To facilitate corrosion detection in difficult-to-access loca-tions such as boiling water reactor drain lines, the PaloAlto, Calif.-based, Electric Power Research Institute

(EPRI) worked with PPL Corp., an electric utility based inAllentown, Pa., to develop and demonstrate a remotely operatedultrasound evaluation system. The tool has been successfullyfield tested during outages at PPL’s Susquehanna plant in spring2007 and 2008.

Preventing Corrosion Failures

Damage to power plant piping from flow-accelerated corro-sion (FAC) can result in costly outages and repairs that affectreliability and safety. FAC is a corrosion mechanism in which anormally protective oxide layer on a metal surface dissolves in afast-flowing fluid. The underlying metal corrodes to re-createthe oxide, initiating a cycle in which the metal loss continues.

Chemistry conditions play a role, as does the fluid velocity.Flow-accelerated corrosion leads to metal loss on the inside

walls of steel piping exposed to flowing water or wet steam. Ifundetected and unmitigated, this damage mechanism can resultin thinning and weakening of the pipe wall, and could culminatein a leak or break.

Because of their configuration and the operating chemistryand flow conditions, the carbon steel drain lines of boiling waterreactors may be susceptible to FAC. Inspecting the drain line forFAC damage, however, is challenging. The line is surroundedby the control rod drive mechanism tubular penetrations andother components that limit access. Adding to the inspectionchallenge, the boiling water reactor drain line can include 90-deg elbows and diameter changes in the susceptible piping sec-tion targeted for FAC inspection.

While boiling water reactor drain lines may be susceptible toFAC, no FAC damage has been reported to date due to the dif-

Based on information provided by the Electric Power Research Institute (EPRI), Charlotte, N.C., www.epri.com.

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System Inspects Nuclear ReactorDrain Lines for Corrosion

The Susquehanna plant of PPL Corp. demonstrated an EPRI-developed remotelyoperated ultrasound evaluation system for the detection of flow-accelerated corrosion

Fig. 1 — The remotely operated inspection tool can be deliveredvia pole past the obstructions of the drain line environment at thebottom of the reactor pressure vessel. Once secured and aligned onthe top of the drain line, the tool employs two ultrasonictransducers to measure pipe wall thickness.

Fig. 2 — Close-up of the drain line inspection tool.

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ficulty accessing and inspecting the pipe location. The potentialconsequences of FAC damage are serious, however, becauseleaks or breaks in the line cannot be isolated. While easily man-aged by the plant safety systems, such a break would result insignificant economic consequences to restore the plant to nor-mal operation.

Once the drain line’s susceptibility to FAC was recognized,industry representatives and EPRI established a plan of actionthat included estimating possible corrosion rates, ranking boil-ing water reactor plants according to level of susceptibility, anddeveloping an inspection technology to verify the in-place con-dition of the drain line in the fleet. EPRI’s NondestructiveEvaluation (NDE) Center led the inspection technology devel-opment efforts, and PPL agreed to host the first use of the tech-nology at its Susquehanna plant.

Technology to Inspect FAC

For more than two decades, EPRI has worked with thenuclear power industry on a broad-based initiative to detect andmitigate FAC before it causes costly failures. EPRI formed a tool

design team with East Hanover, N.J.-based, Nova Technology(www.novatechnologyinc.com), while PPL (www.pplweb.com)provided detailed piping configuration input and design review.

The project team conceived and constructed a remotely oper-ated tool that could be delivered via pole past the obstructions ofthe drain line environment at the bottom of the reactor pressurevessel — Fig. 1. Once secured and aligned on the top of the drainline, the tool employs two ultrasonic transducers to measure pipewall thickness. A geared wheel rotates the transducers aroundthe pipe circumference, while a second gear provides the motionalong the elbow curvature — Fig. 2. The transducers areequipped to permit proper tracking of the surface.

The system also includes ultrasonic data acquisition units, amotion controller to execute positioning instructions, and videounits to monitor the alignment, position, and motion of the tooland transducers.

Full-Scale Mockup

The project team demonstrated the new tool to PPL using afull-scale mockup of the reactor bottom including wooden repli-

14 • INSPECTION TRENDS

Fig. 3 — A full-scale mockup of the Susquehanna drain line andsurrounding obstructions facilitated testing of the inspection tooland development of effective procedures.

Fig. 4 — PPL Susquehanna personnel prepare the inspection toolfor use beneath the reactor.

Fig. 5 — Drain line inspection tool ready to be deployed beneaththe reactor.

Fig. 6 — Inspection tool in position on drain line beneath reactor.

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cas of Susquehanna Unit 2’s drain line and its surroundingobstructions — Fig. 3. The demonstration enabled the projectteam to assess the tool’s performance, gave PPL staff hands-onexperience with the system, and identified issues to beaddressed prior to field deployment.

The demonstration showed that the tool placement proce-dure was satisfactory, and that ultrasonic measurements of pipewall thickness tracked closely with data obtained manually withthe mockup. The demonstration produced one false reading,which the project team traced to improper transducer inclina-tion in the vicinity of the pipe weld; a refined positioning proce-dure addressed the problem.

Susquehanna Field Trial

PPL and EPRI field tested the system at Susquehanna Unit 2during the 2007 spring outage — Figs. 4–6. Following sensorcalibration, the team placed the unit on the drain line, accompa-nied by a video camera to view the drain line and the tool.

The team evaluated the drain line by mapping the wall thick-ness using a grid pattern with a spacing of ½ in. (12.7 mm). Thispattern translated to eight data points around the circumferenceper grid ring. Upon completing the elbow assessment, thedownstream pipe segment was examined in three stages for adistance of 6 in. (152.4 mm).

Results and Conclusions

PPL reviewed the wall thickness data collected by the inspec-tion tool and determined that the drain line met the require-ments for safe operation. Because a measurement at one loca-tion had a value below the nominal wall thickness, PPL will mon-itor this area at a future outage for possible degradation.

The project reached the following conclusions: ◆ The technology provided satisfactory access to the drain line

to perform thickness measurements for piping configurationsrepresentative of the boiling water reactor design atSusquehanna.

◆ Measurement accuracy with the inspection tool is comparableto that obtained with manual techniques.

◆ Video camera support is necessary for proper system operation.◆ Careful video tracking of the transducer inclination angle is

essential because improper surface tracking may lead to falsereadings.

Further Development

PPL successfully deployed the technology a second time dur-ing Unit 1’s spring 2008 outage. To further extend the applica-bility of this technology, EPRI is adapting the tool for use inplants with piping configurations representative of other boilingwater reactor designs.❖

FALL 2008 • 15

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