La Hague - 90000 times increase in air radioactivity -Reply

[Charles Willis <CAW@nrc.gov>]

Bjorn,

A statement that the concentration of Kr-85 in air has increased by a factor of 9 E 4
may be approximately true.  The more general statement that the radioactivity in
air has increased by such a factor is decidedly untrue.  Of course, the reference
date is important.

Kr-85 is a fission product with a 10.76 year half life and a fission yield of  about 1.3
(uranium) or 2.24 (plutonium) atoms per 100 fissions.  Thus considerable Kr-85 is
produced in reactors and in nuclear weapons.  Since it is a noble gas, Kr-85 tends
to escape from the fuel and to remain airborne.  Kr-85 also is produced in nature,
by the neutron activation of Kr-84, but the cross section is small, about 0.09 barns,
so the natural concentration of Kr-85 is small.  Thus, it would not be surprising if
the airborne concentration of Kr-85 in 1998 is dramatically larger than was the
concentration was in 1938.

I have not calculated the concentration ratio for any two years because, from a
health physics perspective, it does not matter.  Actually, both the NRC and the
EPA did such calculations a few decades ago assuming a dramatic increase in
nuclear power that was never realized along with full reprocessing.  The calculated
Kr-85 doses were never important, e.g. about 10 percent of the population doses
from tritium and trivial compared to the doses from nature, principally radon.  For
the curious, some such results are documented in the "Final Generic
Environmental Statement on the Use of Recycle Plutonium in Mixed Oxide Fuel in
Light Water Cooled Reactors,"  NUREG-0002, Vol. 3, September 1976.  

Charlie Willis
caw@nrc.gov
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