Effects of Aging on the Thermal Conductivity of the AP1000® Containment Vessel Inorganic Zinc Coating

Abstract

During a loss-of-coolant accident (LOCA) event the AP1000® passive safety features actuate to provide emergency core cooling (ECC) to the reactor core using passive features that do not rely on electrical power being available. The core makeup tanks (CMTs), and accumulators (ACCs) actuate to quench the fuel rods and refill the reactor vessel. After the CMTs and ACCs empty, the in-containment refueling water storage tank (IRWST) utilizing gravity injects a large volume of sub-cooled fluid into the reactor vessel. This floods the vessel and the lower region of containment (containment sump) initiating gravity induced long-term recirculation cooling. The discharge of high energy fluid during the blowdown, re-flood, and re-fill phases is assumed to condense on the colder structures inside the containment including the containment vessel shell. Heat is transferred through the shell to the film of water from the Passive Containment Cooling System (PCS) applied to the outside of the containment vessel shell. This results in evaporative heat transfer on the outside of the containment vessel. Due to the large heat transfer coefficients on the inside and outside of the shell the heat conduction through the shell is very important to the heat rejection capability of the PCS, and plays a large part in ensuring the containment vessel pressure is not exceeded during design basis events. The AP1000® containment vessel is forged from a high strength carbon steel alloy that is coated with an inorganic zinc coating which protects the containment vessel from corrosion during its design life. The coating acts as a sacrificial anodic layer which corrodes in lieu of corrosion of the substrate beneath it. The corrosion of the coating can potentially lead to degradation in thermal conductivity of the coating due to metallic oxides typically having a lower thermal conductivity than that of the non-oxidized state. A reduction in thermal conductivity of the protective coating will impact the overall heat transfer through the containment vessel during PCS operation. The purpose of this work is to develop a mechanistic model demonstrated against empirical validation for assessing the effects of oxidation on the thermal conductivity of the protective inorganic zinc coating (IOZ) on the AP1000® containment vessel.