Thermodynamic Research On The Deposition Behavior Of Silicates In PWR Primary Circuit

The deposition of crud on fuel rods seriously endangers the safe and economical operation of nuclear power plants.Silicates are trace impurities in the primary coolant of PWR,which can be deposited on the surface of fuel rods during operation.In this paper,the solubilities of silicates as a function of temperature or p H value were calculated by thermodynamic analysis.These silicates were deposited by Al,Ca,Mg and Zn with Si.The high temperature E-p H diagrams of silicates were constructed,and the propensities of various silicates deposition and dissolution were studied.The main research contents and conclusions were as follows:（1）The stable solid phases of metallic silicates were determined,were formed by PWR primary coolant system with four elements（Al,Ca,Mg and Zn）and Si.Firstly,the existence forms of each solid phase and dissolved phase were analyzed,and the precipitation-dissolution reactions of silicates were determined.The thermodynamic data of each substance at 25°C were collected,and the high temperature thermodynamic data at 280°C,310°C and 340°C were calculated.（2）In order to study the dissolution behavior of silicates corrosion products in PWR,the equilibrium constant K of precipitation-dissolution reactions and the relationship between ionic concentration and p H value were calculated by thermodynamic method.The solubility models of substance were established.The solubilities of each substance were calculated.The possibilities of silicates deposition on fuel surface were analyzed.The research results showed that the solubilities of SiO₂and AlOOH increased with the increase of temperature.When the concentration ratio is less than 160,SiO₂and AlOOH were not deposited.Ca,Mg,Aland Simight be deposited as CaSiO₃、Mg₂SiO₄、Mg₃Si₄O₁₀（OH）₂、Mg₅Al（AlSi₃）O₁₀（OH）₈、CaMg（SiO₃）₂ and Ca₂Mg₅Si₈O₂₂（OH）₂.The solubilities of silicates decreased with the increase of temperature,and they had a negative temperature coefficient.These silicates were preferentially deposited on the fuel rods.The solubilities of silicates decreased continuously with the increase of p H value,and the decreasing range gradually became slow.In the PWR primary circuit water environment,zinc injection might be deposited as Zn O and Zn₂SiO₄.（3）The high temperature E-p H diagrams of Si-H₂O,Al-H₂O,Ca-Si-H₂O,Mg-Si-H₂O,Zn-Si-H₂O,Al-Mg-Si-H₂O and Ca-Mg-Si-H₂O were drawn at 280°C,310°C and340°C by thermodynamic analysis.The stable regions of solid phases and the tendencies of precipitations formation and dissolution in high temperature aqueous solution were determined.The research results showed that the dissolved substance Al（OH）₄^-existed stably in Al-H₂O system under normal operation condition of PWR primary circuit,and the other systems were solid substances.With the increase of temperature,the solid phase area of each system in the primary circuit increased.The precipitations had a greater tendency to generate reaction,and were more likely to deposit on the fuel rods.