Pu in U

["Carbaugh, Eugene H" <gene.carbaugh@pnl.gov>]

I wouldn't expect Pu in DU, however it might show up in trace levels in recycled
uranium that has been blended with fresh U during the fuel fabrication process.
To get Pu you have to run the uranium through a reactor first - most likely in
some kind of enriched form.   After irradiation (and presumed processing for
removal of U) the residual uranium is still somewhat enriched and is more
valuable as a fuel material to be blended with freshly enriched U than to
convert back to UF6 for further enrichment.  Assuming multiple exposures of the
fuel, certain impurities will develop in this recycled uranium.  I doubt that
fuel fabricators would return recycled U to an enrichment facility.  More likely
that it would be blended with new material to get the desired total content.

Back when Hanford was in the Pu production business, we had a lot of recycled U,
and our uranium processing facilities had operating specifications.  Our
slightly enriched uranium (0.8 to 1.25% by weight U-235) experienced many
cycles.  The maximum allowed levels and observed ranges of impurities in a few
lots processed in 1988 were:

Plutonium		10 ppbpU  (parts per billion parts U)  - reminder US
billion = 1E+9;   observed was 1-2 ppbpU
Neptunium		not established, observed range was 0.04 to 0.16 ppmpU
(parts per million parts U) 
Thorium			750 ppmpU ; observed 8-10 ppmpU
Tc-99			not established;  observed 3-4 ppmpU
Ru-103,106		<20 microcuries per pound U;  observed < 6 microcuries
per pound U
ZrNb-95			<10 microcuries per pound U;  observed < 4 microcuries
per pound U

We created a reference level of impurity mixture for bioassay monitoring data
interpretation.  (We applied the reference impurity level to uranium bioassay
data to allow for the dose contribution from these impurities which would never
be seen in a bioassay program.)  
The reference impurity level we used was:

Plutonium		0.4 nCi Pu-alpha per g-U  (based on 5 ppbpU)
Neptunium		0.4 nCi Np-237 per g-U	(based on 0.5 ppmpU)
Thorium			5 pCi Th-232 per g-U		(based on 50 ppmp U)
Tc-99			0.2 microcuries Tc-99 per g-U (based on 10 ppmp U)
Ru-103,106		40 nCi Ru-106 per g-U   (based on 20 microcuries per
pound)
ZrNb-95			20 nCi Ru-106 per g-U   (based on 10 microcuries per
pound)

We considered the reference impurity level mixture to be a worst case picture.
The impurities added slightly to the committed effective dose for the mixtures.
For class D uranium mixtures, it increased the dose by about 16%; for class W
the increase was only 6%; and there was no impact on the class Y mixture dose.

Hope this is useful to you.  Again, my key point is that I doubt there would be
any Pu in depleted uranium.

Gene Carbaugh, CHP
Hanford Internal Dosimetry Project
Pacific Northwest National Laboratory
gene.carbaugh@pnl.gov



************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html