ML20153B500
| ML20153B500 | |
| Person / Time | |
|---|---|
| Issue date: | 06/16/1995 |
| From: | Sheron B NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20153B354 | List: |
| References | |
| NUDOCS 9809230155 | |
| Download: ML20153B500 (16) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION BWR CORE SHROUD INSPECTION AND EVALUATION GUIDELINES. REVISION 1 GENE-523-113-0894. DATED APRIL 21. 1995 E
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BWRVIP CORE SHROUD NDE. UNCERTAINTY & PROCEDURE STANDARD.
DATED NOVEMBER 21. 1994 1
l 1.0 REVIEW
SUMMARY
1 By letter dated April 21, 1995, the Boiling Water Reactor Vessels i
j Internals Project (BWRVIP) submitted the report, " Boiling Water i
.r (BWR)
Core Shroud Inspection and Evaluation Guidrunes " Devision 1 (Refersnce 1).
The BWRVIP's report provides guidelines for an strategy and. law evaluation and effectively supersedes the 5 1994 document in these areas. By letter dated September 13, 1994 RC provided a preliminary assessment [ Reference 2) of the original Sepu.-
2, 1994 guidelines (Reference 3), which concluded that the strategy offered was not a prudent way to conduct inspections and that the staff expectation was for examination of 4
all accessible portions of the shroud circumferential welds.
}
On November 9,1994, representatives of the USNRC met with members of both the BWRVIP, Technical Comittee on Inspection and Evaluation and the Electric Power Research Institute (EPRI), to discuss BWRVIP guidelines for performing non-destructive examinations (NDE) of core shrouds in Category "C" BWRs. At i
the conclusion of this meeting, the BWRVIP comitted to providing revised criteria for performing examinations of Category "C" shrouds. The staff issued its Safety Evaluation Report regarding the "BWR Core Shroud Inspection and Evaluation Guidelines," on December 28,1994(Reference 4). This revised criteria was to be submitted to the USNRC in the first quarter of 1995 and was to address a number of items detailed in a March 6,1995, letter sent to the Technical Chairman of the BWRVIP Assessment Comittee (Reference 5). The purpose of the March 6,1995, letter was to provide clarification of the 4
staff's continuing program to address intergranular stress corrosion cracking l
(IGSCC) in core shrouds and other BWR internal components.
I The BWRVIP reports and their supplements are intended to provide guidance to all BWR licensees for the assessment of their core shroud. The submittals address the potential for shroud cracking, possible consequences of failure, j'
procedures to analytically assess any detected cracking. The NRC staff safety recomendations 'for inspection schedule and methods for flaw detection, and evaluation of the subject reports is contained herein.
l 2.0 APPLICABLE REGULATORY RE0VIREMENTS REGARDING EXAMINATION AND EVALUATION 0F CORE SHROUDS I
Title 10 of the Code of Federal Regulations (10 CFR) Part 50.55a, entitled
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" Codes and Standards," requires nuclear licensees to apply the inservice inspection criteria found in Section XI of the ASME Code to systems and components at their nuclear facilities. The scope of the ASME Code,Section XI rules include structural integrity requirements for ASME Code Class I components and their supports, including core shrouds. While not defined under the criteria of the ASME Code,Section III, core shrouds are designated as ASME Code Section XI components (under Class B-N-2 core support structures). Licensee's normally perform VT-3 examinations of the core shrouds every ~3.5 years in accordance with the requirements of Section XI.
VT-3s only require that the licensees confirm that the component is intact.
VT-3 examinations may not pick up flaw indications in the shroud. The General Electric Corporation; therefore, recommended that licensees perform enhanced VT-1 or UT examinations of their core shrouds, since these examination methods were more capable of detecting tightly bound IGSCC cracks. Regulatory requests for augmented inspections (enhanced VT-1 or UT) of BWR core shrouds are covered under the provisions of GL 94-03; however,10 CFR 50.55a(g)(ii) does allow for the Commission to impose additional augmented inspection requirements for the purpose of ensuring the structural integrity of safety related components and their support structures.
The ASME Code,Section XI Article IWB-3000 contains the acceptance standards for ASME Code Class I components, including integrally welded core support structures and reactor ir.terior attachments. One of the options under IWB-3000 allows licensees to use analytical evaluations to prove that adequate structural margin remains in flawed components. These analyses are subject to review by the NRC in accordance with IWB-3000. Licensee are required to implament repairs or replacements of ASME Code Class components for those cases where structural margins are shown to be insufficient to assure the structural integrity of the components.
Another option would be for the licensee to implement a repair to the shroud which assures the required structural margins. Since core shroud are defined asSection XI components, in terms of 10 CFR 50.55a, the repairs for core shrouds are treated as alternatives to the code under 10 CFR 50.55a(a)(3)(i).
This is because repairs of flaws under Section XI are normally accomplished using a code defined weld repair. Since the shroud repairs are not being done in accordance with the provisions of Section XI, the licensees have to submit their repair designs under 10 CFR 50.55a.
