Nations confronted with the safe disposal of high-level radioactive wastes expect to solve that problem by using subsurface repositories in deep (<1,000 meters) geologic formations. Principal rock types being considered include domal and bedded salt, granite, argillaceous sediments, basalt, volcanic tuff, and anhydrite. Geologic and hydrologic characteristics of these rocks are being investigated on a regional scale as well as at several specific sites. Thermal, geochemical, and rock-mechanical properties are also being studied by in-situ and laboratory methods.

Subsurface disposal of radioactive wastes has gained increased acceptance from twelve years of favorable experience at the converted salt mine at Asse, Federal Republic of Germany. That nation has also selected the Gorleben salt dome in which to develop the first repository for high-level wastes. Considerable research has been conducted on bedded salt at the WIPP site in the United States, on the granite at Stripa, Sweden under a joint Swedish-American program, and on Tertiary clays at Mol, Belgium, and Trisaia, Italy. Collective developments from fourteen national programs are summarized and include additional applications of "subsurface space" for the disposal of radioactive wastes other than high-level.

INTRODUCTION

During the decade of the 1970's there developed within those nations confronted with the permanent disposition of radioactive wastes an increased commitment toward safely utilizing subsurface space to solve this international disposal problem. Where intermediate- and high- level wastes, including spent fuel, are involved, the technologic approach that has clearly emerged as the most widely supported, especially by the technical-scientific community, and is the most extensively pursued by every national waste-disposal program involves the engineered containment of solidified wastes within subsurface repositories. Although low-level wastes, including some transuranic elements, continue to be emplaced by several nations in shallow trenches and lined boreholes at land-burial sites, interest has also increased in using shallow-depth, bedrock repositories or converting abandoned subsurface mines to handle this category of wastes.

The only non-repository disposal schemes currently being followed or investieated are injection of liquid wastes through controlled boreholes into deep rock units, and placement of containerized wastes into the soft sediment of the deep-ocean sea floor. For more than ten years, the USSR has injected low- and intermediate-level wastes into confined sandstone aquifers some 1,500 meters deep below a site near Dimitrovgrad. Soviet activities in the design of surface engineered storage however seem to indicate a decidedly lower potential for deep-well injection relative to the disposal of high level wastes. Slurry-cement injection of intermediate-level wastes combined with hydraulic fracturing accomplished beneath the United States government facility at Oak Ridge, Tennessee during the 1960's is presently not practiced.

Research into true seabed disposal, as opposed to conventional ocean dumping, is very preliminary, and only three nations, Japan, United Kingdom, and United States, have seriously investigated this approach. Seabed disposal faces both formidable technical problems as well as numerous international policy issues. In the opinion of the authors, these collective obstacles could well preclude this method from becoming a viable disposal option for any of the more hazardous radioactive wastes.

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