[ RadSafe ] Nuclear Fuel Cycle and Waste

[goldinem at songs.sce.com]

About a week ago, Casper Sun (a very experienced certified HP whom I've met
in years past) asked a relatively straightforward question about the years
required to "cool off" when radiation hazard of spent fuel is no worse than
before irradiating.  I replied that one good source for that data is (based
on Bernie Cohen's work) in Raymond Murray's book: "Understanding
Radioactive Waste."  Page 129 of the latest (5th) edition shows that
irradiated fuel toxicity is equal to the initial uranium at 14,000 years
for reprocessed fuel and 120,000 years for unprocessed spent fuel.  The
figure shown was adapted from B.L. Cohen, Am. J. Phys. 54(1), Jan. '86 and
refers to dose contributions from all radionuclides and probability of
ingestion by humans.  (Note for you techo-weenies, since 1986, ICRP dose
factors for most transuranics were revised.)

Now in my note a week ago, I said that I recalled another graph that
indicated the levels of radioactivity.  I haven't found that graph but did
find useful information in a DOE publication (DOE/RW-0362 TG, Science,
Society, and America's Nuclear Waste, Unit 2, Ionizing Radiation, Teacher's
Guide, no date but also quite a few years old).  The graph on page 85 shows
that "relative hazard" for spent fuel (undefined as to burnup, reactor
type, enrichment and any other detail some folks might want) crosses the
uranium ore line at about 1 million years (greatly dominated by Np-237).
The spent fuel toxicity is within a factor of ten of uranium ore somewhere
around 5,000 years.  Some explanation on page 82 of the guide states that
after about 300 years, the "isotopes with the most intense levels of
radioactivity will have decayed so that they present about the same
relative hazard as uranium ore.  By 10,000 years, isotopes in spent fuel
will present about the same hazard as naturally occurring uranium ore that
is part of our natural environment."   We can argue all day long about the
bases for these statements (whether the 300 year date is based on direct
gamma radiation hazard while the long periods are due to ingestion
scenarios from contamination of water) but without some technical document
to review, the guide is just that - a teaching guide to inform people about
relative risks.  Bottom line for me, is that the 10,000 year compliance
period established by EPA is certainly defensible, along with a 300 year
period for container integrity.

Regards,  Eric

Eric M. Goldin, CHP
<goldinem at songs.sce.com>