Exploration of Hydrogen Gas Evolution from Radioactive Sludges

Abstract

The proposed final resting place for radioactive waste in the UK is a geological disposal facility that will be built at a location in the UK, which has yet to be chosen, at some unspecified time in the future. To safely commit a waste material into this facility requires knowledge about the long-term behaviour of the material in the disposal conditions. It is critical to consider what effect the ionising radiation introduced by contaminants will have. In the case of Magnox sludge, a complex material that exists in large quantities, it is known that hydrogen gas will be produced by radiolysis. It is clear that fundamental understanding of this process is required to build confidence in the proposed disposal plans.

In this investigation, we start with a simple model of bulk brucite, the main component of the Magnox sludge. When a high-energy ionising particle interacts with matter, its initial passage leaves a cylindrical track where there will be a high density of ions and radical species created. This process will spawn many secondary electrons with a lower energy than the initial particle and these will create small secondary tracks, known as spurs, near the main track. Additional electrons will be created at successively lower energies, and this cascade will continue until the electrons can no longer induce ionisations. The end result is that the largest population of electrons contains those with low energies, around the ionisation potential of the material being irradiated. Thus, in our calculations, using density functional theory, the effects of two types of electronic structure defect were considered. The first is the addition of an excess electron into
the system, representing one of these many low energy electrons. The second is the addition of an excess hole, which simulates an ionisation. Molecular dynamics simulations were used to give insight into the effects of the defects on the material at room temperature. Complexity was added to the model by the creation of a surface. Initially, this surface was in contact with vacuum. Subsequently, water was added to the system, first as monolayers adsorbed on the slab surface, and then later as bulk water.

This work shows that excess holes are particularly damaging to brucite, potentially leading to the dissociation of hydroxide groups at room temperature, while excess electrons are relatively inert. This process could be the first step in a reaction pathway that may directly or indirectly lead to molecular hydrogen formation. The presence of a water interface has a considerable effect on the results, the most significant of which is that an excess hole will localise near the surface of the bulk brucite or even in the bulk water, which could reduce damage to brucite mineral. Similarly, an excess electron will localise at the surface and will become solvated by the water monolayer. These results suggest that the water in the sludge, rather than the brucite itself, is a larger problem with regard to the stability of the waste following disposal.

Date of Award	Jul 2019
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Gareth Tribello (Supervisor) & Jorge Kohanoff (Supervisor)