ML20151C689

Forwards Proprietary & Nonproprietary Replacement Pages to Rept Submitted by Re Control Element Assembly Ctr Finger Cracking.Replacement Pages Due to Changes to Proprietary Classifications.Proprietary Pages Withheld

ML20151C689
Person / Time
Site:	Calvert Cliffs
Issue date:	04/06/1988
From:	Lippold W BALTIMORE GAS & ELECTRIC CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
Shared Package
ML19302D421	List: ML20151C689
References
NUDOCS 8804130134
Download: ML20151C689 (13)

Text

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BALTIM O RE GAS AND ELECTRIC CHARLES CENTER.P. O. BOX 1476 BALTIMORE, MARYLAND 21203 W. JWEs LIPPOLD MANAGER PdVCLE An ENceNEEmrNo SERytCE3 DEPARTMENT April 6, 1988 U. S. Nuclear Regulatory Commission Washington, DC 20555 ATTENTION: Document Control Desk SUBJ ECT: Calvert Cliffs Nuclear Power Plant Unit Nos. 1 & 2; Docket Nos. 50-317 & 50-318 Control Element Assembly (CEA) Center Finger Cracking at Calvert Cliffs

REFERENCE:

(a) W. J. Lippold Letter Dated October 2,1987, Same Subject Gentlemen:

The reference letter provided a report detailing an evaluation of CEA cracking at Calvert Cliffs. Discussions with your staff have resulted in changes to the proprietary classifications of some statements in that report. Replacement pages (both proprietary and non proprietary) reficcting the current proprietary classifications are attached.

Should you have further questions regarding this matter, we will be pleased to discuss them with you.

! Very truly yours.

WJL/PAF/cew l 40 1

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Document Control Dask April 6, 1988 Page 2 cc: w/o Attachments D. A. Brune, Esquire J. E. Silberg, Esquire R. A. Capra, NRC S. A. McNeil, NRC W. T. Russell, NRC D. C. Trimble, NRC T. Magette, DNR

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Documsnt Control Dask April 6, 1988 Page 3

~bec: w/o Attachments J. A. Tiernan P. E. Katz/R. C. L. Olson C. H. Cruse /R. F. Ash R. E. Denton/J. A. Mihalcik

-R. M. Douglass/J. T. Carroll ~

T.- N. Pritchett/E. I. Bauereis J. R. Lemons /R. P. Heibel W. J. Lippold/A. R. Thornton F. J. Munno/M. Gavrilss R. B. Pond /R. E. Cantrell L. B. Russell /R. L. Wenderlich C. C. Lawrence, III/M. J. Hiernicki R. C. DeYoung C. M. Rice R. G. Staker L. F. Donatell J. E. Baum D. L. Shaw, Jr.

S. R. Cowne

?. E. McGrane M. J. Warren M. E. Bowman /L. E. Salyards i ',

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Replacemant Pages for CEN-364(B)-P (Non-Proprietary Version) 1

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3.0 Inspection Results and Discussion ,

3.1 Eddy Current Testirig (ECT) of Calvert Cliffs Unit I and Unit 2 CEAs The ECT inspection began with Unit 1 in November, 1986, during the E0C-8 outage and was completed in April, 1987, during the Unit 2 E0C-7 outage. The inspection methods used were eddy current encircling coil and eddy current profilometry to detect defects and characterize dimensions, respectively. All fingers of all CEAs in both units were inspected with some CEAs also undergoing reinspection. There were 67 assemblies inspected from Unit I and 77 assemblies inspected from Unit 2 for a total of 144 assemblies.

The initial analysis of the ECT encircling coil data from the Unit 1 inspection was perfonned off-site, consistent with the BG&E inspection /

l replacement strategy wherein most of the Unit 1 CEAs were replaced by new CEAs. This analysis revealed an indication of a crack on the center finger of CEA C4. Upon returning to Calvert Cliffs for the Unit 2 E0C-7 outage inspection, the ECT data was analyzed on-site using the standard methodology as employed with the Unit I data. At this time cracking l indications were detected on the center fingers of sixteen (16) CEAs.

Visual inspection of two such fingers, however, revealed the presence of cracks on the outside surface, and, from one l

of these, gas bubbles were observed emanating from the crack indicating a through-wall defect.

Because of this discrepancy between the initial ECT results and visual observation, a new analytical procedure was instituted which provided a more consistent description of the crack indications with respect to the visual i observations. This new methodology employed the .

