Radiographic Inspection Technique For Reactor Vessel Nozzle Safe End Weld

The connection between reactor pressure vessel nozzle and primary piping is accomplished by two welds,one is the dissimilar metal weld between the low alloy steel pressure vessel nozzle and austenitic stainless steel safe end,and the other is the same metal weld between safe end and austenitic stainless steel primary piping.The above welds are of the important parts of the pressure boundary of a pressurized water reactor nuclear power plant.Welding defects are frequently produced during manufacture and in the field installation processes.And during the long term operation process,the welds are susceptible to cracking under high temperature,high pressure,and high radiation harsh environment.In accordance with the requirements of the RCC-M standard,radiographic inspection of the weld must be scheduled during nuclear power plant refueling outage.Owing to the special position of the weld,the inspection process must be completed while the piping is fully filled with coolant.Furthermore,it is difficult forinspectors to access the inspection site directly due to the high dose environment.Therefore,the inspection of these welds is very difficult.In this paper,automatic mechanism,radiographic detection technology and detection process were investigated to resolve the difficulties in inspection of the welds.In terms of automatic mechanism,circumferential centering alignment devices,axial alignment devices and radiographic source control devices were developed.The automatic mechanism consists of an airbag,two groups of bevel structure,an axial alignment structure,long handle tools,a radiographic source cassette and a control devices.Radiographic source and cassette are isolated in the mechanism,which is delivered to the designated location to perform circumferential exposure after the mechanism is fixed inside the pipe.Radiographic inspection technology and process,the related parameters were evaluated by experiments and optimized by computer simulation with Monte Carlo method.This novel system can launch radiographic examination of the RPV nozzle safe-end welds automatically according to RCC-M standards during nuclear power plant refueling outages.The device adapted bevel structure has the advantages,such as powerful bearing force,and large variation range of diameters,and prevention of axial sliding along the inner surface of the pipe.Further,the rigid-contact between the retaining wheel and the inner surface of the RPV results in high precision positioning,and the separation of radiographic source from the mechanism increases the safety and reliability of the device.