Corrosion and solubility in a TSP-buffered chemical environment following a loss of coolant accident: Part 3—Calcium

Highlights: • Calcium leaching from NUKON fiberglass in borated TSP-buffered solution is independent of the level of fiberglass destruction. • The initial calcium release rate and the maximum calcium concentration increases with increased fiber concentration. • The calcium release in solution has a repeatable pattern of four distinct regions (prompt release, metastable, autocatalytic drop, and stable region) for all experiments. • Magnesium plays a significant role in initiating calcium precipitation in TSP-buffered environment. • Head loss through multi-constituents debris beds was found to increase progressively in all calcium concentration regions. - Abstract: Calcium that leaches from damaged or destroyed NUKON fiberglass in containment post a loss of coolant accident (LOCA) could lead to the formation of chemical precipitates. These precipitates could be filtered through the accumulated fibrous debris on the sump screen and compromising the emergency core cooling system (ECCS) sump pump performance. Reduced-scale leaching experiments were conducted on three solution inventory scales—bench (0.5 L), vertical column (31.5 L), and tank (1136 L) using three different flow conditions, and fiberglass concentrations (1.18–8 g/L) to investigate calcium release from NUKON fiber. All experiments were conducted in simulated post-LOCA water chemistry. (∼220 mM boric acid with ∼5.8 mM trisodium phosphate (TSP) buffer). Prior to the leaching tests, a preliminary experiment was carried out on the bench scale to determine the effect of the fiber preparation (unaltered and blended) method on calcium leaching. Results indicate that the extent of fiberglass destruction does not affect the amount of calcium released from fiberglass. Long-term calcium leach testing at constant temperature (80 °C) in borated TSP-buffered solution had repeatable behavior on all solution scales for different fiberglass concentrations. The calcium-leaching pattern can be divided into four distinct (and progressive) temporal regions: prompt release, metastable, autocatalytic drop, and stable release region. The prompt calcium release rate and maximum calcium concentration in solution are both dependent on the initial fiberglass concentration. Thermodynamic modeling of calcium orthophosphates determined that the most probable precipitates in the metastable and stable region were calcium-deficient hydroxyapatite (CDHA) and β-tricalcium phosphate (β-TCP). Magnesium that leached from NUKON fiberglass was determined to control the initiation of the autocatalytic drop region; in all long period leaching tests, a Mg:Ca molar ratio of 1.9 existed prior to the autocatalytic drop. Although head-loss resulting from calcium leaching is not the focus of this research, the head-loss change under high-temperature vertical column corresponding to in-situ leaching of calcium from NUKON fiber is reported. The measured head-loss (temperature corrected to 20 °C) steadily increased from 1.1 kPa (4.4′′ H₂O) to 3.1 kPa (12.2′′ H₂O) over an experimental period of 20 days.