Special NDT systems applied to the in-service inspection of a BWR nozzle and safe-end welds

Nuclear Engineering and Design 81 (1984) 77-84 North-Holland, Amsterdam

SPECIAL NDT SYSTEMS AND SAFE-END WELDS

77

APPLIED TO THE IN-SERVICE INSPECTION

OF A BWR NOZZLE

G.M. VAN DIJK KEMA, Utrechtseweg 310, Postbus 9035, 6800 E T Arnhem, The Netherlands

Received 31 January 1984

The present report refers to various dedicated NDT systems applied at 2 particular areas with a BWR, more specifically the inside of a feedwater nozzle for one and safe-end welds as second. At various instances the operating experiences with BWRs have demonstrated the occurrence of actual cracking phenomena in these particular areas.

1. Feedwater nozzle With the feedwater nozzle rapid temperature fluctuations may occur due to inadvertent mixing of hot water and cold water leaking into the vessel alongside the thermal sleeve, particularly when there is insufficient fit of the thermal sleeve. Thus thermal cracking may be induced into the clad layer providing potential nuclei for subsequent fatigue crack growth due to pressure cycling. Experience from BWRs since 1974 indicated prime concern for possible cracking at the inner corner radius and bore region joining the radius. With a Dutch B W R a particular mechanical ultrasonic system has been developed to cover crack detection and subsequent quantitative monitoring at primarily the radius section. A first examination has been carried out in 1977. Since 1977 the examination was performed almost every outage. At a later stage extended capabilities were added to cover various bore sections as well. The detection principle is schematically shown in fig. 1. Coverage of the radius section is accomplished by 2 probes of 1 M H z running at the outside vessel wall, directed to the radius section within a tangent plane, clockwise and anti-clockwise. Circumferential scanning is accomplished through a remote control mechanical system mounted on the nozzle outside. In order to account for geometrical changes related to vessel wall curvature provisions are included for a 20 mm radial probe scan (3 traces) as well as a swivel angle scan over 8 ° (3 angle traces). The entire system thus provides an 18-fold survey of the circumference. Examination data are interpreted relative

to the corresponding calibration traces on a model containing various artificial flaws. The extended ultrasonic system contains a number of 1.5 MHz probes running at the outer nozzle cone and operating in tandem combinations in order to cover the

us - s y s t e m f w - nozzle

Fig. 1. Us-system fw-nozzle.

0 0 2 9 - 5 4 9 3 / 8 4 / $ 0 3 . 0 0 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )

G.M. van Dijk / Special N D T systems applied to in-service inspection

78

various bore sections. Probe configurations are rather complicated, each probe having a specific refraction angle and skew angle relative to the nozzle axis in order to provide the required beam angle within a tangent plane. Throughout the years strong emphasis has been placed on reproducibility. Each examination was preceded by repeated model examination to requalify the system; maximum continuity with equipment and operators was kept up. Of particular interest also is the very limited access at the nozzle area. Radiation levels under normal outage conditions require replacement of the entire system within approximately 10 minutes. Some illustrative ultrasonic traces obtained in 1983

with the radius section probes are shown in figs. 2 and 3. Quite clearly indications can be observed at various positions. Particularly noteworthy also are the differences in traces. Optimum detection apparently requires multiple scanning in order to ascertain sufficient coverage considering the parameters involved. A summary of trend data obtained with the ultrasonic radius section system is presented in fig. 4. Indications appear to develop over the years both in amplitude as well as in number. In interpreting the trend data one should bear in mind the fact that very tight tolerances are to be accounted for as shown in fig. 5. Considering a reproducibility range of examination

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G.M. van Dijk / Special N D T systems applied to in-service inspection

equivalent model flaw depth (mm)

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within 6 dB according to repeated model tests, a detection limit and an allowance limit against possible crack growth, there only remains a very limited range of judgement. In particular, only a 6 dB range appears effective, equivalent to a possible crack development over a 2½ year period. Bearing in mind the seriousness of matters, confirmative data on indication significance were demanded. Such has been fulfilled by a supplementary examination through eddy-current testing, using the socalled "Keel-haul" system. Schematically this system is depicted in fig. 6. This system which is applied from the vessel inside provides a 360 o scan over the nozzle inner radius section and joining vessel wall area over a radial scan width of 40 mm. Access tolerances were extremely tight due to the presence of thermal sleeve, sleeve-to-sparger junction collar with its extensions, sparger and sparger support detect depth (mm) basic calibration basic

