ML17195A740
| ML17195A740 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 03/22/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17195A738 | List: |
| References | |
| GL-84-11, NUDOCS 8503260312 | |
| Download: ML17195A740 (7) | |
Text
UNITED STATES e.
NUCLEAR REGULATORY COMMISSION WASHINGTON, 0. C. 20555
- SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION DRESDEN NUCLEAR POWER STATION, UNIT 2 DOCKET NO. 50-237 INSPECTION AND REPAIRS*OF DRESDEN UNIT 2 REACTOR COOLANT SYSTEM PIPING
1.0 INTRODUCTION
During the current Dresden Unit 2 refueling outage, a total of 102 stainless steel piping welds susceptible to intergranular stress corrosion cracking (IGSCC) were ultrasonically examined in accordance with Generic Letter 84-11.
The initial inspection plan, consisting of 84 welds, was expanded to 102 welds
- to include all accessible reactor water cleanup (RWCU) system piping welds after crack-like indications were detected in that.. p.iping system.
Of the 102 welds inspected, 32 were in the recirculation system, 12 in the residual heat removal (RHR) system, 30 in the RWCU system, 10 in the isolation condenser system, 10 in the jet pump instrumentation nozzle penetration assemblies, five in the control-rod-drive (CRD) return system, and three were head-vent welds.*
The percentage of welds inspected in e~ch pipe size of each system during the outage is shown in Table 1, and varied from 17% to 100%.
The licensee, Commonwealth Edison Company (CECo), indicated that the only variation from the reinsp~ction guidelines in Generic Letter 84-ll was in the inspection of 4" recirculatipn bypass lines. The licensee reexamined four o~.the 20 bypass line welds that were inspected during the 1983 outage, and did not examine any.of the three unexamined *welds~ The three unexamined welds were weldolet branch connections to the 28" piping.
As discussed in a later section~ the staff considers this deviation to be acceptable for this outage.
Personnel from Lambert, MacGill and Thomas (LMT) and Universal Testing Laboratories (UTL)/Kraft Werke Union (KWU) performed ultrasonic testing (UT) for the licensee.
The licensee indicated that Level II and III personnel performing evaluations of crack indications were qualified at the EPRI NOE Center by successfully performing the practical examination and Level I and II UT personnel performing scanning duties were trained by the con-tractor on site. Inspection personnel from Region III of the NRC, based on a review of procedures, equipment, various documentation and field observation of the.examinations, concluded that the UT examiners performing examinations and evaluating indications were qualified by having successfully completed the practical examinations at the EPRI NOE Center for IGSCC detection and sizing and that the same procedures, equipment, and techniques were used in this examination.
. The results of UT inspections indicated that a total of 15 RWCU welds showed crack-like indications~. Of these, 10 (six-8" and four-10 11
) were outside containment and five-8" were inside containment.
One of the cracked welds (8-Kl3) was also inspected during the-1983 outage, and indications identified
- as geometry were reported at that time.
Both axial and circumferential cracks were observed in the cracked RWCU welds.
The deepest axial crack (about 60%
of the wall thickness) was found in weld 8-Kl.
Weld 8-KlO contained the worst circumferential cracks with a reported maximum depth about 43% of the wall thickness and a total length of 12.5 inches (50% of circumference).
Of the 15 cracked RWCU welds, *one (#8-k13) inside the containment was overlay repaired and 14 were replaced.
Because of the extensive cracking observed in the RWCU piping, the licensee replaced all Class 1 RWCU piping with IGSCC resistant materials (304L) except a section inboard of the first isolation valve (M02-l20l-l). There were 4 welds (three-8" and one-6") in the RWCU
- piping system not accessible for UT inspection. The licensee indicated that two inaccessible saddle reinforced welds (#BX-1 and #6X-2) were replaced and the inside surface of an inaccessible-flued-head penetration weld (#8X-l) was liquid penetrant examined during the piping replacement, with no indications reported.
Therefore, during this outage, only one inaccessible-RWCU weld, branching off the shutdown cooling line, was not inspected.
A small circumferential indication was found in one of the 28" recirculation system.. welds (#PDIA-014).
The indication was reported by LMT to be 7/8 inch in length and 4% of wall thickness in depth.
UTL/KWU reexamined the same.
-weld, using shear wave, I.D. creeping wave, and longitudinal wave.
The results of the_ UTL/KWU exa.mination concluded that the indication was caused by I.D. geometry at a window closure made during the welding operation, and the depth of this indication was less than 0.08 inch.
The licensee con-cluded that, even assuming the indication as an IGSCC type defect, it is too sma 11 to be of any major safety concern.
However, the 1 i censee wi 11.
reexamine this weld at the next refueling outage to ensure that the indica-tion is not propag~ting.
