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

K.24
TABLE K.2.1. Near Field Local Thermal Densities(a) for 25-Year
Ready Retrievability of 10-year-old Spent Fuel
Near-Field Allowable Thermal Loading, kW/acre
Salt Granite Shale Basalt
24 53 36 53
(a) These densities are conservative values
that are 2/3 of the calculated densities.
taken in the 5-yr readily retrievable case, the values in Table K.3.1 are two-thirds of the
calculated maximum acceptable thermal densities for 25-yr ready retrievability.
As discussed previously in Section K.1, Thermal Criteria, the criteria controlling
placement of spent fuel in salt with 5-yr ready retrievability is the far-field average
thermal density. However, in the case of 25-yr ready retrievability, near-field local
thermal density becomes the controlling criterion because maintaining room and pillar sta-
bility for 25 years requires a more restrictive thermal density than is needed to limit
long-term uplift.
An additional concern for the repository in salt is the creep closure of rooms over
the 25-yr period of ready retrievability. To compensate for this, room ceiling heights are
increased 7.6 m for 25-yr ready retrievability (6.7 m for 5-yr ready retrievability).
An alternative approach for achieving 25-yr ready retrievability is to provide heat
removal from the mine by continuously ventilating emplacement rooms. This technique could
allow higher thermal densities by removing heat from the rock formation to keep room and
pillar stresses within acceptable limits. Additional details of this approach are provided
in Y/OWI/TM-44 (Union Carbide Corp. 1978).
The unit cost for providing 25-yr ready retrievability for emplaced spent fuel elements
at a repository located in salt is $90/kg HM (mid-1978 dollars) compared to $54/kg HM for
5-yr ready retrievability. The primary reason for this difference in cost is the reduction
of repository waste capacity by about a factor of two for the 25-yr ready retrievability
option. Another contribution to the higher cost is $70 million for additional mining and
backfilling that is necessary as a result of increased ceiling height for the repository in
salt. Use of sleeves for all emplaced wastes also costs an extra $4 million annually. Unit
costs for 25-yr readily retrievable emplacement of spent fuel in the other rock media would
also increase although additional mining to increase ceiling height would not be required.
During the initial phase of readily retrievable emplacement, removal operations are
relatively straight-forward. Because rooms are left open and the lined emplacement holes
are sealed with removable concrete plugs, removal of emplaced wastes simply involves revers-
ing the emplacement procedures. A transporter reenters the emplacement room and positions
itself over the sealed hole. The concrete plug would be removed and the-waste canister
raised into the transporter. The transporter then delivers the canister to a waste receiv-
ing station where it is loaded into a shaft and lifted to the surface. On the surface the
canisters are placed into temporary dry well storage until a new repository is ready.