Criticality safety: investigating the co-mobility of actinides and neutron poisons

To strengthen post-closure criticality safety arguments, understanding the co-behaviour of neutron poison elements (e.g.

Hf, Gd), which have a high propensity for absorbing neutrons, and actinide elements (U, Pu), which are major components of waste, is important.

This includes knowledge of the neutron poison solubility and mobility under conditions relevant to geological disposal of radioactive wastes.

Here, neutron poisons (Hf, Gd) and analogues (Eu) and their co-behaviour with U and Ce (a primary component of criticality-risk wasteforms and as an analogue for Pu respectively) will be investigated. The interactions of Hf, Gd and Eu with representative solid phases (e.g. U/CeO2 MOX simulant), via both incorporation and sorption will be probed.

Further understanding will be developed using both static and dynamic studies investigating the co-behaviour of Hf, Gd, Eu and U/Ce solid phases and their migration in representative geological disposal conditions.

Throughout the study, the chemical speciation (local bonding environment) of the Hf, Gd, Eu, U and Ce will be probed using luminescence and X-ray absorption spectroscopic techniques to underpin the behaviour of these elements with a fundamental understanding of their aqueous geo-chemistry.