3.0 EVALUATION Section 3.1 of this SER provides the staff evaluation regarding the "BWR Core Shroud Inspection and Evaluation Guidelines," Revision 1 (Reference 1), dated April 21, 1995.
The content of Section 3.1 addresses the modifications made to the original "BWR Core Shroud Inspection and Evaluation Guidelines"
[ Reference 3). The changes to the guidelines were prompted by the staff's comments in the staff's SER of December 28, 1994 (Reference 4], and by additional input provided fro m the staff during the November 9,1994, BWRVIP/NRC/EPRI meeting at tia EPRI NDE Center in Charlotte, NC. Section 3.2 of this SER provides the sta 'f's evaluation of the November 22, 1994, submittal of the "BWR-VIP for. Shroud NDE Uncertainty and Procedure Standard,"
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e dated November 11,1994 (Reference 6).
3.1 Staff Recomendations from Oriainal SER on "BWR Core Shroud Insoection and Evaluation Guidelines The staff had several recomendations regarding BWRVIP's inspection scope and requirements which are now addressed by the BWRVIP in Pevision 1.
The following items are discussed below by providing the original staff coments, the BWRVIP response and the staff's current position:
(1)
Previous staff coment:
"The extent of accessible weld area for inspection may vary with the inspection method used. VT generally can access larger weld area than UT. Recently, UT has been the prevalent g
method for the inspection of the core shroud because it takes less time a
to complete the inspection. Therefore, the staff recomends that guidelines should be provided to require the performance of VT for areas not accessible by UT." (Reference 4, pages 11-12, item (1)]
BWRVIP Response to previous staff coment:
"If the cumulative length of weld inspected by automated scanners is less than 50% of the total length of the weld, then supplemental examinations should be performed using either area scanners or VT from both the ID and 00.
These additional inspections will centinue until at least 50% of the weld length has been examined including areas in both halves of the weld, or all accessible areas using these techniques have been examined." (Reference 1, Page 3-9, approach recomended for inspections and evaluations of CATEGORY C plants, item 4.)
Current Staff Position: The Staff considers this supplemental inspection by VT acceptable.
(2)
Previous staff coment:
" Additional guidelines should be provided including how to assess ti. structural integrity of those welds with limited accessibility. Qualitative assessment of structural integrity was discussed in Section 3.0 of the { original) report. The staff position is that when flaws are found in those welds, they should be repaired or evaluated quantitatively.
No credit shall be given for qualitative assessments."
[ Reference 4, page 12, item (2)]
BWRVIP response to previous staff coment:
" Detailed evaluations are performed when it may not be possible to inspect sufficient portions of a weld to assess the structural integrity with the assumption that uninspected regions are fully cracked, due to accessibility limitations or the constraints of the existing inspection technology.
In such instance, inspection of all accessible regions with the best available, qualified technology should be performed. The quantitative. assessment of the structural integrity of the weld will then be based on the inspection results, with a more moderate, but still conservative, assumption for the uninspected regions.
For example, the assumption for the uninspected region might be based on the 3
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deepest indication in the inspected region or on the distribution of indications in the inspected regions. Any assumed flaw would
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be subjected to crack growth as described in Section 4.0 (of the guidelines)."
inspections and(Reference 1, page 3-10, approach recommended for evaluations of CATEGORY C plants, item 8.]
i Current staff position: The Staff concurs that a quantitative l
assessment of the weld's structural integrity based upon inspection results is necessary. Weld areas that are inaccessible j
to inspection equipment or have not been inspected must be assumed j
to be through-wall cracked for evaluation purposes.
l (3)
Previous staff coussent: " Guidelines regarding the frequency for re-j inspection are not provided in the report. The staff recomends that the core shrouds in Category C plants should be inspected every i
j refueling outage since these plants have high degree of ausceptibility to IGSCC. The re-inspection frequency may be reduced when the a
j inspection results would support such a reduction. This reduction of re-inspection may be addressed at a later date." (Reference 4,
]
page12, item (3)]
I BWRVIP response to previous staff coment: The reinspection j
guidelines are still not included in Revision I to the "BWR Core Shroud Inspection and Evaluation Guidelines (Reference 1);
i however, the letter from the BWRVIP to the USNRC does prov(J. T. Beckham, Jr. to A. C. Thadani) on April 11, 1995, ide a tentative schedule for developing these guidelines (Reference 7]. This schedule is in accordance with the BWRVIP Assessment Milestone Number 2. " Guidance for Reinspection of Core Shrouds," with a tentative date for interim guidelines scheduled for September 1995; with a final report scheduled for June 1996.
Current staff position: In regard to the BWRVIP's " Guidance for Reinspection of Core Shrouds," the staff urges the BWRVIP to determine if this document can be submitted at an earlier date, in order to support upcoming reinspections.
