Sodium Sulphate Activated GGBS/PFA and Its Potential as a Nuclear Waste Immobilisation Matrix
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Sodium Sulphate Activated GGBS/PFA and Its Potential as a Nuclear Waste Immobilisation Matrix Yun Bai1, Neil B. Milestone1 and Changhui Yang2 1
Immobilisation Science Laboratory (ISL), Department of Engineering Materials, University of Sheffield 2 Department of Building Materials & Engineering, Chongqing University, P.R.China
ABSTRACT
The UK currently uses Portland cement composite blends to immobilise/encapsulate intermediate and low level radioactive wastes (ILW and LLW). However, among other things, the high pH of these systems causes the corrosion of some metals, which can lead to expansion and excess generation of hydrogen. Therefore, in order to immobilise nuclear waste where corrosion is an issue, a near neutral cementing system is desirable. Among the activators which can be used in the alkali-activated slag (AAS) systems, a solution of Na2SO4 is near neutral but the ground granulated blast-furnace slag (GGBS) itself has a pH about 11, increasing the pH within the Na2SO4 activated AAS. As low calcium Pulverized Fuel Ash (PFA) only has a pH about 9, using a GGBS/PFA blend activated by Na2SO4 offers the potential to develop a near neutral cementing system for nuclear waste immobilisation purposes. In this paper, the replacement of GGBS in the Na2SO4 activated AAS system with PFA at 0%, 10%, 20% and 30% by mass was examined. The pH and corrosion of Al were determined and used as primary criteria for judging the feasibility for further development of a Na2SO4 activated GGBS/PFA matrix for immobilising nuclear wastes. The microstructure of the matrices was studied by SEM. Leaching studies were carried out to examine the possibility of immobilising Cs+ within these Na2SO4 activated GGBS/PFA matrices. The potential of using Na2SO4 activated GGBS/PFA for immobilising nuclear wastes is discussed.
INTRODUCTION
With reduced oil, gas and coal availability and obligations under the Kyoto Protocol, nuclear power is being regarded as an important potential source of CO2-free electricity by the U.K. government [1]. However, some stakeholders consider that the question of how to deal with nuclear wastes safely is one of the main barriers for promoting nuclear power [1]. More than 95% of the bulk of the waste generated at the nuclear power plants is intermediate and low level radioactive wastes (ILW and LLW) which is predicted to rise to 163,000 and 1,490,000 cubic meters, respectively [2]. Currently, the U.K. uses Portland cement composite blends to encapsulate ILW and LLW. These systems offer several advantages over pure OPC including lower heat of hydration, reduced cost as well as incorporation of many ions [3]. However, the high internal pH (typically above 13), can cause the corrosion of metals such as aluminium and
magnesium in ILW and LLW, leading to expansion and excess generation of hydrogen. Although up to 90% of OPC is replaced with blast furnace slag (BFS), the pH still remains about 12.5 [4] so the corrosion of these metals is still a major concern. To safely encapsulate historic ILW and LLW containing th
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