Salt Disposal of Heat-Generating Nuclear Waste

in performance assessment is described in terms of features of the disposal system
that can be used to describe its isolation capability, events that can affect the
disposal system and processes that are reasonably expected to act on the disposal
system. These features, events, and processes (FEPs) are the cornerstone of any
meaningful evaluation of performance of the disposal system.
3.1.1 Catalogs of FEPs
Well-developed catalogs of FEPs exist for the various repository programs around
the world, although these lists are, by design, general in nature (Stenhouse,
Chapman, and Sumerling 1993; Prij 1993; Guzowski and Newman 1993). This
ensures that the list for a specific site is comprehensive and does not overlook
FEPs that might be otherwise omitted based on the characteristics of a particular
site. While the list of possible FEPs is derived independently of the disposal
system, its screening should be based on a basic understanding of the geology,
hydrology, and climatology of the region and the site. Other screening
considerations include waste form and characteristics, disposal concept, and
regulatory and performance standards. The screening process will result in a list
of FEPs that should be represented in performance scenarios for the waste
repository. In this way, FEPs that are relevant to the three questions posed above
are included in the scientific analyses used in making performance predictions.
The process used for the development of a FEPs list is well documented
(Cranwell et al 1982, DOE 1996a, DOE 1996b, Hansen et al. 2010) and could
serve as a model for the development and screening of FEPs for a HLW
repository in salt. For example, the WIPP FEPs list (see Appendix A) is divided
into three primary categories: natural FEPs, human-initiated FEPs, and waste- and
repository-related FEPs. Because the WIPP is situated in bedded salt, the natural
FEPs used for the WIPP could be used for a HLW repository in bedded salt with
minor changes.
Based on the analysis of the FEPs supporting the performance assessment and
compliance certification for WIPP, the primary contributor to any possible release
is human intrusion. Thus, the human-initiated FEPs are studied extensively.
However, they are site specific, stemming largely from the drilling rate associated
with existence of oil and potash resources in the region. A direct human borehole
intrusion through a HLW repository could possibly drill through the entombed
waste. The probability of that event is a function of the drilling rate and the
resulting human exposure of that event depends on the location of the site. Thus,
the human-initiated FEPs determined for a salt formation in southeastern New
Mexico are not necessarily applicable to another HLW repository in salt.
The phenomena caused by heat from HLW would add some FEPs. In addition,
the physical characteristics of HLW are likely to be appreciably different than the
TRU waste that is being disposed in WIPP. Therefore, the waste-related and
repository FEPs would need to be evaluated. Additionally, there could be
differences in the disposal concept, container types, and performance period
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