THE EFFECT OF CHLORIDE AND SULFATE TRANSIENTS ON THE STRESS CORROSION CRACKING BEHAVIOR OF LOW-ALLOY RPV STEELS UNDER SIMULATED BWR ENVIRONMENT

摘要

The adequacy and conservative character of the Boiling Water Reactor (BWR) Vessel and Internals Project (BWRVIP-60) stress corrosion cracking (SCC) disposition lines during and after water chemistry transients were evaluated and assessed in the context of the current Electric Power Research Institute (EPRI) BWR water chemistry guidelines. For that purpose, the SCC behavior of three nuclear grade low-alloy reactor pressure vessel (RPV) steels during and after sulfate and chloride transients was investigated under simulated BWR power operation conditions by tests with periodical partial unloading (PPU) and experiments under constant load. Modern high-temperature water loops, on-line crack growth monitoring (DCPD) and fractographical analysis by scanning electron microscope were used to quantify the cracking response. In oxygenated, high-temperature water (T = 288 deg C, 8 ppm dissolved oxygen (DO)), the addition of 370 ppb sulfate (> EPRI action level 3) did not result in acceleration of crack growth under PPU and constant load in all materials and the SCC crack growth rates (CGR) under constant load during sulfate transients were conservatively covered by the BWRVIP-60 disposition line 2. The addition of 10 ppb (>=EPRI action level 1) to 50 ppb chloride ( EPRI action level 2) resulted in acceleration of the SCC crack growth in all investigated materials by at least one order of magnitude and in fast, stationary SCC under constant load in the investigated stress intensity factor range K_I from 32 to 62 MPa centre dot m~(1/2) with CGR significantly above the BWRVIP-60 disposition line 2. In some cases stable, stationary SCC with CGR above the BWRVIP-60 disposition line 2 could be sustained after severe ( EPRI action level 2) and prolonged chloride transients for much longer periods (> 1000 h) than the 100 h interval suggested by BWRVIP-60.