Without a platform

Oyster Creek Nuclear Generating Station in New Jersey is the oldest operating nuclear power plant in the USA, having generated electricity for over 40 years. It comprises one 630MW General Electric BWR-2, owned and operated by Exelon Generation Company. In April the plant had its operating licence renewed until April 2029 by the US Nuclear Regulatory Commission.

During a refuelling outage (1R22) in autumn 2008, IHI Southwest Technologies (ISwT) was contracted to perform ultrasonic examinations of vertical core shroud welds. The company used an underwater inspection scanner known as AIRIS-CS. This device has been used before, however previous operation of the system included deploying the scanning tool from a work platform or refuel bridge. A hand-held camera, also deployed from a platform or bridge, was then used to visually monitor the scanner while positioned on the outer shroud wall.

Since the Oyster Creek Station did not have a work platform during 1R22 and it was not desirable to deploy the tooling from the refuel bridge because of increased critical path time, ISwT came up with alternative methods for deploying the scanner and the camera.

The AIRIS-CS tool was deployed from the edge of the cavity using a floatation device. These devices have been used many times in the past by ISwT to install a similar tool (AIRIS-21) used for the reactor vessel examinations. Also, a camera mounted to a second tool allowed visual observation of the first tool during the examinations without support from a bridge or platform.

Scanner

The AIRIS-CS scanner is about the size of a pizza box, approximately 15in (38cm) wide, 18in (48cm) tall, and 2in (5cm) thick; it weighs 20lbs. The design makes the device ideal for use in tight spaces such as the annulus between a BWR core shroud and vessel wall. Once the AIRIS-CS is lowered to the approximate location on the shroud, it uses two small thrust propellers to move in contact with the shroud wall. Once on the wall, a flexible skirt makes contact and the thrusters create a partial vacuum between the device and the wall, allowing the drive wheels to maintain contact with the surface.

Two independent drive wheels (travel motion) and one caster wheel allow free manoeuvring around the shroud surface. Independent encoder wheels provide coordinated tracking of device position regardless of motion direction. A linear drive (scanner motion) provides search unit module movement perpendicular to the travelling motion of the device. Both the travelling motion and the scanner motion can be used for ultrasonic testing scanning or incrementing.

AIRIS-CS is positioned using a combination of measurements integrated and displayed by the control system. An origin reference point is established for each examination zone by locating a specific elevation and azimuth. Elevation can be determined by the use of two high-accuracy water depth sensors, one on the device and one located at a fixed location such as the vessel flange, or it can be determined visually by driving the device to a fixed component inside the vessel whose elevation is known. Azimuth reference location is typically accomplished visually by driving the device to a fixed component whose azimuth is known or a physical reference can be obtained from other components such as sparger piping.

Once the device is on the shroud wall in the referenced position, the encoded drive wheels provide travel distance and a gravity sensor keeps track of device orientation. The control unit integrates this information and provides constant positional information relative to the device location. A graphic display unit also shows the orientation of the device and direction of movement.

The device is controlled through a single umbilical cable bundle, which contains all inputs and outputs to the control system as well as the AUT transducer cables. This device control cable also contains a steel braided cable so that the cable can also function as a device retrieval mechanism in the unlikely event of a power failure or thrust propeller failure.

Camera

An AIRIS-21 tool was modified to hold an underwater camera to observe the CS tool during the core shroud examinations. The following modifications were needed:

• Removal of the ultrasonic transducer module (that holds the ultrasonic transducers) and mounting hardware.

• Design, fabrication and installation of hardware to allow underwater cameras (colour CCD- type and radiation hardened black & white), and lights to be attached to the AIRIS-21 tool.

• Additional buoyancy to the AIRIS-21 tool to offset the additional weight of the camera system.

Float

ISwT deployed the AIRIS tools from the edge of the cavity using a tethered cable float. The AIRIS umbilical cable is fed through the centre of the float and the float is moved over the reactor vessel using rope tethers. When in position over the vessel, the AIRIS tool is lowered into the vessel by feeding additional cable through the float.

Demonstration

Working diagram of remote core shroud inspection system, including AIRIS UT detector (green) and second AIRIS unit with camera (purple).

Prior to the Oyster Creek refuelling outage, a demonstration was conducted at ISwT’s maintenance facility in San Antonio, Texas to confirm the capability to deploy the AIRIS-CS tool without work platform or bridge support and test the camera system mounted to an AIRIS-21 tool.

An RPV/core shroud mock-up was fabricated and submerged in one of ISwT’s test tanks for this activity. A demonstration was conducted for members of the Exelon Oyster Creek outage team. It included installation of AIRIS-CS and AIRIS-21 with camera utilizing the cable management float. The AIRIS-CS was manoeuvred on mock-up wall and observed with the camera mounted on AIRIS-21 tool. A simulated examination was setup and carried out to prove that the tool could be properly positioned and is capable of performing the examination. The tools were then removed from the mockup and test tank.

The real thing

Pictures taken of the AIRIS UT unit in situ show the tight gap between the core shroud and the reactor pressure vessel

ISwT conducted the automated ultrasonic (AUT) examination of 10 core shroud vertical welds during the recent refuelling outage at Oyster Creek.

In order to save critical path time, ISwT deployed the AIRIS equipment during the portion of the outage schedule when fuel was being moved and there was no work platform or refuel bridge available to support the insertion or removal of examination equipment into the reactor.

The AIRIS examination equipment was deployed from the edge of the cavity using a cable management floatation ring that allowed the AIRIS scanner to be positioned over the reactor and lowered into the RPV/core shroud annulus region. A second AIRIS tool, with an underwater camera mounted to it was also lowered in a similar manner and used to provide visual feedback to assist with the positioning of the examination tool on the core shroud surface.

Exelon personnel determined that this first of a kind approach and innovative delivery technique resulted in both critical path time and radiation dose savings while providing the necessary AUT data to accurately evaluate the structural integrity of the core shroud. The time/cost savings was estimated to be 15 critical path hours, or $515,000, and the estimated dose savings from deploying the tooling from the edge of the cavity rather than over the reactor from the refuel bridge was 600mrem. The successful execution of this examination has lead Exelon to nominate this project for a 2009 Nuclear Energy Institute award.

The AIRIS robotic equipment is highly transferable for use at all BWR nuclear plants. It was not developed solely for use at Oyster Creek and ISwT has used it at many other nuclear plants in the USA.