ML20084E412

Forwards Rept Re Investigation & Repair of Main Steam Safety Valves

ML20084E412
Person / Time
Site:	Oyster Creek
Issue date:	08/11/1972
From:	Finfrock I JERSEY CENTRAL POWER & LIGHT CO.
To:	Anthony Giambusso US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20084E414	List: ML20084E412 ML20084E873
References
NUDOCS 8304150055
Download: ML20084E412 (11)

Text

{{#Wiki_filter:(. . - c T S p s J A G g Jersey Contral Powe.r & Light Company M ADI5oN AVENUE AT PUNCH Bowl. RO AD e MORRISTOWN, N.J. o796o e 539 6111 Auguat 11, 1972 .!L t Mr. A. Ciambusso j p ',/M,j Deputy Director for Reactor Projects Directorate of Licensing ' fy' United States Atomic Energy Commission _ ' ' { ( g\\ / ,y, Washington, D. C. 20545 iil[QtW

Subject:

Oyster Creek Station Docket No. 50-219 Safety Valve Seat Bushing Cracks I //,/f)). In accordance with my letter to you, subject as above, dated May 1, 1972, I am attaching a report of our investigation and repair of the Oyster Creek main steam safety valves. [ We are enclosing forty copies of this letter.with attached report. Very truly yours, US11 1AW Ivan R. Finfro Jr. Manager, Nuclear Generating Stations IRF/pk Attachments cc: Mr. J. P. O'Reilly, Director Regulatory Operations i 'I 50;y;<,q eso > pygedmih S A m O m .,a. 4

p (, () p/ L.) \\_ 5 OYSTER CREEK NUCLEAR GENERATING STATION Report on Safety Valve Ssat Bushing Cracks Introduction A liquid penetrant inspection of the pressure containing parts from twenty-one safety valves that have been in service at the Oyster Creek Plant disclosed cracks in nine seat bushings. No cracks were observed in the valve discs. Summary One of the cracked seat bushings was destructively examined. The metallographic < examination disclosed the cracks to be low temperature transgranular chloride stress corrosion. The cracks were locuted radially across the seat and longi-tudinally and circumferential1y on the outside of the seat bushing. The maximum crack depth was 150 mils. Material tests confirmed that the seat bushing was 304 stainless steci in the fully annealed condition. The valves with the cracked seat bushings were in the three or four valve positions located closest to the reactor on each steam line. These valves had been leaking in service as determined by radiation measurements and scale deposits on the seat bushings., The environment at the cracked seat bushing was steam and water, free oxygen and chloride contamination. The source of the chlorides is not known. During plant operation, the seat bushing was under stress from the reactor pressure of 1020 psig. Under these conditions, 304 stainless steel is known to be susceptible to chloride stress corrosion cracking. To minimize the possibility of cracking in the future, a) all cracked seat bushings were replaced with new 304 stainless steel bushings; b) all valve parts and the outlet piping were washed with demineralized water to remove chloride; c) a drain hole was added just above the inlet flange of each valve to prevent water from 3 standing in the valve base; and d) the valves were reassembled using lubricants P. free of chlorides. 4 Background Information The Oyster Creek Nuc1 car Power Plant has a total of sixteen spring-loaded safety valves on the main steam lines between the reactor vessel and the isolation valves. The valves are attached to the steam line by flanged connections. The steam line and the inlet valve flanges are insulated. The valve bodies above the inlet flange are not insulated. A drawing of the safety valve identifying the primary parts is shown on Figure 1. As shown in Figure 1, the seat bushing and the disc are the reactor coolant pressure boundary. The seat bushing is machined from 304 stainless steel. The disc is machined from 422 stainicss steel. Both the seat bushing and the disc were ultrasonic and liquid penetrant inspected at the time of manufacture. ~

.. O O .s

s. a.

Oyster. Creek Station -Report on Safety Valve Seat Bushing Cracks Page II August 11, 1972 The Technical Specifications for the Oyster Creek Plant requires e minimum of five of the valves shall be bench checked or replaced with a bench checked valve each refueling outage such that all valves are checked in. l three successive refueling outages to insure setpoints are as follows: Number of Valves - Set Point (psig) i l 4 1212 1 12 4 1221 1 12 4 1230 1 12 4 1239 12 The popping point of the safety valves at the Oyster Creek Plant was set at the valve factory on steam. During the Poison Curtain Removal Outage in October 1971, five valves were removed from a, steam line and replaced with spares. In Apr11-1972, these valves were being disassembled and decontaminated for return to the factory 2for setpoint checks when the cracks were found. The cracks were detected by liquid penetrant examination. f The location of the valves on the steam lines from plant startup in 1969 to the curtain outage in October 1971 is shown on Figure 2. The location of the valves on the plant from October 1971 to the refueling outage in May 1972 is shown on Figure 3. s Examination Results The seat bushings and discs of the five valves removed from service in October i 1971 were liquid penetrant inspected. Two seat bushings were 'found cracked. No cracks were observed in the discs. The seat bushing from valve serial number BK-6247 had radial cracks at the valve seat and circumferential cracks at the fillet radius above the lower threads. The radial' cracks started at the'outside surface and extended' part way across the seat face. The circumferential cracks were located-around a major portion of-the circumference. The maximum depth of the-circumferential cracks was 120 mils as determined by grinding out the cracks. There were no crack indications on'the inside diameter of the seat bushing. h. _.. - ig i n' _h

