washington

The US Department of Energy favours placing nuclear waste inside 8-metre-wide tunnels excavated in a dry part of Yucca Mountain (see above). The tunnels will be 300 metres below the land surface, and 300 metres above a water table. The waste will be encased in steel tubing coated with a two-centimetre-thick layer of a special, corrosion-resistant nickel alloy.

Congress's scientific advisory board on nuclear waste says alternatives to the nickel-alloy coating need to be investigated. The shell is designed to resist the eventual corrosion from the outer steel shell by forming a thin layer between itself and the steel. But the board says that the layer itself may be vulnerable to other forms of damage.

William D. Barnard, the board's executive director, says the board also believes that the waste should be stored in such a way that temperatures can be maintained below 100 °C. Under the DoE's planned design, the waste will be densely stored, generating temperatures of around 200 °C.

These temperatures, the board says, may cause water above the tunnels — possibly having percolated from rain or snowfall — to boil. It would then vaporize, cool down, condense and possibly flow downwards in the direction of the tunnels.

Allen Benson, a spokesman for the department, says that an eight-year research programme is underway to assess the effect of high temperatures on the flow of water through the rock. But he says it is unlikely that the design of the metal tubing will change.