Legacy Ponds and Silo Wastes

Understanding wastes arising from the nuclear fuel cycle is an essential
prerequisite for the deployment of new nuclear generation around the world.
Wastes, once disposed of, need to remain stable in their final package for
thousands of years.The underlying chemical processes that occur in such
wastes, however, are often poorly understood. Molecular modelling and modern
electronic structure calculations present a set of tools that can
investigate the chemistry in these problematic materials.

Our research has focused on one waste stream found at the Sellafield site
known as Magnox sludge. The First Generation Magnox Storage Pond (FGMSP)
represents one of the highest priority targets for risk reduction at the
Sellafield Site. A ‘legacy’ storage pond for spent Magnox fuel, it has
accumulated a deep layer of sludge over many years, formed primarily from
corroding Magnox alloy, but also from wind blown debris and decaying
organic matter. The composition of this sludge is complex and uncertain.
Additionally, it contains dissolved fission products where fuel cladding has
failed and split, or even fragments of spent fuel where cladding has
corroded entirely.

A consequence of the sludge is the emission of methane and hydrogen gas,
complicating its future handling and storage. One possible source of the
gases in the sludge is radiolytic decomposition of the primary brucite mineral.
While a project currently underway will transfer the sludge from the legacy
ponds to an engineered storage facility, permanent disposal will require that
potential gas production in situ is quantified.

Molecular modelling employing quantum mechanical calculations offers a unique
view of the fundamental chemistry at the heart of the matter. Our work uses
electronic structure calculations to visualise the effect of excess electrons,
and electron holes, arising in the waste material as a consequence of radiation.
The ultimate goal is to understand the various possible mechanisms of hydrogen
gas production through a radiolytic route, and to assess their feasibility and
relevance within the legacy ponds context.