Investigations of the Natural Fission Reactor Program. Progress report, October 1976--September 1977 Page: 3 of 27
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can be applied to geologic disposal of radioactive wastes. These
studies are conducted by personnel at the Los Alamos Scientific
Laboratory, the Allied Chemical Idaho National Engineering Laboratory,
and the Department of Geology of thc University of New Mexico.
II. ISOTOPIC DETERMINATIONS OF OKLO SAMPLES
A suite of approximately 47 Oklo samples has been received
through the French Project Franceville. The samples represent seven
different geologic categories of the Oklo system including reactor
zones, traverses across reactor zone boundaries, and regions periph-
eral to the reactor zones. These samples are being studied by petro-
graphic, chemical, and isotopic techniques in order to determine the
extent of radionuclide redistribution. Particular attention is being
given to peripheral samples where it is possible to find material
that has migrated from reactor core regions. An isotopic Pb anal-
ysis is being devised and will be used on these samples to indicate
the possible hydrologic gradient responsible for the dispersion of
such species as 137Cs, 99Tc, and 90Sr.
Ruthenium isotopic analyses conducted on an initial group of
10 peripheral samples are presented in Table I. The samples were
from regions between 2 and lC m from the known boundaries of the
Oklo reactor zones. In all samples, the majority of the Ru was from
the thermal-neutron induced fission of 235U, with small fractions of
natural Ru and Ru from the spontaneous fission of 238U. Initial U
isotopic measurements on these samples show normal 235U/238U ratios
(within 0.002 rel wt%) indicating that the samples are not from
reactor zones. Thus, the fission-product Ru in the samples must
represent material that migrated from the reactor zones.
The 99Ru/10IRu ratio in these samples gives valuable informa-
tion on the migration behavior of Tc relative to that of Ru. The
progenitor of 99Ru is 99Tc,which has a half-life of 2.1 x 105 yr.
As indicated in Table I, the 99Ru content is highly variable in these
samples; the ratio of the fission-product 99Ru/101Ru ranges from
0.74 to 3.8. In contrast, both the 101Ru/102Ru and 101Ru/104Ru
ratios are constant to within about 5%. If Tc behaved similarly to
Ru during the period of redistribution, then the 99Ru/101Ru ratio
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Apt, K.E. (ed.). Investigations of the Natural Fission Reactor Program. Progress report, October 1976--September 1977, report, November 1, 1977; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1057069/m1/3/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.