On 3/20/2013 9:41 AM, dpb wrote:
....

The half-life of 92U238 is ~4.5x10E9 _yrs_...(that's 4 billion w/ a B
years). One doesn't observe much change in specific radioactivity w/
time in a normal lifetime.

And, BTW, the depletion in depleted U is the deficiency of concentration
of U235 relative to that naturally occurring which is roughly 0.72% (or
iow, 99.28% U238). But, the half-life of U235 (also an alpha-emitter)
while shorter than that of U238 is still 700x10E8 yrs so the amount of
additional depletion owing to natural decay is infinitesimally small in
any observable time.

....typo -- 700x10E8 yrs is 700x10E6 or 7x10E8. Correct below...

For example, in 100 yr, N(t)/N0 ~ 2^(-100/700E6) = 0.99999990 for U235.
Add another roughly another couple of nines or a little less for U238
since the half-life is ~10X longer (X (1/2)^ratio half-lives).

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