ML19345F186

History of U-Bend Cracking in Trojan Row 1 Steam Generator Tubes

ML19345F186
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	01/30/1981
From:	PORTLAND GENERAL ELECTRIC CO.
To:
Shared Package
ML19345F185	List: ML19345F184 ML19345F186 ML19345F189
References
NUDOCS 8102090372
Download: ML19345F186 (8)

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{{#Wiki_filter:. h ATTACHME!C 1 HISTORY OF U-BEND CRACKING IN TROJAN ROW l STEAM CENERATOR TUBES The Trojan Nuclear Plant is a four loop Westinghouse Pressurized Water i l l Reactor facility with a Nameplate Rating (Gross) of 1178 MWe and l Mnfmum Dependable Capacity (Net) of 1080 MWe. Initial power genera-i tion took place in December 1975, and the Plant was declared commercial in May 1976. l l The four steam generators designated "A", "B", "C", and "D" are Westing-house Series 51 units, and were fabricated in the Tampa facility. The 3388 Inconal 600 U-bend tubes (0.875-in. OD I 0.050-in. vall) in each generator were produced at the Westinghouse-Blairsville mill. The tubes were shop rolled into the tube plate and later Wextex explosively expaaded in the field. The nominal primary pressure ia 2235 psig. Primary inlet and outlet cesperatures for the steam generators are 616.8'F and 552.3*F respec-tively, and the nominal secondary temperature is 533 3*F. Table 1 summarizes the results of the steam generator eddy current inspections performed to date. All relevant indications not found during pre-service inspection have been located in the Row 1 U-bends, of which there are 94 per stesa generator. Figure 1 shows the con-figuration of the U-bend region for the Row 1 U-bends. 1 Initial leakage was detected in Januarf 1978 after t. bout 10,000 hr of operation. Note that this leak in the "B" stesa generator was not found with the eddy current equipment then available. _ Pressurization of the secondary system revealed the leak to be in the cold leg. This leak was plu5ted, and a long outage due to seismic deficiencies in the Control Building occurred. Leakage was again detected in January 1979 (-12,000 he of operation). Upon shutdown in October 1979, all Row 1 tubes in all four generators were inspected. Operating time then was over 17,000 hr, and a maximum leak rate of about 150 gpd had been observed prior to shutdown. Five leakers and four eddy. current indications'in ~1' 8102 09.0 3~7 9

4 Row 1 U-bends were found. Rl-C91 in the "D" generator was found to be a leaker but no eddy current indication was observed. Leakage was agais observed soon af ter startup in January 1980, and increased to a strimum of 68 gpd just prior to shutdown in April 1980 at about the 20,000 he sark. Leak checks of the "A", "B", and "C" generators revealed five leakers. Eddy current inspection of all Row 1 tubes found five indications. The two indications in the "D" steam generator ray or may not have been leakers. In addition to the indi-cations found in this inspection, R1-C7, which was not inspected prior to removal, had an indication in the cold-leg tangent region upon laboratory aramination. This indication was subsequently determined to be due to cracking which had not progressed through the tube wall. l Table 2 summarizes the overall history of lenkers and eddy current j indications found. The rough balance between hot-leg and cold ?eg occurrences is of interest. It is also interesting to note that if one marks the column numbers off on a tube sheet diagram (Figure 2), no l particular pattern is observed. Some tube marks are displaced one tube i because of duplication among the generators. Eighty percent of the occurrences are located adjacent to flow slots, which happen to be adjacent to 80 percent of the tubes. Pursuant to an agreement with the NRC, it was decided to remove 26 R.w 1 tubes (Cl - C26) and 3 Row 2 tubes (Cl - C3) from the "D" steam generator. This provided one known lenker (Rl-C6) and one indication (Rl-C26) plus comparison tubes from Row 2. Rl-C7 was subsequently identified as a defective tube. Redesign for access to the U-bend region of the secondary side of the "D" steam generator and tube removal procedures were developed by Westinghouse-NSD whose personnel supervised the operation. After drilling a pilot hole to verify location, a weld-deposited buildup was applied and machined to accommodate a flanged cover for.this field j installed hand hole. The buildup was post-weld heat treated to satisfy applicable pressure vessel code requirements. A 6-in. hand hole was L_

