ML18347B273

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Additional Information Re a Steam Generator Eddy Current Testing, Dated Dec. 1974
ML18347B273
Person / Time
Site: Palisades 
Issue date: 01/31/1975
From:
Consumers Power Co
To:
Office of Nuclear Reactor Regulation
References
Download: ML18347B273 (60)
v • d • e


Text

50-255 Palisades Addl info re* A Steam Generator Eddy Current

'Testing....* dtd Dec 1974 1-31-75.**...*.*..*.***.*.* #1315 THE ATTACHED FILES ARE OFFICIAL RECORDS OF THE OFFICE OF REGULATION. THEY HAVE BEEN CHARGED TO YOU FOR A LIMITED TIME PERIOD ANS MUST BE RE TURNED TO THE CENTRAL RECORDS STATlON 008. ANY PAGE(S)

REMOVED FOR REPRODUCTION MUST BE RETURNED

  • ro I TS/THEIR ORIGINAL ORDER.

~ DEADUNE RETURN DATE

-s*------

z MARY JINKS, CHIEF CENTRAL RECORDS STATION 2

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Provide the summer 1974 and December 1974 raw ECT data for the tubes inspected during December 1974.

Answer Enclosed is the raw data taken in December 1974 coupled with the data taken in the summer of 1974.

It should be noted that where duplicate data existed for the summer 1974, these results are averaged.

Readings of less than 20% are denoted by the number 19.

Although the data indicates that the June 1974 reading for tube Line 47, Row 46 in Quadrant III was zero, a rereading of the eddy current tape revealed a defect of 50-60% which had been missed.

Also, the June 1974 values for tubes Line 39, Row 50, Quadrant III and Line 7, Row 102, Quadrant II were found to be

<20% rather than zero by similar rereading of the tapes.

The computer printout does not include the data indicated to be between supports and one tube with a defect of 52% at #10 sup-port plate.

The data between support plates i.s presented below.

Line Row Quadrant II 25 86

<20% Between #3 and #5 Support 24 85

<20% Between Egg Crates 4

85 30% Between Egg Crates 11 100 40% Above #10 58 49 52% Betweep #2 Egg Crate and Top Support Quadrant III 3

96 34% Above #6 11 96 46% Between Egg Crates 14 93 46% Above #10 38 85 45% Below #10 - Multiple 24% Above #10 42 85 45% Multiple Above #10 48 91 52% Above #10 46 89 32% Above #10 34 45 Multiple 49% From #6 to Egg Crate 2 33 44 45% Between #4 and #6 32 45 35% Multiple Above #6 di /3/-5

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2.

Provide a summary of the December 1974 ECT results by defect size for the following ranges of defect sizes:

30-39%, 40-49%, etc.

Answer 21 Attached, in additi.on to the summary, is a listing of data taken in December 1974, broken down into segments between 30%

and 80%.

In listing these data, an effort was made to list a tube only once.

Each tube is listed in the appropriate interval cor-responding to the maximum defect existing on the tube.

Also attached is a listing of tubes whose eddy current indications decreased by 5% or more.

SUMMARY

OF DECEMBER 1974 Eddy Current Testing Results Eddy Current Testing Number Indication of of Defect Size Tubes 30 - 39 215 40 - 49 255 50 -

59 23 60 - 69 3

70 - 79 l

J

22 INCREMENTAL LISTING OF TUBES 12/74 Reading 6/74 Reading Line Row

(%)

(%)

Eddy Current Readings ~70%

Quadrant III 47 46 74 50-60 Eddy Current Readings 2:60 <70 Quadrant II 7

102 69

<20 Ditto 48 61 61 30 Quadrant III 39 50 66

<20 Eddy Current Readings :2::50 <60 Quadrant II 1

88 55

34.

Ditto 2

89 50 29 9

92 52 37 32 53 52 45 33 52 56

<20 38 85 52 48 50 93 50 45 11 100 52

<20 58 49 52

<20 (Between EC2 & Top Supp)

Quadrant III 23 104 57 40 Ditto 25 46 55 42 37 52 52 44 40 45 50 46 40 49 50 30 41 46 50

<20 42 47 52 45 42 87 51 40 47 48 53

<20 49 48 50 34 55 48 56

<20 57 46 51

<20 <4 (34 EC-1) 60 57 52 40 48 91 52

<20 (Above #10)

Eddy Current Readings ~40 <50 Quadrant II 1

90 40 Ditto 2

91 46 2

93 48 3

86 42 4

85 44

23 12/74 Readins; 6/74 Reading Line Row

.(%)

(%)

Eddy Current Readings ~40 <50 (Contd)

Quadrant II 4

89 46 Ditto 4

95 48 II 4

103 45 II 5

100 40 II 6

103 41 II 7

86 48 II 7

92 42 II 7

96 13 II 7

104 48 II 9

88 43 II 11 100 48 II 12 87 48 II 12 89 46 II 12 95 46 II 13 92 45 II 14 85 46 II 14 87 46 II 14 95 49 II 16 51 45 II 17 90 43 II 17 104 49 II 18 53 42 II 18 85 45 II 18 87 45 II 18 93 47 II 18 95 45 II 18 103 46 II 19 90 41 II 20 87 44 II 20 89 42 II 20 99 42 II 21 92 44 II 21 100 40 II 21 104 44 II 22 45 42 II 22 91 42 II 22 99 45 II 23 58 44 II 23 90 44 II 23 104 40 II 24 95 47 II 24 97 49 II 24 105 42 II 25 60 45 II 25 86 46 II 25 106 43

24 12/74 Readi!!ej 6/74 Reading Line Row

.(%)

(%)

Eddl Current Readin~s :!:40 <50 (Contd)

Quadrant II 26 47 46 Ditto 26 95 49

. II 26 97 40 II 27 86 47 II 27 90 44 II 27 92 45 28 49 40 28 89 43 28 97 48 28 99 44 28 105 48 29 50 40 II 29 60 43 II 29 104 46 II

29.

106 41 II

. 30 51 42 II 30 87 44 II 30 89 40 II 30 95 48 II 30 97 40 II 30 99 42 II 30 105 48 II 31 60 44 II 31 86 41 II 31 104 45 II 32 59 40 II 32 61 43 II 32 107 44

,II 33 58 42 II 33 60 42 II 33 88 49 II 34 103 42 II 35 104 43 II 36 59 42 II 36 95 44 II 37 44 42 II 37 46 46 II 38 45 40 II 38 89 42 II 38 95 43 II 39 60 48 II 40 45 46 II 40 87 40 II 40 105 43 II 41 50 48

25 12/74 Readin~

6/74 Reading Line Row

(%).

