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, leach resistant form. AWE has developed such a process using Ca3(PO4)2 as the host material. Successful trials of the process using actinide-doped Type I waste (essentially a chloride-based 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 using a modified MCC-1 test at 40°C/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 (an oxyhydroxide-based waste) is currently underway at PNNL.

Differences in the ionic state of plutonium 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. Early investigations into the use of HfO2 as the surrogate for Pu(IV) oxide in Type II waste showed some apparent differences in the phase assemblages of the surrogate and actinide-based products. However XRD examination of the products at higher resolution has demonstrated there is no significant difference and that for this work HfO2 is a suitable surrogate for PUO2.