Eddy current inspection of AGR fuel channels: keyway cracks and density mapping

Thomas Bloodworth Bloodworth Consulting Limited

107 Trevore Drive, Wigan, WN1 2TT, UK +44 (0)7725 520391

tbloodworth@btinternet.com

Mark Anderson James Fisher Nuclear Limited

York Road Business Park, Malton, N. Yorkshire, YO17 6YB, UK

Matthew Brown and John Williams EDF Energy

Barnett Way, Barnwood, Gloucester, GL4 3RS, UK Abstract There have been efforts to develop eddy-current inspection for the graphite moderator bricks in Advanced Gas Reactor (AGR) cores for a number of years. The only access to the graphite bricks is down the fuel channel bore whilst the reactor is off load and the fuel has been removed from the channel. There is a need for a technique that will detect cracks that have the potential to propagate from the external keyways of the bricks, but do not reach the bore; i.e. the cracks remain subsurface. As graphite is an electrical conductor, albeit with a high resistivity, eddy current testing is a suitable technology to apply to this problem. In 2009 a proof-of-principle eddy current tool was deployed at Hartlepool and has since had three further deployments at Heysham 1 and Hinkley Point B reactors. Variations in electrical resistivity over the bore surface of the bricks are the greatest limitation on defect detection. However, the sensitivity of eddy current probes to resistivity – and therefore capability to map the graphite density - has provided an additional opportunity to gain further information on the condition of the graphite bricks. In March 2012, a prototype tool was deployed for the first time at Heysham 1. This paper provides a summary of the work undertaken from development through to deployment of the prototype tool in 2012.

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1 Introduction The graphite bricks that make up the core of an Advanced Gas-cooled Reactor (AGR) are known to degrade during the course of their operating life. There is potential for the bricks to crack as a result of dimensional change gradients set up within the bricks caused by fast neutron irradiation. The graphite bricks are also prone to material degradation as a result of radiolytic oxidization. A programme of routine inspection is carried out during periodic reactor shutdowns, using tools lowered into the fuel channels. Remote visual inspection is effective at detecting cracks and other discontinuities at the channel bore surface. Samples are removed by trepanning in order to measure the loss in density of the graphite and other material properties. The loss of density (increased porosity) is related to a loss in the strength of the graphite. It is predicted that as reactors get older, the stress fields in the core bricks change. In particular, the bricks become susceptible to cracking initiated at the external keyways, rather than at the bore. If such cracks do not propagate through the brick all the way to the bore, then they cannot be detected by the remote visual inspection deployed within the bore. There is therefore a requirement for an inspection that is capable of detecting these sub-surface ‘keyway cracks’. Graphite is an electrical conductor and so eddy current testing is a suitable technology to apply to the problem. Moreover, using destructive testing to determine the state of degradation of the graphite is not ideal. It would be preferred to complement the trepanning with a non-destructive evaluation of the material enabling complete coverage of selected channels without further degradation of the graphite bricks.

2 Geometry and Access The shape of the fuel channel core bricks varies slightly between stations, but typically they are approximately cylindrical and almost 1 m tall, with an outer diameter of almost 0.5 m. There is a cylindrical hole down the middle of each brick. The bricks are stacked on top of one another in the core, so that there is a continuous channel into which the fuel assembly is positioned. During refuelling operations, the old fuel assembly is lifted out, so that the bore of the channel is accessible for inspection until the new fuel is put in. The start-of-life bore diameter varies between stations, but is typically (Hartlepool - HRA, Heysham 1 - HYA) 270 mm. The core bricks lie between about 14 m and 22 m below the access on the charge face of the reactor. The fuel channel bricks are held in place with the other bricks in the core by means of graphite keys that lock between the axial keyways on the outside of the bricks. In the Hartlepool/ Heysham 1 bricks, the keyways are only present in the top third of the brick. The wall thickness between the base of the keyway and the bore is 54 mm in these bricks. Since the objective is to detect cracks growing from the keyways, the bore-to-keyway distance is an important parameter for inspection design. Many holes are drilled axially through the full height of the brick, to allow gas circulation. These holes provide an obstruction to the circulation of eddy currents.

