A new reactor vessel head inspection robot recently received qualification for use at nuclear power plants in the US.
The reactor pressure vessel head (RPVH) is an integral part of the reactor vessel pressure boundary, and has a key safety function in ensuring the primary circuit integrity. The appearance of the cracks on the RPVHs in nuclear industry have raised concerns about leakage and structural integrity of RPVH and consequently increased the extent of non-destructive examination (NDE) and repair methods. Possible solutions for repairing the RPVH include boring out and re-welding the original J-groove weld, or welding over the cracks. These methods are expensive and time-consuming, thus prolonging the shutdown period of the power plant. Moreover, these actions have caused additional requirements for design verification calculations. It was in this context that ARCHER was developed.
Developed by Croatia’s Institute for Nuclear Technology (INETEC), the ARCHER robot is designed to provide eddy current and ultrasonic examinations of reactor vessel closure head nozzles and J-groove welds. It can also perform automated surface repair of these areas, applications in which manual grinding has been the norm.
ARCHER is a completely-automated, remotely- operated system capable of providing inspection with very little operator intervention.
The ARCHER system consists of the manipulator, a number of modules (also called end effectors), control unit, ET and UT instruments, and PCs loaded with software for manipulator control, inspection management, acquisition and analysis.
It moves in eight motorized axes, including main rotation, horizontal axis, vertical axis, docking and end effector axes. The maximum speed of each linear axis is 100 mm/s with repetitive positioning accuracy of 0.2 mm. The rotation speed is up to 20°/s with repetitive angular accuracy of 0.2°.
The number of driving mechanisms makes the ARCHER system quite complex. One of the big challenges during development was cable management. More than 5 km of wires are carried through the inside of manipulator to avoid tangling and minimize the risk of damage. In addition, due to the complex component configuration, additional cameras and real-time 3D visualization of effector position assist the operator in monitoring all steps of the inspection process.
The ARCHER control system distributes the workload between the low-level control system (LLCS) and high-level software running on a PC.
The LLCS was designed to run all of the time critical operations including motion control, sensor reading and safety procedures were any of the module working parameters to exceed the desired range, which could damage the reactor vessel head.
High-level PC software processes the operator’s commands. Each of the axis motors is driven by a separate digital motor drive responsible for servo loop execution (position, velocity, current). A multi-axis motion controller is used to enable motion control synchronization and trajectory manipulation of the axes. For radiation safety reasons, the PC is located outside of the radiation zone. The PCs are connected to the LLCS through Ethernet connection using the TCP/IP protocol.
Due to the RPVH’s complex geometry, Archer uses a number of modules or end-effectors, each specially designed for a particular type of examination. During an inspection or repair outage, the robot is set up underneath the vessel head stand. The robot mast tilts to horizontal, where a built-in docking mechanism is used to bring modules through a service hatch where operators can safely change them outside the radiation environment. Modules are equipped with a quick-release connection mechanism. Switching modules is often required during an inspection.
Three modules detailed below perform RPVH inspection and repair. Additional modules are also available for vent pipe and funnel guide inspection as well as for visual inspections.
The gap-ID module is used to determine the surface flaws or cracks on inner diameter surface of penetration nozzle for RPVH with thermal sleeves. It guides a slim sword-like array probe (INETEC’s Pro Ultra Sabre), which is able to enter the 3mm-wide gap and provide eddy current and ultrasound inspection simultaneously. This inspection method is Electric Power Research Institute-qualified with scan speed of 3 in/s and scan increment of 2°.
The weld-OD module is designed to fit the geometry of the J-groove weld of the penetration nozzle and funnel guide. An INETEC array eddy current probe (PRO ARCHER) is driven by a motorised axis over the weld covering the region of 45mm on shell side and 11mm on the penetration tube side of the weld. Another PRO ARCHER array probe is used for inspection of an outside surface of the penetration tube over its whole length from the bottom to the weld.
The automated surface repair (ASR) module is positioned and fixed on a CRDM nozzle by a pneumatic centring tool. Crack removal is performed with a grinding tool powered remotely via a flexible shaft. The grinding tool is ball-shaped and 8 mm in diameter. It rotates and if pressed against the surface it removes the surface material. Following removal, a dent with no sharp edges remains, stopping or at least slowing crack propagation.
ARCHER can be used on PWRs as well as Russian-designed VVER reactors. To date it has been used at nuclear power plants in Slovenia and Ukraine.
The system meets nuclear industry standards, including recently American Society of Mechanical Engineers (ASME) code requirements, and the NRC 10 CFR 50.55a rule, and is EPRI-qualified for PWR RVPH examinations. US vendor and contractor Babcock & Wilcox Nuclear Energy says it plans to use the robot to expand its US services portfolio.