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

5.30
TABLE 5.3.1. Conceptual Repository Design Thermal Limits for Spent Fuel
Medium kW/ha(a) kW/acre(a)
Salt(b) 124 50
Granite 320 130
Shale 200 80
Basalt 320 130
(a) Area occupied by the emplacement rooms and their associated pillars
only.
(b) The placement of spent fuel in salt is limited by long-term surface
uplift. The degree of surface uplift is dependent upon the thermal
loading averaged over the full emplacement area (corridor area as well
as rooms and pillars). Two-thirds of the allowable average thermal
loading for spent fuel in salt is 100 kW/ha (40 kW/acre). The thermal
loading listed in this table (124 kW/ha) is the room and pillar area
loading that results in 100 kW/ha average loading. Room and pillar
integrity is the controlling criteria in other rock media and is dependent
upon the room and pillar loadings listed in the table.
repository startup, the earliest date considered in this Statement, the average age of spent
fuel available for the first repository was calculated to fall within the range of 7 to
11 years. For a later repository startup the spent fuel will initially be much older (See
Section 7.3). For the conceptual repository described here we assume that the average age
of the spent fuel delivered to the repository is 6.5 years old. The criteria in Table 5.3.1
were developed for 10-year-old fuel. Using those criteria for 6.5-year-old fuel provides
an additional degree of conservatism since the thermal limit tends to increase for younger
waste. There are also thermal limits for the individual canisters, but for the spent fuel
repository concept used here, where the canisters contain only a single fuel assembly, the
thermal output of the canisters is always well below the limit.
In the absence of detailed site-specific geologic data, optimization of the repository
design to account for the special qualities of each medium is not possible. Instead a
standardized repository design using a conventional underground layout is specified with an
overall area of approximately 800 ha (2000 acre). This area provides reasonable waste capa-
city and is achievable from both construction and operational points of view. Actual repos-
itories may be either larger or smaller than 800 ha depending upon specific site character-
istics and more detailed operations analyses.
Repository design, construction, and operations presented here assume a homogeneous
geologic formation without major flaws or discontinuities. This simplifying assumption is
appropriate for use in this generic analysis; actual repositories will have site-specific
design features. The design may involve preparation of a preliminary repository layout on
the basis of initial site investigations. The preliminary layout would be modified as con-
struction progresses and the formation is more fully explored.
For the conceptual repository described here, excavation of the full underground
repository area is postulated to be completed during the first five years of repository