Reducing in-core zinc borate precipitation after LOCA in pressurized water reactors

During the sump recirculation phase after loss-of-coolant accidents (LOCAs) in pressurized water reactors (PWRs), coolant spilling from the leak in the primary cooling circuit is collected in the reactor sump and recirculated to the reactor core by residual-heat removal pumps as part of the emergency core cooling system (ECCS).
Lab-scale studies within previous research projects have shown that the long-term contact of the boric acid containing coolant with hot-dip galvanized steel containment internals may cause corrosion of the corresponding materials influencing the cooling water chemistry due to dissolution of the zinc coat. Generic experimental investigations regarding the solubility of Zn corrosion products in boric acid solutions resulted in a decreasing solubility with increasing temperature. Thus, precipitation of solid corrosion products (zinc borates) cannot be ruled out if zinc containing coolant is heated up due to its recirculation into hot zones.
Consequently, generic corrosion experiments were carried out in a lab-scale corrosion test facility aiming at the development and test of water-chemical measures to prevent zinc corrosion and zinc borate precipitation in boric acid containing coolants.
The experimental results showed a decreasing corrosion rate with increasing pH value of the coolant. Thus, the risk of zinc borate precipitation can be reduced by addition of alkalizing media to the coolant after a LOCA. However, by adding of a moderate amount of alkalizing media to enhance the coolant pH into the neutral region, the zinc borate precipitation rate can be reduced only to about one third but not fully prevented. Extensive suppression of zinc corrosion and zinc borate precipitation is only achievable from a coolant pH of 7.5 resulting in a lithium concentration of 125 ppm if LiOH is used as alkalizing additive. Furthermore, foaming of the coolant cannot be ruled out if the coolant pH is increased into the slightly alkaline region.