With this approach, the presence of the previously detected cracks in the Unit 2 3-1

l Table 3-2 CEA Visual Examination Results Distance (in.) from CEA Sottom 10 Cracks Width of Comments Crack length Crack CEA Start of Crack Midpoint (in.) (in.)

Bubbles emanating C4 Bubbles emanating 09 Bubbles emanating CK Bubbles emanating BG Y

• > Bubbles emanating (

CA Definite crack,'no bubbles CE BA Definite crack, no bubbles B2 No crack visible AC Small possible crack A6 No crack visible Definite crack, no bubbles AB

Eight of the fingers showed definite cracks with 5 cracks emitting gas bubbles thus supporting them to be through-wall. Figure 3-la shows the widest crack (on CEA C4) and Figure 3-lb shows a tight crack on CEA CE which is typical of the remaining fingers. '

The appearance of t.he cracks is consistent with an intergranular cracking phenomenon having a brittle appearing irregular path on the cladding surface.

A possible crack was detected on one finger (AC). Two CEA fingers showed no visible signs of cracks (B2 and A6). Since the cracks are small and the cladding surface exhibited axial scratches and various wear patterns, the possibility exists of not being able to visually recognize a crack.

Figure 3-2 shows a schematic diagram of the bottom of a CEA center finger on which the axial extent and the midpoints of the visible cracks are located.

All midpoints are within the in the center finger. This location corresponds to the maximum strain region of the >

cladding caused by swelling of the B4 C pellets.

l l

3.3 Strain Data

, The inspection campaigns included measurement of all five fingers of each CEA with both the encircling coil eddy current probe and the eddy current profilometer equipment. Since both inspection techniques provide cladding strain information, the determination of the cladding strain for an individual finger was based on the best available ir. formation from either (or both) technique (s). Because the outer fingers of the CEAs have Ag-In-Cd slugs in l the high exposure portion of the rod (the lower end), there is essentially no strain in any of the outer fingers. The center fingers have B 4C pellets over their full length and had maximum circumferential average strains measured in 35 l

the cold condition of up to 1.3%. The distributions of the maximum strains associated with the center fingers of both Unit 1 CEAs and Unit 2 CEAs are shown in Figure 3-3. Superimposed on the strain histograms is the distribution of the strain of the center fingers that are cracked (both partial-and through-well cracks). Inspection of the figure shows that the distribution of the strains associated with cracked fingers is very similar to the strain distribution for all center fingers. This similarity suggests that the presence of a crack is not highly dependent on the magnitude of the cladding strain, which implies that another contributor (Irradiation Assisted Stress Corrosion Cracking (IASCC), for example) in combination with some minimum strain is necessary for the cracks to form.

3.4 Relationships Among ECT Data, Visual Observations and Strain Measurements The available data from ECT, visual observations and strain 'neasurements on the cracked fingers obtained during the E0C-7 inspections at Calvert Cliffs Unit 2 have been correlated ar.d are summarized in Table 3 3. Also included are data from CEA C4 discharged from Unit I at EOC 8. Of the 17 cracked fingers found at Unit 2 at EOC-7, 7 were also ECT inspected at EOC-5 (A6, AB, BA, BD, C3, CA, and 02). Only one of them, BA, had a crack indication at E0C-5, in this case L part tal-wall crack, wnich subsequently developed into a through-wall crack by EOC-7.

An important result from the analysis of the ECT data was that the partial wall cracks originate on the outside surface of the cladding. This was substantiated by visual observation of the center finger of CEA AB which had a partial-wall ECT crack signal and a crack was observed on the outside surface. In this connection, only cracks with through-wall ECT indications exhibited bubbling which tends to support the accuracy of the ECT designation of "partial-wall cracks".

The measured cold strains in Table 3-3 range from for cracked fingers indicating no obvious correlation between: the CEA finger duty cycle, the measured strain; and the occurrence of cracking.

38

Table 3 3 Sumary of lJnit 2 CEA Examinations at EOC-7 Center Fingers Only heasured Visual Cold LGI Crack observed Strain  !

