Utilization of reverse osmosis and nanofiltration for the separation of SiO2 from boric acid solutions at nuclear power plants

Abstract

Even though the problem with an increasing SiO₂ concentration in the reactor cooling system (RCS) emerges at most nuclear power plants, no publications yet have focused on the separation of SiO₂ from solutions simulating RCS composition regarding boric acid (16 g/L) and silica concentrations (0–12 mg/L). Therefore, we examined the utilization of reverse osmosis and nanofiltration to separate SiO₂ from H₃BO₃ solutions simulating RCS composition to meet the operation limits of SiO₂ and to minimize additional costs for H₃BO₃ refill. The rejection of both H₃BO₃ and SiO₂ by reverse osmosis was generally high (>90%). The maximal separation efficiency of SiO₂ from H₃BO₃ was achieved at low pH and high temperature and was 1.30. The rejection of H₃BO₃ by nanofiltration was around 20% and of SiO₂ around 80%. The highest separation efficiency was achieved at low pH and intermediate SiO₂ concentrations and was 2.23. To achieve the operation limits for SiO₂, a multi-stage nanofiltration system must be used for the solutions with intermediate and high SiO₂ concentrations. Nanofiltration is for the separation of SiO₂ from H₃BO₃ solutions more favorable than reverse osmosis, because of the higher separation efficiency of SiO₂ from H₃BO₃, and thus, lower H₃BO₃ loss.