Environmentally Assisted Cracking Of The Safe-end Weld Joints In Primary Pressurized Water Reactor Environment

Since the critical equipments such as primary circuit and pressure vessel can not be replaced during the whole life cycle of nuclear power plants and it is costly to scram and repair the reactor,the environmentally assisted cracking(EAC)of safe-end weld joint has become a key factor influencing the reliability,safety and economy during the long-time operation of pressurized water reactors(PWRs).The localization and performance assessment of key structural materials are two important parts to realize the autonomous development of the nuclear power industry.However,there is limited systematic investigation on the EAC behavior of domestically manufactural safe-end weld joints.Therefore,it is necessary to reveal the EAC behavior of the weld joints in simulated PWR primary water and to develop the technology for the evaluation of the EAC behavior.The present study designed a crack growth rate test system using reversed direct current potential drop technique to precisely investigate the crack growth rate of the compact tension specimen in primary water.The EAC behavior and microstructure of SA508/309L/308L/316L and SA508/52M/316L weld joints were systematically investigated.The objective is to clarify the EAC behavior and mechanism of the weld joints.A reliable research system,crack growth rate test system using reversed direct current potential drop technique was firstly developed.This system resolved the temperature fluctuation and interferential potential by reversing the direct current,adding a reference electrode and improving the electrode-lead.Since the designed system can automatically measure,collect and handle the data,it can be used to study the crack growth behavior in simulated PWR primary water.The development of this system reaches the international advanced level.Stress corrosion cracking(SCC)behavior of the heat affected zone(HAZ)in a stainless steel 308L/316L weld joint in simulated PWR primary water was investigated.The results showed that the low stress corrosion crack growth rate of the HAZ was due to the low residual strain level of the HAZ.Stress corrosion crack growth in the HAZ was observed in off-normal primary water chemistry with dissolved oxygen,but not in normal primary water chemistry with dissolved hydrogen.Further,the stress corrosion crack growth rate increased with increasing of dissolved oxygen and stress intensity factor.Microstructure analysis of the crack tip suggests that the stress corrosion crack growth in the HAZ of 316L stainless steel follows the slip-oxidation mechanism.Environmentally assisted crack growth behavior of 308L stainless steel weld metal in simulated PWR primary water was investigated.Ultra-low stress corrosion crack growth in the weld metal was observed.However,obvious environmental acceleration of fatigue crack growth was observed,which was increased by more aggressive water chemistry and gentle cyclic loading conditions.Further,the environmental fatigue crack growth rate of 308L weld metal was lower than that of 304 and 316 stainless steels under the same environmental and loading conditions.Since the ferrite phase has a higher Cr content than that of the austenite,the formation of Cr2O3 film at the crack tip was promoted when a propagating crack intersected with the 8 ferrite.As such,a higher film rupture strain as well as a higer repassivation rate at the crack tip can be expected,leading to a lower corrosion fatigue crack growth rate.The SCC susceptibility of a SA508/52M/316L dissimilar metal weld joint in PWR primary water was investigated.The results showed that the sequence of the SCC susceptibility of different regions in the SA508/52M/316L weld joint was as follows:the Alloy 52M buttering>the HAZ of 316L stainless steel>the dilution zone of Alloy 52M adjacent to the stainless steel>Alloy 52M weld metal.The Cr-depletion at the grain boundary was the dominant factor causing the high SCC susceptibility of Alloy 52Mb,which is a synergic effect of Cr carbide precipitation at the grain boundary and the lower Cr content in dilution zone.However,SCC initiation in the HAZ of 316L stainless steel was attributed to the increase of residual strain adjacent to the grain boundary.In addition,the decrease of Cr content and increase of residual strain adjacent to the grain boundary increased the SCC susceptibility of the dilution zone of Alloy 52M adjacent to the stainless steel.Correlation of microstructure and SCC initiation behavior of the fusion boundary(FB)region in a SA508/52M weld joint in PWR primary water was investigated.The Cr content is the primary material factor influencing the overall SCC susceptibility while the stress mismatch in the FB region can lead to a preferential SCC initiation at the FB region.The partially mixed zone between the dilution zone of Alloy 52M and SA508 has a retardation effect on SCC initiation.This was attributed to the decrease of strength mismatch and the alleviation of strain concentration adjacent to the grain boundary in dilution zone.The partially mixed zone containing martensite decreased the tensile stress in the FB region during deformation.On the other hand,the formation of the partially mixed zone decreased the weld shrinkage and thus led to a lower residual strain.Therefore,the SCC was less likely to initiate in FB region where there is a partially mixed zone.EAC behavior of the FB region in a SA508/52M/316L weld joint in simulated PWR primary water was investigated.The crack preferentially initiated at the FB of the SA508/52M weld joint or the interface of low alloy steel and partially mixed zone.The formation of type I and II boundaries led to a high stress corrosion crack growth rate of the FB region(1 × 10-7 mm/s).Further,the crack growth rate of the HAZ of 316L stainless steel with a high residual stress was about 3.3 × 10-8 mm/s?...