Solidification of glass-ceramics of simulated fission products RE3+, Sr2+, Ba2+ in chlorine-containing waste molten salt

Abstract

Embedding nuclear waste in glass-ceramic and immobilizing nuclides in ceramic lattice is an effective way for the disposal of high-level radioactive waste. In this paper, a method of solidification of simulated various nuclides was proposed, i.e., RE3+(RE = La, Sm, Nd, Dy), Sr2+ and Ba2+ precipitated from waste molten salt in the form of REPO4, SrCO3 and BaCO3 were solidified in glass-ceramics. To avoid the decomposition of SrCO3 and BaCO3 at high temperature, SrCO3/BaCO3 containing Cl− salt was further sintered with NH4H2PO4 to form Sr5(PO4)3Cl/Ba5(PO4)3Cl ceramics. It was found that the prepared REPO4 belonged to monoclinic or tetragonal crystal system, while Sr5(PO4)3Cl and Ba5(PO4)3Cl belonged to hexagonal crystal system. REPO4, Sr5(PO4)3Cl and Ba5(PO4)3Cl ceramics were co-solidified in iron phosphate glass. BET results showed that the ceramics had a dense structure without any pore inside. XRD, TEM and HRTEM results showed all ceramics had high crystallinity, and nuclides could enter the lattice structure of ceramics through isomorphic replacement, which made the nuclides stable in the crystal structure. The effects of embedding rate on the volume density, Vickers hardness and wettability of glass-ceramics were explored. It was found that the density of the glass-ceramics gradually increased with the increase of ceramic embedding rate, however, the Vickers hardness firstly increased and then decreased. When the embedding rate reached 20 wt%, the Vickers hardness of the glass-ceramics could reach 583.90 GPa. The water contact angles of glass-ceramics with an embedding rate 0–40 wt% were measured to be 70.45°–84.05°, indicating glass-ceramics having a good water leaching resistance. Furthermore, the normalized leaching rate NRi of La3+, Sm3+, Nd3+, Dy3+, Sr2+, Ba2+, Cl− on the 28th day were estimated to be 7.53 × 10−7, 5.02 × 10−7, 5.12 × 10−7, 4.04 × 10−7, 1.22 × 10−3, 1.59 × 10−4, 1.07 × 10−4 g‧m−2‧d−1, which indicating that all elements remained good leaching resistance.