ML20086S437
| ML20086S437 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 12/16/1974 |
| From: | Stallings C VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | Moseley N NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| Shared Package | |
| ML20086S438 | List: |
| References | |
| 321, AO-S2-74-09, AO-S2-74-9, NUDOCS 8403020362 | |
| Download: ML20086S437 (24) | |
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December 16, 1974 '
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lir. Norman C. !!oacicy, Director Serial'lIo. 321 Directorate of Regulatory Operations POEM /JIS:civ United Statco Atomic Energy Cor: mission Region II - Suite 813 Docket Hos. 50-200 230 Peachtree Street, I!orthwest 50-281 At1cuta, Georgir. 30303 License ::os. DPR-32 DPR-37
Dear l'.r. Moseley:
Purcuant to Surry Pcver Station Technical Specificction 6.6.B.1, the Virginia Electric and Power Company hereby submito forty (40) ccpics of Abnorcal Occurrence Report lio. A0-S2-74-09.
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Upon cespletion of the corrective action stated in the report, we plan ~
to return the units to. power.
The substr.nca of this report has been reviewed by both the Station Nucler.r Safety and Operating Cor.mittee and the System Nucicar Safety and 0;,erating Cemittee.
Very truly yours,
I . . i'l$c<< o ,
C. !!. S'tallinga Vice Prcoident-Peuer Supply and Production Operations Enclosuren cc: Mr. K. R*. Goller, Assistant Director. . s. . i .
for Operating ncactors (40 copica) '/
Mr. H. Fairtile g,2} 23 8403020362 741216 PDR ADOCK 05000280 S PDR COPY SENT REGION 2 7
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ABNORMAL OCCURRENCE REPORT i REPORT NO. A0-S2-74-09 i
4 STEAM GENERATOR COUPLING SUPPORTS j
DOCKET NOS. 50-280 50-281
! LICENSE NOS. DPR-32 DPR-37 I DECEMBER 12, 1974 SURRY POWER STATION i
t VIRGINIA ELECTRIC AND POWER COMPANY 1
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I. INTRODUCTION In accordance with Technical Specification 6.6.B.1 for the Surry
- Power Station, Unit Nos. I and 2, Operating License Numbers DPR-32 and DPR-37, this report describes an abnormal occurrence which was identified on October 17, 1974. The Directorate of Regulatory Operations, Region II, was notified on October 18, 1974.
The occurrence reported herein has been classified as an abnormal occurrence by the Directorate of Regulatory Operations, Region II.
The occurrence described herein involved the discovery of cracks in some of the steam generator lower ring support swivel end couplings on Unit No. 2. Because a similar problem was identified on Unit No. 1, both occurrences shall be discussed in this report.
II.
SUMMARY
OF OCCURRENCE All supports in the reactor coolant system, including the steam generator supports, are designed to withstand the design basis earthquake acting simultaneously with an instantaneously applied pipe break.
l The steam generator supports consist of two (upper and lower) rings and associated suspension rods and snubbers. The lower ring which carries the steam generator weight is suspended by means of.three pipe columns.
Hydraulic snubber cylinders connect the upper and lower rings to the steam generator cubicle walls to allow piping expansion and contraction during heatup, cooldown and power changes while resisting movement during seismic and pipe break conditions. In addition, the lower casting is connected to the cubicle concrete structure by swivel end couplings. The swivel end couplings discussed herein are required only during the
i accident conditions previously described and do not support the steam generator under normal conditions. Additional information may be found in Section 15.6 of the Final Safety Analysis Report.
As shown in Figure 2, there are two swivel end couplings per steam generator.
The swivel end coupling assembly, schematically shown in Figure 1 (3 pages), basically consists of a sandwich plate (Item B) between two end plates (Items A and C) which can rotate in two swivel rings (Items D and E). The end plates are held in position by closure rings (Item F) which are shrink fitted into place. The sandwich plate has two dovetails, perpendicular to each other which slide into each of the end plates. One swivel ring is bolted to the lower ring on the steam generator and the other is attached to the cubicle concrete structure. The swivel ring attached to the concrete structure is actually attached to a six inch embedment plate by means of 24 bolts. The embedment plate is attached to the wall by 10 studs which are embedded in the structural wall.
