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

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Since long-term repository containment cannot be demonstrated by short-term test,
mathematical models must be relied on to predict the long-term behavior of the repository.
Risk assessment is thus dependent on the development of reasonable predictions of the long-
term behavior of the processes and phenomena that could occur within the repository system.
The risk assessment under development for geologic isolation is taking the form described
in the following methods.
3.4.2.1 Disruptive Events
Many geologic events and processes occur because of the long-term motion of the earth's
plates with their associated stresses and strains, and by the action of long-term weather
patterns associated with a variety of astrophysical and earth phenomena. Many of these phe-
nomena are predictable (usually with an element of randomness); others can only be assigned
an estimated site-dependent probability of occurrence. More specifically the key interest
in predictive modeling is whether a site (selected by virtue of historical stability) will
change to an unstable area (e.g., active faulting, volcanism, significant ground- and/or
surface-water activity, etc.).
Potential disruptive phenomena that could affect a repository have been categorized as
natural processes, natural events, man-caused events and repository-caused processes and
are listed in Table 3.4.2.
The science of geology has tended to concentrate on predicting the location of ores and
fossil fuels and to explain the structure of the earth. Nuclear waste isolation appears to
be the first subject of large interest in long-term predictive geology. Many geologists
have recently been engaged in the development of suitable predictive geologic models and/or
scenarios. This research is concentrating on specific sites as well as global processes.
To be complete, risk assessment must include all significant sources of risk and must
predict the condition of the repository and surrounding area following failure, the time of
failure occurrence and its probability of occurrence. This evaluation is called "Scenario
Analysis" (Burkholder 1978, Greenborg et al. 1978). In general, these evaluations employ
models that are very complex and require the capabilities of electronic.data processing.
Confidence in the models can be increased by comparing the results of the models to natural
systems which exist and adjusting the models until a reasonable degree of conformance is
reached. This concept of calibration and verification has been employed in the hydrology
models discussed below.
3.4.2.2 Lithosphere/Atmosphere Transport
This risk assessment process includes both lithospheric (by ground water) and atmos-
pheric (by airborne and other surface processes) radionuclide transport analysis. The
physicochemical processes governing ground-water movement and transport of pollutants are
sufficiently understood that mathematical models can be formulated. However, these models
require measured physicochemical parameters representing the specific site in order to simu-
late the system. These data are seldom adequate in terms of quantity and quality. However,