Contribution to understanding of stress corrosion cracking of Alloy 600 in PWR primary water

Abstract

The stress corrosion cracking mechanism of Alloy 600 in pressurised water reactor (PWR) primary water has been investigated. U-bend specimens (extracted from tubular products) were tested in simulated PWR primary water containing H₂¹⁸O. After testing, crack tips was characterised by nanosecondary ion mass spectrometry (nanoSIMS) and unstressed areas were analysed by standard SIMS. The results allowed oxygen (¹⁶O, ¹⁸O) penetration into the material to be evaluated and the oxide present at crack tips to be characterised. Experimental data were compared with results obtained by calculations performed according to Fickian diffusion laws and Whipple Le Claire theory.     

Initiation of SCC in nickel base alloys in primary PWR environment: studies at Studsvik since mid 1980s

Abstract

Investigations on initiation of primary water stress corrosion cracking (PWSCC) in alloys 600 and 690 in simulated primary pressurised water reactor (PWR) environments, carried out since 1985, are reviewed. A large number of tests, mostly on reverse U bend specimens, some from steam generator tubes from operating PWRs, have been conducted for periods up to 33 000 h (4 years). Most exposures were at 365 or 330°C in either high purity water with hydrogen addition or simulated primary water (representing beginning of cycle conditions). Parameters investigated include: material (composition, heat treatment (mill annealed or thermally treated), carbide distribution, yield strength, grain size, etc.), environmental (hydrogen content, boron–lithium–pH, zinc) and experimental environment control techniques. Although the PWSCC mechanism has yet to be fully explained, these studies provide an overview of important parameters for crack initiation. The present review aims to survey the evidence for PWSCC initiation in nickel base alloys. It is concluded that, within the normal range for operating PWRs, the influence of dissolved hydrogen on initiation is small. However, a weak maximum in crack growth rate is observed at 15–25 ml H₂/kg H₂O; a corresponding minimum in crack initiation time has not been confirmed but cannot be excluded. It is concluded that hydrogen must be reduced to well below 10 ml H₂/kg H₂O to achieve significant benefits. Increasing the Li content from 2–2·5 to 3·5 ppm is shown to reduce crack initiation time by 30–50%. Lithium in the range 2·2–7·5 ppm has little effect on crack growth rate at ～1200 ppm B. Boron appears to have little influence on crack initiation or growth.