(%)

Eddy Current Readin~s 2:40 <50 (Contd)

Quadrant II 42 51 42 Ditto 42 61 40 II 42 101 40 II 43 60 42 II 44 61 40 II 45 58 46 II 45 102 40 II 46 45 40 II 46 91 45 II 46 95 44 II 47 48 44 II 48 59 47 II 49 92 44 II 50 87 44 II 51 86 48 II 51 92 46 II 51 94 40 II 51 96 41 II 51 98 47 II 52 57 42 II 52 87 46 II 52 89 44 52 91 48 II 52 97 46 II 54 87 44 II 54 89 45 II 55 46 40 II 56 53 40 II 56 55 43 II 60 51 48 60 61 40 Quadrant III 2

95 43 Ditto 2

97 43 II 3

90 43 II 4

87 48 5

86 40 5

90 40 7

86 40 8

93 46 8

97 42 8

103 40 9

88 48 9

92 48 II 9

94 48 II 10 85 45 II 10 95 46

26 12/74 Reading 6/74 Reading Line Row

(%)

(%)

Eddy Current Readin~s ~40 <50 (Contd)

Quadrant III 11 94 40 Ditto 12 85 44 II 12 87 42 II 13 60 41 II 13 86 43 II 14 61 40 II 15 86 40 II 15 88 40 15 90 40 15 92 40 17 46 40 17 64 41 17 88 48 17 90 48 17 92 42 II 17 106 47 II 18 51 43 II 18 87 45 II 18 89 46 II 19 56 40 II 19 90 42 II 19 96 47 II 19 106 44 II 20 103 46 II 21 44 40 II 21 58 40 II 22 95 48 II 22 103 42 II 24 45 42 II 24 85 45 II 24 87 40 II 24 101 45 II 25 44 48 II 25 98 48 II 28 103 40 II 30 49 45 II 32 45 49 II 32 91 40 II 32 103 49 II 32 105 43 II 33 44 45 II 33 46 46 II 33 48 48 II 33 52 47 II 33 58 41 II 34 51 47

27 12/74 Reading 6/74 Reading Line Row

{%)

(%)

Eddy Current Readings ~40 <50 (Contd)

Quadrant III 35 86 42 Ditto 35 88 40 II 36 43 42 II 36 51 48 II 36 87 42 II 36 89 42 II 37 44 44 II 37 94 44 II 38 47 44 38 51 46 38 91 47 38 95 42 38 103 45 39 44 46 40 61 40 40 85 43 40 87 46 II 40 89 48 II 41 50 41 II 41 96 40 II 42 85 47 II 43 50 45 II 43 88 40 II 43 90 49 II 43 98 43 II 43 100 48 II 44 85 43 II 44 95 47 II 45 44 45 II 45 50 40 II 45 90 48 II 45 92 41 II 45 94 49 II 46 87 43 II 46 89 46 II 47 50 40 II 47 90 48 II 47 92 44 II 47 94 42 II 47 96 42 II 48 87 45 II 48 91 46 II 49 94 40 II 49 96 40

28 12/74 Reading 6/74 Reading Line Row

(%).

(%)

Eddy Current Readings ~40 <50 (Contd)

Quadrant III 50 49 49 Ditto 50 87 46 II 50 89 43 II 50 99 45 II 51 48 46 II 52 49 45 II 52 63 41 II 52 91 44 II 52 95 44 II 53 90 41 II 54 49 40 II 55 50 42 II 55 52 40 II 55 58 43 II 55 88 42 II 55 92 48 II 56 45 46 II 56 51 42 II 56 89 46 II 58 45 44 II 58 55 45 II 60 49 45 II 60 53 42 Total 255 Eddy Current Readings ~30 <40 Quadrant II 1

100 33 Ditto 7

94 36 II 8

105

\\ 35 II 9

86 30 II 10 87 35 II 10 91 33 ll 90 36 II 11 106 35 II 12 105 35 II 13 58 34 II 13 60 37 II 13 88 37 II 14 59 32 II 14 105 34 II 15 104 39 II 16 53 39 II 16 61 30 II 16 91 34 II 16 103 38

29 12/74 Reading 6/74 Reading Line Row

{% ).

(%)

Eddy Current Readings ~30 <40 (Contd)

Quadrant II 18 59 30 Ditto 18 63 36 II 18 89 38 II 19 54 38 II 19 64 32 II 20 51 30 II 20 63 33 II 20 93 38 21 44 38 II 21 62 30 II 21 88 37 21 106 38 II 22 57 38 II 22 59 38 22 87 30 II 22 105 35 II 24 49 33 24 61 34 24 85 36 25 46 30 25 104 39 26 49 39 26 89 39 27 58 32 27 88 38 27 104 38 28 85 36 28 95 38 29 86 38 30 59 39 31 98 38 32 51 35 32 87 30 II 32 89 39 32 95 39 32 99 37 II 32 101 31 33 96 38 33 100 33 34 57 35 34 85 35 II 34 87 30 II 34 89 39 II 34 97 39 II 34 107 30

30 12/74 Reading 6L74 Reading Line Row

(% ).

(%)

Eddy Current Readings ~30 <40 (Contd)

Quadrant II 35 96 32 Ditto 35 98 35 II 35 100 36 II 37 48 34 II 37 94 36 II 38 101 39 II 39 46 38 II 39 54 30 II 39 62 30 II 39 86 38 II 39 94 34 II 4o 61 35 II 40 85 32 II 40 107 30 II 41 60 30 II 41 94 31 II 41 96 37 II 41 104 37 II 42 57 34 II 42 107 30 II 43 50 36 II 44 57 32 II 45 60 35 II 46 57 34 II 46 61 35 II 47 106 35 II 48 65 39 II 48 95 35 II 49 48 36 II 53 92 38 II 54 95 37 II 54 97 36 II 54 103 38 II 55 90 39 II 57 62 32 II 58 49 32 II 58 61 30 Quadrant III 2

85 35 Ditto 3

44 33 II 3

86

.35 II 3

94 36 II 4

47 37 II 4

85 30 II 5

88 38 II 5

92 35

31 12/74 Readine; 6/74 Reading Line Row

(%)

{%)

Eddy Current Readings ~30 <40 (Contd)

Quadrant III 6

99 34 Ditto 6

103 36 II 8

85 39 II 8

87 39 II 8

99 35 II 12 99 30 II 14 87 30 II 14 93 30 II 14 95 32 II 14 97 32 II 15 46 39 II 17 44 35 II 17 54 30 II 17 56 36 II 17 100 31 II 18 91 31 II 18 95 31 II 18 97 36 II 19 64 37 II 19 98 30 II 20 47 30 II 20 49 35 II 20 55 30 20 105 34 II 21 88 30 II 22 55 30 II 22 61 30 23 44 30 II 23 56 38 II 23 58 30 23 68 30 II 23 88 38 II 24 55 37 II 24 57 31 II 24 97 38 II 25 58 32 II 25 60 31 II 25 94 39 II 26 61 36 27 46 35 II 27 94 34 II 28 57 31 II 28 59 36 28 61 36 II 28 89 35 II 29 90 36 II 29 94 31

32 12L74 Reading 6/74 Reading Line Row

(% ).