(4)
Previous staff comment: "In Tables 3-2 and 3-3 of the report, VT/ET is recomended to supplement UT examination for detection of near-surface flaws when performing re-inspection during every other outage; however, it is not recomended for reinspection performed every outage. BWRVIP did not provide any basis for such recomendation. To ensure the detection of near-surface flaws, the staff recomends that supplemental examinations should be performed for all UT examinations. Recently, the UT creeping waves techniques qualified at EPRI NDE Center are used for detection of near-surface flaws at several plants. Because of the lack of service experience in the use of UT creeping wave technique near-surface flaw examination, the staff recomends that limited VT on a sampling basis should be performed to verify the inspection results.
This recomendation should also apply to eddy current examination (ET) when it is qualified for near-surface flaw examination."
(Reference 4, page12, item (4)]
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BWRVIP response to the previous staff comment: The original Tables 3-2 and 3-3 have been eliminated in Revision 1 to the "8WR j
Core Shroud Inspection and Evaluation Guidelines" (Reference 1).
There is no guidance in the document with regards to the inspection intervals or the specific use of supplemental VT for j
Current staff position: The Staff finds that creeping wave examinations are acceptable based on the additional information provided by the i
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.(5)
Previous staff comment: "In Tables 3-2 and 3-3 of the report, all UT j
inspections are recommended to be conducted from the shroud outside surface. This recommendation may not be suitable for the inspection of j
H3 weld for some plants. For example, the recent inspection of H3 weld 3
at OCNGS plant was performed from the shroud inside surface as the shroud outside surface at this location is not accessible due to the 4
presence of core spray piping. Furthermore, the recent industry inspection results have shown that the major cracking at weld H3 is initiated from the inside surface of the top guide ring. Considering j
the geometry of the top guide support ring and the incident beam angle, j
it is not clear to the staff that cracks initiated from the ring inside surface could be easily detected from the ring / shroud outside surface.
The staff recommends that additional inspection guidelines be provided 1
l to emphasize the importance of selecting appropriate inspection methods for achieving the best inspection results by considering the plant-i specific accessibility, component geometry and the flaw locations at 1
each weld." (Rtference4,page12, item (5))
l BWRVIP response to previous staff comment: Table 3-2 of the "BWR Core Shroud Inspection and Evaluation Guidelines, Revision 1,"
i entitled " Typical NDE Methods for BWR Core Shroud Welds H1-H7.."
i provides additional guidance on the through-weld UT techniques i
which may be used to inspect the H3 welds in BWR core shrouds j
[ Reference 1,pages3-16and3-17).
The revisions to the guidelines also state:
... Plant specific accessibility, component geometry, and location of potential flaws, I
should be considered when determining the inspection method (s) to be used on a given weld, in order to achieve the best inspection results."
[ Reference 1, page 3-1, second paragraph)
Current staff position: These changes adequately respond to the Staff's original recommendations.
(6)
Previous staff comment:
"The staff recognizes that it is difficult to inspect weld H7 with a backing ring from the outside surface of the shroud. The BWRVIP should provide additional information regarding the detection of cracks initiated from the weld root.
This is presently unclear since the backing ring appears to interfere with such an examination. Also provide a discussion of the reliability and accuracy of such an UT inspection." (Reference a, page 13, itet (6))
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BWRVIP response to previous staff coment: Table 3-2 of the "BWR Core Shroud Inspection and Evaluation Guidelines, Revision 1,"
entitled " Typical NDE Methods for BWR Core Shroud Welds H1-H7.,"
provides diagramatical depictions of the two UT techniques, 45' shear wave UT and creeping wave UT, that the BWRVIP considers are applicable for examinations of H7 welds with backing rings.
(Reference 1,pages3-16and3-17]
Additionally, page 2-2 of the "BWR-VIP Core Shroud NDE Uncertainty &
Procedure Standard," dated November 22, 1994, states that "... a mock-up of the lower cylinder-to-shroud support weld, the H7 weld dissimilar metal fleid weld with a backing ring on the outside surface... is being designed" [ Reference 6).
Current staff position: Qualification of the ins)ection methods utt11 zing mock-ups of H-7 welds with and without Jacking rings should be expedited by the BWRVIP.
(7)
Previous staff coment:
"The BWRVIP should provide a date regarding when the inspection of other shroud support assemblies and attachment welds will be addressed. Also include any on-going development program or program to be implemented in facilitating such an inspection.
(Reference 4, page 13, item (7))
BWRVIP response to previous staff coment: Revision 1 of the "BWR Core Shroud Inspection and Evaluation Guidelines" (Reference 1) does not address this issue; however, the letter from the BWRVIP to the USNRC, dated April 11,1995 (Reference 7, from J. T. Beckham, Jr. to A.C.Thadant],doesprovideascheduleforthefollowingBWRVIP inspection mi,estones:
Inspection Milestone Number 4: Training and Qualification of Inspection Personnel (Vessel Internals) Begin training Sept. 95; to be completed December 96.
Inspection Milestone Number 5: NDE Quantification for Other Internal Components December 96.