The swivel ring assembly was manufactured from Vascomax 300 and 350 CVM 18 Ni maraging steel heat treated to provide a minimum yield strength of 270 ksi. Based on actual chemical analysis of the material, in May of 1969, the material contained the constituents shown in Table 2. The analysis of the closure rings before installation showed that the parts were made from metallurgically sound material, the heat treatment was good, and the chemical and physical properties were typical for the grade of steel.
The purpose of the " swivel" feature in the couplings was to allow for slight rotational and parallel misalignment during installation.
I o
O O The purpose of the dovetails is to allow for thermal expansion during heatup and cooldown of the reactor coolant system.
During a maintenance outage on Unit No. 2 to perform turbine repairs, a routine inspection of the containment and primary systems was performed.
This inspection disclosed the presence of minor cracks in the concrete cantilever to which the outboard lower ring support swivel end coupling of steam generator "A" was attached. An investigation was initiated to determine the cause of the cracks and their effect on the structural integrity of the wall.
The cause of the concrete cracks in the concrete cantilever was a lateral load in the direction of the reactor pressure vessel, indicating that the swivel end coupling was bound and not operating properly during steam generator cooldown. Visual examination of the swivel end coupling in place showed that the sandwich plate, Item B, Figure 1, page 15, was at a slight angle of about 5 degrees to the centerline. The swivel rings, Items D and E, were horizontally aligned. In the vertical plane, Item D on the steam generator support ring was approximately k inch higher
~
than Item E on the foundation base plate. The end plates, Items A and C, had rotated in the rings so that portions of the spherical surfaces were exposed approximately 3/16 inch. From appearances, the dovetail joints between Items A and B, and between B and C, were not sliding.
In order to determine whether the swivel end coupling was properly working or whether it was bound, the assembly ring was removed for examination. Removal of the assembly was difficult and it was evident that the swivel end coupling had seized. The 24 bolts (l\" UNF) which attach the coupling to the baseplate and support ring were heavily
O O side-loaded in shear, making then very difficult tc remove. Most of the bolts removed exhibited flattened threads which had rubbed on the inside of the drilled hole of the coupling flange. Visual examination of the holes after bolt removal showed about 1/16 inch horizontal dis-placement of the concentric holes of the flange and tapped holes. The two halves of the swivel end coupling were displaced in opposite directions.
In addition, there was separation of the swivel end coupling from the foundation embedment plate as bolts were removed, indicating heavy tensile as well as shear loads on the cap screws. Both the tension and shear loads were relieved, but not eliminated by loosening the 10 anchor stud nuts on the foundation embedment plate.
While still in the tapped hole about ik inch, the last bolt in the embedment plate was difficult to remove by turning. It was sawed off using a hacksaw blade inserted in the gap between the embedment plate and the swivel end coupling. The gap was about k inch.
Af ter removal of the swivel end coupling, the embedment plate, the support ring and the coupling were examined, and all three components exhibited evidence of binding. The corners of both end plates, A and C, had chipped Heresite coating over about 180 degrees of are where the end plates had rubbed against the support ring and embedment plate. The i
support ring and embedment plate were both "brinelled" over an 8 inch long are where the end plates had cut into them. One of the dovetail joints 4
between the sandwich plate and end plate was seized to the point where it was not possible to slide the dovetail by hand. The spherical surfaces I
between the end plates and swivel rings (between A and D, 'and between C and -
E) were free and could be easily rotated by hand. !
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O O The dovetails appeared dry and there was evidence that some Heresite had been inadvertently applied in small areas, as evidenced by an area l
l where Heresite had been scraped off an end plate by the movement of the sandwich plate B. This indicates that the dovetail sliding joints were
- operational at one time.
i-As part of the investigation, the first swivel end coupling was removed and inspected. Visual and dye penetrant inspection of the swivel ring revealed three major cracks. These cracks extended from the sharp j fillets in the bottoms of the grooves where the closure rings had been installed (shrink fitted) to the spherical surfaces of the swivel rings.