(%)

Eddy Current Readings 2:30 <40 (Contd)

Quadrant III 30 91 39 Ditto 30 105 34 II 31 58 36 II 31 94 36 II 31 106 38 II 32 51 39 II 32 53 34 II 33 54 30 II 33 88 34 II 34 45 30 II 34 59 31 II 35 46 35 II 35 58 33 II 35 94 38 II 36 55 30 II 37 54 30 II 37 60 39 II 38 85 37 II 39 56 34 II 39 60 30 II 39 86 37 II 39 90 34 II 40 47 30 II 40 95 32 II 41 52 31 II 41 92 35 II 41 94 30 II 41 98 38 II 42 51 31 II 42 61 30 II 42 91 38 II 42 105 37 II 43 52 34 II 43 92 35 II 44 61 30 II 44 99 30 II 45 48 34 II 45 98 38 II 45 100 36 II 46 47 30 II 46 97 35 II 48 89 38 II 48 95 38 II 52 53 30 II 52 59 38 II 52 61 30 II 53 54 37

33 12L74 Reading 6/74 Reading Line Row

{%) *.

(%)

Eddx Current Readings >30 <40 (Contd)

Quadrant III 54 57 30 Ditto 55 54 32 II 55 60 35 II 58 47 34 II 58 51 31 II 58 61 34 II 58 63 37 II 58 85 38 II 59 44 38 II 59 48 37 II 59 60 34 II 60 51 39 Total 215 Tubes Whose Eddy Current Readings Decreased by 5% or More 12/74 Reading 6L74 Reading Line Row

(%)

(%)

Difference Quadrant II 7

94 36 41

-5 Ditto 11 100 38 49

-11 II 19 54 38 43

-5 II 20 51 30 40

-10 II 20 55 29 39

-10 II 21 44 38 48

-10 II 22 45 42 48

-6 II 22 59 38 44

-6 II 26 97 40 47

-7 II 27 104 38 48

-10 II 32 99 32 45

-13 II 34 57 35 42

-7 II 35 96 32 42

-10 II 36 59 42 48

-6 II 37 48 34 40

-6 II 38 89 42 48

-6 II 38 95 43 48

-5 II 39 60 30 46

-16 II 39 86 38 48

-10 II 39 94 39 45

-6 II 40 87 40 46

-6 II 40 105 43 49

-6 II 40 107 30 35

-5 II 41 46 30 35

-5 II 41 96 37 42

-5

34 Tubes Whose Ed Current Readin s Decreased b;y:

5~ or More Contd 12/74 Readine; 6/74 Reading Line Row

(%)

{%)

Difference Quadrant II 42 61 35 47

-12 Ditto 42 107 30 42

-12 II 43 64 22 30

-8 II 44 61 32 40

-8 II 45 60 30 40

-10 II 46 57 34 49

-15 II 46 61 29 44

-15 II 47 48 44 49

-5 47 106 35 43

-8 II 49 50 25 48

-23 II 50 87 40 49

-9 II 51 96 29 36

-7 II 51 102 28 33

-5 II 52 57 42 48

-6 II 56 59 25 38

-13 II 57 62 32 46

-14 II 58 61 30 48

-18 II 60 61 40 46

-6 Quadrant III 5

88 35 44

-9 Ditto 15 60 24 34

-10 II 15 62 25 30

-5 II 17 44 35 40

-5 II 17 46 40 48

-8 II 17 54 30 45

-15 II 17 56 36 45

-9 II 17 96 25 40

-15 II 19 64 37 44

-7 II 20 47 30 40

-10 II 22 87 25 35

-10 II 23 56 38 48

-10 II 23 60 24 30

-6 II 23 88 24 32

-8 II 23 92 25 35

-10 II 23 96 29

,34

-5 II 24 57 31 45

-14 II 24 85 20 30

-10 II 25 88 22 30

-8 II 26 57 26 44

-18 II 27 46 35 48

-13 II 28 57 31 45

-14 II 28 59 36 45

-9 II 29 92 27 34

-7 II 29 94 31 40

-9

35 Tubes Whose Ed Current Readin s

(

Decreased by 5* or More Contd 12/74 Readin~

6/74 Reading Line Row

(%)

(%)

Difference Quadrant III 30 55 20 30

-10 Ditto 30 97 20 30

-10 II 30 105 34 40

-6 II 31 94 36 45

-9 II 31 96 20 30

-10 II 31 106 38 45

-7 II 32 59 25 30

-5 II 34 35 30 35

-5 II 35 60 24 32

-8 II 35 86 42 48

-6 II 35 88 40 48

-8 II 36 87 42 48

-6 36 89 42 48

-6

  • 38 95 42 47

-5 39 48 28 42

-14 39 90 34 40

-6 40 95 32 47

-15 40 99 28 35

-7 II 41 52

.29 35

-6 II 41 92 35 45

-10 II 41 94 30 44

-14 II 43 92 35 40

-5 II 43 98 43 48

-5 II 44 99 30 36

-6 II 45 44 29 44

-15 II 45 48 20 25

-5 II 45 50 40 46

-6 II 46 47 30 35

-5 II 47 94 42 48

-6 II 48

.91 22 35

""'.13 II 51 52 22 32

-10 II 52 33 29 48

-11 II 53 64 29 35

-6 II 54 57 30 35

-5 II 57 64 25 34

-9 II 58 63 37 48

-11 II 59 52 28 40

-12 II 59 60 34 42

-8 II 60 51 39 48

-9 II 60 53 42 47

-5

3.

Identify which tubes were plugged in the December 1974 outage in accordance with your 50% plugging criteria.

Answer 36 As a result *of the eddy current inspection conducted in December 1974, one tube was plugged. *This tube is located in Quadrant III of the "A" steam generator at Line 47, Row 46.

This tube showed an eddy current testing indication of 74% through wall penetration at the #2 egg crate.

Subsequent investigation revealed that this defect had been missed in the June 1974 eddy current ins~ection. A review of the tape containing the eddy current trace for this tube showed that a defect was present in June 1974, and that the defect amounted to a through-wall penetration of 50% to 60%.

As this defect exceeded the plugging criterion established for the June 1974 eddy current inspection, it was plugged.

Based upon the results of a statistical analysis of the eddy current data taken in December 1974 compared to that taken in June 1974, it was concluded that little, if any, additional wastage has taken place in the time period between these two inspections.

The results of this statistical analysis were reported in a January 3, 1975 submittal.

A statistical investigation was launched into the eddy current testing techniques following the implementation of the 50% plugging criteria in the summer of 1974 because:

a.

