[en] The solidification of high-level wastes is being evaluated as a method for final control of high-level nuclear wastes. The high temperatures required in the solidification process cause the volatilization of small amounts of fission products, primarily ruthenium and cesium. In order to determine cleanup requirements, a good understanding must be gained as to the quantities of fission products volatilized and how these quantities relate to operating conditions. The Pacific Northwest Laboratory, operated by Battelle-Northwest for the U.S. Department of Energy, has been investigating ruthenium and cesium volatilization in various calciners. Studies have been made using both simulated (nonradioactive) and radioactive feedstock. The ongoing investigations have indicated that the semivolatile fission product losses are small enough to be controlled by off-gas cleanup treatment. During recent spray calcination of simulated high-level liquid waste, not more than 0.2 percent of the ruthenium and 0.1 percent of the cesium were lost to the off-gas system. Decontamination factors of about 600 for ruthenium and 1150 for cesium have been obtained across the spray calciner