Inspection Milestone Number 6: Other Internals NDE Development-Advanced Techniques Uncertainty measurement determined / visual training complete by August 96. Advanced procedures and scanners developed by December 96.
Inspection Milestone Number 7: Other Internals Mock-ups December 96.
Inspecticn Milestone Number 8: Training and Qualification of Inspection Personnel Visual training complete by December 95, Qualification and demonstration protocol complete by December 96.
i Current staff position: These dates are consistent with the latest BWRVIP schedule for accomplishing the BWRVIP inspection milestones 4-8.
The staff will review these items upon receipt 6
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(8)
Previous staff coment:
"The BWRVIP inspection guidance does not address the inspection of core shrouds w1ere a repair has been implemented. The staff recently issued a safety Evaluation Report (Reference 8' for the BWRVIP shroud repair design criteria (Reference 9' Pre-installation or post-installation inspections of welds structurally replaced by the repair option are not required.
j However, other welds not affected by the repair were not discussed.
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Future revisions of the BWRVIP inspection guidelines should address the t
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inspection of core shroud welds considering the possibility of a j
repair." (Reference 4,page13, item (8))
BWRVIP response to previous staff coment:
It does not appear that this recommendation was responded to in Revision 1 to the "BWR Core Shroud Inspection and Evaluation Guidelines,"
[ Reference 1) or Revision 1 to the "8WR Core $hroud Repair Design i
criteria"(Reference 8)
Current staff position: The BWRVIP should develop inspection guidelines which address the inspection of core shroud welds that are considered to be critical in terms of assuring the structural integrity of core shroud repair assemblies. These guidelines should be submitted for NRC review prior to the first reinspection of a repaired BWR core shroud.
(9)
Previous staff coment:
" Sections 5.0 through 5.2 of the BWRVIP guidelines do not address the possibility of not detecting shallow IGSCC surface flaws when using volumetric examination methods.
Due to the difficulty in detecting shallow cracks using volumetric a
NDE methods the existence of such flaws cannot be precluded.
Therefore, if fillet weld cracking is confirmed using a volumetric examination technique, a minimum detectable flaw size must be assumed."
[ Reference 4,page18)
BWRVIP response to previous staff coment:
Page 5-2, Revision 1 submittal, last saragraph, last sentence states:
"If any cracking i
is detected at tie fillet weld, the minimum detectable fillet weld crack must be assumed to exist as a 360 degree crack, and that assumed crack must be assumed to grow at an appropriate rate for structural analysis purposes."
[ Reference 1,page5-2,last paragraph)
Current staff position:
In regard to performing limit load or linear elastic fracture mechanics evaluations of 8WR core shrouds, the staff has not yet given credit for fillet welds in the shroud designs.
Further justification and guidelines must be provided should licensees seek credit for core shroud fillet welds. The content of the justification and guidelines should include supporting finite element modeling and should address the capabilities and limitations (e.g.
threshold of detection) justification and guidelines should also address of the inspection method 'Jsed for these fillet weld inspections.
The flaw evaluation methods and criteria should cracking be detected in the 7
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1 fillet welds. Where appropriate qualified field or test data on shroud 5 x 10' geld crack growth rates is lacking, a bounding growth rate of fillet inches /hr should be assumed.
3.2 Evaluation of BWR-VIP Core Shroud NDE Uncertainty & procedure Standard The staff has not previously issued an SER regarding the "BWR-VIP Core Shroud NDE Uncertainty & Procedure Standard" (Reference 6 henceforth abbreviated as i
theNDEguidelines).
The following evaluation in this Section paraphrases the content of the NDE guidelines, and then provides the current staff position 1
regarding the content.
3.2.1 NDE of Core Shroud Welds Sumary of the content in the NDE guidelines: The BWRVIP has recognized the fact that accurate NDE measurement is necessary to provide accurate input into the fracture mechanics calculaticos for the Shroud inspections / evaluations.
The BWRVIP has determined that the uncertainties for any given NDE technique will depend on the physical access and the type of NDE method employed for any given weld. This submittal has summarized the access and the applicable NDE methods available to inspect the horizontal shroud welds on most BWR plants.
Series of measurements have been performed to determine the uncertainties of each NDE method to be utilized: visual examination (VT), ultrasonic examination (UT),andeddycurrentexamination(ET)ldeliverysystemposition taking into account that uncertainties can arise from two sources: mechanica uncertainty, and the uncertainties inherent in any given NDE method.
The BWRVlP has derived, evaluation factors from these uncertainties for the given techniques and deitvery systems evaluated.
If a shroud is inspected by a technique / delivery system which has not been evaluated by the BWRVIP, then default evaluation factors of 4T for length and 0.3 inch for depth have been assigned by the BWRVIP.
Current staff position: The Staff finds the NDE uncertainty values for inspection delivery systems that haven't been evaluated by the BWRVIP to be reasonable based upon the inspection experience to date and experience with IGSCC.
