PNNL

Abstract

The presence of halide anions in four types of wastes arising from the pyrochemical reprocessing of plutonium required an immobilization process to be developed in which not only the actinide cations but also the halide anions were immobilized in a durable waste form. At AWE, we have developed such a process using Ca3(PO4)2 as the host material. Successful trials of the process with actinide- and Cl-bearing Type I waste were carried out at PNNL where the immobilization of the waste in a form resistant to aqueous leaching was confirmed. Normalized mass losses determined at 40°C and 28 days were 12 x 10-6 g·m-2 and 2.7 x 10-3 g·m-2 for Pu and Cl, respectively. Accelerated radiation-induced damage effects are being determined with specimens containing 238Pu. No changes in the crystalline lattice have been detected with XRD after the 239Pu equivalent of 400 years ageing. Confirmation of the process for Type II waste (a oxyhydroxide-based waste) is currently underway at PNNL. Differences in the ionic state of Pu in the four types of waste have required different surrogates to be used. Samarium chloride was used successfully as a surrogate for both Pu(III) and Am(III) chlorides. Initial investigations into the use of HfO2 as the surrogate for Pu(IV) oxide in Type II waste indicated no significant differences.