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

5.20
Potentially active faults can be deterministically identified from geologic, geophysi-
cal, seismic, and natural stress data. Standard earthquake-hazard assessment provides prob-
abilistic estimates of expected return periods at specific sites for ground motions of
various magnitudes. These methods are used in conjunction to help determine appropriate
seismic design requirements. Similar methods are evolving for volcanic and diapiric
phenomena.
The consequences of tectonic events must also be estimated. Observations of
earthquake-related damage, both at surface facilities (Lew et al. 1971) and in mine tunnels
(Pratt et al. 1978 and Dowding et al. 1978), provide empirical data for substantiating cal-
culations based on the physical response properties of the structures of interest.
The consequences of such intrusive processes as salt diapirism and volcanism are esti-
mated by studying the geometry, disruption zones, and chemical alterations associated with
existing intrusions. Where conditions allow current study, the movement of faults, intru-
sions of material, and tectonics are evaluated also in terms of their effects on hydrologic
systems and erosional processes. Impacts of faulting, erosion, and intrusion are estimated
parametrically by assuming various event-scenarios and analyzing their effects on the hydro-
logic flow models.
The prediction of tectonic events and their potential impacts over periods of tens of
thousands of years is an advancing capability. Careful selection of repository sites can
reduce the likelihood of tectonically induced disruptive events to almost zero. The poten-
tial impacts of postulated events will be defined by scenario analysis in order to assess
their effects on containment and isolation.
Resource Studies
The potential for exploiting mineral, energy, water, and subsurface land-use resources
both now and in the future will be assessed throughout the site-selection process. Geolo-
gic, geophysical, borehole, and geochemical studies conducted during site exploration and
qualification provide data for evaluating the potential for resource development. The
exploration and ultimate selection of a repository are the converse of seeking an ore body
or an oil field, in that investigations are conducted to locate areas with a low resource
potential. If any characteristic, including thermal gradients, in the site location signi-
ficantly exceeds the crustal average, its potential value to future generations needs to be
carefully considered. The consequences of inadvertent human intrusion into the repository
due to resource exploration at some future time must also be considered.
Status of Ongoing Exploration Programs
Preceding sections have described the factors of the natural system important in site
selection, design, and construction of deep geologic repositories; the requirements that
must be satisfied by a repository site; and the methods available or being developed for
characterizing and assessing the natural system.
This section identifies site-specific geologic investigations conducted over the last
several years. The site characterization process, described in Section 5.1.1.2, will be