It is unclear, however, from the submittal what procedures are available to evaluate or certify a given delivery system /HDE technique for use. Additional guidelines should be established to clarify what sort of actions and steps are necessary in order to qualify NDE inspection technologies for use.
3.2.2 Nockups Sumary of the content in the NDE guidelines A number of shroud weld mock-ups have been developed for this program. These mock ups are designed with fatigue cracks in lieu of IGSCC or IASCC cracks due to the fact that laboratory production of thete cracks is both time consuming and not very controllable with respect to the shape and dimension of the produced cracks.
These fatigue cracks are similar to IGSCC cracks with respect to their width (tightness) and are located in orientations similar to those seen in actual shroud examinations. Detection of these cracks is qualitatively similar to IGSCC cracks when subjected to ultrasonic examination methods.
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types of rack-ups have already been developed for the core shroud inspection program (1) mockup of the Brunswick submergrJ arc welded (SAW), single-J, e
ring-ta-cylinder H3 weld with saw cut and EDM notches, and solidificattor.
cracks near the weld surface, (2) single-J, ring-to-cylinder SAW welds with fatigue cracks, fatigue cracks.an(d3) double-V, single sided ring-to cylinder SAW welds with following mockups ar(4) GE mockup of the Brunswick's H3 and H4 welds.
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e in the process of being developed:
(1) cylinder-to-
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cylinder mockup containing double-V, double-sided, SAW horizontal and vertical beltline welds with five fatigue cracks, and (2) lower cylinder-to-shroud mockup an H7 dissimilar metal field weld with a backing ring on the inside surface and containing fatigue cracks.
The following mock-ups are also either being prepared or borrowed from other sources in order to define specific sources of NDE uncertainty:
Crack branching: The NDE center has initiated the borrowing a set of stainless steel plates from Savannah River Laboratory which contain IGSCC which are severely branched.
Surface roughness: Hock-up to imitate the surface conditions observed on boat samples removed from Dresden and Quad Cities.
Weld profile of shroud block BWRVIP-B: Full-scale weld profile to evaluate fusion line location.
Multiple parallel cracks: Multiple cracks (simulated by EDM notches) to imitate field observations.
First mock-up contains four notches.
EDM notches in weld toe: BWR recirculation outlet piping sample used to evaluate effectiveness of ET in detection of flaws located in weld crown toe. Some shrinkane and undercut at the weld crown toe. EDM notches were cut into this area. The notches were 0.02-inch. 0.05-inch and 0.20-inch deep parallel to the weld.
Current staff position: The staff agrees that full size mock-ups for assessing the performance of NDE techniques will enhance core shroud evaluations. The staff requests that a schedule for completion and evaluation of these mock-ups be provided.
3.2.3 Ultrasonic Examination Procedure Standard and Uncertainties Sumary of the content in the NDE guidelines: BWRVIP investigations involving shroud Of have included a review of field UT data, UT experiments on shroud weld and other mock-ups; and bench marking of scanner accuracy. These investigations have evolved a dhtabase supporting the use of UT for aluation of shroud welds and the uncertainties involved in flaw characterization, and a procedure standard.
This procedure standard uses a 45 degree shear wave, 60 degree longitudinal wave and creeping wave probes. The 60 degree longitudinal wave probe is used for detecting and sizing cracks connected to either the scanning surface or the opposite surface. The 45 degree probe allows detection of shallow cracks, 9
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s connected to the scanning or opposite surfaces.
It is also used to confirm both the size and depth of the cracking detected by the 60 degree scans. The creeping wave probe allows detection of shallow cracks connected to the examination surface. 'It is useful primarily for crack detection.
The primary uncertainties associated with the shroud UT inspections are the technique uncertainties inherent with the ultrasonic techniques involved and the delivery system uncertainties. The BWRVIP evaluations have included crack length measurement accuracy; crack depth measurement accuracy; the effects of multiple / parallel cracks; the effects of surface roughness and detectability of shallow, near surface flaws. The investigations conducted to date by the BWRVIP have utilized 45 (455) degree shear wave, 60 degree longitudinal wave (60L) and creeping wave probes with the same nominal parameters as those utilized in field examinations. The 60L 2 robe has two 4 MHz, 0.6 inch by 0.23 inch rectangular elements in a side )y side pitch-catch configuration, arranged for a nominal focal depth of 0.8 inch. The 455 probe has a single 2.25 MHz, 0.4 inch by 0.38 inch element. The creeping wave probe is composed of two 2 MHz,10 mm by 15 mm rectangular elements in a side by side pitch-catch arrangement with a focal distance set at 12 mm.
Data acquisition was made utilizing the NDE Center's Tomoscan computerized data acquisition and imaging system. The system utilized is essentially identical to the systems in use for shroud inspections at the time of the submittal. All data was acquired using a water coupling which is the method used for plant inspections.
Crack length and depth measurements were performed on all cracks in the single-J, SAW, ring-to-cylinder weld mockup (BWRVIP-A mock-up). Crack length was measured between the probe positions at which the crack response dropped to zero (this tends to oversize the crack length by one beam's width) and crack depth was measured at the deepest detectable feature of the crack face.