Wide variations both positive and negative in two sets of eddy current testing data obtained one day apart on 87 tubes in a foreign steam generator.

b.

Wide variations both positive and negative in two sets of eddy current testing data obtained approximately one month apart (during plant shutdown) on approximately 110 tubes in the Palisades steam generators.

c.

Wide variations both positive and negative in the data sets obtained during the full testing of the Palisades Plant steam generators.

37

d.

The conclusion of no increased wastage from the metallographic examination of a tube. extracted from Palisades steam generators that, according to the eddy current testing evaluation, had shown a large.increase in wastage between fall 1973 and summer 1974.,

During this.period the plant was shut down.

The metallographic conclusion was, "There has been thought that*wastage has pro-gressed during shutdown.

The observation that an indigenous oxide film persisted on wasted areas examined from tubes removed in June 1974 indicates wastage had terminated, rather than c~n-.

tinued during the shut.down period.

Moreover, for wastage to have continued, one would expect phosphate salt deposits immediately adjacent to wastage regions, which were considered to have propa-gated during shutdown.

Such was not the case.

Rather, these surfaces were effectively clean."

{R. C. Youngdahl transmittal to Directorate of Licensing dated August 20, 1974, Attacbment B, Page 4.)

Attacbment A to the August 20, 1974 submittal summarized significant (<15%) increases in wastage that eddy current testing data indicated had occurred during the shutdown period from fall 1973 to summer 1974.

Similar data for significant decreases were not included in this attacbment because it was recognized that no reasonable mechanism for tube healing existed and statistical evaluation tools had not yet been developed to evaluate 4he significance of the data.

The tubes indicating significant (>15%) increases (approximately 250) to above 50% as well as other tubes having increases.to above 50% and tubes that had previously indicated above 50% through-wall degradation were plugged.

Following this plugging, the results of.the statistical investigation showed that no significant wastage had occurred betwe.en fall 1973 and summer 1974.

These ~Qnclusions and the techniques used to arrive at the conclusions and the evaluation of the December 1974 eddy current testing results are detailed in the attached (and pre-viously informally submitted) August 8, August 27 and September 26, 1974 letters from J. Jaech to D. Noble and the eddy current testing

38 report submitted January 3, 1975.

Also following the tube plugging, the results of the metallographic examination described above became available.

The attached* letters also conclude that, based on both physical and eddy current measurements, there is about a 12. 7%

conservative bias in the eddy current measurement.

The fact that some tubes do show an indication, based upon the December 1974 inspection, of more than 50% through-wall penetra-tion, may be attributed to several different sources.

One may conclude that isolated locations exist where wastage is still taking place.

Because the mean wastage was essentially zero, however, one must then also conclude that isolated locations exist where the tubes are healing

  • themselves. This second conclusion, clearly, is not realistic. While a large number of the measurements taken in December 1974 agree very well with previous measurements, isolated occurrences of substantial deviations, both plus and minus, do exist.

These substantial devia-tions appear as tails to the bell-shaped normal distribution curve expected from measurements of finite accuracy.

Consumers Power Company has concluded that good reasons exist for not plugging tubes that may have shown greater than 50%

through-wall penetration in the December 1974 inspections.

a.

In all cases, except for the tube missed in the June 1974 in-spection and subsequently plugged, the indications are still less than 73% through~wall penetration required to withstand an LOCA.

This level of defect is identified by Regulatory Guide 1.83 as the limit of acceptability.

The water chemistry environ-ment within the steam generators has been changed from coordinated phosphate to volatile treatment.

Flushing reports submitted in accordance with the Technical Specifications show, that the conversion to volatile treatment has been achieved and the secondary water can be maintained fully in accordance with volatile treatment specifications.

b.

Industry experience, both actual operating experience and pot-boiler testing, has shown that volatile water treatment does

not attack the steam generator tubes at as rapid a rate as phosphate treatment has in various plants.

39

c.

No known mechanism exists for causing wastage at a rate which would have to be assigned to a few of the Palisades tubes based upon the difference in observed eddy current indications and the period of operation between measurements (7.5 effective full power days) *

d.

These tubes may provide valuable assistance in determining the limitation of eddy current testing in steam generator tubes if they are saved for subsequent inspections.

  • If they are plugged, their usefulness is destroyed.
e.

There is no known wastage mechanism for only a few isolated tubes to undergo attack.

Therefore, no tubes other than previously described were plugged.

.~ E'}(ON~NUCLEAR COMPANY, Inc..

\\

40 2101 Horn Rapids Road; Richland, Wpshington 99352.

ll PHONE* (509) 946-9621 fr r

Ml:. Daniel M. Noble Nuclear Operations Consumers.Power Company 1945 W~ Parnall Road Jackson, Michigan 49201

Dear Dan:

August 8, *1974 This summarizes my findings as a result"of my visit to Jackson on August 6-7, 1974 to explore the statistical aspects of the Eddy current testing of Palisades generator tubes.

The initial problem as defined is to evaluate the measurement error associated with a reported tube "wastage" value* as determined by Eddy current testing.

More generally~ the problem is to distinguish between actual tube wastage and reported wastage, where this latter

.value includes the effects of measurement er*rors.

Statistical Model In.. many measurement situations, the problem of evaluating measure-ment errors is relatively simple.

This is true when there exist good calibration standards and when a measured value is not too dependent on the human element.

Unfortunately, with Eddy.current testing neither of these conditions exist.

The calibration standard is of *questionable quality, and the human interpretation of the source data plays a major role in determining the reported value.

Thus, reliance must be placed on making inferences about the sizes of measurement errors based on data that consist of reported wastages for individual tubes.

The largest data base consists of paired measurements.

That is, measurements are made of a large number of tubes at one point _in time and repeated later on the same tubes.

Unfortunately, actual corrosion may take place between tbe two time periods which complicates the problem.

However, such data can be useful if some assumptions can be made *

. This situation is modeled.

Let*

x. = reported wastage at time 1 for tube i (% wast~ge)

J..

yi = reported wastage at time 2 for tube i Throughout this discussion, restrict attention to value of xi and Yi that are above the detection limit of 20% *.

AN AFFILIATE OF EXXON CORPORATION

~"

v.

0 A

~

.41 Mr. Daniel M. Noble 2 -

August 8, 1974 Assume the following models where µi Is the "true" wastage at time 1 for tube.i, e:i is a random error.

of measurement,.wi is the true. incremental wastage _for tube i that-occurs between times 1 and 2, and ni is also a random error of measurement.

Note that at time 2, the true wastage for tube i is t_hen µi + wi.

Before proceeding further, a comment on this model is in order. It is assumed that wi, the incremental wastage, is independent of.lli, the initial wastage.

The adequacy of this assumption caµ. be checked in a gross way now (more on this later), and when data are.taken at a third time period, the assumption can be relaxed.