j .. O o ,s S . Oyster Creek Station Report on Safety Valve Seat Bushing Cracks Page III August 11, 1972 4 The seat bushing from valve serial number BK-6268 had approximately fif teen radial cracks across the seat, numerous longitudinal cracks on the outer diameter and circumferential cracks at the fillet radius above the lower threads. No crack indications were found on the inside diameter. This seat bushing was shipped to Vallecitos Nuclear Center for destructive examination. The metallography examination disclosed that the cracks were transgranular chloride stress corrosion cracking. A fracture surface of a crack was examined by microprobe in two areas.

Chloride, sulphur and potassium were detected in both areas. The material was tested and found to meet the requirements of ASTM A-182, 304 stainless steel. No evidenca of carbide precipitation was found.

When the plant was shut down for refueling in May 1972, the sixteen valves in service were removed, disassembled and inspected. A dye penetrant inspection of the seat bushings and discs from the valves disclosed that seven of the seat bushings were cracked. On three of the seat bushings there were cracks across the seat and on the outside surfaces. Two of the seat bushings had cracks only across the seat, and two of the seat bushings had cracks only on the outside surfaces. The Jocation and appearance of the cracks were the same as previously described for valves BK-6247 and BK-6268. A summary of the inspection results on the sixteen valves is given in Table 1. Prior to disassembling the valves, the discharge area of the valves vss _ examined. All valves had scale or rust deposits on the outside of the seat bushings. There was a water level ring even with the low point in the outlet discharge pipe indicating that all valves had leaked. Valves were conducted at a pressure of 1020 psi after maintenance on the reactor coolant systems. 1 On the cracked seat bushings, there was c hard scale about 1/32-inch thick that had flaked off in places. The scale was light tan in color and was p located below the water level mark on the base. The contact gamma radiation dose rate at the thick scale was 200 to 2000 mr/hr. Three of the valves with the heavy hard scale (BK-6312, UK-6314 and BK-6323) were on the plant a short time from November 1971 to May 1972. With the exception of the cracks across the scating surfaces, the longitudinal and circumferential cracks on the outside of the seat bushings were located below the water level mark. As indicated in Table 1, four of the seat bushings had uamaged threads at the seat bushing to valve base connection. Metallurgical examination of two of these seat bushings indicated that the thread damage was mechanical in nature and probably occurred either during valve assembly or disassembly. L ~.

,. n .. O O Oyster Creek-Station Report on Safety Valve Seat Bushing Cracks Page IV August 11,-1972-Discussion The high gamma dose rates and the hard scale deposits indicate the valves with the cracked seat bushings were leaking steam in service. The scale deposits are believed to have been deposited by a condensing and boiling cycle inside the valve body. There is evidence that steam was condensing. on the inside of the uninsulated valve body and the resulting water was running down in the area between the valve base and the seat bushing. The water was boiling at the side of the seat bushing depositing the scale on the hot surface. The seat bushing would be heated by the reactor steam. The valves with the cracked seat bushings were the valves located closest to the reactor as shown in Figures 2 and 3. Since these valves were'not randomly located on the -steam line, the steam leakage is believed to be a significant factor in causing the cracking. The microprobe examination of fracture surfaces and the analysis of a crud sample taken from inside a valve base indicated chloride contamination was present on the surfaces of the valve parts exposed to the drywell ambient. Possible sources of chlorides include chloride contamination of the valve parte and'outict piping during fabrication and installation and concentra- ' tion of chlorides in the reactor water by boiling. The chloride concentra-tion in the reactor water is normally 1 css than 0.1 ppm with an operating limit of 1.0 ppm. Some free oxygen is present at the outside of the seat bushings from the nominal two percent oxygen in the drywell during plant operation. In summary, the environment at the seat bushing during plant operation was hot steam and water, free oxygen, chloride contamination and stress in the seat bushing from the reactor pressure of 1020 psi. Under these conditions, stress' corrosion cracking of 304 stainless steel could' occur. 9 The valve manufacturer has indicated that cracking of seat bushings has l previously occurred in only three or four instances out of thousands of dimilar valves in service. According to the valve manufacturer, in each of the previous cases, the cracking was attributed to chloride contamina-tion of the valve. In no instances had e seat bushing failed catastrophi-cally. In previous cases, the corrective measure was to replace the defective parts and to control further exposure of the valves to sources of chloride contamination. [ Conclusions and Corrective Measures Based on the above information and the experience at other plants, the cracks in the seat bushings at the Oyster Creek Nuclear Plant are the result of chloride contamination. L.

