Inspecting Chinese AP1000s

THE AP1000 NUCLEAR POWER PLANT is a third generation passive two-loop PWR. China’s Sanmen 1 in Zhejiang province is the first AP1000 nuclear power unit in the world. Compared with the existing M310 and VVER reactors in China, the AP1000 has significant difference in the layout of nuclear island, the structure of key components and the manufacturing process of many components (see Figure 1).

In the AP1000, the centres of the inlet and outlet nozzles in the reactor pressure vessel (RPV) are not on the same horizontal plane. The steam generator is directly connected to the main pump. There are new passive safety systems, including a passive residual heat removal heat exchanger, core makeup tank and so on. The RPV closure head uses a Quickloc build-up structure.

Because the main types of PWR in China are M310 and VVER types, local existing NDT technology cannot meet the requirements of PSI and ISI for AP1000s. The existing technology must be improved and new technologies developed.

Inspection requirements

According to the requirements of in-service-inspection (ISI) for the RPV of an AP1000, components such as the shell circumferential weld, the inlet nozzle weld, the outlet nozzle weld, the direct vessel injection (DVI) pipe weld and the integrated closure head should be examined using non-destructive testing (NDT). Because the centres of the inlet and outlet nozzles in the AP1000 are not on the same horizontal plane equipment requires new functionality, alongside its regular functions such as axial scanning and circumferential scanning.

Forged steel has been used for the AP1000 primary pipe and there is no transition wrapper between the primary pipe and the steam generator. The ISI for AP1000 primary pipes requires that volumetric NDT technology is used on the whole area and there is a volumetric examination of the primary pipe dissimilar metal weld at the 3rd year, the 7th year and the 10th year during each inspection interval, according to ASME code. This requires dedicated examination equipment, due to the examination frequency and the possibility of independent examination. In contrast, the primary pipe weld in M310 can be examined in the same refuelling outage as other components in the RPV.

The DVI pipe has to be examined from the inside surface due to the surrounding conditions, but it has a special necking structure, which makes it difficult to approach and position the examination equipment. The inside surface of the weld is not regular, which produces signal interference easily. Consequently, traditional contact ultrasonic inspection cannot be used and new technology is needed.

The AP1000’s integrated closure head requires volumetric examination for the J-type weld. And whereas other types of PWR only require surface examination for the closure head, in the AP1000 the Quickloc head has a build-up welding structure and special technology is needed for the examination.

The AP1000 main pump shell is connected to the steam generator directly. The PSI and ISI has to be implemented inside the main pipe to steam generator weld because of the external environment.

Because of personnel accessibility restrictions during the ISI, special automatic examination equipment has to be developed to position, scan and exclude foreign material. What is more, because the weld is coarse crystal dissimilar metal with a thick wall of 141.2mm, special technology is also needed for efficient detection and accurate sizing.

In addition, there are larger passive safety systems such as the residual heat removal heat exchanger and core makeup tank. For these upsized systems and components, there is a similar need for new technology.

In conclusion, the AP1000 differs from existing reactors in key component structures, systems and components. This requires new examination technologies and equipment for the PSI and ISI, eventually meeting the need for automatic examination, accessibility, detecting and sizing.

NDT technology and equipment research and development has three parts:

• Improving existing equipment;
• Developing new examination equipment;
• Developing new technology.

Improving existing equipment

To solve the problem of the inlet and outlet nozzles being on different planes a single assembly was developed to lift and position the inspection platform automatically. This allows both nozzles to be examined without the need to change multiple fixed platforms. This reduces reliance on the ring crane, reduces the contamination and safety risk that arises for workers in the event of frequent lifting of multiple traditional fixed platforms, and also reduces the inspection time (see Figure 2).

Along with an intelligent control system the improved examination equipment can automatically generate a scanning track for the complex curved surface inside the pressure vessel and acquire the data using three- dimensional visualisation modelling technology.

The steam generator tubes in the AP1000 are arranged in a triangular array, unlike the square arrangement in other reactors. To survey this structure the crawling robot requires new motion controlling modules. It can be automatically positioned to examine tubes at the edge, by optimising the robot gait and the path control algorithm, reducing operator error. Reliability is also increased by the structure of the robot crawling toe.

Combined with eddy current data aquisition and analysis, planning and management software, this equipment allows for real-time transmission of data and synchronous analysis. This is an efficient integrated examination process.

Developing new equipment

Primary pipework connected to the AP1000 pressure vessel must undergo either ultrasonic examination or radiographic testing, according to ASME codes, and can be examined independently. In contrast, M310 plants require both ultrasonic examination and radiographic testing of primary pipe welds, with no possibility of independent examination.

New equipment has been developed (Figure 3). It adopts peristalsis stretching positioning technology and has a special hoisting mechanism for remote installation and positioning underwater. In this way it is not necessary to have large equipment or to remove lower internals. Integrated probe trays carry out parallel scanning of the nozzle to safe-end weld and the safe-end to pipe weld.

The AP1000 vessel has an integrated closure head. Volumetric examination is required for the base material of the penetration and surface examination is required for the internal surface of the penetration and the external surface of the J-type weld, according to ASME codes.

