  As an individual cylinder is reused, these
solid, radioactive residues can accumulate (the half-life of the Th-234 is
24 days). This accumulation creates a substantial source of photon
emissions, gamma-rays and bremsstrahlung photons.  In the "UF6 industry" it
is a common practice to clean the cylinder interiors periodically in order
to control or limit the buildup of these gamma-emitting residues.


Milton McLain
Affiliate, LANL