Three cracked welds left unrepaired in 1983 were UT reexamined by the same contractor (LMT), using the same equipment and procedures.
The results of*
the reexamination indicated* that there was no significant crack growth in these three unrepaired welds.
All the cracks were in the circumferential orientation.
The depths of *these cracks were reported to be about 16% to 17% of the wall thickness, and the longest total crack length in any df the three unrepaired welds was no more than o~e inch long.
The licensee reported that the overlay design for weld 8-Kl3 was determined by using the same methodology employed at other CECo units.
The flaws were assumed to be through wall at the reported length.
Ferrite content measurements were performed after the completion of the first layer.
The overlay, including the*
first layer, was designed to have a minimum thickness of 0.17 inch and a minimum length of l.5 inches.. The licensee reported that the measured ferrite content of the first la~er was about 6.5 ferrite number (FN) and the *as-built overlay thickness was approximately 0.22 inch.
2.0 EVALUATION The staff has reviewed the licensee's submittals, including the ultrasonic -
examination results and the weld overlay design to support the continuing service of Dresden Unit 2 fo~ an 18-month fuel cycle at the present configuration.*
- 2. 1 Scope-of-Inspection The percentage of welds inspected in each pipe size of each piping system is presented in* Table 1.
Except for the 4" bypass line welds in the recirculation piping system, the inspection performed by the licensee during*this outage met the guidelines in Generic Letter 84-11.
The licensee did not inspect the remaining three 4" bypass line weldolets not examined during the 1983 outage.
We consider this deviation is acceptable in this case because the ultrasonic examination of weldolets is* not meaningful. This conclusion is also based on the considerations that the pipe size of the bypass line is small (4") and no cracks were reported in the other 20 welds examined.
Therefore, cracking in the three unexamined welds to the extent that.the safety of plant operation is comprised is unlike~y.
The licensee was committed to inspect all 14 furnace sensitized safe ends (FSSE) in Dresden Unit 2 during *each refueling outage; however, only 5 FSSE (one-28 11 outlet, two-12 inlets and two-4" jet pump instru-mentation nozzle) were inspected during this outage.
The licensee's justification for not inspecting all FSSE was based on the following considerations:
(1) All 14 FSSE, including th~ entire volume of each safe-end and the attachment welds, were examined during the 1983 outage.
No.
crack indications were reported.
(2)
The radiation exposure level was high in the general areas of the safe-ends and a saving of radiation exposure of 31.5 man-rem was anticipated by not examining all FSSE.
(3)
Hydrogen water chemistry will continue to be implemented during the next fuel cycle.
The laboratory test results as well as limited field experiences have shown that hydrbgen water chemistry is effective in inhibiting crack initiation and growth.
In view of the above discussions, especially the ALARA consideration and the demonstrated integrity of FSSE through the 1983 inspection, the staff concludes that the licensee not inspecting all 24 FSSE in this outage is acceptable.
. 2.?.
UT Indication in Weld PDEA-014 The licensee indicated that the circumferential indication reported by LMT on the. 28" recirculation weld #POIA-014 was believed not to be IGSCC.
This is based on the results of extensive reexamination performed by UTL/KWU, using advanced UT techniques.
However, the staff does not believe that the exact nature of the reported indication can be. conclusively detennined by nondestructive methods in this case. Nevertheless, the staff agrees with the licensee's conclusion that, because the indication is so small (7/8" 1ong and 4% deep), even if it is assumed to be an IGSCC, it does not constitute a ~afety concern during future operation.
2.j Unrepaired Welds The crack growth in the three unrepaired welds {two-12" riser welds and one-28" outlet weld) was not significant during the last fuel cycle. There was no reported growth in crack length and the reported growth in crack depth was no more than 3% of the wall thickness.
In view of the insignifi-cant crack growth during the last fuel cycle and the continuing
- implementation of hydrogen water chemistry in the next fuel cycle, the staff does not anticipate crack growth in the three unrepaired welds during the next fuel cycle to the extent that the safety of plant operation is compromised.
. containing the 24 cracked welds were replaced with stainless steel 304L
{Carbon *content e~ual to or less than 0.02%) materials. The replaced piping materials are acceptable because the 304L materials are considered to be resistant to IGSCC.
In the RWCU piping system, there was only one weld which was not UT inspected because of access limitations. This 8 11 uninspected weld
~as a ~addl~ reinforc~d trinn~ction weld branchin~-6ff the sh~tdowri * -
- cooling line.
The subject weld was liquid penetrant examined and no indications were reported.