Equipment was developed for automatic ultrasonic examination of the base metal of the penetration and automatic eddy current examination of the internal surface of the penetration and the external surface of the weld.

For the small internal diameter control rod drive mechanism (CRDM) penetration and vent pipe penetration, an integrated probe assembly is used. This consists of a single-element ultrasonic probe, dual-element longitudinal probe, TOFD (time-of-flight diffraction) probe and eddy current probe. The stretched external diameter of the probe assembly is close to the internal diameter of the penetration to ensure the probe assembly can access the surface to be examined.

The AP1000 steam generator is directly connected to the main pump. The position of the main pump only permits in-service inspection via automatic examination from the internal surface. Equipment was developed to examine the steam generator to main-pump weld, consisting of a scanner carrier, scanner, three dimensional control system, video monitor system and data acquisition and analysis system (see Figure 4). All the examination processes were considered in the development, including inserting the equipment via the manway, adjusting it in the water chamber, reaching position, starting the foreign material exclusion device, scanning according to a preset trajectory, and removal. The scanner completes axial and circumferential scanning using ultrasonic probes and it is connected to the carrier by a quick-connection mechanism. Three dimensional control system and motion simulation technology is used to generate the scanning trajectory, identify the collision risk, simulate the three dimensional motion state, avoid obstacles, and monitor the whole process with the help of the monitors inside the closed environment.

In addition, equipment was developed for other key components such as pipe welds, the passive residual heat removal heat exchanger and the core make-up tank.

Developing new technology

The weld in the direct vessel injection (DVI) pipe has a special necking structure, with a minimum internal diameter of 101mm. This is difficult for NDE equipment to approach.

The internal surface of the weld is not regular, so it produces signal interference easily, and it cannot be examined by the traditional contact ultrasonic test methods. Instead, water immersion reflector examination technology was used, with a special device to adjust the probe angle (see Figure 5). During the examination, a reflector is used to increase the sound path in the water, making the reflector signal appear prior to the secondary water/steel interface signal, which can eliminate extraneous signal interference in the analysis. An angle adjustment device can adjust the probe with 0.5° accuracy, so one probe can look at multiple angles, which reduces the complexity of the mechanical design and improves examination efficiency.

In contrast to other designs, the AP1000 steam generator to main pump weld has a thick wall (up to 141.2mm) and a preload build-up weld edge on both sides of the weld (edge width up to 38.1mm). The base metal on the main pump side is cast stainless steel.

The weld structure and the base metal material mean there is serious energy attenuation during the ultrasonic transmission process and the interface reflection in the different material of the weld is complex, which makes ultrasonic examination of this weld very difficult.

The following technical solutions were employed: dual element longitudinal contact probes were used to detect from the internal surface; low frequency probes were used to reduce the attenuation impact of the coarse grain; probes with different focal depths were designed to detect different depth zones to increase the signal-to-noise ratio of acquisition data; sound field simulation helped find the optimal probe parameters; a whole set of characterisation methods was built and verified.

Experimental results on a test block showed 100% detection rate for the pre-embedded true defects in the block. Statistics show that the root mean square deviation of measured defect length is 6.4mm, smaller than the required value of acceptance criteria (19mm). The root mean square deviation of measured defect height is 2.0mm, smaller than the required value of acceptance criteria (3.2mm). The developed examination technology passed performance verification by China’s National Nuclear Safety Association (NNSA).

Qualification of NDT technology

In-service inspection for AP1000 components complies with ASME codes. The qualification methodology follows ENIQ methodology on the qualification thinking and conforms to ASME XI appendix VIII (1998) on the sizing acceptance criteria and other requirements for specific programmes.

Independent qualification requirements for AP1000 nuclear equipment are classified as specific qualification, general qualification, conventional qualification, expert assessment and applications not requiring qualification, according to RSE-M code.

The choice of specific qualification programme refers to ASME XI appendix VIII (1998), covering programmes suitable for the AP1000, feedback from operating experience in China’s nuclear plants and the technical characteristics of the AP1000. In Sanmen 1, for example, inspection programmes require qualification. The specific qualification includes technical evaluation, equipment function test, open test and blind test.

Application in AP1000s

Between 2013 and 2018, new technologies and equipment were successfully applied at three AP1000 units: Sanmen 1&2 and Haiyang 2.

Sanmen 1 (the first AP1000) started eddy current examination for steam generator tubes at the end of 2013. In 2015 it finished ultrasonic examination of nuclear class 1, 2 and 3 piping welds, which was divided into four stages using conventional automatic ultrasonic examination technology and phased array ultrasonic examination technology. In 2016, Sanmen 1 finished automatic ultrasonic examination for the weld connecting the steam generator to the main pump – the first time automatic ultrasonic examination for this type of weld was carried out from the internal surface.

In summary, the NDT technologies developed fully meet the PSI and ISI requirements for key components in the AP1000 nuclear island and have been successfully applied in China. 

Reference: Discussion and analysis of the practices of non-destructive testing qualification for Sanmen AP1000 nuclear power plant, NDT, volume 36, issue 5, 2014. 

Authors are with: China Nuclear Power Operation Technology Corporation, Ltd