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

5.6
geologic setting may require extensive geologic mapping, some field exploration, and remote
sensing surveys, especially in areas that are not yet thoroughly studied.
5.1.1.3 Generic Basis for Repository Design
Several conceptual designs for repositories have been proposed. The surface structures
of a repository do not present unique engineering problems. The typical conceptual design
of the underground portion of a repository consists of numerous excavated storage rooms (at
one or more levels) interconnected by tunnels which serve as transportation and ventilation
corridors. The undisturbed rock masses that separate the storage rooms are called pillars.
Boreholes will be drilled into the floors (and possibly walls) of the rooms. The waste will
be placed in these boreholes. The repository levels are reached from the surface handling
facilities through vertical shafts.
The integrity of a repository will depend largely on the state of stress level in the
rock, the ground-water flow, the strength of the rock, heating and radiation effects from
the wastes, and the layout of the excavations and the disposition of the waste within them.
A large body of pertinent data exists which presents and analyzes each of the above factors.
The results indicate that thereare no fundamental geological or mechanical reasons why
excavated repositories should not be used at suitable sites in rock.
The cost of excavating the repository and the cost of rock support systems depend on
several interrelated geologic factors: rock strength, rock fractures, rock hardness, rock
permeability, rock heating by decay of radioactive nuclides, the state of rock stress, the
depth of waste placement, and others. The extent to which these factors influence cost is
difficult to determine in advance of construction; unforeseen rock conditions are often
encountered in conventional mining operations and in some cases can significantly change the
design and the predicted cost. Cost estimates for geologic repositories are given in Sec-
tion 5.6 of this Statement.
5.1.2 Engineered Barriers
The multiple barrier concept of waste isolation and containment includes both natural
or geologic and engineered barriers. Various aspects of engineered barriers are discussed
in this section.
5.1.2.1 Engineered Barriers--Waste Package System
The term "waste package" as used in this Statement includes everything that is placed
in the waste emplacement hole, e.g., the solidified waste form, canister, overpack, filler
and backfill materials, and hole sleeve. The function of the waste package is to:
* Contain the waste for periods sufficient to allow most of the fission products to
decay to very low levels.-
* Limit the rate of release of radionuclides to the near-field (within the reposi-
tory proper, see p. K.4) host rock system.