Experimental studies of boric acid precipitation in nuclear power plants during the post-LOCA long-term cooling period

Abstract

Boric acid precipitation, which recently was considered as a generic safety issue by NRC, could happen in the pressurized water reactor during the post-LOCA long-term cooling period. Chemical additives and containments debris mix together with boric acid in the sump pool, then generating new insoluble chemical products and reducing boric acid solubility. This study is to provide the experimental data base and theoretical method for the boric acid solubility in the mixture solution, and develop screening process to select key containment debris for in-depth study. Low temperature experiments for boric acid solubility in NaOH and TSP mixture solution were performed, and the physical model was proposed to predict the boric acid solubility under various temperatures and fractions of NaOH and TSP. Boric acid solubility stably and obviously increases in the mixture solution as temperature and the fractions of NaOH and TSP increase. Importance Factor (F) for the screening process is obtained in order to reflect the total effects of the dissolution, transport, chemical and physical influence of debris on precipitation. Through the screening process, aluminum, concrete, NuKon fiberglass, calcium silicate, and epoxy were selected out as key debris for further study. Dissolvability tests of concrete, calcium silicate and aluminum were performed to investigate their dissolution characteristics. The sensitivity study on pH, temperature, buffer agents and geometry for precipitation was discussed. AlOOH, $Na_3AlSi_3O_8$ and $Ca_3(PO_4)_2$ were recommended as the most possible precipitates and their solubility was investigated.