For the safe disposal of radioactive wastes in deep boreholes the hole must be properly sealed above the disposal zone so that it does not provide an easier route back to the biosphere than the surrounding geology for any radionuclide bearing fluids. Conventional hydrocarbon and geothermal well seals are unlikely to last the 10~5 or more years required for the isolation of high-level radioactive wastes. Also, they cannot eliminate the zone of damaged rock around the borehole that could act as a by-pass for any radionuclide bearing fluids. We propose instead an innovative concept for sealing deep borehole disposals - "rock welding". This process involves partially melting crushed granite backfill and the granitic wall rock with energy from a down-hole electric heater. Controlled cooling allows the melt to recrystallize to a holocrystalline rock identical to, and continuous with, the host rock in almost all its properties except grain size. Experimental work confirms that such melting and recrystallization are possible under achievable conditions in a deep borehole. Further, thermal modeling demonstrates that rock welds large enough to seal the hole and locally eliminate the zone of damaged rock can be created on a time scale consistent with a disposal program.
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