The staff does not consider that possible
- cracking in this.weld will have significant impact upon the safe operation of the Dresden Unit 2 plant, because the pipe size is small (8
11
) and the loss of reactor coolant due to through-wall cracking is within the coolant make-up capacity.
The licensee's overlay design for weld 8-K13 takes credit for the first layer of overlay.
The staff's earlier position was that this was not permitted.
The purpose for not allowing credit for the thickness cf the first layer was to provide additional conservatism in the overlay design. These additional design margins were considered desirable because at the time the requirement*was introduced, there were substantial uncertainties in the UT crack depth sizing, and experimental measurements had not been performed to support the presence of beneficial residual *stresses resulting from weld overlay repair. Since then, the
- quality of UT crack depth sizing has been greatly improved and the extent of the beneficial compressive residual stresses from v1eld overlay repair have been substantiated by both analytical and experimental measure-ments.
Therefore, the staff considers that the original additional design conservatism for overl~y is not needed if full IGSCC resistance of the first layer was demonstrated.
The licensee reported that low carbon stainless steel was used for weld overlay and the ferrite
. content of the first layer was measured to be 6.5 FN.
The staff concludes that the licensee has demonstrated adequate IGSCC resistance in the first layer and, therefore, the licensee's overlay design thickness, which includes the first layer, is acceptable.
Inspection personnel from Region III of the NRC also concluded that the modification and repair programs performed for the RWCU system were in accordance with.the required Codes and Standards, and NRC requirements.
2.5 Augmented leakaqe limit and Monitoring In their submittals, the licensee did not discuss what actions will be taken to upgrade the leak detection capability in the plant to ensure timely investigation of unidentified leakage.
Since there are three cracked welds not being repaired and not all non-conforming stainless steel piping welds wer~ examined or mitigated, the staff concludes that the licensee should implement the augmented leakage limits and monitoring in accordance with the guidelines in Attachment l to Generic letter 84-11.
\\
3.0 CONCLUSION
S Based on the review of the licensee's submittals, the staff concludes that the Dresden Unit 2 plant can be safely returned to power and operated in its present configuration for an 18-month fuel cycle provided that the aug-mented leakage limit and monitoring are satisfactorily implemented in accordance with the guidelines in Attachment l to the Generic Letter 84-11.
Nevertheless, there remains a residual concern regarding. the long term*
growth of small IGSCC cracks that may be present but not detected during this inspection. Therefore, the staff requires that plans for inspection and/or. modification of the recirculation and other renctor coolant prP.ssure boundary piping systems during the next refueling outage be submitted for staff review at least three months before the start of the next refueling outage.
4.0 ACKNOWLEDGEMENT This safety evaluation was prepared by W. Koo.
Dated:
March 22, 1985.
e TABLE l THE SCOPE AND THE RESULTS OF UT EXAMINATIONS OF DRESDEN UNIT 2 REACTOR COOLANT SYSTEM PIPING Number of Welds Examined No. of Not Total Cracked System Previousl~ Examined Previousl~ Examined Examined.
Welds No.
No.
No.
Recirculation 4"
4 20%
0(7) 0%
4 17%
0 12 11 9 ( 1) 20%
5 100%
14 28%
2 22" 2
33%
4 33%
6 33%
0 28" 3(2) 38%
5 20%
8 24%
l RHR
. 14" *'
0 0%
2 50%
2 50%
0 16" 2
50%
8 21%
10.
23%
0 CRD Return
'\\
4" l
100%
4 80%
5 83%
0 Heat*Vent 4"
0 0%
3 100%
3
.100%
0 Isolation Condensor 12 11 2
100%
- 4.
36%
6 46%
0 14" 0
0%
4 25%
4 25%
0
'.. k>. of Not Total Cracked System Previouslx Examined Previously Examined Examined Welds No.
- No.
No.
Llet Pump Instrument ion 4
100%
6 100%
10 100%.
0 RWCU 6"
0 0%
1(5) 50%
1 50%
0 8"
4 100%
20( 6) 87%
24 89%
1l ( 9)
.lQ" 0
0%
5 100%
5 100%
4( 10)
Total Welds 31 71 102 18(8)
Notes:
1 -*Includes 2 unrepaired cracked welds and 10 SE/nozzle welds.
2 - Includes l unrepaired cracked wel~ and 2 SE/nozzle.welds.
3 - Includes 2 unrepaired and 7 repaired cracked welds.
4 - Two inaccessible welds (saddle reinforcements).
5 - One inaccessible weld (saddle reinforcement).
6 - Three inaccessible welds (2 saddle reinforcement welds and 1 flue head penetration weld)
- 7 - Three welds not examined.
8 - Three unrepaired welds from 1983 outage.
9 - All replaced except one weld overlay repaired.
. 1.0 -- All rep 1 aced *