Caustic recovery from alkaline nuclear waste by an electrochemical separation process

Abstract

Abstract Large quantities of highly-alkaline, radioactive liquid wastes are stored in underground tanks at the Department of Energy's Savannah River, Hanford and Oak Ridge sites. These wastes will be pre-treated to separate radionuclides and the remaining decontaminated liquid wastes will be incorporated into solid wasteforms for permanent disposal. Significant savings in disposal costs could result by recovering the sodium hydroxide for re-use. Laboratory tests demonstrated an electrochemical separation process for the recovery of sodium hydroxide from simulated and radioactive Savannah River Site (SRS) wastes. The separation process uses an electrochemical cell separated into two compartments with a cation-selective membrane. The waste solution enters the anolyte side of the cell. Sodium ions migrate across the membrane into the catholyte. Hydroxide forms at the cathode by the reduction of water. The catholyte product is thus sodium hydroxide or caustic. Hydroxide oxidizes at the anode to form oxygen and water. The anolyte product is thus a sodium hydroxide-depleted waste stream. If nitrite is present in the waste solution, the nitrite also oxidizes to nitrate. Two different membranes were evaluated: an organic-based membrane, Nafion® Type 324 or 350, and an inorganic-based membrane, Ceramatec® NASD. Initial tests with the Nafion® membrane with simulated waste solutions indicated that the migration of sodium across the membrane was not significantly affected by changes in the current density, temperature and the concentrations of nitrate/nitrite, hydroxide and aluminate. Both membranes successfully separated caustic from radioactive SRS waste.