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

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sealing of the breach line by further earth movement, healing because of the nature of the host
rock or because of plugging of the water path by silt carried by the stream.)
Several studies have been performed to estimate the leach rate of waste by water. Two
important factors affecting leach rate of a waste material are the waste form (chemical
nature) and the temperature of the solid-liquid interaction zone. Data reported by Ross
(1978), under repository conditions much more severe than would exist a thousand years after
closure, indicate leach rates ranging from 10-8 to 10 -5 g/cm 2 -day for reactions between
aqueous solutions and waste glasses in a devitrified and fractured state. Other studies by
McCarthy et al. (1978), with conditions of 300°C and 300 atmospheres, have suggested changes
in waste form properties which might lead to higher leach rates for some radionuclides in
borosilicate glass. The same processes also caused recombination of some of the radionuc-
lides with the immediate environment to a more stable form with a lower leach rate. Other
studies in field situations at lower temperatures and pressure with the ground saturated
with water have shown rates as low or lower than 10-10 gm/cm2-day for radionuclides in neph-
eline syenite glass (Merritt 1976). The leach rates used in consequence analyses,
Table 5.5.6, are considered highly conservative in view of these studies and the likely
temperature of the water contacting the waste.
TABLE 5.5.6. Estimated Leach Rates for Various Forms of Radioactive Wastes
Used in Consequence Analyses.
Number of
Waste Form and Assumed Geometry Leach Rate gm/cm2-day Canisters Contacted
High-level waste glass (assumed to be 1 x 10- 4 for first 10 days 210
devitrified and fractured, and without any
protection from the canister--l-cm cubes) 1 x 10-5 thereafter
Spent fuel (1-cm-dia spheres)(a) 1 x 10 -5 1230 PWR
1320 BWR)b)
Fuel residue 1 x 10- 5
Other TRU wastes 1 x 10- 4 480, 560
(a) The fuel pellet simulating a combination of PWR and BWR fuels is taken to be a cylinder
1.16 cm in diameter by 1.16 cm long. Since the spent fuel dose calculations were made,
the determination has been made that spent fuel may be fragmented following irradiation
and that the area subject to leaching may be about 5 times that used in the original
calculations (Pasupathi 1978). This factor has been applied to doses in this section.
(b) Subsequent to the calculations made for this Statement on the basis of 1230 PWR and
1320 BWR canisters (816-MTHM) contacted by water and subjected to leaching, the contents
of the repositories in the various media were changed. The amounts of spent fuel
contacted by water following a 12-m-wide fracture along the diagonal of a repository
were estimated to be: salt; 340 MTHM, Granite; 870 MTHM, shale; 390 MTHM and basalt
810 MTHM (DOE/ET-0029). For all practical purposes the doses that follow would apply
to the breach of granite and basalt repositories. Doses should be multiplied by a factor
of 0.4 to obtain doses reflecting a breach in a salt repository and by a factor of
0.5 for a shale repository.
For dose calculations for spent fuel and vitrified high-level waste (the major contri-
butor to dose from reprocessing wastes), doses may be calculated for other leach rates by
multiplying the tabulated dose by the ratio of the assumed leach rates to the listed leach
rate.
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