For now, however, assume that wi and llf are independent.

Assume that µi, e:i, wi, and ni are random variables with mean

  • values llo1 Eo, wo, and no and with variances crB, cr~, cr~, and cr~ respectively.

A number of statistics can be calculated from the data~ These include the means of the x and y values, denoted by x and y respectively, their variances, denoted by s~ and s 2 respectively, and the covariance between y

the x and y values, denoted by sxy*

sxy =

where n is the total number of paired tubes.in question.

Now, it follows that x

estimates llo + Ea y

estimates µo + WO + no s2 x

estimates cr2 + cr2

µ e:

s2 y

estimates cr2 µ + a2 +

w cr2 n Sxy estimates cr2

~

µ From the above, not~ that (Y -,X> estimates w0 + (n0

~0 ).

Thus, the observed change in average'wastage estimates the true average

  • increase for the tubes in question plus the difference in measurement biases.

If the measurement bias is the sp.me at *both periods of time, then this has *no affect on determining w0 since (n0 -

e:0 ) becomes zero *

~

v

~-

v G

.42 Mr. Daniel August 8, 1974 However, in order ~o estimate the true average_wastage (as opposed, to incremental wastage) at any point in time, there must be no measurement bias, i.e., n0 and _e:0 must be zero.

Note also that for data of the type

  • under discussion, it is impossible to distinguish between actual.incre-mental wastages and* measurement biases.

Supplementary*data must be* made available (more on this later).

2 2

Turning your attention to*sx, sy, and sxy' note that there are 3 statistics and 4 unknowns.

However, it is not unreasonable to ass~me that cr~ = a~, i.e., the measurement precision is the same for both time periods *.

  • When* this assumption is made, then we obtain the following estimates of the parameters:

"'2 O'µ = sxy

"'2 = "'2 = s2 - Sxy O'e:

crn x

"2 = s2 - s2 crw y

x These results are now applied to some data sets.

Japanese Data For purposes of our discussion, the first data set is referred to as the Japanese data.

For these data, the "ti.file 2" data were taken after a period of plant operation.

The data are somewhat different from those just discussed in the sense that at time 2, duplicate wastage measurements were taken.

Di.r_ect use is made of these duplicate measure-ments momentarily, but for the present_, use the method of analysis just described_ except that Yi is now the average of 2 measurements.

This means that s~ then estimates a5 + a~ + a~/2, a~d the estimate of a~ j,s changed accordingly.

For the 87 data points in question, the following results are obtained.

y --x = 12.3 (% wastage increase) s2 = 123.73 x

s2 = 146."83 y

Sxy = 108.25

"'2 aµ = 108.25

+

O"µ

= 10.40 "2

"'2 O'e: = crn = 15.48-

-+

cre: = 3.93 "2

crw = 146.83 - 108.25 - 7.44. = 30.84

+

crw =

5~55

0 0

~

~

Mr. Daniel August 8, 1974 The key parameter is OE (or on), the measurement error s_tandard deviation for a given measurement.

This is about 4% wastage for these data so that roughly speaking, a true wastage is within +/- 8% of the reported wastage for a given tube with about ?5% confidence.

This data set also provides a direct estim*ate of. oe: because of.

43 the duplicate measur~ents made at* time 2.

In general,. duplicate measure-ments made at a given point in time will.tend to depict less _scatter

  • than at different points in time because of more stable measurement conditions.

This is true here also where the direct estimate of Oe: is 2.38% (as opposed to 3.93%).

I regard this lower value as a limiting value and would accept the 3.93% value as being more descriptive of actual measurement performance.

  • Before leaving the Japanese data, we investigate briefly the validity of the assumption that wi is independent of µi, i.e.' that the.

increase in wastage is independent of the wastage at time 1.

This data set affords a good opportunity to do this because the µi values vary over a wide range, from 25% to 80%.

The data were divided into 3 groups on the basis of the µi values, and values ot" fj - X) were computed for each group with the following results:

Group xi~ 43%

44% ~xi~ 53%

xi~ 54%

<Y-X)

  • 12. o*

13.7 8.6 Since the fj - X) values are fairly constant and independent of xi, it follows that for these data, the assumption that µi and wi are

. independent is reasonably valid..

I hestitate t;:o generalize on this,.

however, and at this point would caution.that the assumption bears watching.

The Palisades data to be taken within the next few months will.be_very useful in determining if those tubes that show the highest wastage between times 1 an4 2 will continue to exhibit such accelerated wastage rates.

Palisades Data The Pali~ades data that were analyzed consisted of wast~ges for a subset of tubes in the A generator measured in June Of 1974 and remeasured several weeks later. During this period of time, the plant was not in-operation.

The data had been written on data sheets, and as I transcribed the per!inent da~a for analysis, a definite lack of randomness was noted in.

the (yi :_ xi) values.

The firs*t subset of the data consisted of values that were largely positive, while a second subset contained a large number of negative values.

~

v c

e

.44 Mr. Daniel M. Noble

- 5 August 8, 1974 In returning to the original source data, it was found.that. the demarcation point in the data coincided with*two different sets of.

original data, one.set with pages numbered'l-5, and corresponding to Quadrant II, and the second set with pages numbered 1-4 and corresponding to* Quadrant III. It is apparent that some relative bias exists between the two data sets; and if the reason(s) for this bias can be determined~

it should. hopefully.lead to measurements of improved quality *. The relative bias in the.two sets of data in question is over.7%, and hence,

. is not inconsequential.

For purposes of this discussion, the data were divided into the two subsets and the parameters were estimated separately.

The results are as follows:

Set 1 Set 2 n

57 48 x

52.9 54.1 y

57.4 51.6

<Y-X) 4.5

-2.5 s2 x 7.11 lE>.87 s2 y

35.17 40~25 Sxy 5.33 1.62

<1µ 2.31 1.27 crw 5.30 5.42 C1E 1.33 3.04 Bias in Eddy Current Results The best way to estimate the Eddy current measurement bias is to compare Eddy current results with those based on direct physical measure-ment of tubes removed from the generator~ This was done for seven tubes with the results as follows

  • 0 0

0 45 Mr. Daniel M. Noble 6

August 8, 1974

% Wastage Eddy Current Physical*.*

Difference 50 37 13 30 18 12 45 30 15 61

  • 41 20 71 60 11 72 65 7

76 65 11 The average difference is 12.7%, with the Eddy current results on the high side.

If the random error of measurement.for the. physical measurement is assumed to be small relative to that for Eddy current, then the estimate of this error standard deviation for Eddy current is 4.03%.

This corresponds to Oe;*

Summary (1)

The random measurement error standard deviation for a single measurement is about 2-4%.

(2)

The Eddy current method is biased high by about 13% (absolute).

(3)

There is an apparent shifting bias of sqme magnitude in the Eddy current method.

