Chemical Durability of Yttria-Stabilized Zirconia for Highly Concentrated TRU Wastes
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ABSTRACT Neptunium has an extremely long half-life and will be the main toxic element in the later stage of disposal. Yttria-Stabilized Zirconia (YSZ), with a fluorite structure, samples doped with Np-237 in high concentration (20, 30, 40 mol %)were fabricated (sintered in Air or Ar, at 1773 K, for 80 hours), and their leaching test (in deionized water, at 423 K, for 84 days) was carried out. The results indicated that the obtained leaching rates were much smaller than those of the Synroc and glass waste form, and that the increase in Np content did not cause any drastic changes in the leaching rates of Zr, Y, or Np. They were also compared to the results of YSZ doped with Ce or Nd used as surrogates for actinides. The work showed that YSZ doped with Np in high concentration has an excellent chemical durability. INTRODUCTION Isolation of hazardous radionuclides is one of the most important issues in nuclear waste management. In particular, TRU elements should be treated with special concern because their half-lives are substantially longer than those of other radionuclides. The most common and practical strategy is to fix these hazardous radionuclides into a suitable solid matrix with high stability and high resistance to ground water attack, in order to retain them in underground repositories. Yttria-stabilized zirconia (YSZ) with a fluorite structure has been studied for use in the immobilization of partitioned and highly concentrated TRU waste [1]. The biggest advantage of YSZ lies in the fact that it can form a solid solution with tri- and/or tetra-valent TRU elements and can maintain the cubic system [2-4]. Cubic-zirconia is known to have a higher Possible advantages resistance to a -decay damage than other ceramics such as _A110 3 [5]. from the YSZ waste form are, therefore: (i) the accommodation of TRU elements in high concentration; and (ii) the production of very stable waste forms. In addition, it has been proved that YSZ has an excellent stability, as an inert matrix, to the irradiation of neutron and fission fragments [6]. The resultant high quality waste form therefore could be utilized in other applications, such as TRU burning [7]. The general purpose of the present work is to study the applicability of YSZ as a host matrix for the waste form of highly concentrated TRU. The study was conducted focusing on the chemical durability of YSZ doped with Np. Neptunium has an extremely long half-life (2.14 million years) and will be the main toxic element in the later stage of the disposal. The chemical durability was evaluated by a leaching test. The present evaluations were also conducted using data of Ce or Nd-doped YSZ for comparison. Cerium and Nd are often used 393 Mat. Res. Soc. Symp. Proc. Vol. 608 ©2000 Materials Research Society
as surrogates for tetra- and tri-valent actinides, respectively [1-4]. EXPERIMENT Sample Preparation In the present study, YSZ samples doped with Np-237 in high concentration (20, 30, 40 mol %) were fabricated. The procedure is: (1) an Np solution (Np in nitric acid: 3 or
