Caustic recycle from high-salt nuclear wastes using a ceramic-membrane salt-splitting process

Abstract An electrochemical salt-splitting process, based on sodium-ion selective ceramic membranes, is being developed to recover and recycle sodium hydroxide from high-salt radioactive tank wastes in the U.S. Department of Energy complex. The ceramic membranes are from a family of materials known as sodium (Na), super-ionic conductors (NaSICON). Two membrane compositions based on the rare-earth elements, neodymium and dysprosium, and a new proprietary material, NAS-D, have been fabricated as disks and are currently being tested with waste simulants. The membranes have been incorporated into a polyethylene scaffold for implementation into commercially available plate-and-frame electrochemical cells. A purified caustic product with a sodium hydroxide in excess of 3 M was produced from waste simulants with the Dy- and Nd-NaSICON membranes. This is the nominal concentration for onsite recycle and higher concentrations are expected. Membrane fouling was not observed, even though gibbsite {A1(OH) 3 } was precipitated in large amounts during some of the runs. Preliminary testing of the NAS-D material indicates that a sodium current density of 38 mA/cm 2 with a sodium current efficiency of approximately 90% is achievable over 1000 h of operation with an applied potential of 4.5 V.