That' is, the bias depends on the set of measurement conditions that exist at the time of measurement.

As I indicated to you, I will try and contact Clyde Denton of Zetech and obtain further information on measurement precision and accuracy.

I will keep you informed.

Sincerely,

~ech Staff Consultant JLJ:jak

-~ '.E*-ON NUCLEAR COMPANY, Inc.*

2101 Horn Rapids Rood, Richland, Washington 99352

<it. PH.ONE: (509) 946*9621.

46

~-

Mr, Daniel M. Noble Nuclear Operations Consumers Power Company 1945 W. Parnall Road Jackson, Michigan 49201-

Dear Dan:

August 27, 197 4 As we had pl~nned, I ~isited Zetec on August. ~6 frir discussions with Clyde Denton on Eddy current inspection of generator tubes, especially from point of view measurement errors. Clyde was most cooperative and

  • eager to assist in the evaluation of such errors,.and in reducing their effects to the extent feasible.

With reference to my report of August 8, you wi 11 rec a 11 that we were bothered by the non-randomness in the Palisades inspection results of June-July, 1974.

Clyde explained that the June data had been created by a reader who had the results of earlier inspections before him and who had been hesitant to report decreased wastages in the June data.

On the tither hand, Clyde himself generated the July data completely independent of prior knowledge.

Thus, the June data are perturbed by biases while the July data form the more realistic set.

To correct this situation, Clyde volunteered to re-read the June tapes himself.

He will then give me the corrected results and I will redo the analysis reported in the August 8 letter.

I also discussed some of the other points we had raised durin.g my visit with you; The following are Clyde's reactions.

  • With respect to the advisability of routinely taking duplicate measurements, he does not feel that this is worthwhile.

He fe~ls that the trace data themselves are quite reproducible. Although this point did not occur to me at the time, it would seem that the use of two readers, operating independently, might be beneficial if the incentive is there to reduce the error.

The human element has been removed to the extent possible.

The key tasks in reducing a trace to a percent wastage involves locating the two points on the pattern that define the slope.

Once this is done, all else is mechanized and is independent of the.operator. There seems to be no way to. lessen f.urther operator influence* on a reported result.

AN AFFll.IATE OF EXXON CORPORATION

(~

~

Mr. Daniel August 27, 1974 I brought up the question of adequacy of calibration standards.

Clyde does feel that he needs improved standards and intends to move in this direction.

He feels that this need has become more apparent now that wastage surveillance has become so important.

However, with respect to the 13% estimated bias between the Eddy current data and the physical measurements, Clyde does not feel that the bias is this large.

In his opinion, physical measurements are also subject to large errors and will tend to be biased on the low side.

With respect to probe*changes, the operating procedures call for probe replacement upon breakage. Apparently probes do not wear out as such but rather break first because of the tight bends they must traverse.

He also feels that the trace data are not much affected by*which probe is used.

Finally, I asked about the advisability of indicating on the data sheets those instances when traces are especially difficult to interpret.

Clyde feels that this serves no useful purpose because of the freque.ncy with which this occurs, and because operator judgement as to which traces fit this category becomes so important.

I will keep you informed as more information develops on this subject.

JLJ:jak Sincerely,.

~aech.

Staff Consultant

. E*-ON NUCLEAR COMPANY, Inc.

48 ml Horn Rapids Road, Richland, Washinglon 99352 PH_ONE: (509) 946-9621 Mr. Daniel M. Noble Nuclear Operations Consumers Power Company 1945 W. Parnall Road Jackson, Michigan 49201

Dear Dan:

September 26, 1974 With reference to my earlier communications on the subject of Eddy currenttesfing of Palisades generator tubes, Clyde Denton has completed.

re~reading the June tapes and ~upplied me with a corrected set of data.

Although this removed some of the apparent anomalies in the June-July, 1974 data, there is still some strong evidence of nonrandomness in the data which makes it very difficult to sort out the effects of measure-ment error from actual tube wastage.

The problem is portrayed in Table I which gives.frequency distri-butions of wastage for quadrants II and III.

Percent Wastage 23 to 25*

20 to 22 17 to 19 14 to 16 11 to 13 8 to 10 5 to 7 2 to 4

..:1 to l

  • -4 to -2

-7 to -5

-10 to -8 TABLE I Frequency Distributions of Percent Wastage Quadrant II Quadrant III I

II 11 IH-l 11 Nil 1111 m.!

11 1i{l 1111 l'NJ. Nil 1111 1111 I

IHI rtU 1111 flit hit lrkl f1il !NJ. N-1-!

tN.J. I II II AN AFFILIATE OF EXXON CORPORATION l

Mr. Daniel M.

Nobl~

- 2*-

September 26, 1974 There are two conclusions to be drawn on the basis of the Table I data. First, it is evident that the data for quadrants II and III are different for some reason. Second, it i~ also ~uite obvious that within the quadrant II data~ there is more than one population present, i.e.,

the distribution is bimodal.

It is the second point that is particularly bothersome, although the facts that the distributions for the two quadrants differ.also poses a problem because there is no apparent reason why actual wastages should differ.* Thus, the problem reduces*to trying to uncover the reason-(s) for the.nonrandomness.

Further investigation suggests that the data of Table ~ might perhaps be categorized by date of measurement for the last set of measure-.

ments made around July 1, 1974.

Table II prese!1ts th_e* data in this way.

  • Percent Wastage 23 to 25 20 to*22 17 to 19 14 to 16 11 to 13 8 to 10 5 to 7 2 to 4.

-1 to 1

-4 to -2

-7 to -5

.;.10 to -8 TABLE II Wastage Data Measurement Date ~ 6/28 11 Ill f1il lH.t I INJ ~ tHl !HJ. 1111 mt II 11 11 Measurement Date > 6/28 I

II

.II tH-tlll tHJ 1111 1111 111

. liil* II Nil M-U N-W mJ.

111 This partitioning of the data. presents the same sort of problem

  • as *the partitioning by quadrant, since most of the measurements made in Quadrant II occurred on or after June 29 while most of those made in Quadrant III occurred on or.before June 28.

I can offer no ~xplanation for the nonrandomness in the data but can nnly observe that there must be reasons for it, and that uncovering the.reasons might prove to be a very important result frem point of view of mak1ng inferences about tube wastage.

Jn the meanwhile, it is necessary

  • 50 Mr. Daniel September 26, 1974 to group the data using some criterion in order to obtain valid estimates of measurement errors.

In fanning the grouping, I note fran Table I that all the quadrant III data and part of the quadrant II data fonn one group while the remaining part of the quadrant II data fonn the second group.

There appear to be obvious time trends in the quadrant II data. Referring to -the original 5 data sheets of quadrant II data, form a* group consisting of th~ first 8 obs~rvations on page l (42-79 to 11-104}, obs~rv~tions 4 through 9 on page 2 (10-97 to 19-96}, and observations 7 on page 4 through 5 o.n page 5 *

(56-45 to 36-91}.

