Salt Disposal of Heat-Generating Nuclear Waste

Considerable qualitative support for this strong impression derives from pertinent
analogues.
Many anthropogenic and geologic analogues provide insight into permanent
nuclear waste disposal in salt. Anthropogenic analogues derive from over 7,000
years of salt excavation by mankind and wide use of salt formations, including
storage of fluid hydrocarbons. Fifty years ago the unique sealing capability of salt
was dramatically demonstrated by the most severe test possible: containment of
nuclear detonations in salt horizons. In addition to anthropogenic evidence from
mining experience and nuclear detonations, nature itself showcases the
encapsulating ability of salt formations penetrated by high-temperature magmatic
dikes. The analogues summarized here provide qualitative evidence that salt
formations have the capacity to contain a wide variety of severe conditions
permanently.
Sealing a nuclear waste repository and waste corrosion are considered two
primary issues with salt disposal. Anthropogenic evidence associated with vast
and pertinent mining experience provides important qualitative assessments of
preserved artifacts. The Hallstatt area in Austria supported prehistoric salt
mining. Archeological re-excavation has recovered organic material such as
leather, wood, clothing and even an unfortunate Celtic miner preserved in the salt
for 3,000 years. More recent analogues spanning 50 or 100 years of mining
experience offer convincing evidence of structural, mechanical, and hydrological
behavior of the underground salt environment. The salt and potash mining
industry has placed functional seals in underground workings, backfilled, and
reconsolidated crushed salt in active mining operations. Within the potash basin in
southeastern New Mexico mining operations, machinery which is not brought
back to surface suffers almost no corrosion after more than 50 years.
The most dramatic man-made analogues—considered beyond-worst-case
analogues—are nuclear detonations in salt (Rempe 1998). The USA has
detonated three nuclear devices in salt: one at the Gnome Site near WIPP and two
at the Salmon Site at Tatum Dome in Mississippi. The 3.1 kiloton Gnome shot,
which unintentionally breached drift seals and sent radioactive steam up the shaft,
was cleaned up by dumping the surface material down the shaft and thereafter
recommended for public release (Gardner and Sigalove 1970). The Gnome shot
was situated near the top of the Salado Formation, roughly half as deep as the
WIPP repository. No migration has been detected outside the experiment’s
boundary for half a century. The Tatum tests involved two sequential devices; the
second was detonated in the cavity created by the first. By virtue of monitoring
results, the radioisotopes were determined to be confined to the test cavity, i.e.,
the test cavity was determined not to be leaking radioactivity (DOE 1999). The
Tatum shots were executed at a depth of 2,700 feet. Salt formations have been
shown to seal and confine nuclear detonations.
Geologic analogues also supply strong evidence of the confining nature of salt
formations. Examples of natural geologic analogues include salt formations in
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