Near side measurements of crack length were generally conservative. The average length measurement error was +0.60-inch with a standard deviation of 0.43 inch. Only one length measurement was shorter than the actual crack length. This occurr in a non-preferred e.ed when measuring a 0.6-inch deep crack with a 45S probe xamination orientation. Near side crack depth measurements were also conservative. The average depth raeasurement error was
+0.60-inch with a standard deviation of 0.ll-inch. Again, only one measurement was non-conservative (45S probe measurement). The crack measurement uncertainties from the weld far side is still in question since insufficient data has been generated to provide firm conclusions. The BWRVIP also conducted comparisons of UT crack depth estimates performed on boat samples before their removal from shrouds and subsequent destructive examination.
The sensitivity of UT to detect shallow reflectors (cracking) is extremely important in assessing a core shroud's safety margins for operation. The creeping wave probe was used to detect EDM notches of 0.05 inch and 0.02 inch depths in a shrink area of a stainless steel pipe weld. The signal to noise ratio (SNR) was at least 14 dB providing a sensitivity comparable to flaws
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connected to the opposite side when scanning the shroud cylinder. The 60L probe was capable of detecting the shallowest crack in BWRVIP-A.
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of the 60L probe indicated that a drop in the probe's detection sensitivity occurs when scanning for flaws less than 0.2 inches deep, and that the probe's detection sensitivity was only marginally capable of detecting flaws less than 0.1 inch deep.
R The BWRVIP determined that cracks up to approximately 35 degrees off-axis can be detected with a creeping wave probe. Additionally, multiple defects were sized and detected. A 0.01 inch deep notch was not detected with the 60L probe in this series cf investigations. Surface roughness effect on UT i
measurement s were made using the 60L, 45S and creeping wave probes.
It was j
determined that surface roughness did not degrade detection and sizing of EDM notches.
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The only delivery system evaluated for this submittal for uncertainties was i
the GE system for UT and ET. This system utilizes two main scanning systems.
These systems are the "00 Tracker" and the " Suction Cup Scanner". Uncertainty evaluations were perfomed on these scanners on September 13,.1994. The uncertainties associated with the "0D Tracker" was determined to be 0.037 inch t
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out of total travel of 6.935 inches (about 1/2%). Uncertainties in regard to the evaluation of the " Suction Cup Scanner" were comparable. The largest single uncertainty associated with this system is in the scanner placement on the shroud. The "00 Scanner" had azimuthal uncertainties of approximately one inch. The " Suction Cup Scanner" uncertainties can be somewhat larger.
Current staff position: The staff agrees that the UT sizing and detection methods employed by the BWRVIP appear to be conservative. The staff is in agreement that UT creeping wave detection and sizing capabilities are reasonably accurate down to 0.02 inch crack depths. However, the lack of confidenceinthefarsideUTmeasurementsneedstobeevaluatedlnatimely manner.
3.2.4 Eddy Current Examination Standard and Uncertainties Sununary of the content in the NDE guidelines: The BWRVIP ET standard utilizes a flat-surface, cross-point probe operating in the frequency range of 50-600 kHz and in the driver / pick-up mode. The shroud weld ET evaluations were conducted at a primary test frequency of 300 kHz using a computerized data acquisition and imaging system. At the time of this submittal. ET has not been performed during a plant shroud examination. The uncertainties involved in ET examination are similar to those of UT e.g., technique inherent uncertainty and delivery system uncertainty. Experimental results from the NDE center show that the techniques are capable of detecting the end of a flaw within I coil diameter. These evaluations determined that cracks up to about 40 degrees off axis can be detected with the cross-point probe in a single scan. All notches were detected in the automated scans with surface roughness being a variable. The delivery system uncertainty was evaluated at the same time as the UT delivery system (September 13, 1994) and is the same.
Current staff position: The staff has reviewed the ET standard of the "BWR-VIP Core Shroud NDE Uncertainty and Procedure Standard," and does not have any unresolved issues in regard to its content at this time. However at this time ET has not been qualified as an acceptable method for performing examinations 5
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'l of core shrouds. It appears that the schedule for q' alification of ET u
techniques for internals is covered by the scopes of BWRVIP Inspection Milestones Numbers 2-8 (Reference 7). If not, the staff requests that a schedule be provided for ET qualification milestones. Should ET inspections i
be performed on core shrouds or other BWR internals in the field prior to qualification of the technique by the BWRVIP, the staff requests that the details of the technique, and appropriate field data and experience be included in future submittals to the staff.
3.2.5 Visual Examination Procedure Standard and Uncertainties Summary of the content in the NDE guidelines: Personnel are required to 1
receive at least four hours of shroud specific training, including review of actual shroud inspection tapes containing the flaw types of interest. The i
standard requires a 0.0005-inch diameter wire as a sensitivity, resolution, j
and contrast standard.