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immediately next to the nozzle area. Sample eddy-current traces with one of the "Keel-haul" probes are shown in fig. 7. The relevance of the ultrasonic indications with respect to occurrence and nature was confirmed beyond any doubt. The nozzle appeared to contain a number of cracks. A repair was concluded mandatory. An inspection-file overview over the years before repair and during the repair is presented in fig. 8, including ultrasonic testing of the radius section, ultrasonic testing extended to the bore region, eddy-current testing by "Keel-haul", eddy-current extended testing (entire nozzle surface), high resolution photographic examination and on-line ultrasonic. The nozzle configuration after repair is shown in fig. 9, including removal of clad layer and crack affected material by cutter machining, local removal of crack affected material by

80

G.M. van Dijk / Special N D T systems applied to in-service inspection area D

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crack runout at 2 locations outside the cutting range was removed by spark erosion. Continued plant operation under the renovated crack-free conditions will be monitored by regular examinations with both ultrasonic systems and the "Keel-haul" eddy-current system. Operation is scheduled to be resumed at 10 april 1983.

2. S a f e - e n d

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electrical discharge machining and installation of a redesign triple sleeve integral with the safe-end. Some illustrative examination data showing the extent of defects existing prior to repair are shown in the next figures, in particular: - eddy-current scan prior to machining (fig. 10); - eddy-current scan at intermediate repair phase (fig. 11). These figures and additional visual examination clearly indicate the presence of multiple cracking, thermal fatigue induced and a limited number of cracks fatigue related at greater depth. A cross-section of the major defects as established by the various examinations during repair is presented in fig. 12 (eddy-current 7 × , ultrasonic on-line at 15 cutting sections). At the final repair phase after cutting a

welds

With austenitic safe-ends and joining welds stress and environmentally supported cracking near the welds has been demonstrated an area of concern particularly when the component is furnace-sensitized. With a Dutch BWR various safe-ends have been replaced in 1974. One particular safe-end, specifically joining the recirculation nozzle, has not been replaced and furthermore is not accessible from the vessel outside (embedded in concrete). An attempt to examine the safe-end by conventional ultrasonics provided ambiguous results. Both the data interpretation and the scanning reliability were judged doubtful. A refined test-system has been developed to allow redundant examination. Examination has been performed in 1981 and 1982. Schematically the safe-end configuration and inspection probes are shown in fig. 13. Particularly the safe-end/pipe assembly weld has a highly complicated configuration. Prime items with the refined test-system were: - tolerance with respect to eccentricity and ovality; - r e d u n d a n t detection techniques, as schematically shown in fig. 14. With respect to the detection technique the crucial element is provided by the so-called "CETUS" probes. These probes integrally comprise both an ultrasonic creep-wave element and an eddy-current element. Probe configuration has been designed to provide simultaneous detection of cracks (if present). Correlated presentation of both ultrasonic and eddy-current data on a XY-scope allow a clear distinguishment between actual crack indications (both UT and ET) on the one hand and indications of a different nature on the other hand (non-relevant UT indications with improper surface conditions or non-crack deficiencies; non-relevant ET indications with material changees or probe-surface gap). The eddy-current technique is sensitive to surface cracks and does provide supplementary data on probe-to-surface contact; the ultrasonic creep-wave technique is particularly sensitive to near-surface cracks in thickness direction. Sample records on a safe-end model indicating the ultrasonic

G.M. van Dijk / Special N D T systems applied to in-service inspection

81

ultrasonic on-line ~ photographic eddy-current extended

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creep wave detection capabilities are shown in fig. 15. All artificial defects, even at the assembly weld, down to 1.5 mm depth appeared detectable. Optimum detection occurs for defects somewhat inclined towards the probe. So far application did not reveal crack-life deficien-

cies. Besides it has been shown that due to significant scaling at approximately 40% of the safe-end inner surface proper examination is entirely impossible, therewith providing a definite explanation for the ambiguous results obtained with the previously applied system.

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Fig. 9. Feedwater nozzle after repair.

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G.M. van Dijk / Special N D T systems applied to in-sertdce in.q~ection

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G.M. van Dijk / Special N D T systems applied to in -service inspection

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84

G.M. van Dijk / Special N D T systems applied to in -ser~,ice inspection

Scale removal by using brushes so far appeared unsuccessful. Further measures are in course of preparation.

3. Concluding statements (1) Examinations should be accomplished up to performance criteria attuned to actual flaw potentials rather than to formal general sensitivity criteria. (2) It is recommended to enhance reliability of flaw detection by the inclusion of redundancy features such as multiple techniques a n d / o r multiple scanning.

(3) It is recommended to promote an inspection approach based upon flaw potential dedicated early detection and subsequent quantitative monitoring rather than one based upon general examination rules and judgement against accept/reject criteria with single examinations. (4) Development of methods capable of producing reliable sizing results are to be encountered. So far the sizing performance of conventional methods is considered insufficient for a proper subsequent evaluation. (5) Code rules and technical background are to be developed in accordance with the foregoing statements.