Two of the major cracks were in the top half of the swivel ring, and.the other was in the bottom half. Other dye penetrant indications of cracks were observed in the fillets of the closure. ring grooves, but these cracks did not extend to the spherical surface. Minor pitting corrosion was observed on the surfaces of the closure ring grooves, in the fillets where the cracks were detected, and on the spherical surfaces of the swivel rings.
i The fracture surfaces of the three major cracks had a deposit of Heresite on them. .Some of the Heresite could have run'into the cracks when the closure ring grooves were cleaned with a solvent prior to the dye i
penetrant inspection. However, the heavy layer of Heresite observed on one i
of the fracture surfaces (the crack in the bottom half of the swivel ring) suggests that the crack may have been present before the coating was applied after the closure rings were shrink fitted in place, j The remaining swivel end. couplings on both Unit Nos. I and 2 were ultrasonically testedito determine if they had experienced' cracking. This examination indicated that 20 of 24 halves exhibited cracks. Disassembly i
of several couplings arx1 visual examination-verified the presence of, I
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( v cracks similar to those previously described. A summary of these inspections
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is shown in Table 1.
III. ANALYSIS OF OCCURRENCE The cracks which were initially discovered in the concrete were apparently caused by the binding of the sliding dovetails due to the lack of sufficient lubrication. The binding of the dovetails and cracking of the concrete were not related to the cracked swivel plates. The basis for this conclusion is that cracks were found in many of the couplings, whereby significant corresponding concrete cracking was not present. In addition, cracks were discovered in swivel rings which had freely moving dovetails.
The binding of the dovetails was not considered to be a significant occurrence since the cracks in the concrete were nominal. Their ability to perform their intended design function, to withstand the design basis seismic event simultaneously with a pipe rupture was not affected. In order to evaluate the effects that the binding of the dovetails may have had, a stress analysis of the main reactor coolant loop was performed to evaluate the stresses that were generated by the malfunctioning of the swivel end couplings. This was achieved by evaluating the magnitude of force that was generated by the binding of the coupling and using that force as input to a computer model of the piping. The results of the analysis revealed that an overstress condition did not occur from the malfunction, and the integrity of the coolant pipe was not compromised.
Most of the cracks observed in the concrete while the coupling was in place were less than approximately 0.002 inches wide. Since removal of the swivel end couplings, the cracks have closed up. The massiveness of the structure and the reinforcing steel present is sufficient so that the
~
structural integrity is not affected.
The first cracked swivel end coupling removed was sent to Battelle .
Columbus Laboratories for analysis. The analysis has not been completed, with the chemical analysis of the steel and evaluation of the micro-i structure yet to be performed by Battelle.
The previous description of the cracks in the swivel ring was based on the Battelle preliminary report. Examination of the fracture surfaces after they were cleaned with solvents and a fiber brush to remove the Heresite revealed that there was very little corrosion product on the surfaces. In addition, the examination showed that the major cracks contained multiple crack origins. These origins were located in the fillets of the closure ring grooves. Thus, crack growth was from the closure-ring fillet toward the spherical surface in each case.
Replicas of the fracture surfaces from several origin regions were examined with the transmission electron microscope and the tracture surfaces in the regions of the origins were examined with the scanning electron microscope. It was found that the predominant fracture mode was intergranular fracture, with some transgranular cleavage. These fracture modes indicate a brittle failure mechanism and suggest that the cracks were initiated either by an environmentally induced failure mechanism, such as stress-corrosion cracking or hydrogen-stress cracking, or that the material was inherently brittle because of its composition or prior processing.
The mechanical properties of the steel were evaluated with specimens from the top half of the swivel ring. The properties measured were the tensileproperties,observedwithbothsmoothandnotched(Ky5) specimens; plane-strain fracture toughness (K ); and the energy absorbed during l
i 1
O O fracture at room temperature of Charpy V-notch specimens. The properties obtained are summarized in Table 3.
