Research on the positioning strategy for CRDM penetrations in reactor vessel head

The reactor vessel head is a core component of the nuclear reactor. Internally, multiple penetrations are connected through interference fit and welding, which are prone to generate Primary Water Stress Corrosion Cracking (PWSCC). ASME codes mandate non-destructive testing (NDT) of penetrations and their J-welds. However, the eccentricity between thermal sleeves and penetrations, coupled with the larger diameter of guide cones connected to thermal sleeves compared to penetrations, creates significant challenges in positioning inspection equipment for penetrations, adversely affecting NDT efficiency and quality.To address this issue, this paper proposes a precise positioning strategy for penetrations: 1) establishing a coordinate mapping relationship between the inspection equipment's coordinate system (X'O'Y′) and the reactor vessel head's coordinate system (XOY), (2) achieving millimeter-level of the guide cone positioning via machine vision-based object detection, and 3) executing final penetration alignment using the ultrasonic ranging sensor.Experimental validation on a mock-up demonstrates a positioning accuracy of ±0.5 mm, fully complying with NDT requirements. This methodology enhances the reliability of in-service inspection for reactor vessel head penetrations under PWSCC-sensitive conditions.