ML082170723
| ML082170723 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 04/29/2008 |
| From: | Arizona Public Service Co |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| TAC MD9329 | |
| Download: ML082170723 (20) | |
Text
Summary of Summary of SG Inspections in U2R14 SG Inspections in U2R14 April 29, 2008 April 29, 2008 P
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E
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E Table of Contents Table of Contents
- 1) OVERALL ECT RESULTS
- 2) CENTRAL CAVITY WEAR PHENOMENA
- 3) BW2 WEAR IN COLD LEG PERIPHERY OF SG22.
- 4) HISTORICAL UNIT 2 PLUGGING
SUMMARY
- 5) CONDITION MONITORING
- 6) LOOSE PARTS/FOSAR
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E Overall ECT Results Overall ECT Results PVNGS U2R14 PVNGS U2R14 Significant amount of wear found in Central Cavity Wear Region (both SGs).
A fairly large group of BW2 wear calls were made in a concentrated area on the cold leg periphery of the tube bundle in SG22.
The remaining wear calls were found in the non-Central Cavity Wear Region portions of the SGs. These were a combination of Diagonal Support and Vertical Support wear with relatively low wear rates.
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E Central Cavity Wear Phenomena Central Cavity Wear Phenomena
- Most significant group of wear calls during U2R14 was diagonal support (batwing) and vertical support wear in the central cavity region.
First identified in Replacement Steam Generators (RSGs) in U2R12.
Similar mechanism appeared in Original Steam Generators (OSGs).
Westinghouse/Ansaldo incorporated design changes into the RSGs to eliminate this mechanism but it appears that the design changes may not have been effective (at least in Unit 2).
The relatively high velocity flow through the central cavity of the SG impinges upon the portions of the diagonal (batwing) and vertical supports that extend across the central cavity, resulting in vibration that causes the supports to rub against the tubes causing wear.
Entire region consists of 219 tubes (in each SG).
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E Central Cavity Wear Region Batwing Supports above stay cylinder
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E Central Cavity Wear Region Affected Tubes
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E Central Cavity Region Wear Found During U2R14 Central Cavity Region Wear Found During U2R14 SG21-U2R14
- 89 tubes with wear (66 new, 23 repeat).
- Largest repeat reading is 45%.
- Largest new reading is 35%.
SG22-U2R14
- 94 tubes with wear (57 new, 37 repeat).
- Largest repeat reading is 59%.
- Largest new reading is 47%.
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E U2R14 Central Cavity Region Wear U2R14 Central Cavity Region Wear-- SG21 SG21
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E U2R14 Central Cavity Region Wear U2R14 Central Cavity Region Wear-- SG22 SG22
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E Revised Central Cavity Region Wear Growth Rate Using Revised Central Cavity Region Wear Growth Rate Using U2R14 data U2R14 data Per the U2C14 Operational Assessment (performed after U2R13), the upper 95% Central Cavity Region wear growth rate was calculated as 31.65% through wall (TW) per year.
Using the large amount of new data from U2R14, a more refined estimate of the wear rates has been calculated (see the next two slides). It is now clearly established that the Central Cavity Region wear growth rate is row dependent.
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E Revised Central Cavity Region Wear Growth Rate Revised Central Cavity Region Wear Growth Rate Using U2R14 data Using U2R14 data Analysis of measured wear rates shows significant decrease with distance from stay cylinder Wear rates appear to decrease to normal levels by 4th row from stay cylinder
- Normal observed levels are approximately 10% TW/EFPY
[upper limit value].
Implications:
- Less restrictive repair requirements than currently envisioned may be possible for rows from stay cylinder >2
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E PALO VERDE 2 WEAR RATE VERSUS ROW DISTANCE FROM STAY CYLINDER R14 DATA 95%
MEDIAN 0
1 2
3 4
5 ROWS - FROM STAY CYLINDER 2
4 6
8 10 12 14 16 18 20 22 24 26 28 30 32 GROWTH RATE -%TW/EFPY Revised Central Cavity Region Wear Growth Rate Using U2R14 data Revised Central Cavity Region Wear Growth Rate Using U2R14 data
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E Revised Plugging Criteria in Central Cavity Wear Region Revised Plugging Criteria in Central Cavity Wear Region Since the central cavity wear rates are still fairly high, it was determined that the plugging criteria should be revisited.