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Figure 1 shows a section of a fuel channel brick with a sub-surface keyway crack and a simplified view of how access to the bore of the fuel channel bricks can be achieved.

Figure 1: Fuel channel brick section and schematic view of inspection access

3 Development of eddy current inspection

3.1 Definition of eddy current problem The objective of the inspection was to detect cracking that grows from the corner at the base of the keyway. Significant eddy current penetration is therefore needed at the radial distance from the bore corresponding to the base of the keyway. To achieve penetration to a depth of 54 mm from the bore (as in Heysham 1/ Hartlepool design), a coil with a large diameter is needed. The resistivity of virgin Gilsocarbon graphite is about 10 µΩm but quite variable. It is known that the resistivity of core graphite increases with age, as it becomes more porous. The porosity and hence resistivity is greatest in the first 10 mm or so of the surface at the bore. Estimates of 30 to 50 µΩm have been made for the resistivity of the surface material. The dependence of penetration depth of eddy currents in the graphite on resistivity and coil excitation frequency is given approximately by the skin depth relation; penetration is proportional to the square root of the ratio of resistivity to frequency. The standard penetration depth in virgin Gilsocarbon (10 µΩm) at 4 kHz is about 25 mm. In material of resistivity 50 µΩm, the standard penetration depth increases to about 56 mm. In the true geometry, penetration is influenced by the size of the coil, the curvature of the brick surface and the presence of the gas-circulation holes.

Reactor core

Inspection tool

Hoist and control

Support chains and umbilical

Charge face

Empty fuel channel

Channel bore

Keyway

Keyway crack

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3.2 Early Development In the mid-1980s an interest in the problem of keyway cracking led to some preliminary investigations at CEGB(1,2) and then Nuclear Electric(3); the idea of treating the graphite brick as a large tube and testing it with a co-axial bobbin coil was proposed. It was not until 2000 that further work was done by John Turner(4) of Phoenix Inspection Systems in collaboration with NNC. Turner rejected the single axial bobbin coil idea as being insufficiently sensitive, in favour of surface coils of 70 mm diameter that could be used to resolve defect signals around the brick circumference as well as axially. Further laboratory studies(5,6) investigated the detection of discontinuities following the expected cracking direction between the holes in slices of virgin bricks. The sensitivity of the response to crack tightness was demonstrated.

3.3 Phase 1 Core Inspection Tool Development In 2007, the scanning of inspection probes over the bore surface became the focus of development. Phoenix Inspection manufactured a manipulator that was able to perform rotational, axial or helical scan patterns. The 70 mm-diameter coils were used in impedance bridge mode, with a local reference coil. The manipulator was used to scan the bore of stacks of two or more virgin core bricks in Amec’s Risley laboratory facility. Figure 2 shows the manipulator in use in a two-brick stack. The mechanism is held centrally by spring-loaded PTFE feet, which slide over the bore surface. The probe is held slightly away from the surface below one of the feet.

Figure 2: Phoenix laboratory manipulator in use at Amec facility in June 2007 Several bricks containing cracks and slots to simulate keyway cracking were examined using the manipulator. The performance of different scanning patterns was compared.

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The simplest way to get complete coverage of the brick was to perform a helical scan. One option considered was however that rotation could be avoided by using a multi-coil array. Coverage equivalent to that of a 24-coil array was achieved on the experimental rig by making axial raster scans with a single probe at a 15° pitch. Material variation in the bricks (heterogeneous resistivity) caused large interfering signals. Two-frequency mixing was employed to reduce the effect of resistivity variation. The mixed channels showed improved capability for detection of the deeper simulated keyway defects. Figure 3 shows a C-scan image of the mixed and high-pass filtered data from two bricks. The simulated defects are labelled with the through wall extent from keyway to bore. As part of the phase 1 work, mathematical modelling of the inspection was performed using the Vector Fields Opera 3D electromagnetic modelling software. Responses from a number of simple slot defects were modelled and the modelling results were validated by comparison with actual measurements on specimens of the same defects. Once validated, the model could be used for a range of parametric tests that are difficult or impossible with real samples.

