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

5.87
the repository that the dose would occur are as follows. Assuming that 129I removed from
dissolver off-gas is sent to the repository and is leached at roughly the same rate as from
spent fuel, the doses are essentially the same for either fuel cycle option.
Radionuclide Dose, rem Time, yr
14C 90 1 x 10 4
9Tc 22 4 x 103
1291 990 1 x 104
(to the
thyroid)
135 Cs 0.2 1 x 106
237 Np 440 1 x 106
The probability of the event is estimated to be on the order of 4 x 10 - to 2 x 10
(a )
over a 10,000-yr period.
Because of the extremely small probability of occurrence, and because of the very
limited number of individuals that could be contaminated by such a well, the societal risk
is believed to be insignificant.
5.5.4 Repository Breach by Drilling
In this scenario, about 1000 years after repository closure an individual (or group)
drills 600 m into a waste repository in search of a mineral resource or for geologic study
itself. Repository markers are no longer evident, are misunderstood, or are ignored. These
individuals, while having the technology to drill to repository depth, do not possess or do
not apply the knowledge and apparatus to assay material brought up in the drilling process
and to discover its radioactive properties.(b)
Because a probability for exploratory drilling could not be determined, an overall
probability was not assigned to this event. In qualitative terms, someone could be explor-
ing for potash, oil, etc.(c) in the area of a repository in salt based on the same explora-
tion principles that established the presence of the formation in the first place. In other
formations such as granite, shale and basalt, associations with any particular resources
are not as strong as in the case of salt. The probability that drilling will occur some-
where on the repository site is highly uncertain. If drilling occurs on the property, the
(a) Probability of faulting over a 10,000-yr period of 4 x 10 -7 was taken from EPA comment
#113 on the draft to this statement. The probability of 2 x 10 - ' over 10,000 years
was developed from Claiborne and Gera (1974).
(b) The drill crew may not be aware of radioactive material in the drilling mud as it is
brought up; however, once samples are sent to their assay laboratory, the drillers
would soon know of the radioactive nature of their exploratory effort. If the assay
were crude they might conclude, in the case of drilling through a spent fuel element,
that they had struck uranium, but very little sophistication in assay would be required
to recognize that the radiation spectrum was not at all like that of natural uranium.
The radiation characteristics of material brought up after passing through a solidified
high-level waste canister would resemble natural ores even less.
(c) Because of the frequent occurrence of salt deposits at depths much shallower than 600 m
the explorer would not likely be drilling to 600 m in search of salt.