Salt Disposal of Heat-Generating Nuclear Waste

placed over the waste canisters for radiological shielding. The operation of
placing the crushed salt over the waste would involve remote controlled loadhaul-dump
machinery similar to that used in salt mining today.
Width of the main disposal drifts and alcoves was selected for mining
convenience. The typical continuous miner used at WIPP today cuts nominally
11-foot swaths. Therefore, the mining system would involve essentially one-pass
mining. The stand-up time under ambient conditions associated with roughly 11-
foot dimensions was not calculated, but based on WIPP experience structural
stability would endure for many years without bolting.
The width-to-height ratio, extraction ratio, depth, and geometry govern
underground standup time. Pillars take up the load when excavations are created.
The width-to-height ratio of the pillar controls the pillar strength. Shorter pillars
are stronger than taller pillars because of the confining effects experienced at the
ends. For this generic salt repository layout, a nonspecific room height from seven
to ten feet could be readily mined with current equipment.
Other recommendations included avoiding sequential co-disposal of remotely
handled waste and contact-handled waste. The disposal sequence should proceed
from the most distal excavations inward. This avoids having to transport past
filled disposal rooms and promotes modular panel isolation. The concept of
disposal in salt can be flexible. Heat generation, though important, can be readily
accommodated by design.
2.3 Seals
This section describes a shaft sealing system design for the WIPP, which has been
reviewed and certified by the EPA regulator. The system is designed to limit entry
of water and release of contaminants through the existing shafts after
decommissioning. The design approach applied redundancy to functional
elements and specifies multiple, common, low-permeability materials to reduce
uncertainty in performance. The system comprises 13 elements that completely
fill the shafts with engineered materials possessing high density and low
permeability. Laboratory and field measurements of component properties and
performance provided the basis for the design and related evaluations.
Hydrologic, mechanical, thermal, and physical features of the system were
evaluated in a series of calculations. These evaluations indicated that the design
effectively limits transport of fluids within the shafts, thereby limiting transport of
hazardous material to regulatory boundaries. Additionally, the use or adaptation
of existing technologies for placement of the seal components combined with the
use of available, common materials ensure that the design can be constructed
(Hansen and Knowles 2000).
The design of the seal system for a HLW salt repository would benefit from
design and performance calculations on seal systems developed for the WIPP,
which were subject to extensive technical peer review and comprise published
20