The world in general, and the UK in particular, has belatedly woken up to the fact that we must remove over 50 years backlog of radioactive waste (radwaste) from the biosphere. Much radwaste is already in forms suitable for deep geological disposal but some, such as the slurries and supernates currently stored in tanks at Hanford in the US, requires extensive treatment before it can be safely removed from our environment.

The Immobilisation Science Laboratory (ISL) at the University of Sheffield in the UK is a British Nuclear Fuels (BNFL) university research alliance specializing in the incorporation of radioactive waste into solid, durable matrices for eventual repository disposal. Current research is examining ways of improving technology and wasteforms for vitrification (in glass) of high level waste (HLW) and cementation (in composite cements) for intermediate level waste (ILW), as well as developing new matrices for wastes with no current wasteform such as ceramics for plutonium.

What is immobilization and why do we need it? Some of the radionuclides generated by producing power from nuclear fission are particularly unpleasant and long-lived. For example, 129I has a half-life of 15.7 million years and, if ingested, accumulates in the body and damages the thyroid gland. As a result, it is necessary to make any wastes containing such radionuclides into a durable form in which such elements are securely locked. Many wastes are gases or liquids that must be converted into solids. Waste is immobilized by chemically incorporating it into the structure of a suitable matrix (glass or ceramic) so it is captured and unable to escape. Waste may also be encapsulated or physically surrounded by a material that isolates it and prevents radionuclide escape such as composite cements (based on ordinary Portland Cement, blast furnace slag, and pulverized fuel ash), which may also contain phases capable of chemically immobilizing many radionuclides. Borosilicate glasses are particularly good hosts for radwaste, as they can accommodate most elements, but multiphase ceramic systems based on phases such as zirconolite, hollandite, and perovskite, e.g. Synroc (synthetic rock), have also been developed to immobilize complex waste streams.

An important aspect of immobilization is the need to understand the hierarchy or levels of immobilization ranging from the atomic level (the location of radionuclides in the glass or crystal structure) through the microstructural level (controlling defects or grain boundaries in ceramics and spent fuel), the packaging system (the metal canister in which the wasteform is placed), the near-field repository environment (including the metal and concrete engineered structure and back-fill materials (such as clay and concrete), and the far-field geosphere and biosphere. All of these aspects are important to retain radionuclides in a repository for as long as possible, and much research is being done to find the most stable glass, ceramic, and cement waste hosts, the most stable and nonwasteform interactive metal containers, and nanoparticles for the back-fill systems, which can sorb any radionuclides that eventually breach the container.

Many international research programs, such as the four-year European NF-PRO Integrated Project, are aimed at understanding the key interaction processes in the near field, and their coupling, for different host rocks and repository strategies. This is also a massive modeling challenge because of the length and time scales involved. The radionuclides must remain isolated for at least 100?000 years, a period that will typically span several ice ages. One problem is that some countries (such as the UK) have not decided on a repository location and so cannot be sure of the nature of the surrounding rock on which to base the models.

The last five years have seen much progress in cleaning up radwaste sites. Important milestones include emptying the liquid from all of the Hanford tanks and initial decommissioning of several of the UK's redundant reactors and nuclear research laboratories. The UK government has set up the Nuclear Decommissioning Authority (NDA) to clean up its civil nuclear legacy in ways that safeguard the environment. In addition, the Committee on Radioactive Waste Management (CoRWM) will report to the UK government in July 2006 on options for long-term disposal of HLW and ILW. CoRWM is likely to recommend that a deep level geological repository is the best option, but its location is not within its remit. Allied to these steps has been an equally dramatic change to the UK's nuclear industry and research infrastructure to support nuclear topics including waste clean-up. The research councils have started to fund significant levels of research such as the Keeping the Nuclear Option Open (KNOO) consortium of seven universities led by Imperial College London.

International efforts include the formation of the World Nuclear University (WNU) to support the study of nuclear science and technology on a global scale. Clearly, the infrastructure is in place to deal with the world's radwaste; it is not an obstacle to building new reactors.

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DOI: 10.1016/S1369-7021(06)71423-7