Calculation of uncertainty in EU determination

["Abelquist, Eric" <AbelquiE@orau.gov>]

Hans and others-

I do not understand why the determination of uncertainty in the %U-235
enrichment calculation requires Monte Carlo or other special techniques.
Why does the fact that U-234, U-235 and U-238 add up to 100% pose a problem?
It seems that standard propagation of error techniques can be used.

The following approach is used to calculate enriched uranium (%U-235):

1.  By alpha spec, the activity concentration (usually in pCi/g) is measured
for each of the 3 isotopes.  The total propagted uncertainty (TPU) is
provided with each analytical result.

2.  The activity concentrations can be converted to a mass basis by dividing
each isotope concentration by its specific activity.  Assuming the specific
activities to be without error, the individual uncertainties for each
isotopic mass are easily determined.

3.  The %U-235 enrichment is then the U-235 mass divided by the total U mass
(U-234 + U-235 + U-238).  The overall uncertainty in %U-235 enrichment can
be propagated by standard propagation of errors techniques.    

I do not see the difficulty in propagating the error for the U-235
enrichment.  What am I missing?

Regards
Eric Abelquist

abelquie@orau.gov
865  576-3740
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