The yield strength of the steel exceeded the minimum value specified for the part and the strength properties exceeded the minimum values usually reported for this grade of maraging steel. The ductility of the steel, particularly the reduction in area, was lower than is generally expected for this grade of steel. Similarly, the notched-bar tensile strength, plane-strain fracture toughness (K ), and Charpy V-notch-specimen energy absorption were-lower than the typical values reported for this grade of maraging steel.
One of the important reasons for determining the mechanical properties was to determine the fracture mode of the ' steel under the different testing conditions. Examination of the fracture surfaces of the various types of specimens with the scanning electron microscope revealed that the fracture mode was predominantly ductile, with only isolated regions of brittle cleavage fracture; no intergranular fracture was observed in any of those samples. Thus, those studies showed that the material does not fail in a brittle manner under normal testing conditions at ambient temperature.
An additional experiment was performed in which one of the fracture-toughness specimens was loaded in bending to an initial stress intensity of 18 kai (about one-third the K value determined in air) and placed in distilled water. The specimen failed in about 2-3/4 hours. The behavior shows that this steel is sensitive to environmentally induced crack growth.
In the region of slow crack growth (prior to final failure), the fracture
! mode was predominantly intergranular.
In addition to the metallurgical analyses described above, a stress analysis was performed for the fillet region where the cracks initiated.
l I
l
It was found that stresses induced by the shrink fit would be at least 229 ksi, assuming the radius of the fillet to be 0.010 inch, the maximum radius specified on the drawing.
_ The information obtained from the Battelle study suggests that the cracks in the swivel ring may have been caused by an environmentally induced cracking mechanism. Conditions that lead to such cracking processes are a combination of a sustained tensile stress, a source of atomic hydrogen or a suitable corrosive environment, and a, susceptible metal. The shrink fit provided high tensile stresses in the origin region adequate to initiate cracking. The behavior of the bend-test specimen in the water environment proved that the metal is susceptible to environmental cracking, and the presence of the corrosion pits near the fracture origins indicates that moisture was present which could provide the necessary environment. Actually, other studies have shown that the corrosion process itself can cause atomic hydrogen to be introduced into the metal in sufficient quantities to cause hydrogen-stress cracking in high-strength steels.
The investigation did not indicate the presence of a specific corrodent, only the possibility of the atmosphere containing unknown corrodents. As-previously stated the cracks contained Heresite. The origin location of the cracks in the closure ring grooves does not lend itself to exposure.to the atmosphere once the swivel end coupling is assembled since the spherical
! surfaces of the end plate and swivel ring mate. In addition, the spherical surfaces are lubricated with Molykote 8800. The above facts indicate that-l the cracks probably began before the couplings were shipped to the station l
during initial construction.
1 I
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- _9_
O O High stresses were calculated to exist in the swivel ring because of the shrink fit of the closure ring. This stress was calculated to .
be 229 ksi based on a 0.010 inch radius at the base of the closure ring groove. The 0.010 inch radius was specified as maximum radius on the machine drawing. A smaller radius, which would have met the performance requirements as established on the fabrication drawing, would cause higher stresses to exist. The actual measurement of this radius on the swivel end coupling analyzed by Battelle is not yet availabic; however, visual inspection of the swivel rings from which the closure rings have been machined off, indicate little or no discernabic radius.
The history of the swivel end couplings prior to installation on the units indicates that several closure ring failures occurred. The original design of the swivel end couplings provided for holes in the closure ring to allow bolting into place. While being stored and waiting to be installed, the closure rings developed cracks in them. A thorough investigation revealed that the only corrodent detected was atmospheric moisture. The investigation included thorough efforts to identify any other corrodent which might have been present, including investigation of items such as cutting oil used, cleaning agents, furnace gas analysis, etc.
No other corrodent was identified. The analysis of the Vascomax material indicated it met the design specifications. The final conclusion was that cracking of the closure ring was caused by high stresses at the holes in combination with an environmentally induced cracking mechanism. The corrective action was to climinate the holes in the closure ring which eliminated the stress riser and to coat the assembly with Heresite to protect it from the environment.
The circumstances surrounding the cracks in the swivel rings, as reported herein, bears a similar relation to the previous failures.
