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

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5.5 LONG-TERM ENVIRONMENTAL CONSIDERATIONS OF GEOLOGIC DISPOSAL OF RADIOACTIVE WASTES(a)
The objective of disposal of radioactive wastes in deep geologic repositories is to
provide reasonable assurance (b ) that the radionuclides contained in these wastes in biologi-
cally significant concentrations will be permanently isolated from the human environment.
The following presentation examines the likelihood and consequences of events that could
compromise this objective over the millenia following repository closure.
No significant long-term physical impacts are expected to result from having placed the
heat-emitting radioactive wastes in geologic repositories as described previously in this
Statement whether located in salt, granite, shale or basalt formations. Although heat from
decaying radionuclides will ultimately reach the surface of the earth via conduction through
overlying rock, temperature rises at the surface were estimated to be less than 0.5 0 C in all
cases. Such a temperature rise is insignificant. Heat flowing into and through the rock
surrounding repositories will cause expansion of the rock and would result in some uplift
at the surface. The largest uplifts (over several centuries) are expected to be on the
order of 0.3 to 0.6 m (1 to 2 ft) in shale, and 1.2 to 1.5 m (4 to 5 ft) in salt at the cen-
ter of the 800 ha (2000 acre) repository area.
Subsidence of the formation containing a waste repository following closure or collapse
of the void spaces that remained after the mine has been backfilled (backfilled to 60% of
volume) might occur at repositories in salt and shale. Uplift and subsidence are expected
to occur over very long time periods, and as a consequence no impacts associated with earth
movement are expected to result. For repositories located in granite and basalt, subsidence
or uplift is believed unlikely.
Nuclear waste repositories will be sited, loaded, and sealed with every expectation
that long-term radiological impacts will be nonexistent. There are, however, a few highly
improbable events that can be postulated to take place singularly (or in combination with
smaller probability events) that might result in radioactive wastes reaching the biosphere.
Three kinds of events leading to release of some of the repository contents were postulated:
* direct release of contents to the atmosphere: Such release could follow volcanic
activity, impact of a large meteorite or large nuclear weapon, or, on a much
longer time scale, denuding of the earth to the depth of the repository by erosion
or glaciation. Releases and consequences of these events are believed to be ade-
quately represented by those of a meteorite strike; however, the probability of
occurrence could be substantially different.
(a) "Long-term" as used here means hundreds to tens-of-thousands of years after the repository
has been closed.
(b) "Reasonable assurance" is admittedly a subjective expression. While DOE believes that
shallow land burial for spent fuel, HLW, remotely handled TRU or fuel reprocessing
wastes would not give such reasonable assurance, DOE believes that at some depth isolation
is reasonably assured. Depths on the order of hundreds of meters are believed to
meet this requirement.