Eddy current detection of cladding defects due to pellet imperfections

Transcription

1 Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 1 Eddy current detection of cladding defects due to pellet imperfections D. Papaioannou 1, R. Nasyrow 1, N. Niagolova 1, V.V. Rondinella 1 and W. Goll 2 1 : European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, Karlsruhe, Germany 2 : AREVA NP GmbH, FDWM-G / Materials & Thermal-Mechanics, Paul-Gossen-Str. 100, Erlangen, Germany

2 Layout Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 2 Presentation of the small EC-device and some cold tests; Application on irradiated fuel segments: defect detection and sampling; Metallographic analysis on fuel samples and defect observation.

3 Defect detection device Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 3 sensor Sample

4 Defect detection device Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 4 eddy current sensor Manual (dynamic) crack detection; small surface cracks on high-alloyed in unspecified locations (independent of the direction of the inspection). PROBE SYSTEM: Absolute, ferrite core, transformer FREQUENCY RANGE: 100 khz - 3 MHz ACTIVE AREA: Approx. 1.0 mm PENETRATION DEPTH: Low CABLE: EK-X-HF/1, EK-X-007 HOUSING: Plastics (Delrin); pencil housing # 2 DIAMETER: 9.5 mm LENGTH: 75.0 mm WEIGHT: 10 g

5 Calibration standard Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 5 Internal grooves at the top side

6 Calibration standard Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 6 External grooves at the bottom side

7 Calibration standard Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 7 Simulated defects (slits and hole) in the middle zone

8 Calibration standard Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 8 Hologram showing the positions and size of the detected defects Internal double groove each, 0.16 mm deep Hole mm Slit 0.3 mm Internal groove 0.16 mm deep Slit 0.6 mm The smaller internal defects could not be detected Detection limit (for the applied measuring parameters) seems to be defects with depth >0.1 mm 0.1 mm 0.2 mm 0.3 mm 0.4 mm External grooves

9 Preliminary testing Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 9 A non irradiated creep test cladding sample was used for further cold tests

10 Preliminary testing Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 10 NI1 NI2 NI3

11 Campaign of defected fuel rods Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 11 A series of irradiated fuel rods were delivered to ITU for PIEs with strong indications of clad defects; The rods were packed in tight capsules to avoid any contamination risk. Fuel rod Burn-up [GWd/tU] Comment Indication of primary defect at about 2137 mm Indication of primary defect at about 2153 mm Indication of primary defect at about 2370 mm Indication of primary defect at about 2218 mm (short fuel rod)

12 Axial Gamma Spectra Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 12 Fuel Rod No. 1 Fuel Rod No. 3 Suspicion of clad defect Suspicion of clad defect Fuel Rod No. 2 Fuel Rod No. 4 Suspicion of clad defect Suspicion of clad defect

13 MPS and cladding defects Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 13 Chip off pellet In addition to uniform stresses caused by pellet expansion, localised stresses are built up during power changes in regions where a piece of pellet is missing (missing pellet surface, MPS); Gaseous fission products move from the hot centre of the pellet (the movement is facilitated by eventual fuel cracks) to fuelcladding interface; At initiated cladding cracks the ductile metal is converted to the brittle ZrI 4. The crack is then easily propagating.

14 Sampling on the defect fuel rod No. 4 (1/3) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 14 a b Sample preparation: a) defect detection via EC examination; b) careful visual inspection.

15 Sampling on the defect fuel rod No. 4 (2/3) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 15 Detailed drawings (cutting plans) are carefully prepared for precise cutting and specimen preparation. A pellet-pellet interface (dishing) at the end of the segment is used as reference to count the fuel pellets up to the cladding defect

16 Sampling on the defect fuel rod No. 4 (3/3) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 16 Photographs of the cut sample before embedding in a metallographic holder. Left: In absolute conformity to the cutting plan, the bottom end has been cut just before the dishing. The white paint is put by the operator to distinguish the sample orientation Right: The clad defect is clearly visible.

17 Microscopic examination (1/4) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 17 Sample from fuel rod No. 4, Level-1 Left: overview slightly above the pellet dishing Right: a close up of the clad defect

18 Microscopic examination (2/4) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 18 Schematic drawing showing the different levels of grinding and observation of the sample from fuel rod No. 4 for a systematic MPS and defect growth study.

19 Microscopic examination (3/4) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 19 Sample from fuel rod No. 4, Level-2 Left: sample overview bellow the pellet dishing Right: a close up of the clad defect

20 Microscopic examination (4/4) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 20 Sample from fuel rod No. 4, Level-6 A close up of the clad defect and MPS.

21 Summary of results Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 21 MPS confirmation and cladding failures Fuel rod No. 1 Axial position:2137 mm Fuel rod No. 3 Axial position:2370 mm Fuel rod No. 2 Axial position:2153 mm Fuel rod No. 4 Axial position:2218 mm 44 42

22 SEM examination (1/2) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 22 SEM macrograph of the sample from fuel rod No. 1 showing the MPS and the cladding defect Cladding defect area after chemical etching on the sample from fuel rod No. 1

23 SEM examination (2/2) Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 23 a b a) Cladding defect area after chemical etching on the sample from fuel rod no. 1; b) Precipitated hydrides are observed at the crack tips (precipitation during cool down in the stress field);

24 Conclusion Hotlab 2011-IAEA TM, 23 rd - 27th May 2011,Smolenice / Slovakia 24 A small EC-device has been successfully tested for detection of clad defects and installed in a hot cell; A series of irradiated fuel rods with clad defects were analyzed; The small, in many cases invisible defects, were localised very well; Careful sampling and metallographic analysis confirmed that the clad defects were due to initial fuel pellet imperfections; Pellet regions with missing fuel parts (Missing Pellet Surfaces, MPS) are potential sources of clad ruptures during irradiation at power changes.