The distributions of th~ wastage data are then as follows.

Percent Wastage 23 to 25 20 to 22 17 to 19 14 to* 16 11 to 13 8 to TO

  • 5 to 7 2 to 4

-1 to 1

-4 to -2

-7 to -5

-10 to -8*

TABLE *I II Grouped Wastage Data Group l I

11 11 tH-! II m.t II I 111 I

11 I

I 111 Im I mJ tH-l rH-J I Group 2 rNJ Mil !HJ INJ Mi! Nil J'l{J f1il rN{ 1111 Im mt 11 II

.. The nonrandomness has now been removed from the data. I repeat, however, that the real key to evaluating the wastage data consists in uncovering the reason(s) why the group l and group 2 distributions are different.

The methodology discussed in my August 8~ l974 letter to you is now applied to the Table III data.

The results are su1TJTiarized in Table IV using the same notation as in the August 8 lett~r.

51 Mr. Daniel M. Noble

': September 26, 1974

  • TABL.E IV Parameter Estimates Group l Group 2 n

25 84

-x 51.0 51'.8 y

63.7 52.6 (y - x) 12.7 0.8 s2

. x 28.08 26.4l s2 y 28.23 35.88 sxy

17. 54 24.38

" au.

4.19 4.94.

CJW 0.39 3.08 (J

3.25 1.42 e:

First, consider the Group 1 data. It is quite evident that s~ and sj are not really different and therefore, ow is not truly different from

  • zero. This says that all 25 tubes involved 11wasted11 by the same amount between times land 2; i.e., by 12.7%.

Such an occurrence is difficult to accept~ and it seems more reasonable to me to suspect that the 12.7%

incre~se in wastage reflects ~ measurement bias rather than an actual

  • wastage.

Consider Group 2.* In this instance, the average wastage is only 0.8%, cJearly not d_ifferent from zero. Although the best estimate of ow is 3.08 for this set of data, the facts that the average wastage is essentially zero and thats} and s§ are not statistically different suggest~

that, as with Group 1, ow does not differ from z~ro. Thus, revised estimates of the parameters are in Table V.

t

  • I 52 Mr. Daniel September 26, 1974 TABLE V

. Revised Parameter Estimates Group l Group 2

4. 19 4.94 C1 u C1.

w 0

0 3.26 2.60 C1 e:

In summary, I suspect that there was no actual tµbe wastage during the one month period of non-operation, but that the average incremental wastage of 12.7% for the 25 tubes that comprise Group l was the result of a measurement bias.

The random error.of measurement is about 3% (one standard deviation).

I plan on doing nothing further with these data unless you wish my assistance in trying to uncover reasons for the nonrandomness in the data.

Sincerely,

~aech Staff Consultant

. JLJ:jak

53

4.

Provide sufficient evaluation of the wastage observed in the summer 1974 and the December 1974 inspections to verify that wastage has not progressed in the Palisades Plant steam generators.

Answer The raw data obtained during the December 1974 ECT of the "A" steam generator shows wide variation both positive and negative in some cases from the data previously obtained during the summer 1974 inspection.

These variations are similar to variations observed between data collected previously at the Palisades Plant (fall 1973 to summer 1974 data and data which compared approximately 110 tubes tested about 30 days apart during the summer i974) as well as similar to variations observed in two sets of data obtained in a foreign steam generator.

Two alternatives exist with regard to evaluation of the data.

One alternative is to evaluate the data using statis-tical techniques to determine if a pattern exists in the data which indicates that wastage has taken place during the interval of interest.

The second alternative is to accept the data as is, assuming that the*

significant variations in both the increased wastage and decreased wastage directions are not appreciably caused by the ECT technique.

The purpose of ~he statistical analysis (the first alternative) is to determine the true wastage of a given tube.

The problem is complicated by the fact that the measurement error with ECT is quite large.

On top of this random error of measurement, with the collecting, interpreting, and processing of such a volume of data, there is the likelihood that "mistakes" beyond tb,e range of expected errors will be made.

These can occur due to misidentification of tubes, misreading of the ECT tapes, or mistakes in. recording the wastage value.

This problem is common in statistical analysis with values that are un-explainable being identified as "outliers." The fact that outliers can be expected to occur with ECT is important to keep in mind, for it may not be possible to find a rational explanation for a small number of tubes that may appear to exhibit a large increase in wastage over a short period.

On the other hand, the data would indicate that some tubes apparently experience appreciable "healing." These data points

can also be identified as outliers.

Since outliers have been shown to occur in both directions, it seems*that the rational explanation is that they are due to "mistakes" in the measurement process.

The random error of measurement can be evaluated based on data that consist of paired measurements.

That is, if a tube is measured twice, then the paired data can be used to estimate the measurement error. If, in fact, actual wastage occurred, this can be taken into account.

Consider the following model:

then:

Let xi = Reported Wastage at Time 1 for Tube i yi = Reported Wastage at Time 2 for Tube i

µi = True Wastage at Time 1 for Tube i

w. = True Incremental Wastage Between Times 1
l.

and 2 for Tube i Thus, for example, if there were no error of measurement, xi = µ.

l.

yi = µi + wi However, there is a measurement error, which we will depict by e. at Time 1 and by n. at Time 2.

The model becomes:

l.
l.

Xi = µ. + E *

. l.

1 yi = µ. + w. + n.

l.
l..

1 It is assumed that *e. and n. both have mean values of zero,

l.
l.

ie, on the average, the true value will equal the observed value.

{This appears to be on the conservative side since a relative bias of about 13% has been demonstrated between ECT results and those found by metallographic examination, with ECT results on the high side.)

Further, assume that both £. and n. have the same variance, designated

l.
l.

by cr2 *

µ The problem is to estimate the' parameters from the data.

It is easily shown that:

(y - x) estimates w 0

, the average increase in true wastage.

s 2 estimates cr2 + cr2 x

µ n

s 2 estimates cr2 + cr2 + cr2 y

µ w

n s

estimates cr2 xy

µ

55 where s2 is the sample variance among the xy values, s2 is the sample x

y variance among they. values, ands-

  • is* the sample covariance between

].

xy the x. and y. values.

].

].

This model is applied to the inspection data of June 1974 (Time l) and December 1974 (Time 2).

The results are as follows:

{y x) = 0.62% (Estimated Increase in Wastage) s2 = 47.81 x

s2 = 55.95 y

s 2 = 28.67 xy Therefore, the following estimates are found for the standard deviations:

a = 5.35

µ a = 2.85 w

a = 4.37 n

The best estimate of the average increase in wastage is 0.62%.

The best estimate of the standard deviation of how much individual tubes might have differed from this average of 0.62% is 2.85%.