A decision was made to reduce the plugging limits within the stay cylinder wear region from 20% TW to plug on detection (for diagonal support wear) and from 40% TW to 20% TW (for vertical support wear).
The revised plugging criteria will have a very favorable affect on the U2C15 Operational Assessment.
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E Plugging Performed in the Central Cavity Wear Region during U2R1 Plugging Performed in the Central Cavity Wear Region during U2R144 SG21
- Using revised plugging criteria, 83 tubes were plugged in the Central Cavity Wear Region.
SG22
- Using revised plugging criteria, 89 tubes were plugged in the Central Cavity Wear Region.
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E BW2 wear in cold leg periphery on SG22 BW2 wear in cold leg periphery on SG22
- The next most significant group of wear calls during U2R14 was a group of wear calls in a concentrated area on the cold leg periphery of the tube bundle above row 150 in SG22.
- A total of 47 wear calls were made in this area, ranging in size from a low of 7% through-wall (TW) to a high of 32% TW.
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E BW2 wear in cold leg periphery on SG22 BW2 wear in cold leg periphery on SG22
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E BW2 wear in cold leg periphery on SG22.
BW2 wear in cold leg periphery on SG22.
- The structural integrity impacts of the worst case tube (tube R168C81, which went from 8% to 32%TW in one cycle) were estimated using the ProfilerAx module of the Aptech OPCON software. Using a very conservative method, the module calculated a 95/50 value of 4251psid for the end of cycle 15 burst pressure for this tube. Although this value is acceptable (since it is well above the PVNGS 3NDP value of 3633psid), we decided to plug this tube as a conservative measure.
- The issue was entered into the PVNGS corrective action program and will be addressed in the U2C15 Operational Assessment.
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E Unit 2 Historical Plugging Summary Unit 2 Historical Plugging Summary 126 90 17 7
12 SG22 Total 14 1
0 1
12 SG 22-Misc 3
0 3
0 0
SG22-Loose Parts 2
0 1
1 0
SG 22-Preventative 107 89 13 5
0 SG22-BWSC Wear 116 83 14 8
10 SG21 Total 11 0
0 0
10 SG 21-Misc 1
0 1
0 0
SG21-Loose Parts 3
0 0
3 0
SG 21-Preventative 101 83 13 5
0 SG21-BWSC Wear TOTAL U2R14 U2R13 U2R12 U2 Preservice Description
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E U2C14 Condition Monitoring U2C14 Condition Monitoring Worst case wear call was 59% TW (bobbin). This initial call was later resized at 55% bobbin and 56% +pt using the large wear standard.
The plus point probe data determined that the wear scar was slightly tapered wear with a length of 0.32.
The structural integrity impacts of the worst case wear scar were estimated using the ProfilerAx module of the Aptech OPCON software (as a conservative measure, the 59% TW wear call was used). Using a very conservative method, the module calculated a 95/50 value of 3979 psid burst pressure for this tube. Since this value is above the PVNGS 3NDP value of 3633psid, it was determined that In-Situ testing was not required.
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E Loose Parts/FOSAR Loose Parts/FOSAR FOSAR was performed in the following areas on each SG:
- Tube sheet (hot and cold leg annulus).
- Peripheral cold leg region above feedwater distribution plate (FDP).
Two sludge rocks, one piece of wire and a piece of gasket material were present upon initial inspection. The wire and gasket material were left in the SGs after U2R13. The wire and one of the sludge rocks were gone upon the final inspection and the other sludge rock was removed. The gasket material remained in the SG, but was located one row deeper into the tube bundle (in a lower flow area). No evidence of wear was seen in any of the locations associated with the loose parts.
There were no PLP calls with wear.