Figure 3: Helical scan of two virgin bricks – 4/20 kHz mixed and filtered result

The aim of the phase1 work was to determine what would be a suitable strategy for the design of a tool to be used in-reactor. The intention was to adapt one of the existing core inspection hoists to carry an eddy current tool for measurements in the reactor core. Phase 1 showed that the helical scans produced the better data, but axial raster scans with a 15° pitch were shown to be a possible solution if helical scans proved not to be technically feasible with the hoist units.

51%

49%

metal

62%

100% crack

9%

39%

rotation

axial

Interface & end-face keyways

Interface & end-face keyways

Brick with crack

Brick with slots

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It was shown that it is possible to detect open slots simulating sub-surface cracks in virgin graphite, with suitable mixing and filtering. At this stage there were still many unknown factors to be considered: • In-core graphite was known to have a higher resistivity than virgin graphite. • The amount of variation in resistivity within the channel and the extent to which it

would obscure sub-surface crack signals were unknown. • If sub-surface cracks are very tight, then the size of their eddy current response

will be very much reduced.

4 Proof-of-principle eddy current tool In late 2008, British Energy invited tenders for the supply of a proof-of-principle eddy current tool (PoPECT) for reactor trials. James Fisher Nuclear Limited was awarded the contract. PoPECT was designed to be lowered and raised into and out of the fuel channels using the existing hoists used for other core monitoring tools. The hoists are large pieces of equipment; almost 4 tonnes (see Figure 4). The hoist mechanisms are enclosed so can be sealed to the open channel and be matched to reactor pressure during deployment. The inside of the enclosure is a contamination control C2 area.

Figure 4: Core monitoring tool delivery hoist

PoPECT is a robust mechanism encased in stainless steel for ease of decontamination (Figure 5). The tool is suspended by chains from the hoist. The bottom half of the tool can be rotated. The eddy current probe is deployed and retracted from an opening in the casing. An identical 70 mm coil was used in the probe as for the phase 1 tests, but the probe and the deployment mechanism were manufactured from polyether ether ketone (PEEK), for improved radiation tolerance. The reference coil of the balanced-bridge pair is mounted within the nose of the tool, on a piece of virgin gilsocarbon.

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Figure 5: PoPECT August 2009

The probe itself does not touch the surface, but is mounted in a foot that makes contact with the bore surface with ball transfer units. The probe is maintained 2.5 mm from the surface. The tool hangs freely on chains, so the alignment of the tool is determined as it is for other core monitoring tools by viewing an LED arrow on the top of the tool with a camera mounted in the hoist. The resistivity of the graphite bricks that would be encountered was predicted to be between 15 µΩm and 50 µΩm , a range of frequencies for detecting sub-surface defects was used; 2, 4 and 8 and 16 kHz. In addition, 40 kHz excitation was also used, to give the opportunity for two-frequency mixing to suppress signals from varying resistivity. The PoPECT tool was first deployed in reactor 2 of Hartlepool power station in November 2009(8). The tool was deployed in two fuel channels. After some modifications, PoPECT was also deployed at Heysham 1, reactor 1 in June 2010. The tool was deployed again at Hartlepool in reactor 1 in June 2010 and finally at Hinkley point B, reactor 4 in December 2011. For the first deployment, the R/DTech TC5700 eddy current instrument was used, with Multiview software. For subsequent deployments the Zetec MS5800 instrument was used, with ECVision software. • It takes about 45 minutes for both the test and reference coils to warm up to the

channel temperature, so that the signal output stops drifting. • Axial scans were used to identify brick interface heights. The large brick-to brick

difference in resistivity is revealed.

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• The quality of rotating scans was compromised in the first deployment, because the main part of the tool would rotate. For subsequent deployments, an inflatable silicone seal was installed around the top of the tool. The seal would be inflated for circumferential scans, so that the top part of the tool would remain stationary, while the bottom part of the tool, including the probe was rotated.

