The effects of boric acid on corrosion of steels in simulated pressurized water reactor primary coolant.

摘要

The effects of boric acid on corrosion of steels in Pressurized Water Reactor (PWR) primary system are currently of interest due to their effects on the primary system pressure boundary integrity and on activity transport that leads to out-of-core radiation fields. This thesis focuses on the effect of boric acid on corrosion of type 316 stainless steel (SS) and on the boric acid corrosion of reactor pressure vessel (RPV) steel.;The boric acid corrosion of a reactor pressure vessel (RPV) head steel is studied by an impingement of a simulated primary coolant, issuing from an orifice at high temperature and high pressure, onto RPV steel samples at atmospheric pressure.;Flow rate, jet velocity, heat flux, water chemistry, oxygen and jet length are found to be important factors affecting corrosion. The experiments showed that increasing flow rates result in increased volumetric metal removal. However, higher localized penetration rates are seen in tests with low flow rates but high jet velocity (due to a decreased orifice size). With applied heat flux, local volumetric mass loss and penetration rates increase significantly. Without applied heat flux, coolant survives and flows across the sample and this causes a fairly high general corrosion. Water chemistry is important in that the local corrosion rate caused by an impingement of coolant with pH300°C 6.9 is significantly higher than that of coolant with pH300°C 7.4. At the same pH300°C, the higher boron-to-lithium concentration ratio increases the corrosion rate. Therefore, not only pH 300°C but also the boron-to-lithium concentration ratio is important to corrosion mechanisms. The jet length also affected the corrosion rate. As the distance between jet orifice and sample strip increases, the local wastage rate decreases. Oxygen in the atmosphere is an important factor in the corrosion rate. Corrosion in the nitrogen-purged run is significantly below that in the air-purged run. In a small gap of horizontal impinging jet, liquid hold-up does not have significant effect. The wastage mechanisms were discussed based on the surface analysis and profilometry of the damage area. Clear evidence for flow-accelerated corrosion was apparent by the scalloping seen on the surface.;The effect of high concentration of boric acid on oxide formation on type 316 SS is examined by using a high velocity impact of submerged jet of simulated PWR primary coolant. Results indicate that high boric acid content could suppress the deposition of the outer oxide layer on stainless steel. It can be postulated that the oxide suppression may increase the corrosion product transport in PWR primary system. The progression of oxide formation on type 316 SS, exposed to simulated PWR coolant over extended time, was characterized by neutron powder-diffraction techniques. The oxide phase compositions and precise corresponding material corrosion rate were obtained, which is generally difficult using traditional weight loss techniques, especially the corrosion rate under thin oxide film. It is significant that boron sequestration by the oxide films was not detected.