The Research on Ultrasonic Testing Technology for Bottom Part of Reactor Pressure Vessel
Authors: Liang Ge , Xuguang Xu , Jiafan Cai , Zhaoyu Cheng 
Published: October 2017

Reactor Pressure Vessel (hereinafter RPV) is the core component of PWR nuclear power plants. The nuclear power plants operation experience shows that the dangerous defects could produce in the material of Bottom Mounted Instrument and J type weld (hereinafter BMI). In early nuclear power plants, the BMI is made of corrosion-sensitive materials (such as Alloy 600). The stress corrosion crack is easy to grow up and destroy the integrity of RPV in operation. Therefore, inspection for BMI during the service stage is necessary to ensure the integrity of the primary circuit pressure boundary.

The BMI is general made of Alloy 600 or Alloy 690 and penetrate the reactor pressure vessel lower head vertically to form J type weld on the inner wall of the RPV lower head. The inner diameter is generally 15 to 16 mm and the wall thickness is about 15 mm. In order to improve the welding performance, the dissimilar metal J-type weld groove is usually overlaid with nickel-based pre-heap edge. The component structural and material characteristics have brought great difficulties to the test technology research as well as the location.

In this paper, the main failure modes of BMI are introduced and the technical requirements of in-service inspections are presented. The technical scheme and inspection system are designed according to the above requirements and weld structural characteristics. Both UT and ET methods are used for the inspection. UT is based on the time of flight diffraction (hereinafter TOFD) technique. According to the structural and material characteristics of the BMI, many researches have been done on block with simulated defect, and characteristic rule of defect indication signals in different areas are summarized. Then, the optimal design parameters of the TOFD probe are gained and the disadvantages of ultrasonic inspection are overcome. The eddy current inspection system adopts the inspection technology with the Bobbin probe and the rotary point probe combined together. The use of its surface and near the surface of the detection ability has become a powerful complement to ultrasonic testing. From the test result, TOFD probe has high defect height measurement accuracy. Measurement error isn’t more than 1mm and mostly is positive deviation. The surface defects with a height of not less than 2 mm can be measured.

Considering the complex geometry of BMI, the multi-mode multi-axis scanning program and remote automatic docking device are used in the system. The multi-mode multi-axis scanning program can be achieved axial and circumferential rotation of the probe with the axial positioning accuracy within 2mm and circumferential positioning accuracy within 3°. The remote automatic docking device can be used to connect the bottom of the mechanical device and the top of the BMI in fast and seamless security to avoid the mechanical device bumping of the component. The probe connecting shaft is made of flexible material, which avoids the violent bumping of the tested parts and greatly enhances the safety of the equipment operation. The system simulates the actual scanning environment by establishing three-dimensional model and testing it on the simulation body.