Effect of Environment on Stress Corrosion Cracking of Structural Materialsin Nuclear Power Plants (I)
—Effect of Flow Rate on Crack Growth Rate—

Abstract

  We studied the effect of the flow rate of reactor water on crack growth rates (CGRs) in a boiling water reactor (BWR) coolant system using 1/4TCT specimens made of sensitized 304 stainless steel (SUS304). Specimen electrochemical corrosion potentials (ECPs) in each oxidant concentration increased with increasing of water flow. This was caused by the increase of the mass transfer amount from bulk water to the specimen surface through a diffusion layer determined by the water flow. A relative CGR was defined as the ratio of the CGR under a given set of conditions to that under simulated normal water chemistry (NWC) at a low flow rate. The relative CGRs at high flow rates were higher than those at low flow rates under NWC. However, when the concentration of oxidants was below 10 ppb and the specimen ECP was below −0.15 V vs. Standard Hydrogen Electrode, the relative CGRs at high flow rates became as low as those at low flow rates. The mass flux of the oxidant was one dominant parameter, which controlled the CGRs at high flow rates. Although the flow rates in a BWR vessel and piping are very high, the CGRs should decrease when the oxidant concentration is adequately reduced by applying hydrogen water chemistry.