The use of cement based materials could be widespread in the long term management of radioactive materials in the United Kingdom. In the Geological Disposal Concepts proposed by the Radioactive Waste Management Directorate of the Nuclear Decommissioning Authority (NDA), several cement based materials are used in the long-term management of intermediate-level wastes. Much of the waste will be immobilised within stainless steel containers using cement grouts based on ordinary Portland cement (OPC) blended with blast furnace slag (BFS) or pulverised fuel ash (PFA). The resulting waste packages will be placed underground in a Geological Disposal Facility (or Repository) after a period of storage at the waste producers’ sites. The repository will then be filled with cement based backfill. The encapsulation grouts and the backfill materials will perform as both a physical barrier and chemical barrier for confining the radioactive wastes. During storage and disposal, some wastes may generate carbon dioxide from the degradation of organic materials and this will react with the cement based materials. Therefore, carbonation of the cementitious encapsulation grouts and backfill materials is of interest because of the resulting changes to their physical and chemical properties and also because of its ability to remove carbon-14 labelled carbon dioxide from the gas phase. It is also important to understand the reaction kinetics under a range of conditions, due to the long-term nature of storage and disposal. In this work, the carbonation progress of one backfill material and of two encapsulation grouts used in the UK has been studied in batch reactors. These materials are known as Nirex Reference Vault Backfill (NRVB), 3:1 PFA/OPC and 3:1 BFS/OPC. Based on the single dimensional carbonation experiments, fundamental parameters affecting the rate of carbonation were investigated and the carbon dioxide uptake capacity of each material was determined. For these three materials, an increase in relative humidity (75% to 100%) decreases the carbonation rate. A higher reaction pressure can facilitate the carbonation, but its effect was less obvious than the effect of relative humidity. The progression of the carbonation fronts have also been observed by various techniques and the shape of carbonation front was proved to be influenced by the relative humidity. Special attention was given to the modelling of the kinetics and mechanism of the carbonation reaction of these materials. This work provides fundamental understanding of the carbonation reaction of NRVB, 3:1 PFA/OPC and 3:1 BFS/OPC of relevance to the future optimization of a geological disposal facility in the UK and to assessments of the performance of such a facility.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:565259 |
| Date | January 2011 |
| Creators | Sun, J. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1306875/ |
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