Development of zirconium magnesium phosphate composites for immobilizationof fission products

Novel chemically bonded phosphate ceramics have been investigated for the c apture and stabilization of volatile fission-product radionuclides. We have used low-temperature processing to fabricate zirconium phosphate and zirco nium/magnesium phosphate composites, A zirconium/magnesium phosphate compos ite has been developed and shown to stabilize ash waste that has been conta minated with a radioactive surrogate of the Cs-137 and Sr-90 species, Excel lent retention of cesium in the phosphate matrix system was observed in bot h short- and long-term leaching tests, The retention factor determined by t he USEPA Toxicity Characteristic Leaching Procedure was one order of magnit ude better for cesium than for strontium, The effective diffusivity, at roo m temperature, for cesium and strontium in the waste forms was estimated to be as low as 2.4 x 10(-13) and 1.2 x 10(-11) cm(2)/s, respectively. This b ehavior was attributed to the capture of cesium in the layered zirconium ph osphate structure via an intercalation ion-exchange reaction, followed by m icroencapsulation, However, strontium is believed to be precipitated out in its phosphate form and subsequently microencapsulated in the phosphate cer amic. The performance of these final waste forms, as indicated by the compr ession strength and the durability in aqueous environments, satisfies the r egulatory criteria.