Toward Sustainable Cementitious Radioactive Waste Forms: Immobilization of Problematic Operational Wastes

Developing effective radioactive waste management practices is essential for ensuring the sustainability of the nuclear industry. The immobilization of radioactive wastes is one of the main activities conducted during the management of these wastes; it aims to produce a durable waste form that has sustainable performance over long periods of time. In this work, the challenges that face the design of durable cementitious waste forms are addressed for problematic operational wastes. In this respect, the problematic characteristics of evaporator concentrates, spent ion exchangers, and organic liquid wastes are overviewed, and the factors that affect the durability of their cementitious waste forms are identified. A summary of potential conventional and innovative cementitious matrices is presented by reviewing the cementation practices in national programs and recent research devoted to developing durable matrices. Finally, a guide to optimize the mix design of these waste forms was proposed that includes the selection of the testing procedure, factors that affect the waste form performance, and the optimization technique. This guide was presented with special focus on leaching tests, which are a means to test the stabilization performance of nuclear waste forms.

Immobilized nuclide and leaching mechanism of zircon waste forms efficient synthesized by molten salt

Molten salt has rapid mass transfer and nucleation process, so it can synthesize ceramic solid solution of immobilized high-level radioactive waste at low temperature. The chemical stability in the process of interaction with groundwater determines the ability of matrix phase to prevent radionuclides from entering the biosphere and the release form of radionuclides. Nd-doped ZrSiO4 ceramics with different sintering temperature (1100-1500 ℃), sintering time (3-24 h) and molar ratio of salt to oxide (3:1, 7:1 and 10:1) were prepared by molten salt method. The sintered ceramic is Zr1−xNdxSiO4−x/2, where x is the solubility of Nd in ZrSiO4. The results show that the optimum molar ratio of molten salt to oxide is 10:1, which can quickly synthesize zircon waste form at low temperature. The chemical stability test shows that the normalized leaching rate of trivalent nuclides in zircon structure is in the order of ~10−5 g·m−1·d−1, and the surface layer is dissolved. The experimental results show that zircon structure is an excellent waste form.

Study on Structure and Properties of La2O3-Doped Basaltic Glasses for Immobilizing Simulated Lanthanides

The La2O3-doped basaltic glass simulated high-level waste form (HLW) was prepared by the solid-state melt method. The simulated waste La2O3 maximum loading and the doping effect on structure, thermal stability, leaching behavior, density, and hardness of basaltic glasses were studied. XRD and SEM results show that the simulated waste loading of La2O3 in basaltic glass can be up to ~46 wt.%, and apatite (CaLa4(SiO4)3O) precipitates when the content of La2O3 reaches 56 wt.%. Raman results indicate that the addition of La2O3 breaks the Si–O–Si bond of large-membered and four-membered, but the number of A13+ involved in the formation of the network increase. Low content of La2O3 can help to repair the glass network, but it destroys the network as above 26 wt.%. DSC results show the thermal stability of simulated waste forms first increases and then decreases with the increase of La2O3 content. With the increase of La2O3 content, the density of the simulated waste form increases, and the hardness decreases. The leaching chemical stability of samples was evaluated by the ASTM Product Consistency Test (PCT) Method, which show that all the samples have good chemical stability. The leaching rates of La and Fe are three orders of magnitude lower than those of the other elements. Among them, L36 has the best comprehensive leaching performance.