A giant radioactive souffle is rising toward the top of a million-gallon tank of nuclear waste buried in the desert near Richland in the U.S. Whipped up unexpectedly by a pump that was supposed to dissipate pockets of hydrogen gas, the waste has smothered one tube for vapour sampling, threatens other instruments and could eventually overflow, according to officials of the Department of Energy and the contractor in charge of the tank, Lockheed Martin Hanford Corp.