Understanding nuclear waste glass durability as a function of groundwater composition

In the UK, spent nuclear fuel has been reprocessed and the radioactive by-products vitrified. The resulting glass is intended to be disposed of in a geological disposal facility (GDF) for tens of thousands of years. There is an expectation that groundwater will eventually ingress into the GDF so understanding how the glass interacts with its aqueous environment over that time scale is important for the geological disposal safety case.

This thesis has studied the dissolution behaviour of simulant nuclear waste glass under various aqueous environments including alkali metal and alkaline earth cations common to UK groundwaters. Durability studies were conducted to investigate a link between physical properties of the cations and how that behaviour changes with time, temperature, and glass surface area to solution volume ratio (S/V). In static tests, the alkali metals cations generally increased dissolution compared to pure water whereas the alkaline earths decreased dissolution. The new Stirred Reactor Coupon Analysis (SRCA) methodology was utilised to study the effect at very low S/V and high pH. A relationship was observed between a larger hydrated ionic radius and decreased dissolution rates
within Group 1 and 2 cations at early reaction progress.

In addition to simple solutions, durability tests were carried out in more complex solutions, containing combinations of the same cations, that were representative of deep subsurface groundwaters. The aqueous environment of the GDF may also be highly alkaline and saturated with calcium from co-located intermediate level waste (ILW) and a cement backfill. The durability of simulant calcium and zinc containing glass compositions was studied under high pH and calcium saturated conditions and found to be similar.

This thesis shows that the chemistry of the aqueous environment can have an effect on the dissolution behaviour of simulant HLW glass and should therefore be considered in the GDF safety case.