_ = _ .m Oyster Creek Station Report on Safety Valve Seat Bushing Cracks Page V August 11, 1972 To eliminate the possibility of water standing in the lower part of the valves where cracking occurred, a drain hole was added inat above the inlet flange. All cracked seat bushings were replaced with new tr:m Ma ai inless steel bushings of the same design as originally provided. After parts were machined and dimensionally checked, the parts were cleaned with alcohol and demineralizer water, and the valvec were assembled using graphite and caster oil as a lubricant. All materials were checked for chloride and fluoride contamination. The inlet and outlet flanges on the valves were covered during shipping, handling and installation. The outlet discharge pipe for each valve,' which consists of a flange, a piece of pipe and a pipe tee was washed with demineralized water prior to installation. To prevent the leakage of water during leak testing of the reactor coolant systems, the safety valves will be gagged above a pressure of 900 psi. Future Action At the next refueling outage, selected valves from locations where seat-bushings were previously found cracked will be removed, disassembled and the seat bushings inspected for cracks. The leakage of safety valves during plant operation will be investigated. i L

p p 4 s l ~~. g J ^ a Top Lever 8elease Rut-isp Leves d tsapressies $ctes Eut g~t A L 3rra teser Pan # -Q' @7[t Mp u take .tenpressica scoes ,.1 top sprang tastiesq Irsp Levet f Spring Spiedle / / e .i 'i Teke ta$s\\ lettee $pting f ast'er N ~ Cover Plate N /{$ tad Lif t 8tsN >t Bise Estder - k %~ Stud Net f Sise Gaid KQ l Dese tells by lLT hpper 18. Rio j 3 bppes Adj. test PLN..i., y "g Bis 9 g nm .Leest Adj. Sitt Pi ~uggEg' y) to.e,ui.... -1 9 1 f sist ensning !j 9 ~ ~ ( 1 I -,t s. s 's ~ s,_ FIGURE I-MAXIFLOW. SAFETY VALVE i 4

'l ~ t (,3 ...Bk 63 /l BK G2 G4' ~ Bk6279 Bk G3/5 ip \\ p BK G324 Sk Gl534 - gg ggzo Q _ M 6265('T Eg6533 / t L Bk 62.46(') ~Bxsz7o f , BkG278

• N ggggag'X2) l Bk 6 slo BKS 259 0

. Bk 62 47 y O )O k REACTOR - m (((). ( )) ) SOUTH STEAM UNG OORTH S TEAM L /NG (!) Valves Removed Qet 177/ and Inspected (2). Cracked Seat Bus &y ( .Fiyure 2. s fety valves in Service -from Aug tese to oet 197/ - a 1 f b

_~ .w. ~ N N 0 . _BN G3 /l ) \\ / BkG269 ~ BK6279 Ek03IO 'A \\ gg ggg g, 9 ~ J' Ek &534 .gggggo Bk6326 Bk 6533 'Bk 6170 " % EkG323(' aksz7e'

• % 'gpg3jy )

U Bk 6310 BKS 259 -8k&3/$ ~_y O O t' REACTOR ) ~ s SOUTH STEAM UNE ~ NORTH STEAM LINE 03 Cracked Seat Bushiyd Impected ail' Va/ves Removed an fi ure 3. Saf~e -ty Va/vcs 'in S e e vic c of Reocfor Shufdown May /772 7 1 ~

e- - TABLE 1 EXAMINATION RESULTS OF SEAT BUSHINGS INSPECTED MAY 1972 mr/hr Valve Serial Set Pressure Radiation ] Nucher* psig Liquid Penetrant Inspection Thread Damage Scale Deposits Measure =ents BK 6259 1230 small linear cracks on outside none hard scale 2000 BK 6310 1212 four radial cracks on seat none hard scale 2000 many linear cracks on outside' l 1 BK 6278 1212 twenty-four radial cracks on threads damaged hard scale 1000 seat g linear cracks on outside W j BK 6320 1239 no cracks none medium rust scale 50 i DK 6324 1239 no cracks none light rust scale 50 i BK 6533 1230 no cracks none ' light rust scale 75 ) l, BK 6274 1221 no cracks none light rust scale 50 BK 6311 1221 no cracks threads damaged light rust scale 35 l cVnives from north steam line BK 6312 1239 ten radial cracks on seat none hard scale 500 0 BK 6314 1212 small linear cracks on outside threads damaged hard scale 200 l BK 6270 1212 nine radial cracks on seat none hard scale 300 BK 6323 1221 twenty-three radial cracks on seat none hard scale 500 many linear cracks on outside circumferential cracks above threads f BK 6534 1221 no cracks none light rust scale 70 BK 6315 1230 no cracks threads damaged light rust scale 40 cValves from south steam line 5

w. 1 \\ TABLE 1 EXAMINATION RESULTS OF SEAT BUSHINGS INSPECTED MAY 1972 cr/hr Radiation 3 Valve Serial Set Pressure Thread Damage Scale Deposits Measurements Number

• psig Liquid Penetrant Inspection BK 6326 1230 no cracks none not examined low BK 6269 1239 no cracks none very light rust scale 75
• Valves from south steam line e

i l' 1 I l l I 5 O I =5}}