Compliance of the Waste Isolation Pilot Plant with 40 CFR 194.24(b) Page: 4 of 18
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DRAFT -- R. WEINER -- November 7. 1996
HLW repository, the fraction numerator is much more heavily influenced by fission products, not TRU isotopes, and
the waste unit factor is relatively more important.
The second impact is on modeling or calculation of possible releases, and potentially includes all radionuclides in the
repository. However, most of these are present in the waste in such small quantities that their impact on long-term
performance is negligible For a mix of radionuclides, as in the WIPP, each radionuclide is normalized with respect
to its release limit (1), and the sum of all releases must have
- less than one chance in 10 of exceeding the release limit, and
- less than one chance in 1,000 of exceeding ten times the release limit
where the sum of releases in EPA units expressed by
(1) R. - 1 + +.. = 1 (.0)
()f Lt L i= f Li
in which Ri is the total release in EPA units under scenarioj, fW is the waste unit factor, Qj is the cumulative release
for radionuclide I under scenarioj, Li is the EPA release limit for radionuclide I, and the EPA release limit nR is the
number of radionuclides contributing to the release. The EPA unit provides a more precise description of waste
radioactivity than a unit like curies or becquerels, because it addresses regulatory compliance directly. The
calculation of possible releases and the associated probabilities of release (under various scenarios) is presented as a
complementary cumulative distribution function (CCDF), as is shown in Figure 2.
The three potential release mechanisms: direct release in the drill mud, direct brine release on drilling, and brine
release to an overlying stratum, include different radionuclides. Modeling of direct release by drilling (including
caving) includes Pu-238, Pu-239, Pu-240, Am-241, Cs-137, Sr-90, U-233, and U-234. The isotopes Pu-241 and
Cm-244, although not regulated by 40 CFR Part 191, are included because their daughters, Am-241 and Pu-240,
respectively, are significant. Direct release of brine to the surface can result in the release of isotopes mobilized-in -
the brine. In modeling this type of release, all of the actinide isotopes (isotopes of thorium, uranium, neptunium,
plutonium, americium, and curium) present in the waste are included.
All isotopes of an a particular actinide exhibit the same solubility, and can therefore be assumed to dissolve in the
same mass proportion as the inventory. That is, if the uranium in the repository is 85% U-238 by weight, 85% of any
dissolved uranium will also be U-238. U-238 has an exceedingly long half life (4.9 billion years) and its specific
radioactivity is very low (13 Ci/gm) compared to, for example, U-233 (about 300,000 Cilgm). In a solution, a
preponderance of U-238 will effectively dilute the radioactivity of other isotopes in that solution.
Brine released to an aquifer in an overlying stratum like the Culebra aquifer (see Figure 1) can also carry mobilized
radionuclides. The eight radionuclides used in calculation of brine release, all actinides, that dominate the total
potentially released EPA unit for all but the earliest part of the regulatory period are Pu-239, Pu-240, Pu-242, Am-
241, U-233, U-234, Th-229, and Th-230.
In sum, the radioisotopes that may influence repository performance are Pu-239, Pu-240, Pu-242, Am-241, U-233,
U-234, Th-229, and Th-230; for potential releases in the first few hundred years of the regulatory period, Pu-238,
Sr-90, and Cs-137 are included in the calculation (Table II). When potential releases are modeled, however, the
only radioisotopes that appear to be significant are Pu-239 and Pu-240 throughout the regulatory period, and Pu-238
and Am-241, during the first thousand years after closure. The WIPP is, in this respect also, quite different from a
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Chu, M. S. Y.; Papenguth, H. W. & Stockman, C. T. Compliance of the Waste Isolation Pilot Plant with 40 CFR 194.24(b), article, November 7, 1996; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc690392/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.