In supporting the UK government in progressing a final decision on plutonium disposition, the Nuclear Decommissioning Authority has commissioned a substantial research programme to determine the quantity of plutonium unsuitable for reuse in MOX fuel and to develop the technology for immobilisation1. Given the diversity and characteristics of UK plutonium, which span contaminated residues to fuel quality material, at least three approaches to immobilisation and disposal are under consideration. One method is to immobilise the separated plutonium and residue material in titanate ceramics and glass-ceramics, and recent investigation using plutonium and surrogate species has developed confidence in this approach9,10,11,12,13,14. These wasteforms target zirconolite, prototypically CaZrTi2O7, as the plutonium host phase, which is known to have excellent aqueous durability and radiation tolerance15,16. It has been demonstrated that waste packages could be effectively manufactured by hot isostatic pressing, with the advantage of batchwise processing in a hermetically sealed container, produced to near net shape specification10,11,12,13,14. Recent research has focused on the development of a furnace containment system to enable the hot isostatic pressing of small scale waste packages incorporating UK plutonium, building on laboratory demonstration studies17. Alternatively, a variant of the MOX fuel fabrication process could be utilised to immobilise plutonium in such a ceramic material or in “disposal MOX”, sometimes referred to as “low specification MOX”, which would be disposed without irradiation18.
Whilst there is confidence in the technical feasibility of immobilisation approaches, considerable research and development remains to be undertaken, before such technology could be deployed, which would be “first of a kind” at industrial scale. For example, the wasteform formulation must be adequately underpinned by extensive surrogate and plutonium active studies, at laboratory and demonstration scale, to understand the phase diagram, define the operational envelope and recovery from mal-operations. The post-closure safety assessment for the disposal of such wasteforms is complex, due to the coupled nature of the degradation processes, which will progress over 105 years15,16. For example, self-radiation damage will induce a crystalline to amorphous phase transition during the period of container integrity. This may result in micro-cracking of the wasteform, increasing the surface area available for dissolution reactions, which could plausibly result in differential release of fissile material and neutron poisons incorporated within the wasteform as a safeguard against criticality. Understanding of the impact of these coupled degradation processes on the long term wasteform evolution and alteration, remains fragmented, but is clearly of crucial importance for post closure safety and criticality assessments. In this context, a review and further investigation of relevant natural analogue systems will provide useful long term insight into wasteform degradation mechanisms, at realistic rates, in addition to accelerated laboratory studies. To ensure that the necessary holistic understanding and the evidence base is developed, a roadmap has been developed to plan and guide the UK research programme.