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

6.117
The geologic formation in which a well injection repository would be located would have
to be bounded by impermeable strata and free of water-transmitting faults. Such formations
occur in the sedimentary basins in the U.S., and it is these basins that oil and gas compa-
nies are exploring for petroleum and natural gas. This exploration could cause a major
safety problem by connecting waste disposal zones with aquifers.
Potential Impacts
As with the mined geologic repository, the principal pathway for release of radio-
nuclides to the biosphere in the long term would be by ground-water transport. It is be-
lieved, however, that the likelihood of ground water reaching the injected waste is extremely
small.
The only quantitative estimates on the movement of radionuclides via ground water trans-
port are from ORNL's experience with grout injection of remotely handled TRU waste into shale
(ERDA 1977).
The maximum quantity of activity that could be leached from a single grout sheet was
calculated, using data presently available (ERDA 1977). This sheet would have a volume of
about 28,300 m 3 (1 million ft 3 ) and could contain as much as 500,000 Ci of 9 0 Sr (if a
maximum waste concentration of 5 Ci/gal is assumed) and an equal amount of 1 3 7 Cs. Leach
data indicate that the 6-month leach rate of radionuclides from cured grouts would not exceed
6.2 x 10- 5 Ci/month of 1 3 7 Cs per sq ft of leached area, 1.7 x 10-3 Ci/month-ft 2 of
9 0 Sr, 5.5 x 10- 7 Ci/month-ft 2 of 2 4 4 Cm, and 5.6 x 10-10 Ci/month-ft 2 of 2 3 9 Pu.
If the entire grout sheet surface were exposed to water flow, a maximum of 62 Ci/ month of
1 3 7 Cs, 1700 Ci/month of 90 Sr, 0.6 Ci/ month of 24 4 Cm, and 6 x 10- 4 Ci/ month of
2 3 9 Pu would be leached. If the water flow is assumed to be 0.5 ft/day, the calculated
concentration of 23 9 pu in the water would be approximately 1 x 10-6 Ci/ml (less than the
concentration guide for this isotope in uncontrolled areas). The shale surrounding the grout
sheets has considerable ion-exchange capacity for cesium and strontium; a calculation yields
rate of movement of leached cesium and strontium through the shale that would be so low that
these nuclides would be transmuted by radioactive decay long before they approached the
surface. The small quantity of 2 4 4 Cm that might be leached would also be retained by the
shale.
6.1.6.6 Cost Analysis
Capital, operating, and decommissioning costs of well injection disposal have not been
estimated. However, since well injection disposal would not require costly mining opera-
tions, it could offer a low-cost means of disposal compared to mined repositories.
Cost data are available from ORNL (ERDA 1977) for a site-specific application of grout
injection disposal of RH-TRU. Estimated capital costs for a new waste shale fracturing dis-
posal facility, adjusted to 1978 dollars, are $6.0 million. Annual operating costs are es-
timated at $110,000. No data are given for decommissioning costs. The costs are estimated