A Cathodoluminescence Investigation of Pyrochlore, (Ca,Gd,Hf,U,Pu) ₂Ti₂O₇, Doped with ²³⁸Pu and ²³⁹Pu

Abstract

Crystalline ceramic based on the pyrochlore structure [(Ca,Gd,Hf,U,Pu) ₂Ti₂O₇], has been proposed in the U.S. as a candidate waste form for the immobilization of weapons grade plutonium. Several samples of synthetic polycrystalline pyrochlore doped separately with ²³⁸Pu and ²³⁹Pu were studied using cathodoluminescence (CL) and electron probe microanalysis (EPMA) immediately after synthesis and then after 175, 245 and 405 days. The CL spectra of pyrochlore doped with ²³⁹Pu and ²³⁸Pu obtained immediately after ceramic synthesis and 175 days later were nearly the same and characterized by a typical broad band emission with a maximum peak centered at 2.5 eV. New CL peaks with maximum intensities at 2.0 and 2.3 eV in the emission spectra of ²³⁸Pu-and ²³⁹Pu-doped pyrochlore were observed 245 days later. It was determined that this newly formed CL peak at 2.3 eV is similar to one in CL spectrum of artificial standard glass doped with the uranyl ion, (UO₂) ²⁺. It was suggested that CL band with maximum at 2.0 eV is caused by the U tetrahedral complex (UO₄) ^2-. Also, the same peaks were observed in the CL spectrum of natural U-rich pyrochlore containing approximately 23 wt.% U (mainly in the form of uranyl ion). The results obtained allow us to conclude that the radiation damage of pyrochlore-based ceramic waste form is accompanied with conversion of tetravalent uranium incorporated into pyrochlore structure to the mobile uranyl ion, (UO₂) ²⁺, and complex (UO₄)^2-.