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

K.8
In the very-near-field analyses, the baseline canister-emplacement design was a single
overpacked canister placed in a hole. In general, the void space between the sleeve and
the hole was assumed to be backfilled with crushed rock. In each of the HLW calculations,
a 15-cm annulus of crushed or fractured rock was assumed.
K.1.2 Thermal Loadings Achieved in Conceptual Repository Designs
Engineering or operational constraints may restrict any of the thermal loadings dis-
cussed in the above section to values lower than the limits presented in Table K.1.2. These
constraints include such factors as reasonable HLW concentration in canisters, available
canister sizes, permissible hole spacing, and room stability limitations on hole arrange-
ments. Spent-fuel canister loading is limited in this Statement to a single PWR or BWR
spent fuel assembly so that canister heat loads are below limiting values. The HLW canister
diameters are reduced as necessary in each case so that the canister loadings are below the
limits of Table K.1.2. Alternatively the waste could be diluted with inert material without
reducing canister sizes to achieve the same result.
As a hedge against uncertainties in the criteria and other factors and to ensure a con-
servative estimate of repository capacities, the design areal thermal lodaings for both
spent fuel and HLW were established at 2/3 of this limiting areal loading parameter in
Table K.1.2. The age of both the spent fuel and HLW were assumed to be 6.5 years. Using
the criteria in Table K.1.2 for 6.5-year-old waste provided a further degree of conservatism
since the criteria were developed for 10-year-old waste (the thermal limits could be
increased for younger wastes). The resulting thermal densities actually achieved in the
first conceptual repositories are listed in Table K.1.4. The limiting thermal parameter,
i.e., near-field or far-field, is denoted by an asterisk. In the case of BWR fuel in shale
and the RH-TRU waste in all media except salt, structural limitations on canister place-
ments limit thermal loading.
Temperature profiles calculated for the conceptual repositories using the achieved
loadings are shown in Figures K.1.1 through K.1.8. The profiles show temperature increases
above ambient temperature as a function of depth at several times after the repository is
loaded, for both spent fuel and HLW and for the four geologic media. For example, the pro-
files for a spent fuel repository at a depth of 600 m in salt with the average loadings of
Table K.1.4, are shown in Figure K.I.1. The figure shows that the temperature at the repos-
itory depth reaches a maximum value about 70 years after emplacement. The calculation is
made assuming that the heat source is uniformly dispersed at the repository level. The tem-
perature is calculated along a line perpendicular to the plane of the repository and passing
through the center of the emplacement area. Actual temperatures in the vicinity of the
repository level will vary with the discontinuities of the temperature profile around each
canister.
Figure K.1.2 gives the profiles for the repository in salt for the high-level waste
from the reprocessing cycle. Corresponding profiles for each cycle are shown in Fig-
ures K.1.3 and K.1.4 for granite, K.1.5 and K.1.6 for shale, and K.1.7 and K.1.8 for basalt
repositories.