Abstract
Cementitious materials are one of the essential components for low- and intermediate-level waste disposal sites. Low-level nuclear waste from power plants consists of highly concentrated (~25 wt %) Na2SO4, and the wastes are solidified with cementitious materials. Degradation of cementitious materials that result from chemical and physical sulphate attack is a major concern in the safety of the waste disposal. In this study, hydration and reactive transport models, developed in previous works by the authors, were applied with Pitzer interactions coefficients to evaluate the long-term performance of Portland cement (PC) solidified with high concentration of Na2SO4. Expansive sulphate-bearing products of ettringite and mirabilite were formed and filled the pores in the hydrating PC with 25% of Na2SO4 by weight, but they were destabilised as temperature increased. Influence of Na2SO4 concentration and temperature on mineralogical changes is discussed. The simulation results from the reactive-transport model showed that the degradation of solidified Na2SO4 waste by cementitious materials exposed to 10% Na2SO4 for 1000 years is due to dissolution of mirabilite and secondary formation of ettringite, but not Na2SO4 crystallisation. The phases and porosity became stable close to exposure surface after 10 years, although the deterioration progressed from the surface to core with exposure time.
Highlights
Portland cement (PC) is an important material at several low- and intermediate-level radioactive waste disposal facilities in most countries, because of its high-quality performance in various environments and its ability to act as a chemical barrier to prevent or retard the transport of radionuclides [1,2]
The simulation results from the hydration model were given to the reactive transport model, together with boundary conditions for thermodynamic equilibrium and multispecies transport calculations
The cementitious material environment considered in this studyin consists of solution a high concentration of the cementitious material environment considered in this study consists of a high concentration of
Summary
Portland cement (PC) is an important material at several low- and intermediate-level radioactive waste disposal facilities in most countries, because of its high-quality performance in various environments and its ability to act as a chemical barrier to prevent or retard the transport of radionuclides [1,2]. Seawater has been used as a cooling water in many nuclear power plants, and the resultant precipitates in the pipes, including NaCl, have been removed by sulfuric acid This process produces highly concentrated sodium sulphate, up to 25% by weight, as a low-level radioactive waste needing disposal [7,8]. Hydration and reactive-transport models are essential tools to evaluate the performance of waste storage systems, because the degradation of the materials needs to be predicted precisely as a function of time and depth. The interaction of sodium sulphate in the hydration and degradation of cementitious materials is studied, to evaluate PC on its performance in low- and intermediate-level radioactive waste. Performance of the sodium sulphate solidified waste due to external sulphate attack is evaluated using a reactive transport model [25], considering phase-equilibrium reactions and multi-species transport
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