Lis_! Partial Thru Wall .Y.g1 Eq ,1 A6 x(1/2 signal) x AS x(1/2 signal) x AC x(small) x(possible)

B2 x x BA x x BD x N/A BG x(large) x(bubbles)

C1 x N/A C: x N/A C4+ x(large) x(bubbles) 4 CA x(large) x(bubbles)

CC x N/A CE x(possible) x CF x N/A CK x .v(bubbles)

D2 x N/A DS x N/A

, D3 x(large) x(bubbles) ll/A = Not examined. *

+ Discharged from Unit I at E0C8, originally in Unit 2. Visually examined with Unit rEA: . '

3-10

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A was used to fabricate the center all-B 4C fingers of Calvert Cliffs Unit 2 CEAs. The center finger cladding of forty nine (49) CEAs (including C4 which had been transferred to Unit 1) came from the item 1 tubing lot -

and twenty (20) were from the item 2 tubing lot . Manufacturing records showed that fifteen out of the eighteen fingers with ECT crack indications were fabricated with the item 1 tubing. This tubing had a in comparison to the Item 2 lot and several other lots

-used to fabricate CCAs that have shown satisfactory performance. The ASTM Specification requires . Both the ASTM chemistry and SPI tensile requirements have been met for the Calvert Cliffs tubing as well as for all other tubing ever received by C-E for CEA fabrication.

The remaining three cracked fingers in the Unit 2 CEAs are traceable to the Item 2 tubing which had a relatively . The four cracked fingers in the Unit 1 CEAs are traceable only to a group of tubing lots that include material with a Recent inspection data on other lots at other plants include those from Reactor B, where tubing exhibited no cracks and some measured strains were greater than 1%. If per se, were a factor in the cracking phenomenon, some cracking of the 21 Reactor B CEAs, which were inspected, would probably have occurred and been detected. It should be noted that crack-free operation of CEA fingers at other plants at comparable strain levels is associated not only with the

! The tentative conclusion is that high impurity content, strongly influenced the susceptibility of the Inconel 625 caterial to cracking. High mechanical strength,

. , also appears to contribute to cracking susceptibility.

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l 4.0 Cladding Failure Mechanism 4.1 Significance of the Observations ,

The key characteristics of the cladding cracks observed on the CEA fingers are (1) their location near the point of maximum cladding strain / stress and (2) their initiation on the outside surface of the cladding. These are consistent with an irradiation assisted stress corrosion cracking (IASCC) mechanism where the stress is induced by the inexorable swelling of the B4 C pellet in contact with the claddtng .

The inspection results, in the form of a histogram of cracking frequency observed at both Calvert Cliffs ur,its, are summarized in Figure 4-1. These results indicate that the highest frequency is associated with the

. However, a significant number of cracked fingers were also observed from at least one other lot.

Most of these cracked fingers sre traceable to

. The Figure 4 1 histogram also suggests an effect of irradiation exposure that is consistent with the IASCC mechanism, i.e., susceptibilit/ increases with time in core / fluence.

Previous inspections of tmilarly designed CEAs at other C-E reat -

otr, have not revealed any evidence of cladding crasking even though strains were measured on the order of the mechan cal design limit of

. The cladding for el'. of these CEAs is inconel 625 procured to meet th'e star.dard industry chemical composi-tion and mechantral property requirements for the material in tubing form. To understand why cladding lots used in both Calvert Cliffs Units, were apparently more susceptible to failure by IASCC, it is important to review pertinent industry experience with this phenomenon including C-E's tests of Inconel 625 cladding materials in which data were obtained on the effects of impurity level and mechanical properties.

4-1

Figure 4-1 '

HISTOGRAM OF CENTER FINGER CRACKING FREQUENCIES AT CALVERT CLIFFS UNITS 1 AND 2

. j 31%

30 -

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15%

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08 ~

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m 8 10 2

U o 6%

-3%

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1

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1 I I I t 1 2 3 4 5 6 7 8 9 UNIT 2 UNIT 2 UNIT 1 EXPOSURE (NO. OF CYCLES) 4

o The ECT and visual observations of cladding cracks, especially the indications of partial through wall cracks initiated on the outside surface of the cladding, are consistent with an IASCC' phenomenon.

o Analogous to the BWR absorber rod failure case, it appears that the irradiation-induced swelling of the highest exposed 4B C at the tip of the CEA fingers can result in the application of persistent stresses leading to the cracking of susceptible Inconel 625 cladding, o The finding of crack indications in a significant fraction cf the fingers from the cladding lot having a generally higher impurity level, e.g.,

is consistent with the HNO3 -Cr+6 test results showing that impurity level can influence the suscepti-bility of Inconel 625 to IASCC.

o The fact that some cracking was also observed in other lots indicates that mechani-cal strength (with associated microstructures) was also a factor.

It is concluded that the unusual observation of cladding cracks in the Calvert Cliffs units is related to the ese of IASCC susceptible material caused by (1) the presence of a relatively high impurity and/or (2) a relatively high mechanical strength. It is likely that there exists a spectrum of susceptibility to IASCC with the degree depending upon impurity level, mechanical strength, microstructure, and exposure in reactor.

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