Management of Commercially Generated Radioactive Waste - U.S. ...

7.35
reprocessing cycle wastes are compared in Table 7.3.17. The index employed here is the
amount of water required to dilute one MTHM equivalent of the waste to drinking water stan-
dards (10 CFR 20) divided by the amount of water (8.7 x 10 m ) required to dilute the
original uranium ore to drinking water standards.(a) An index of 1.0 means the toxicity
hazard is equivalent to the original uranium ore. Detailed tables summing the dilution
hazard-index for all of the significant fission and activation products and the actinides
and their decay products are presented in Appendix A.4
The data in Table 7.3.17 show essentially equivalent relative hazard indices for all
of the once-through cycle cases. Equivalence (index = 1) with uranium ore is reached after
about 10,000 years.
Except at the beginning where they are closely similar, the reprocessing waste indices
are somewhat lower than the once-through indices and reflect sensitivity to the amount of
plutonium recycle achieved as identified by the reprocessing date. Equivalence with
uranium ore is reached between 1000 and 2000 years after repository closure.
Nuclides that account for 90-plus percent of the hazard index are listed in Table 7.3.18
for several time periods. Only Case 3 is shown for the once-through cycle since all
once-through cases are similar.
Initially, in both cycles, 90 Sr accounts for 95+% of the hazard index. At 1000 years
the principal contributors in the once-through cycle are 241Am, 240Pu and 239 Pu and in the
reprocessing cycle are 241Am, 24 3 Am and 240Pu. At 10,000 years the principal contributors
in the once-through cycle are 239 Pu and 240Pu, while in the reprocessing cycle they are
243Am, 24 0 Pu and 23 9 Pu. For the 100,000- to 1,000,000-year period in the once-through
cycle, 226 Ra and 2 10 Pb (both daughters of 238 U) are the principle hazards, while in the
reprocessing cycle, the principle contributors include 229 Th, 129 , and 23 7 Np in addition
to 226 Ra.
It should be noted that although this index is one way to measure relative toxicity of
the wastes it says nothing about the complex pathway for a release or the probability of
actual release of these materials to the biosphere. This is discussed in Section 5.5.
(a) Based on 0.2% uranium ore and 3% 235 U fresh fuel.