Pentti Kauppinen, Harri Jeskanen, VTT, Finland Raimo Paussu, Fortum Nuclear Services, Finland

ULTRASONIC INSPECTION OF THE NOZZLES OF CONTROL RODS AND WELD IN REACTOR CORE AREA OF PRESSURE VESSEL OF LOVIISA

NPP (VVER440)

Pentti Kauppinen, Harri Jeskanen, VTT, Finland

Raimo Paussu, Fortum Nuclear Services, Finland

Bernhard Elsing, Fortum Power and Heat, Loviisa Power Plant, Finland

VTT TECHNICAL RESEARCH CENTRE OF FINLAND

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Control rod penetrations to the closure head

• Target of the inspection

• Structure of the nozzles

• Ultrasonic technique applied

• Performance of measurement

• Results


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Upper part of the RPV closure head with control rod housings


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Target of the inspection

• The target of the inspection is to assure that water has not penetrated in the narrow gap between the ferritic nozzle penetration tube and the inner corrosion protection tube

• Water can lead to corrosion of the ferritic tube and corrosion products can cause the bulging of inner tube → the operation of control rods is disturbed

• The upper weld inside the penetration nozzle is the susceptible area for leakage


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Control rod nozzles: flange between the ferritic penetration tube and austenitic control rod housing


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The structure of the control rod penetration nozzle


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The structure of the control rod penetration nozzle


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Principle of the ultrasonic inspection technique

• Test arrangement for laboratory simulations

ferritic plate 15 mm

austenitic plate 3.3 mmWater gap

Transducer 0°


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Ultrasonic signals from simulation test and real inspection

• Test with 0.6 mm water gap (above)

• Result from real inspection; water gap 0.75 mm

• Echo on the left is the bottom echo from 15 mm thick nozzle and the echo on the right its multiple

• Measurement is based on the interface echoes (in the gates)

• The width of the gap is calculated based on the distance between the two interface echoes and on the sound velocities in steel and water


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Dependence of the interface echo amplitude from the ultrasonic frequency used in inspection

0

5

10

15

20

25

30

0,1 1 10 100

Frequency, MHz

Inte

rfac

e an

d f

law

ref

lect

ion

(d

B)

• At certain frequencies the amplitude drops and the gain setting of the UT-device has to adjusted properly for reliable detection

• Calculation made for 0.2 mm water gap; first drop of amplitude at 3.7 MHz

• 5 MHz transducer was selected for measurements


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Performance of inspection: Access to the nozzles


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Access to the nozzles


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Access to the control rod nozzles


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Performance of inspection; access to the nozzles


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Measurement signals on the screen of UT-equipment


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Opening of a defect detected in the upper weld


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Defects on the surface of the corrosion protection tube


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Linear crack in the corrosion protection tube


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Conclusions from inspections performed

• Based on the measurements performed at Loviisa in 2004-2006 two leaking nozzles were detected

• In both cases the reason for leakage was a manufacturing defect in the upper weld of the corrosion protection tube

• The location of the leakage point could only be detected by helium-test

• The sealing weld at the lower end of the tube has been inspected by eddy current testing and no defects have been found

• The reliability of the inspection is decreased because the measurement of all nozzles can’t be performed along the whole circumference

• In order to have access to the lower part of the nozzle the heat insulation material on the cover should be removed


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Inspection of the weld in reactor core area (weld 4)

• The target of the inspection is to assure that the outer surface of the reactor pressure vessel shell is free from defects

• The soundness of outside surface is especially important in possible emergency cooling situation due to the irradiation embrittlement of the reactor pressure vessel material

• Thermal shocks in emergency cooling situation might lead to growth of cracks existing on the outer surface of the shell


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Reactor pressure vessel of VVER440

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3

4

7

6

5

Weld 1

Reactor core area


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Inspection volumes of weld 4 of RPV shell

• Volume A-B-C-D from the outer surface of RPV

• Volume E-F-H-G from the inner surface of RPV


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The transducer packages used for inspection of inner (left) and outer (right) surface of RPV


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Telescope mast and the transducers


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Telescope mast used in the inspection


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Qualification of the technique; detection of 3 implanted defects in a test block. Defect depths 8 , 5 and 3 mm


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Qualification of the technique with test block


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Inspection technique applied

• For inspection of outer surface area a 70° transmitter-receiver L-wave, 2 MHz probe is used

• The sound beam of the transducer is directed just below the surface in order to detect even very low defects opening to the surface

• For detection of surface opening defects the creeping waves created on the surface


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D- and B-images of indications recorded in the inservice inspections in 2006 (above) and 1999 (below)


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Indications on the surface and in the volume of RPV


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Conclusions

• The two techniques presented have been developed to complete the basic inservice inspections performed according to ASMEXI

• The feed-back from RPV inspections performed has been good; the reproducibility of results between different years has been excellent

• The opening and repair of control rod nozzles based on the inspection results have verified the conclusions made from UT results

• The inspection of control rod nozzles has certain limitations that should be overcome to improve the overall reliability of the method

Pentti Kauppinen, Harri Jeskanen, VTT, Finland Raimo Paussu, Fortum Nuclear Services, Finland

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