Understanding the Behavior of Radioactive Cesium duringthe Incineration of Contaminated Municipal Solid Waste and SewageSludge by Thermodynamic Equilibrium Calculation

摘要

Following the nuclear accident at the Fukushima Daiichi Nuclear Power Plant in 2011, even the municipal solid waste (MSW) and sewage sludge (SS) in northeastern Japan became contaminated by radioactive nuclides such as ¹³⁷Cs and ¹³⁴Cs. To understand the state of radioactive cesium (r-Cs) in the incineration residues of the municipal wastes, research groups studied the concentration and the chemical form of r-Cs in the residues, as well as its water-leaching behavior. In the present study, we conducted thermodynamic equilibrium calculations to estimate the possible chemical forms of r-Cs in the incineration residues. Thermodynamic data for cesium oxides and aluminosilicates were collected and compiled into a new database to perform equilibrium calculations for systems that include Cs. The calculation results suggested that Cs (radiocesium and stable cesium) in municipal solid waste was transformed into gaseous CsCl or crystalline aluminosilicate at incineration temperatures and, when a molten aluminosilicate phase (i.e., slag phase) was generated, a proportion of the Cs species wasdissolved into the slag phase. In the case of sewage sludge, Cs wascalculated to be transformed mostly into crystalline aluminosilicateat incineration temperatures, whereas by analogy with the behaviorsof Na and K, Ca,Cs-phosphate double salts were also potential incinerationproducts. These results could account for the high leaching ratesof r-Cs from the MSW incineration fly ash and the low leaching ratesfrom the MSW incineration bottom ash and SS incineration fly ash reportedin earlier studies. In the case of dewatered SS that included a largeamount of slaked lime as a flocculant, it was exceptionally difficultfor the calculation to represent the fate of Cs, and we needed toinclude the contribution of silica sand in a fluidized-bed combustorin the equilibrium calculation to represent the low leaching ratesof alkali species from the dewatered SS fly ash. From the resultsof the thermodynamic equilibrium calculations and also from the calculatedstandard Gibbs energy of cesium aluminosilicate formation/decompositionreactions, the effects of waste composition and incineration temperatureon the fate of Cs were examined: High incineration temperature andlarge amounts of Ca and Cl in the waste composition increased thefraction of gaseous CsCl in the furnace and thus resulted in the highdistribution ratios of Cs in the fly ash of MSW and the high leachingrates of Cs from the fly ash.