• Rotating scans in the top part of the brick showed evidence of signals from the keyways, indicating that deep penetration is being achieved.

• Axial raster scans (lower-raise-rotate) have enabled full coverage of portions of fuel channels.

• As expected there are clear signals from surface cracks. • The raster scans show the variation in resistivity clearly. The left of Figure 6 shows the combined image from remote visual inspection of a ‘lasso’ crack in a graphite brick. The image on the right shows the eddy-current C-scan image of the same crack. The whole circumference of the brick is shown.

Figure 6: Lasso crack – remote visual and PoPECT eddy current images Figure 7 shows a C-scan image of the raster scan over the bore surface of four brick layers. The variation in graphite resistivity is evident as the different colours, blue being lower and red higher resistivity. The variation in resistivity makes it more difficult to see weaker signals, so the variation needs to be minimized using two-frequency mixing to improve the capability for detecting sub-surface cracks. The capability to map resistivity is however potentially very useful. Resistivity is related to the porosity and therefore the strength of the graphite. Eddy currents therefore offer the possibility of an indirect, but non-destructive means of determining the mechanical properties of the core bricks. In some cases, trepanned samples have been taken from the bricks after the channels were examined with PoPECT. The density of the trepanned samples was subsequently measured, so it was therefore possible to relate aspects of the eddy current response to the density. The relationship was then used to make predictions of density in other channels where measurements with PoPECT were made. The predictions were compared with the measurements of the trepanned samples and typically agreed to within 10%.

axial

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Figure 7: PoPECT C-scan of four brick layers

5 Prototype eddy current inspection tool (PECIT) Following the successful deployment of PoPECT, a prototype eddy current inspection tool (PECIT) was manufactured. PECIT has wheels that hold the body of the tool centrally within the brick as it is raised and lowered in the channel. There is a central section which can be rotated. The rotating section has three eddy current probes instead of the one probe used in PoPECT. The three probes are: • An impedance bridge probe equivalent to that used in PoPECT, • A transmit-receive ‘gradiometer’ probe, • A differential transmit-receive probe All probes have 70 mm outer diameter. The transmit-receive probe types were selected in order to reduce the dependence on temperature, which causes signal drift of the impedance bridge probe. The differential probe was used as an alternative means for reducing sensitivity to resistivity change and lift-off noise. All probes are operated at 2, 4, 8 and 40 kHz. In order to avoid any possibility of cross-talk when using all three probes simultaneously, an external multiplexer is required with the Zetec MS5800 instrument. PECIT is deployed using the same kind of hoist as used for PoPECT and the other core monitoring tools. The key decision was made to achieve full coverage scanning by helical scanning of the bore. The alternative option of using arrays to achieve full coverage was rejected because it would require increased tool complexity and result in poorer circumferential resolution. In order to achieve the desired axial pitch in the helical scan (≤ 20 mm), the hoist was modified so that the tool could be raised at a lower speed.

L5

L6

L7

L8

+ρ

+ρ -ρ

axial

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Figure 8: PECIT

PECIT was deployed for the first time at Heysham 1 in March 2012(9). The transmit-receive probes proved to be much more temperature stable than the impedance bridge probes. A full helical scan was made in one fuel channel. The C-scan of the whole channel is shown in Figure 9. The blue indications in the top layer are from bore cracks. The orange/red bands that appear at the same position in each layer are areas of lower resistivity – opposite to the colours in Figure 7, because the rotation of the eddy current signal was changed for PECIT. No signals attributable to keyway cracks were observed. It was however possible to make assessments of the noise levels so that estimates could be made of the extent of keyway cracking that should be easily visible were it to be present. As well as detection of keyway cracking, estimation of graphite resistivity is an objective of the PECIT inspection. Resistivity estimates for the Heysham deployment were made by comparing the operating point obtained from scan data with that obtained on reference blocks of known resistivity. In addition, lift-off measurements have been made at some locations within the channel, so that the lift-off phase angle can be used to estimate resistivity. The lift-off angle method is a standard technique for identifying and sorting metals. The operating-point method does however allow resistivity to be determined at any point on the scan that is far enough away from defects or the brick interfaces.

