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

K.5
Step 5: Make near-field rock mechanics calculations to determine the areal thermal
loading that assures room and pillar stability.
Step 6: Determine maximum canister load from Step 4 data for the areal thermal load
from Step 5.
Step 7: Make far-field thermal and rock mechanics calculations to assure that far-
field design limits are not exceeded.
If any of the tentative limits in Table K.1.1 are exceeded in any of the above steps,
the previous steps are revised and repeated until the calculational results indicate that
the limits are not exceeded.
For spent fuel repository analyses, the above procedure was modified slightly. Because
it was decided to place PWR or BWR spent fuel assemblies in individual canisters, the thermal
load for a given canister was determined, and Step 6 above was not required. Steps 1
through 3 were followed by Steps 5 and 7. Very-near-field heat transfer calculations were
then performed to determine if canister or spent fuel temperature limits were exceeded.
This iterative procedure results in baseline thermal load design values for the canis-
ters in terms of kW per canister at waste emplacement and for the loading of a repository
room (local areal thermal load) in kW/acre. The canister load must be sufficiently low so
that the waste and canister temperatures do not exceed the values in Table K.1.1. The local
areal thermal load must be sufficiently low so that rock mechanics analyses predict room and
pillar stability throughout the readily retrievable period, and so that near-field hydraulic
conductivities are not significantly increased and long-term.as well as far-field restrictions
are not exceeded.
The design thermal limits generated by these analyses depend strongly upon character-
istics of the repository site and formation. These characteristics include media strength,
stress-to-strain ratio, heat capacity, thermal conductivity, overlying strata and their
characteristics, etc. The following simplifying assumptions were made for these analyses:
* Only high-level waste and spent-fuel canisters are considered.
* The entire repository is assumed to be loaded simultaneously and instantaneously.
* Thermal properties of geologic media and other materials are based on reasonable
estimates.
* The effects of stress upon thermal properties are not included.
* The presence of water is neglected in the thermal analysis.
* Only simplified horizontal stratigraphies are assumed.
* No compaction or subsidence of the formation is considered.
The analyses utilize cylindrical symmetry to describe the temperatures within the waste
package. Details of the waste package including overpack and other contents of the
emplacement hole are taken into account. In the case of spent-fuel canisters, details of
the assemblies, radiation and convection are explicity included in the calculation. The