then cut in ene steam generator vessel and the tube bundle wrapper just above the sevanth support plate. Af ter measuring the leg spacing on each tube, the tube was cut above the seventh support plate by a plasma are TIG torch. The U-bends were then packagad and removed from the Trojan facility. The Plant resumed operation in mid-July 1980. Af ter over 2 months of very minimal leakage, discrete leakage events began to occur. Leakage events were heralded by a spike, which then decayed back to a. new and increased steady-state leak rate. Figure 3 summarizes this behavior. The most plausible hypothesis for this phenomenon is that af ter initial opening, the small and tortuous leakage paths act ac filters for particulate matter in the primary system, resulting in partial plugging. At the date of this report the estimated leakage distribution is steam generator "A" 20 percent, steam generator "B" 10 percent, steam genera-tor "C" 10 percent, steam generator "D" 60 percent. The leakage distribu-tion may or may not accurately reflect the number of leaking tubes in the respective generators. That is, each leakage event may not make an equal contribution to the overall leakage, and each leakage event may not j derive from a previously non-leaking tube. l l l l l l DRS/4138A12...

TABLE 1 TROJAN STEAM GENERATOR INSPECTIONS

SUMMARY

l PRE-SERVICE: April, 1975 100 percent of " accessible" tube area; 400 kHz. May-July 1977: SG/C 900 tubes @ 400 kHz through first support. { 278 tubes @ 400 kHz through U-bend. i 144 tubes @ 400 kHz through seventh support. 1 tube @ 100 kHz through U-bend. No lenkers. SC/A 450 tubes @ 400 kHz through first support. 214 tubes @ 400 kHz through U-bend. No lenkers. APRIL 1978: SC/B 507 tubes @ 400 kHz around U-bend. 68 tubes - small radius at 100 kHz. 28 tubes @ 100 & 400 kHz.through U-bend (supplemental). Leak 9 RIC57 not located by eddy current. OCTOBER 1979: All Steam Generators 94 tubes (all Row 1) SG/A, C, D @ 100 kHz absolute. 93 tubes (all Row 1) SG/B @ 100 kHz absolute. 5 leakers: One (1) in SG/A; four (4) in SG/D. All verified by ET. One (1) ET indication in each SG which did not leak during visual an=1 nation. 720 111 ball gauging in Row 1, SG/D: All tubes passed. JUNE 1980: All Stesa Genarators Row 1 in all four (4) SGs. Regulatory Guide Program in SG/D. 5 leakers: Three (3) in SG/A; two (2) in SG/B. All verified by ET. ET Indicatione: -One (1) each in SG/A, B, C which were not visually observed leaking and two (2) in SG/D. (possibly leakers) DRS/41m8A15

TABLE 2 LEAKERS AND FIELD EDDY CURRENT INDICATIONS Date Sj[ Column Elevation Finding Plugged A 68 CL-Tangent L 11/79 A 38 EL-Tangent I 11/79 A 48 CL-Tangent L 6/80 A 80 CL-Tangent L 6/80 A 87 CL-Tangent L 6/80 A 7 EL-Tangent I 6/80 B 57 CL-Unknown L 6/78 3 47 CL-Tangent I 11/79 3 48 EL-Tangent L 6/80 B 51 EL-Tangent L 6/80 B 50 EL-Tangent I 6/80 C 34 EL-Tangent I 11/79 C 68 CL-Tangent I 6/80 D 6 CL-Tangent L 11/79 D 62 EL-Tangent L 11/79 D 70 EL-Tangent L 11/79 .D 91 CL-Unknown L 11/79 D 26 CL-Tang-at I 11/79 D 31 CL-Tangent I, L? 6/80 D 55 EL-Tangent I, L? 6/80 l D 7 CL-Tangent I [a] [a] Not inspected prior to removal; laboratory finding 12 cold leg - 9 hot leg. DRS/41m8A16 i

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