Feasibility Limits in Using Cerium as a Surrogate for Plutonium Incorporation in Zircon, Zirconia and Pyrochlore

Abstract

Cerium is a well-known surrogate element for Pu that is widely used in experiments on the synthesis and study of ceramic waste forms for actinide immobilization. Cathodoluminescence (CL) images and emission spectra from synthetic zircon, (Zr,…)SiO₄, cubic zirconia, (Zr,Y,Gd,…)O₂ and pyrochlore, Ca(Hf,Gd,…)Ti₂O₇, individually doped with Ce and Pu were investigated. It was determined that some of the Ce, which was added as Ce⁴⁺ in the form of CeO₂, was converted to the 3+ valence state during synthesis of zircon-based ceramics by sintering in air and in argon gas. However, Pu-doped zircon synthesized in air under the same conditions does not exhibit the CL emission of Pu³⁺. Analysis of CL spectra of cubic zirconia doped with Ce as well as Pu suggests that both ions are in the tetra-valent state in these synthetic materials. However, the CL peak of Ce-doped zirconia is almost identical to that of pure zirconia (2.6 eV). The CL peak for Pu-doped zirconia is shifted to 2.4 eV. No CL emission of Ce³⁺ or Pu³⁺ was observed in pyrochlore samples. The CL spectra for Ce-and Pu-doped pyrochlore samples were almost identical. We suggest that under oxidizing conditions Ce is good surrogate of Pu for experiments on the synthesis of pyrochlore-based and acceptable enough for zirconia- based ceramics. The use of Ce as Pu surrogate for zircon-based ceramics should be considered as limited.