Ren Morinaka

Prediction of Residual Stress Improvement by Water Jet Peening Using Cavitating Jet Simulation With Bubble Flow Model

We developed the new method for predicting a region of compressive residual stress on the weld surface after water jet peeing (WJP), which is a preventive maintenance technology for nuclear power plants. A cavitating jet is impinged on the weld surfaces of structures in a nuclear reactor. Bubble collapse impact causes plastic deformation of the weld surface, and changes the residual stress from tensile to compressive. Compressive residual stress prevents the occurrence of stress corrosion cracking (SCC) on the weld surface. A cavitating jet vertically injected into a submerged flat plate was investigated. Tensile stress was introduced onto the surface of the stainless steel plate by grinding before WJP in the experiment. We numerically simulated impulsive bubble pressure that varied by microseconds in the cavitating jet with the “bubble flow model”. The bubble flow model simulates the abrupt time-variations in the radius and inner pressure of bubbles based on the Rayleigh-Plesset equation in a cavitating flow. The cavitation collapse energy was estimated based on the bubble pressure. The cavitation collapse energy was compared with the measured compressive residual stress on the flat plate after WJP. The radial range of the compressive residual stress from the jet center axis is one of the most important measures of performance of WJP. The radial range of the cavitation collapse energy corresponded to that of compressive residual stress with a prediction error of +/− 20% under different conditions of jet velocity and the distance between the jet nozzle and plate surface. The results confirmed that the method we developed for predicting the region of compressive residual stress after WJP was valid.Copyright

Application of Water Jet Peening on Pre-Cracked Nickel Based Alloy and Stainless Steel

Water Jet Peening (WJP) has been widely applied to Japanese nuclear power plants since 1999 in order to mitigate stress corrosion cracking (SCC) generation on SCC susceptible material used their reactors as an approved mechanical mitigation technique against SCC [1].Laboratory data shows that WJP has an effect to mitigate or suppress shallow crack propagation if the achieved compression depth is deeper than the crack, and that WJP has no adverse effect on pre-existing cracks on the mitigated surface, such as crack propagation during WJP treatment or acceleration effect after its treatment [2]. However, there was no field experience of applying WJP on cracked surfaces of operating reactor internals since we removed the crack(s) as the practice in Japan was to remove the crack prior to WJP implementation.When observed crack can be left as is according to its profile, it is required to perform the crack sizing periodically by Ultrasonic Testing in order to monitor its propagation. In order to apply WJP on the cracked surface, additional testing was needed to clarify that no adverse effect on UT sizing capability after WJP will be observed. The Japan Atomic Power Company (JAPC) and Hitachi-GE Nuclear Energy, Ltd. (HGNE) have performed several tests about this UT crack sizing test before and after WJP treatment and other related test before its field application.Copyright