Simultaneous immobilization of radionuclides Sr and Cs by sodium zirconium phosphate type ceramics and its chemical durability

Sodium zirconium phosphate (NaZr2(PO4)3, NZP) type phosphate compounds have been considered as a candidate material for the immobilization of radionuclides. In this work, the highly densified NZP-type ceramic waste forms for immobilizing simulated radionuclides Sr and Cs, which were designed as the formula of Cs1-2xSrxZr2(PO4)3 (x = 0, 1/12, 2/12, 3/12, 4/12, 5/12, and 6/12), were prepared by microwave-assisted solid-state sintering method. The effects of Sr and Cs incorporation on the phase composition, microstructure, densification, and chemical durability of Cs1-2xSrxZr2(PO4)3 ceramic waste forms were systematically discussed. It was shown that the single CsZr2(PO4)3 (CsZP) phase was generated in the samples when x ≤ 2/12, while two phases of CsZP and Sr0.5Zr2(PO4)3 (SrZP) were formed when 3/12 ≤ x ≤ 5/12. The Rietveld refinement results revealed that Sr/Cs could be incorporated in the NZP crystal structure. The as-prepared samples all presented a well dense microstructure, whose relative density reached up to approximately 98% with Sr incorporation. In addition, the Product Consistency Test (PCT) leaching results demonstrated that the ceramics waste forms simultaneously immobilizing Sr and Cs exhibited superior leaching resistance, and the leaching rates of Sr and Cs elements were in the order of 10−3-10−4 g m−2 d−1. The increase of Sr incorporation brought about the decreased leaching rates of ceramic samples.