Combustion synthesis of Hf-doped zirconolite-rich composite waste forms and the aqueous durability

Abstract

Zirconolite is recognized as one of the most durable waste matrices for the disposal of high-level radioactive wastes (HLWs). In this study, HfO 2 was employed as the surrogate of tetravalent actinides. Hf-bearing zirconolite-based composite waste forms (CaZr 1-x Hf x Ti 2 O 7 ) were rapidly prepared by combustion synthesis (CS) using CuO as the oxidant, where quick pressing (QP) was introduced to obtain densified samples. Similar as solid state reaction process, the Zr site of zirconolite can be totally occupied by Hf (x = 1.0) under the CS reaction. The original 2M zirconolite structure was maintained and a small amount of perovskite impurity phase was generated in the final products. The aqueous durability of representative sample (Cu-Hf-0.6) was tested, where the 42-day normalized leaching rates (LR i ) of Ca, Cu, and Hf are 0.25, 3.10×10 -2 , and 1.11×10 -8 g·m -2 ·d -1 .glasses is generally recognized as the first generation waste management method. In recent decades, highly durable ceramic materials have been proposed as an alternative medium for HLW immobilization [4][5][6]. The naturally existed zirconolite and pyrochlore are important actinide-bearing ceramic materials, which exhibit excellent chemical and physical durability over geological time scales [7][8][9][10][11][12]. Zirconolite (ideally CaZrTi 2 O 7 ) was also developed as a crystalline phase to sequester Pu from surplus nuclear weapons, which makes it receive great attention as a potential waste form [13].Zirconolite is formed by the stacking layers of edge shared Ti-O polyhedra (TiO 5 and TiO 6 ) and layers of Ca 2+ and Zr 4+ cations [14]. Due to that the Ti 4+ and Zr 4+ ions can be mutually occupied, the zirconolite J Adv Ceram 2019, 8(3): 448-455 449 www.springer.com/journal/40145

Investigation of the Effect of Milling Duration on a Ce-Gd Doped Zirconolite Phase Assemblage Synthesised by Hot Isostatic Pressing

“…A key design requirement for waste forms containing a considerable actinide fraction is the ability to co-incorporate a suitable inventory of neutron absorbing additive (typically Gd and/or Hf) to reduce the neutron flux in the disposal environment. Zirconolite has demonstrated high solubility of Gd and Hf across various solid solutions, e.g., Ca 1−x Gd x ZrTi 2−x Al x O 7 and CaZr 1−x Hf x Ti 2 O 7 [16][17][18].…”

Effect of improved trialkyl phosphine oxides waste content on phase composition and density of spark plasma sintered Gd 2 Zr 2 O 7 ceramics

“…12 Among these ceramics, pyrochlore is commonly employed to immobilize nuclear waste due to its high thermal stability, 13,14 high thermodynamic stability, 15,16 high chemical durability, 17,18 and physical stability. 15,[19][20][21] In recent years, a series of treatment methods have been developed for incorporating TRPO waste using pyrochlore, including solid reaction method with simple operation, 22,23 self-propagating high-temperature synthesis plus quick pressing method with fast reaction, 24,25 microwave sintering with fast heating rate and low energy consumption, 26 flash sintering with extremely short sintering time, 27 and spark plasma sintering (SPS). [28][29][30][31] SPS has the advantages of fast heating rate and short holding time, which makes it widely used to solidify radionuclides.…”

Synthesis of nearly spherical AlN particles by an in-situ nitriding combustion route

“…Combustion synthesis (CS, also known as self-propagating high temperature synthesis (SHS)), is considered as a promising synthesis route to fabricate AlN powders because of its low processing cost, high energy efficiency and short reaction period [12][13][14][15]. However, because of the high thermal-gradient, high combustion temperature and fast reaction speed, the morphology of the AlN powders is difficult to control and various grain morphologies, such as whiskers, faceted particles, agglomerated particles are often observed [16][17][18][19], which impedes the CS process for wide application.…”