Industry practice is generally to use a 0.001-inch i
diameter wire. Detailed surface cleaning requirements are provided. System i
resolution demonstrations are required, in addition to water clarity j
monitoring. Two uncertainties are associated with crack length measurement:
a) only the visible portion of the crack length can be measured; b) the
.i manipulation of the viewing equipment at long distances can significantly affect length measurements when determining distance from visible landmarks.
I 3
l Detectability uncertainties were evaluated utilizing a 0.0005 inch diameter wire and cameras.
In each case the wires were detected. Further experiments are planned when actual IGSCC samples will be evaluated remotely and a comparison made between crack depths and detectability. The uncertainties associated with the delivery system were evaluated and show that in general, i
the indications tend to be undersized. The average error in 25 measurements a
was approximately 0.79 inches, while the average of the absolute value of the i
errors was 1.81 inches. The average undersize error was 1.91 inches, with the maximum undersize error being 5.81 inches out of a true length of 71.06 1
inches. The average oversize was 1.6 inches, with the maximum oversize error being 14.75 inches out of a true length of 47.5 inches.
Current staff position: The current BWRVIP position in regard to performing i
examinations of BWR core shrouds is that UT testing is the preferred method of inspection. Should VT be chosen as the primary examination method, the staff I
requests that the BWRVIP establish further criteria to account for VT sizing (measurement uncertainties.
minimum accep) table VT adjustment value that shall be added to the VTThese cr i
determined flaw indications (flaw lengths).
4.0 CONCLUSION
S The staff has reviewed the "BWR Core Shroud Inspection and Evaluation Guidelines," Revision 1 (Reference 1), and "The BWRVIP Core Shroud NDE Uncertainty & Procedure Standard," (Reference 6). The subject reports and
(
their supplements are intended to provide guidance to all BWR licensees for the assessment of their core shroud.
12
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The staff's evaluations of these BWRVIP documents are found in Sections 4.1 l
and 4.2 of this SER. The staff has noted several items in "The BWRVIP Core Shroud NDE Uncertainty & Procedure Standard,' [ Reference 6) and in Revision 1 1
to the "8WR Core Shroud Inspection and Evaluation Guidelines" (Reference 1]
{
which are in need of further clarification or guidance. These issues are provided in the Current Staff Position paragraphs located in Sections 4.1 and 4.2 of this report. For ease of review, these unresolved items and staff i
concerns are repeated in Attachment I to this SER.
5.0 REFERENCES
I 1.
April 21, 1995 - Letter from the BWRVIP to the NRC, submitting the "BWR 1
{
Core Shroud Inspection and Evaluation Guidelines, Revision 1."
1 1
2.
September 13, 1994 - Letter from the NRC to the BWRVIP forwarding its j
preliminary " Evaluation of BWR-VIP Report Entitled 'BWR Core Shroud Inspection and Evaluation Guideline,' GENE-523-ll3-0894, j
September 2, 1994."
{.
3.
September 2, 1994 - Letter from the BWRVIP to the NRC submitting the
- I "8WR Core Shroud Inspection and Evaluation Guidelines."
i 4.
December 28, 1994 - Letter.from the NRC to the BWRVIP submitting the i
staff's " Evaluation of 'BWR Shroud Cracking Generic Safety Assessment, i
Revision 1,' GENE-523-A107P-0794, Augv't 5, 1994 and 'BWR Core Shroud
]
Inspection of Evaluation Guidelines,' GENE-523-ll3-0894, September 2, 1994,"
4 l
S.
March 6, 1995 - Letter from the NRC to the BWRVIP submitting the staff's
" Clarification of Staff Position Regarding Inspection and Evaluations of
{
BWR Core Shrouds and Other Internal Component."
i 6.
November 22, 1994 - Letter from the BWRVIP to the NRC submitting the l
"BWR-VIP Core Shroud NDE Uncertainty & Procedure Standard."
l 7.
April 11, 1995 - Letter from the BWRVIP to the NRC submitting the j
" Milestones for the BWR Vessel and Internals Project Program Plan."
a j
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8.
September 13. 1994 - Letter from the BWKVIP to the NRC submitting the i
BWRVIP ' Response to NRC Request for Additional Information (RAI)
Regarding Boiling Water Reactor BWR) Core Shroud Repair Design Criteria," and including "Attachm(ent 1 - Response to NRC Question i
lI
' Attachment 2 - Revision 1 to the Core Shroud Repair Design Criteria."
i 9.
September 29, 1994 - NRC letter to the BWRVIP submitting the staff's
" Safety Evaluation on Boiling Water Reactor (BWR) Core Shroud Repair Design Criteria."
i l
PRINCIPLE CONTRIBUT0f1-l Carl J. Czajkowski BNL j
(516)282-4420-James Medoff, MRR (301)415-2715 i
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ATTACHMENT 1 i
LIST OF STAFF CONCERNS AND UNRESOLVED ITEMS IN REVISION 1 TO THE "BWR CORE SHROUD INSPECTION AND EVALUATION GUIDELINES."
