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

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be from 4 x 10 - 3 to 3 x 10 - 2 health effects from the largest dose to the population as pre-
sented in Tables 5.5.3 to 5.5.5 over one million years. By way of perspective, in the
United States the societal risk of death by lightning is about 120 per year, or about
1 x 108 deaths per million years (Accident Facts 1974). Thus, in this framework, the socie-
tal risk from a meteorite breach of a repository is about 3 x 10-10 that from lightning
strikes. Even if the estimate of probability of this meteorite event was in error by a fac-
tor of a billion (as might be the case for the probability of a nuclear detonation over the
repository), the risk to society remains less than that from lightning and can hardly be
considered significant.
5.5.2 Breach of Repository by Fault, Fracture, and Flooding
This scenario is a combination of improbable events: first, a fracture or series of
fractures either from the surface or from near an aquifer penetrates to the repository, sec-
ond, the fractures are connected and permit water to reach the wastes. Two cases are
presented, one where a fairly large stream of water penetrates the repository and leaches
out radionuclides and then, following an assumed conduit, returns to the surface to form a
stream. The second case presumes water reaches the repository and leaches out radionuclides
and transports them beyond the boundaries of the host rock; some of the nuclides are then
held up by adsorption on soils outside the repository area before slowly working their way
to the biosphere. Such scenarios are presented as being independent of host rock
properties.
These scenarios involve improbable combinations of events with very low probabilities
of occurrence, and in some cases are contrary to the evidence available. For example,
faulting of thick salt units does not generally lead to formation of permeable zones, and
the plastic behavior of salt tends to heal any opening. Most of the known faults in salt
formations confirm this self-healing behavior of salt (Claiborne and Gera 1974). Also,
massive salt units generally occur in a geologic environment that contains clays, shales and
argillaceous units that again tend to deform plastically. Faults in rock material that
yield by brittle fracture (granite, basalt, some carbonates) are more likely to form perme-
able zones of crushed, broken rock than faults in salt. However, even in brittle rocks a
fault zone may, through the grinding and crushing of the material, form a zone of very low
to essentially no permeability. That any fault would form a continuously permeable conduit
to the repository is doubtful, even if a fault should occur through the repository to the
land surface.
In this scenario the repository is assumed to be breached by fracturing either at
1000, 10,000 or 1,000,000 years after repository closure. Water in the form of a stream of
2.8 m3/sec ( a ) (100 cfs) invades the repository, flows among the wastes and enters the refer-
ence environment in the R river about 10 km from the repository center. The stream is
assumed to be in contact with the wastes for one year. (This case simulates the subsequent
(a) Several comments were received on the draft Statement that such a large flow of water
was unreasonable. However, the scenario is not all that unreasonable, at least in the
long term. One can envision stream displacement as a result of ice dams, glaciation,
or land slides to where the scenario becomes plausible at least to the extent of entry
of water. Return of water to the biosphere is harder to imagine.