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

Kuibao Zhang,Zongsheng He,Baozhu Luo,Haibin Zhang,Dan Yin

doi:10.1007/s40145-019-0339-z

Abstract

Zirconolite is recognized as one of the most durable waste matrices for the disposal of high-level radioactive wastes (HLWs). In this study, HfO2 was employed as the surrogate of tetravalent actinides. Hf-bearing zirconolite-based composite waste forms (CaZr1−xHfxTi2O7) 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 (LRi) of Ca, Cu, and Hf are 0.25, 3.10×10−2, and 1.11×10−8 g·m−2·d−1.

Highlights

The disposal of high-level radioactive wastes (HLWs) has long been a great and urgent challenge for the nuclear industry [1,2]
The as-synthesized specimens were pulverized into fine powders, which were characterized by X-ray diffractometer (XRD; Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation to obtain the phase compositions
The concentrations of Ca and Cu in the leachates were obtained by inductively coupled plasma (ICP) analysis using an iCPA 6500 spectrometer, while Hf was tested by inductively coupled plasma-mass spectrometry (ICP-MS) analysis using an Agilent 7700× spectrometer

Summary

Introduction

The disposal of high-level radioactive wastes (HLWs) has long been a great and urgent challenge for the nuclear industry [1,2]. Zirconolite-based waste forms were mainly synthesized using traditional methods, such as liquid phase synthesis (hydroxide and sol–gel methods) and solid state reaction sintering [20,21,22]. These approaches are usually time consuming and costly. High temperature (1400 °C or higher) sintering is an indispensable procedure to obtain crystal lattice disposal of actinide-bearing wastes, which inevitably induce the evaporation of volatile nuclides (such as highly radioactive U, Tc, Sr, and Cs elements). The HfO2-doped zirconolite-rich waste forms were prepared by the CS/QP process using CuO as the oxidant and Ti as the reductant. The aqueous durability of representatively solidified Hf-doped sample (Cu-Hf-0.6) was evaluated using standard Materials Characterization Center (MCC-1) leaching test [38]

Experimental details

Results and discussion

Conclusions