Radionuclide alteration behaviour in engineered subsurface environments

Abstract

Nuclear activities over the last 75 years, both in the UK and globally, have resulted in a legacy of contaminated land and structures at nuclear sites. Over the next 100 years, many of these sites will progress through various stages of decommissioning, producing significant volumes of radioactive waste that will require careful management. These wastes will largely comprise concrete, contaminated land, as well as steelwork including concrete reinforcing bar (rebar) and pipelines. One emerging option in the UK to manage the large volumes of radioactive waste produced during decommissioning is in-situ disposal, where low-level radioactively contaminated land and subsurface structures may be safely left in place under the assumption that sites meet strict regulatory requirements. Many radionuclides are likely to be present in these radioactive waste disposal scenarios, including uranium (U), a chemotoxic as well as radiotoxic element. As such, it is important to develop an understanding of these radionuclide interactions with engineered components and their alteration products to underpin any future site environmental safety cases that may include in-situ disposal as an optimised solution for radioactive waste management. In this thesis, the transport and speciation of U in a number of engineered subsurface systems was investigated. Field lysimeter experiments were chosen to explore these systems as they offered a unique opportunity to research U behaviour in environmentally relevant conditions that were representative of the subsurface at nuclear sites. A multi-technique approach was utilised to analyse and build a picture of the altered lysimeter samples post-field emplacement including X-ray absorption spectroscopy techniques, inductively coupled plasma mass spectrometry and environmental scanning electron microscopy. In the first study, the fate of U(V)-incorporated in magnetite, a common zero-valent iron corrosion product, was explored using a field lysimeter set up. The results showed limited to no transport of U away from the originally emplaced U(V)-magnetite source horizons and retention of U(V) incorporated into the structure of the iron (oxyhydr)oxide despite oxidation of both U and the magnetite itself over 12 months. The second study explored the fate of U in U-contaminated sediment and concrete subsurface lysimeter systems. Here, U speciation was found to be the defining factor in the extent of transport within the systems, with greater U migration in the system without concrete, where uranyl speciation dominated. In contrast, the formation of insoluble uranate phases in the systems containing concrete resulted in significantly reduced U transport over 13 months. These studies provide much needed insight into the transport and speciation of U in field scale engineered subsurface environments and contaminated land scenarios.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Samuel Shaw (Supervisor), Katherine Morris (Supervisor) & Jonathan Lloyd (Supervisor)