Technical demonstration of zirconolite glass-ceramics processed in a hot isostatic press: An option for immobilisation of actinide containing residues at Sellafield

Abstract

For the past three years the Australian Nuclear Science and Technology Organisation (ANSTO) has been collaborating with Nexia Solutions (formerly BNFL Nuclear Science and Technology Services (NSTS)) to define technical options for the immobilisation of heterogeneous plutonium and other actinide-bearing residues stored on the Sellafield site. The goal is to create a stable matrix that immobilises the residues in a waste form that is suitable for long term passive storage and eventual disposal in a repository. As a result of this collaboration, a Hot Isostatic Press (HIP) processing option is being developed that will enable the material to be successfully immobilised in a highly chemically durable and proliferation resistant glass-ceramic. The use of a glass-ceramic waste form is proposed for these highly heterogeneous residues because it provides the advantage of combining the necessary processing flexibility of glasses with the chemical durability of ceramics, which are most suited for immobilising actinides. We have successfully demonstrated the viability of the baseline zirconolite glass-ceramic waste form and HIP process, and studied the effects of in-can redox, temperature, and Pu concentration on phase development, Pu partitioning, and aqueous durability. Resistance to retrieval of the Pu from the waste form when simple crushing and dissolution in concentrated sulphuric or nitric acids was used has also been demonstrated. These initial studies have yielded the following main outcomes: HIP processing of the glass-ceramic has been successfully demonstrated at the -260 g scale with U+Th as Pu surrogates, and validated at the -35 g scale with Pu. Pu partitioning into zirconolite over glass in excess of 100:1 has been achieved. Extremely low 28 day normalised Pu releases of 10-4 to 10 -5 g.m-2.d-1 have been obtained in modified MCC-1 leaching tests, consistent with actinide releases, from synroc-type titanate ceramics. Excellent Pu proliferation resistance has been demonstrated. Plutonium retrievability tests indicate that between 11 000 kg and 20 000 kg of glass-ceramic would need to be diverted, finely crushed up, and leached in nitric acid to recover ∼1 kg of PuO2. Copyright © 2005 by ASME.