AND THE "BWR-VIP CORE SHROUD NDE UNCERTAINTY AND PROCEDURE STANDARD" This list of staff concerns and unresolved items is presented in the order found in the staff's evaluation (Section 4.0 of this SER).
Item 1.
In regard to the BWRVIP's " Guidance for Reinspection of Core Shrouds," the staff urges the BWRVIP to determine if this document can be submitted at an earlier date, in order to support upcoming reinspections.
stem 2.
Qualification of the inspection methods utilizing mock-ups of H-7 welds with and without backing rings should be expedited by the BWRVIP.
Item 3.
The BWRVIP should develop inspection guidelines which address the inspection of core shroud welds that are considered to be critical in terms of assuring the structural Integrity of core shroud repair assemblies. These guidelines should be submitted for NRC review prior to the first reinspection of a repaired BWR core shroud.
Item 4.
In regard to performing limit load or linear elastic fracture mechanics evaluations of BWR core shrouds, the staff has not yet given credit for fillet welds in the shroud designs.
Further justification and guidelines must be provided should licensees seek credit for core shroud fillet welds. The content of the justification and guidelines should include supporting finite element modeling and should address the capabilities and limitations (e.g. threshold of detection these fillet weld inspection).of the inspection method used for s
The justification and guidelines should also address flaw evaluation methods and criteria should cracking be detected in the fillet welds. Where appropriate qualified field or test data on shroud fillet weld crack growth rates is lacking, a bounding growth rate of 5 x 10'5 inches /hr should be assumed.
Item 5.
The Staff finds the NDE uncertainty values for inspection delivery systems that haven't been evaluated by the BWRVIP to be reasonable based upon the inspection experience to date and experience with IGSCC.
It is unclear, however, from the submittal what procedures are available to evaluate or certify a given delivery system /NDE technique for use. Additional guidelines should be established to clarify what sort of actions and steps are necessary in order to qualify NDE inspection technologies for use.
15
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LIST OF STAFF CONCERNS AND UNRESOLVED ITEMS IN REVISION 1 TO THE l
"BWR CORE SHROUD INSPECTION AND EVALUATION GUIDELINES
- 4 AND THE "BWR-VIP CORE SHROUD NDE UNCERTAINTY AND PROCEDURE STANDARD" i
l Item 6.
The staff agrees that full size mock-ups for assessing the i
performance of NDE techniques will enhance core shroud evaluations, j
The staff requests that a schedule for completion and evaluation of j
these mock-ups be provided.
Item 7.
The staff agrees that the UT sizing and detection methods employed i
by the BWRVIP appear to be conservative. The staff is in agreement i
that UT creeping wave detection and sizing capabilities are j
reasonably accurate down to 0.02 inch crack depths. However, the lack of confidence in the far side UT measurements needs to be evaluated in a timely manner.
Item 8.
The staff has reviewed the ET standard of the "BWR-VIP Core Shroud
}
NDE Uncertainty and Procedure Standard." and does not have any unresolved issues in regard to its content at this time. However at this time ET has not been qualified as an acceptable method for i
performing examinations of core shrouds.
It appears that the schedule for qualification of ET techniques for internals is covered by the scopes of BWRVIP Inspection Milestones Numbers 2-8
[ Reference 7.
provided for)ET qualification milestones.If not, the staff requests that a Should ET inspections be performed on core shrouds or other BWR internals tr, the field prior to qualification of the technique by the BWRVIP, tM staff requests that the details of the technique, and appropriat6 (teld data and experience be included in future submittals to the staff.
Item 9.
The current BWRVIP position in regard to performing examinations of BWR core shrouds is that UT testing is the preferred method of inspection. Should VT be chosen as the primary examination method, the staff requests that the BWRVIP establish further criteria to account for VT sizing These criteria shouldincludeestablis(measurement) uncertainties.
hment of a minimum acceptable VT adjustment value that shall be added to the VT determined flaw indications (flawlengths).
16 m ur
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j' 9.
The current BWRVIP position in regard to performing examinations of BWR 5-core shrouds is that UT testing is the preferred method of inspection.
Should VT be chosen as the primary examination method, the staff i
requests that the BWRVIP nstablish further criteria to account for V1 q
sizing (me'asurement) uncertainties. These criteria should include establishment of a minimum acceptable VT adjustment value that shall be added VT determined flaw indications (flaw lengths).
These items are more comprehensively discussed in the staff's Safety i
Evaluation Report enclosed with this letter. Please address any questions i
regarding the staff evaluation to Mr. Robert A. Hermann of my staff at (301) 415-2768.
s 0 ICI SIGNEDBY BRANW.SHDW Brian W. Sheron, Director Division of Engineering Office of Nuclear Reactor Regulation cc:
E. G. Carpenter Distribut'on:
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DOCUMENT NAME: G:\\ SHROUD \\BWRVIP.EVL
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