A steamy end to seals.

Bentonite clay swells when it comes in contact with water. This property makes it very useful in science and industry as a sealing agent. This also makes bentonite attractive as an impermeable barrier surrounding high-level nuclear waste packages or to fill in tunnels, shafts and rooms in deep, underground geologic repositories. Recent experiments, however, show that bentonite, under the conditions likely to be found in a repository, loses its ability to slow the trickle of groundwater toward waste packages.

In mined repositories like those in basalt, the waste package would sit below the water table in fractured rock. Temperatures could rise to 300[deg.]C. Any water present would be turned into steam. Reporting in the Nov. 7 NATURE, Rex A. Couture of the Argonne (III). National Laboratory says that contact with steam at 150[deg.] to 250[deg.]C rapidly and irreversibly reduces bentonite's ability to swell and fill fractures. Reduction of the swelling capacity may also occur at temperatures below 150[deg.]C, especially after long periods of time, says Couture. Bentonite used to fill tunnels in nuclear waste repositories could reach high enough temperatures to be adversely aflected by water vapor.

"The effects observed here need to be addressed for safe design of nuclear waste repositories," says Couture, "and additional work on the mechanism of alteration may be required."