Feasibility Limits in Using Cerium as a Surrogate for Plutonium Incorporation in Zircon, Zirconia and Pyrochlore
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Feasibility Limits in Using Cerium as a Surrogate for Plutonium Incorporation in Zircon, Zirconia and Pyrochlore M.V. Zamoryanskaya and B.E. Burakov V.G.Khlopin Radium Institute, 28, 2-nd Murinskiy Ave., St.Petersburg, 194021, Russia Email: [email protected] 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 zirconiabased ceramics. The use of Ce as Pu surrogate for zircon-based ceramics should be considered as limited. INTRODUCTION At present, the development of optimal synthesis conditions for crystalline ceramics based on zircon, (Zr,Hf,…)SiO4, cubic zirconia, (Zr,Hf,Gd,…)O2 and pyrochlore, Ca(Zr,Hf,Gd,…)Ti2O7, which have been suggested for the immobilization of weapons-grade Pu, is the subject of intensive study [1-10]. In order to reduce the cost of experiments, nonradioactive Ce is widely used as a surrogate of Pu. An acceptable strategy for employing a surrogate for Pu, based on using Ce incorporated into the crystalline structures of host-phases, can be achieved under or because of the following conditions: Chemical behavior of Ce under ceramic synthesis conditions is similar to Pu; Ce incorporated into the structures of host phases has a valence state the same as Pu; Ce4+ has a similar ion radius to Pu4+; Lattice capacity of zircon, zirconia and pyrochlore to Ce4+ incorporation is similar to incorporation of Pu4+ in those materials; • Stability of the crystalline structure doped with Ce is similar to one doped with Pu. The main goal of this paper is to investigate the feasibility limits of using Ce as a surrogate of Pu in the structures of zircon, zirconia and pyrochlore. In order to do this the ceramic samples • • • •
doped with Ce were studied in comparison with those doped with Pu. Cathodoluminescence (CL), which is a technique that is sensitive to ion valence state and defects in cryst
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