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Figure 9: Helical Scan of L5 to L11 – Gradiometer 8 kHz Horizontal

It is planned that the keyway crack inspection will be qualified. The qualification will include the detection of sub-surface cracks and a capability to determine the through-wall extent of the crack in terms of the size of the remaining ligament to the surface. Such estimates are likely to be based on the phase angle of the signal locus. The relationship between phase angle and depth is however dependent on the resistivity, so reliable measurements of ligament size will require accurate measurements of the local resistivity.

6 Conclusions

Eddy current inspection of the graphite bricks that comprise AGR fuel channels has progressed from laboratory demonstration, through several deployments of a ‘proof-of-principle’ tool, to deployment of a prototype inspection tool. With the prototype tool, helical scans covering the entire bore surface of the channel can be made. Although the eddy current probes are designed for detection of subsurface cracks, known bore cracks are reliably detected. The penetration depth of the technique appears to be sufficient, because the pattern of keyways can be detected. The greatest obstacle to the detection of keyway cracks is the noise level produced by the variation in graphite resistivity within bricks in the axial and circumferential directions. Two-frequency mixing and filtering help to improve the signal-to-noise ratio.

L5

L6

L7

L8

L9

L10

L11

+ρ -ρ

axial cracks

rotation

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The eddy current responses from variations in resistivity are in themselves very useful; they offer a way of measuring the resistivity and therefore possibly the material porosity, which needs to be known for structural integrity calculations. Methods for obtaining the resistivity measurement are being developed.

7 Acknowledgements Most of the eddy current laboratory work that led to the development of the in-core tools was done by John Turner, then at Phoenix ISL, Warrington. The authors are indebted to John for his great contribution to this project. The mathematical modelling using Opera was begun by Chris Holt of ESR Technology and then taken over by John Taggart of Serco (now Amec). The engineers at Zetec (now Eddyfi) have been helpful and patient in their response to questions about our unorthodox uses for their equipment over the course of the project. In particular the authors are grateful to Michael Sirois.

8 References 1. J P Forrest, W A Hammond and O Lloyd (1985): ‘In-situ integrity monitoring of

graphite core blocks; preliminary experiments.’ CEGB report TPRD/M/TD1062/M85.

2. J P Forrest (1988): ‘Eddy current inspection of AGR graphite core blocks.’ CEGB report RD/M/1752/RR88.

3. M C Warnes (1991): ‘Design of eddy current coils for the inspection of graphite core blocks.’ Nuclear Electric Technology Division Report TD/IRB/REP/0614.

4. J L Turner, ‘NDE of graphite AGR core blocks: Final Report (Project T537)’ Phoenix ISL Report P7774 / FR2, 6 June 2000.

5. B F Eccleshall and J L Turner, ‘RACSC Core Inspection techniques: Eddy Current Technique Development – up to April 2001’ (NNC Title: Revised AGR Safety case: Cracked brick detection using an Eddy Current Technique – developments up to April 2001), NNC Report C6508/TR/002, Issue 01, November 2001.

6. J L Turner, A C Holden and R Smith, ‘Eddy Current testing of AGR Graphite Core Bricks (Work up to June 2003)’ PISL Report P9107/R1 Issue 01, June 2003.

7. J L Turner, T J Bloodworth, C C Holt, B France and S Pountney, ‘Eddy-Current Core Inspection Tool Development Phase 1’, Amec Report 14119/TR0001 Issue 1, July 2007.

8. T Bloodworth, ‘Hartlepool in-core PoPECT testing’, James Fisher Nuclear Ltd. Report TR-1272-02-066, January 2010.

9. T Bloodworth, ‘Prototype Eddy Current Inspection Tool for AGR Graphite Core Bricks: Heysham 1 R2 PECIT Deployment’, JFN report in progress, 2012.