The best estimate of the random error standard deviation is 4.37%.

The estimate of actual wastage increase is very small and is of questionable statistical significance.

In fact, s 2 and s 2 will x

y both estimate the same quantity if a 2 is deemed to be zero.

In this

µ case, the random error standard deviation is 4.82%.

Some additional data confirm the above results.

In June 1974, there was a number of tubes that were measured in duplicate.

Based on 80 such pairs, the estimated a = 4.78%, in excell~nt agree-n ment with the June 1974 December 1974 data.

The conclusion is that the agreement between the June and December data was the same as the agreement between the two sets of June data, confirming the conclu-sion that no wastage occurred between June and December.

A similar conclusion follows if the September 1973 data are compared with the June 1974 data.

For the 80 sets of duplicates just m~ntioned, the June readings were also available. *The differences, June 1974 minus September 1973, are given in the frequency distribu-tion table.

56 6/74 - 9/73 Wastage (%)

Number of Tubes ll - l3 2

8 - lO 3

5 - 7

7.

2 4

l2

-l - l 31 -2 13 -5 10 -8 1 -11 1

The average is 0.0, implying no actual wastage.

The estimate of a is ~ = 3.66, in qualitative agreement with the previous results.

T]

T]

It is concluded that very little, if any, wastage has oc-curred between September 1973 and December 1974.

The few tubes that seem to have wasted can be explained as being due to "mistakes" in the measurement process.

If the tubes measured in December 1974 are regarded as a sample of the tubes in the generator, then on the basis of the December measurements, the best estimate of the distribution of ac-tual wastages is that the mean is 37.81 and the variance is (28.67 +

8.14) = 36.81, or the standard deviation is 6.07.

Thus, the estimate of the percentage of actual wastages that exceed 50% is found from a table of the normal distribution (t value is (50-37.81)/6.07 = 2.01) to be 2.2%.

The estimated percentages that exceed other true wastage values are as follows:

True Wastage 50%

55%

60%

65%

70%

73%

"t" 2.01 2.83 3.65 4.48 5.30 6.12 Percentages That Exceed This 2.22%

0.23%

<0.1%

This applies to the population from which the tubes were sampled, ie, those considered to be the most prone to wastage.

Assuming the observed variations in data were not appreciably affected by the ECT technique (the second alternative), one must ex-plain the increases and decreases irt the tube wastage indications.

The decreased wastage indications show that some tubes are healing

57 themselves.

No known mechanism exists for this phenomenon and it is difficult to postulate a mechanism.

It is easier to postulate a mechanism that would cause wastage increases to occur in the interval between tests although it is not possible to postulate it occurring on only a few tubes.

Postulated mechanisms are a shutdown wastage mechanism, the previously observed wastage mechanism during power.

operation and possibly fretting due to flow induced vibration.

Evaluation of these postulated mechanisms shows that none were likely to have occurred.

A summary of these evaluations follows:

a.

Shutdown Mechanism - The plant was in a shutdown the majority of the time during the interval between eddy current tests.

Because of large increases in wastage indications (based on ECT results) *during a previous inspection interval,, detailed in-vestigations have already been performed to determine a shutdown wastage mechanism.

These investigations failed to identify a shutdown wastage mechanism for Inconel tubing.

These results were also confirmed by metallographic examination.

The con-clusion of the metallographic examination was, "There has been thought that wastage has progressed during shutdown.

The ob-servation that an indigenous oxide film persisted on wasted areas examined from tubes removed in June 1974 indicates wastage had terminated, rather than continued during the shutdown period.

Moreover, for wastage to have continued, one would expect phos-phate salt deposits immediately adjacent to wastage regions which were considered to have propagated dur~ng shutdown.

Such was not the case.

Rather, these surfaces were effectively clean."

(R. C. Youngdahl transmitted to the Directorate of Licensing dated August 20, 1974, Attachment B, Page 4.)

Wherever and whenever possible the generators have been operated so as to minimize the effect of a shutdown mechanism if one is occurring.

This has been done by keeping the generators hot (300°)

and under continuous or periodic blowdown.

Therefore, we have concluded there is no shutdown wastage mechanism.

58

b.

Wastage During Operation - Wastage during power operation has been shown to occur only in the presence of very high phosphate ion concentrations during power operation and to be stifled by all volatile water treatment.

The "Steam Generators - Secondary Water Chemical Flushing Report" submitted to the Directorate of Licensing, November 6, 1974 shows that the steam generators were converted to operation within the specifications for all volatile chemistry control shortly after resuniing power operation following the summer 1974 eddy current testing.

The report shows that power operation with secondary water phosphate ion concentrations of greater than 1 ppm was limited to approximately one day and operation with concentrations of greater than 5 ppm was limited to about two hours.

The results of pot-boiler tests performed by Nuclear Steam System Suppliers and industry experience in the conversion from phosphate to volatile secondary water chemistry treatments shows that wastage is stopped once all volatile secondary water specifications are achieved.

Assuming the indications of increased wastage are not a product of the ECT techniques and are, in fact, actual wastage, a wastage rate can be calculated. If the time period for the rate is assumed to be that which corresponds to power operations with phosphates greater than 1 ppm, the wastage rate calculated is more than 100 times greater than that previously observed through industry experience or induced in pot-boiler testing. If the total time of power operation (including the period within volatile specifications) is used to determine the was.tage rate, the rate calculated is still in excess of 10 times greater than any rate previously observed.

Further, the tubes tested in December 1974 were selected because they had previously experienced wastage.

No mechanism is known by which only a few of these tubes could selectively experience active wastage during this time interval of power operation.

As there is no known mechanism for experiencing wastage at the rates described above and selectively as described above, we have

59 concluded that significant wastage is not likely to have occurred during the power operation in October 1974 and that the observed increases in the ECT data will have to be explained by some other phenomenon which involves the eddy current method.

c. Fretting Due to Vibration - The possibility of fretting due to vibration has been considered. It was concluded that this was not the cause because. of the short interval of operation between the test intervals. It is not conceivable that, during power operation in excess of 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />, fretting would. produce less than a 20% through-wall penetration and yet in the next 600-700 hours, 40% additional through-wall penetration. Further, there is no geometrical pattern consistency as would be expected from fretting.

Also, numerous tube samples have been previously removed from the generators and examined with no observation of fretting damage.

In addition, the possibility of intergranular (IG) attack has been considered.

The tubes showing large increases in ECT indica-tion were not previously affected by IG attack.

The ECT patterns were typical of wastage and not IG-type defects.

Further, as con-cluded in the January 3, 1975 ECT report, the previously experienced IG attack has been shown to have been stifled.

Based on investigation conducted and considerations sum-marized above, we have concluded that reasonable assurance exists that wastage observed in the Palisades Plant steam generators has not progressed since summer 1974 and, in addition, since the fall of 1973*

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