Proper implementation of in-service inspection (ISI) requires minimum design and access provisions. For the Westinghouse AP1000 PWR, such requirements are defined in a formal design-for-inspectability programme.
The programme uses current inspection technology and strategies to define inspection equipment, personnel access and component design. It aims to accommodate, as much as reasonably possible, newly emergent technologies. It coordinates ISI design requirements and their implementation with other design considerations. It makes ISI design decisions in-process rather than as an after-the-fact reconciliation of the design to the requirements.
The AP1000 ISI design evaluation factors include the examination requirements, examination techniques, accessibility, component geometry, and material selection. Each of these factors has identified features that are of importance.
This philosophy is formally documented in a designer’s guide. Intermediate assessments such as formal design reviews and informal in-process discussions are conducted with NDE practitioner input. Final design-for-inspectability assessments, ranging from simple reviews to 3D modelling of the component and inspection tooling and personnel, are performed to identify specific design/fabrication or ISI implementation actions that result in the maximum level of inspectability.
Specific successes include entire main coolant piping runs with no intermediate welds, fewer safety-related valves, safety-related piping, and pumps. For the reactor vessel, optimisation of design for NDE includes the location of circumferential shell welds and nozzle welds for complete and unobstructed robot access. Other improvements include widening of access doors in the integrated head package for personnel access and removal of obstructions that will cause reductions in inspection coverage in the core make-up tank.
This new plant design has also resulted in new component configurations. These configurations require the implementation of unique tooling, inspection methodology, and inspection qualification changes. These needs are identified in the final assessment for each component and are sorted out before the pre-service inspection is implemented.
Manufacturing NDE
The role of NDE continues during the manufacture of an AP1000 component. AP1000 design specifications define NDE criteria to comply with industry codes and standards and regulatory body commitments. In general, this means that the materials and welds are inspected using a combination of visual, surface and volumetric examination methods. The intent is to detect inherent defects that could impact the structural integrity of the component during its service life with high reliability.
Materials are primarily examined using magnetic particle or liquid penetrant test methods to detect surface defects, and ultrasonic test methods to detect embedded defects. Welds are generally examined using magnetic particle or liquid penetrant test methods to detect surface defects and radiographic test methods to detect weld flaws.
For critical components such as vessels and tanks, Westinghouse requires extra inspections. Generally, ultrasonic test methods are applied in these instances in order to obtain key information on flaw shapes, orientations and dimensions for comparison against fracture mechanics-based acceptance standards. For the most critical reactor vessel welds and nozzle inner radius regions, ultrasonic inspections are performed from both the inner and outer diameter surfaces after welding and after the post-weld heat treatment.
Pre-service Inspection
The finishing role of NDE prior to operation of the AP1000 is to inspect welds and critical regions using inspection processes consistent with those to be applied in future in-service inspections. These processes include equipment, procedures and personnel that are qualified for such inspections, and in many cases also include automated inspections using robots and special scanning devices. For example, the WesDyne SUPREEM ROSA V has been instrumental in inspecting current PWR reactor vessels including the first 40-month RVISI and the first 10-Year RVISI at the UK’s Sizewell B in 1998 and 2005, respectively.
During the service life of the AP1000, in-service inspections will be conducted to provide confidence that service-induced degradation will not impact the safety of the plant. Such inspections are made possible through the diligence applied during the design-for-inspectability phase. The significance of manufacturing flaws to plant safety will be expected to have already been eliminated by the manufacturing NDE and pre-service inspection phases.
Author Info:
Rick Rishel, David Kurek and Daniel DeStephen are of power industry inspection services company WesDyne International, a Westinghouse Electric Company subsidiary. Waltz Hill, I-70 Madison Exit 54, Madison, Pennsylvania, 15663
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