Currently in the U.K., higher activity nuclear waste is kept in engineered interim above ground product stores. The long term solution for this waste is disposal in a Geological Disposal Facility (GDF), which is a highly engineered below ground facility. Current timelines for the readiness of a GDF mean that interim storage will be needed for longer than originally planned; this could take place in the pre-existing stores or require new interim stores to be built. Concrete is used extensively throughout the interim nuclear waste stores. So, understanding any changes in the microstructure of the concrete from ageing and/ or gamma radiation will be invaluable in aiding decision making when comparing the feasibility of extended use of the existing store to building a new one. Sellafield Ltd. provided both a concrete mix specification for one of the interim stores and several historic in-service concrete samples that were cast in 1985. The concrete mix specification was used to make fresh samples, which were used to investigate how carbonation and gamma radiation interact. The historic samples were used to investigate whether the conditions an interim nuclear waste store experiences would affect the microstructure of the concrete. It is noted that these samples were a very valuable asset to this project, with no comparative samples within the literature in terms of sample age and gamma radiation exposure. The characterisation of 30+ year old irradiated concrete samples retrieved from an in-service engineered interim higher activity nuclear waste store showed no changes that would affect engineering substantiation. It was therefore possible to recommend life-extension being a valid option, from the perspective of the microstructure of the concrete. This study was part of a larger body of work being carried out by Sellafield Ltd. to assess the viability of life extension. To reach this conclusion, the in-service samples were compared to unirradiated control samples which were produced at the same time. A dose rate comparison was conducted by irradiating a sub-set of the control samples. An increased gamma radiation dose rate was used to achieve a total dose comparative to the in-service samples within several months. A closer examination of the compressive strength data showed that a trend seen in the literature, where compressive strength decreases with increasing gamma radiation dose, is conservative and not applicable to real world scenarios. To complement the characterisation of the historic samples, fresh samples were produced using the specification provided by Sellafield Ltd. These fresh samples were used to investigate long term degradation mechanisms and the effects these have on the hydrated cement phases. Exposure to an increased carbon dioxide concentration atmosphere was carried out using a bespoke benchtop carbonation chamber, which was produced in-house. Gamma radiation dose rates were used that were greater than the real-world conditions of an interim nuclear waste store. Exposure to gamma radiation prior to accelerated carbonation altered the carbonation polymorph formed compared to the unirradiated control samples, which only been subject to accelerated carbonation. This experiment used a sequential experimental methodology which allowed one existing hypothesis within the literature to be ruled out. The use of irradiated, heat control and control samples showed that gamma radiation had an effect separate to that of thermal dehydration. A new hypothesis based on elemental substitution into the calcium carbonate lattice was presented as a potential mechanism contributing to the calcium carbonate polymorph switch. A consequential experiment examined the effects of hydration age; this showed that younger samples displayed more carbonation ingress whilst older samples resulted in a structure containing more cracks once carbonated.
|Date of Award
|1 Aug 2022
- The University of Manchester
|Francis Livens (Supervisor) & Laura Leay (Supervisor)