High temperature boric acid solubility from plant condition during post-LOCA long-term cooling period

Abstract

This study examines boric acid solubility in six different mixture solutions simulating plant conditions over a temperature range of 60~140℃. For preparing experiment, screening process and scaling analysis was performed. We selected five key additives reflecting the physical and chemical priority: tri-sodium phosphate (TSP), NUKON, calcium silicate, sodium aluminum silicate ( $NaAlSi_3O_8$ ), and calcium phosphate ( $Ca_3(PO_4)_2$ ). The masses of the additives were scaled for the experiment. The scaled-down plant condition refers to Kori-1. KAIST high pressure and high temperature experiment facility was made for the experiment. Test solutions were heated and mixed to produce homogeneous solution and cooled to observe precipitation. Snow-like white precipitates were found mostly at interfaces between liquid and steam in the early of cooling phase in the interior structure of the facility. TSP (14g) in the solution showed a slightly increased solubility of boric acid under 80℃, but had little effect at 80-140℃. Also, it turned out that other additives slightly decreased the boric acid solubility. We suggested a physical model to predict boric acid solubility considering the effect of additives. The model includes two empirical constants which estimate activity coefficient as a function of temperature. We suggested two empirical models: best-estimation and conservative models.