Status of Technology for Ultimate Disposal in Underground Repositories Several methods have been considered for the ultimate disposal of encapsulated waste, including ejection into extraterrestrial orbit by rockets, disposal on or under the seabed, nuclear transmutation of long-lived actinides, and isolation at depth in suitable continental geological formations. Only the last of these options, using conventional underground mining technology, is believed to be practical in the near future.

Desirable geological properties for an underground repository include absence of groundwater, low permeability, high plasticity, freedom from joints and faults, good ion-exchange capability, and location in an area of low seismic activity. Rock types that exhibit some or all of these desired qualities include bedded evaporites such as salt and potash, marine shales, unjointed and unfaulted crystalline rocks (igneous and metamorphic), and limestone in arid regions.

Since percolation of groundwater is the only significant mechanism for releasing waste forms from their matrix, evaluation of the suitability of an area depends substantially on properly modeling the transport of radioactive atoms once dissolved. The highly active fission-product waste is hardly at issue here. The matrix in which it is incorporated is expected to be at least mostly resistant to leaching over the period (300–1000 years) during which the waste—principally 90Sr and 137Cs—decays away. (But see below for the U.S. Geological Survey’s reservations on this point) The actinides of medium half-life, such as 239Pu (24,000 years), 240Pu (6500 years), and 241Am (450 years), are the critical nuclides. This underscores the significance of dealing properly with alpha-active waste. There is a good chance that if groundwater were to intrude upon the repository, the matrix could be leached out before these nuclides decayed.

Most actinide transport studies that have been conducted indicate that migration in groundwater will be very slow, being governed by absorption-desorption equilibria with solids in the aquifer, rather than solution transport mechanisms.89 This could be the controlling process in return to the biosphere, and indicates that the ion-exchange behavior of the disposal environment for actinide ions is the most important selection criterion. The resistance to leaching of the glass and the absence of groundwater serve only as “insurance” factors.

This question has recently been reviewed by the American Physical Society (APS) and the U.S. Geological Survey (USGS).90 Although the tones of the reports are different, the APS being generally optimistic and the USGS emphasizing reservations, our study of these documents indicates that their findings are similar. In both cases, there is confidence, primarily based on past experience with radium as a natural tracer of mineral migration in groundwater, that a site chosen with reasonable care will provide the necessary holdup of waste radioactive nuclides. Both reports



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