COPRECIPITATION OF MONOVALENT, DIVALENT, TRIVALENT, TETRAVALENT AND HEXAVALENT IONS WITH NA-POLYURANATES

The objective of this work is to study the solubility and coprecipitat ion phenomena related to spent fuel dissolution under anoxic condition s in order to identify potential upper limits of radionuclide release. In the present work it is attempted to identify those elements for wh ich coprecipitation phenomena are significant. The pH dependent precip itation behaviour of minor elements with Na-polyuranates is studied in carbonate-free 5 m NaCl solutions. Three different systems were inves tigated: ''System A'' NaCl-Cs-Sr-Eu-Mo-Th-U(VI)-H2O. ''System B''' NaC l-Cs-Sr-Eu-Mo-TH-H2O. ''System C'' an aqueous solution of dissolved SI Mulated FUEL (SIMFUEL) containing: NaCl-Sr-BaY-La-Ce-Nd-Zr-Pd-Mo-Rh-Ru -U(VI)-H2O. The experimental results obtained in this work show that: 1) The presence of minor elements do not influence the precipitation o f Na-polyuranate. 2) Solution concentration of trivalent rare earth el ements reveal a strong decrease with pH, possibly controlled by coprec ipitation. 3) Tetravalent elements such as Th (Sys. A and B) and Zr (s ys. C) show a strong decrease in their concentration independent of pH . Th concentrations were not controlled by coprecipitation but probabl y by formation of individual solubility controlling phase such as Th(O H)(4). 4) Sr and Cs stay in solution during the precipitation process.