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

5.36
emplacement. Beyond this initial period of operation, canisters are emplaced without
sleeves and rooms are backfilled. During this later period the wastes are considered to be
recoverable at considerably greater effort than emplacement.
For the conceptual repositories, readily retrievable emplacement spans the initial
5 years of operation. Repository excavation is completed during this period, and no wastes
are emplaced nonretrievably until after the full extent of the repository has been explored.
This provides a period for observation of waste-rock interactions when waste and local rock
temperatures reach their maximum. Repository operations would also be evaluated during this
period and adjustments made if necessary.
The NRC has recently proposed (Federal Register 19 80 )(a) that the repository should be
designed to allow retrieval of wastes for a period of 50 years after termination of waste
emplacement. Whether this proposal might lead to a requirement that the wastes be readily
retrievable for this period of time or recoverable has not yet been determined.
Although the specific requirements for 50-year retrievability have not yet been deter-
mined, requirements for 25-year retrievability have been estimated and the general nature
of requirements for 50-year retrievability can be described. The 25-year retrievability
requirements are described in Appendix K. They include use of sleeve-lined holes and con-
crete plugs and reduced thermal loadings for all of the spent fuel canisters. For 50-year
retrievability the thermal loadings would probably have to be further reduced. An alterna-
tive approach would be to provide continuing ventilation for heat removal to reduce the rock
stresses.
A particular concern for a repository in salt is closure of rooms over long retriev-
ability periods due to accelerated "creep" deformation of the salt caused by the waste's
heat. This can be compensated for, at least to some extent, by increasing ceiling heights
within the repository (7.6 m height for 25-year retrievability versus 6.7 m in height for
5-year retrievability) but this may be a difficult problem for 50-year retrievability.
After repository performance has been adequately verified (after the initial 5 years
of operation for these conceptual repositories, or longer if required), it was assumed that
wastes would no longer be emplaced in a readily retrievable manner. For the remainder of
repository operations, wastes may be emplaced in holes without steel sleeves. As the wastes
are emplaced, the holes are filled with crushed rock or some specially selected backfill
material. The backfill material may be an adsortive material selected to increase the
probability of long-term waste isolation. After a room is filled with waste, it is back-
filled with previously excavated crushed rock or with specially selected backfill material.
During this period of repository operations, the wastes are considered to be recoverable
from the backfilled rooms. Recovery operations are more difficult and costly than retrieval
because of the need to remove room and hole backfill. The nature of these operations
increases the possibility of waste canisters being damaged before or during recovery opera-
tions but conventional techniques should be adequate. It is possible that this condition
(a) Federal Register, Vol. 4, N.94, May 13, 1980, page 31400.