Specifically, cracks appeared; no specific corrodent other than atmospheric was identified; high stresses were present; and an adverse geometry in the failed piece was present. The lleresite coating was not applied prior to the first failure (the failure before initial installation caused by holes in the closure ring) and was not applied in the closure ring groove where the crack initiated af ter the corrective action was implemented for the first failure.
Based on the information available, it can be concluded that the cracks in the swivel plates as reported herein were caused by high stresses induced by the shrink fit of the ' closure ring in conjunction with some environmentally induced cracking mechanism. Based on evidence obtained to date, the cracks were present prior to initial installation on the unit as evidenced by the presence of IIeresite in the cracks.
IV. CORRECTIVE ACTION The primary corrective action implemented was to redesign the steam generator support couplings, eliminating the swivel feature which eliminates the closure ring, the closure ring groove and the shrink fit. The source of high stress has been climinated, as well as the stress riser. The new supports are manufactured of material whose chemistry is in accordance with ASME SA 540 with a minimum yield strength of 160,000 psi.
Figure 3 (3 pages), shows a sketch of the seismic support coupling. As can be seen, the assembly essentially consists of two end plates (ltem X and Y) with machined dovetails, which when installed are O O i
V. ANALYSIS AND EVALUATION OF SAFETY IMPLICATIONS OF THE OCCURRENCE The support couplings discussed herein do not carry any operational loads and do not support the steam generator. They serve only to restrain i
i the steam generator in the improbable event of a pipe break of a feedwater, steam or reactor coolant system pipe simultaneously with a design basis seismic event as defined in the Final Safety Analysis Report.
^
The conditions described above did not exist and there was no
! requirement to utilize the coupling. Therefore, there were no safety implications associated with this occurrence.
VI. CONCLUSIONS The licensee concludes that:
- 1. The cracking of the swivel ring was caused by the high stresses present at the radius in the closure i
ring groove, in combination with an environmentally induced cracking mechanism.
- 2. The integrity of the reactor coolant system piping was not affected as a result of the cracking of the <
swivel ring or binding of the sliding dovetails.
- 3. The cracking of the concrete did not affect the concrete support structure's load carrying capability.
- 4. The modifications described herein provide assurance that the original design requirements of the seismic support coupling are satisfied.
- 5. The occurrence reported herein did not affect the safe ,
operation of the station.
- 6.
- The occurrence reported herein did not affect the health l
i- or safety of the general public.
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TABLE 1
SUMMARY
OF CRACKED COUPLINGS STEAM GENERATOR SEISMIC SUPPORTS STEAM CONDITION CENERATOR LOCATION
- UNIT NO. 1 UNIT NO. 2 A Inboard-Wall side Cracked Cracked A Inboard-SG side Cracked Cracked A Outboard-Wall side Cracked Cracked A Outboard-SG side Cracked Cracked
, B Inboard-Wall side Cracked Cracked B Inboard-SG side Cracked Sound
- B Outboard-Wall side Cracked Cracked 1
B Outboard-SG side Cracked Sound C Inboard-Wall side Cracked Cracked C Inboard-SG side Cracked Cracked ,
C . Outboard-Wall side Cracked Sound C Outboard-SG side Cracked ' Sound 4
- The location of the support is relative to the reactor vessel, i.e. the
, inboard support is radially. closer to the reactor vessel than the outboard, support. ,
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TABLE 2 .
1 I TYPICAL ANALYSIS OF
, VASCOMAX 350 CVM i
! (Based on actual analysis of closure ring-May 1969) t 4
r Carbon .007 1 Silicon .02
- Manganese .02 l Sulphur .005
! Phosphorus .002 l
1 Cobalt 11.90 i
Nickel 18.54 Molybdenum 4.66 i
Titanium 1.34 i
Aluminum .10 Boron .0016 Zirconium .009 4
r p
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e a
n, e .
TABLE 3
~
MECHANICAL PROPERTIES OF THE STEEL FROM THE TOP HALF OF THE CRACKED SWIVEL RING .
Ultimate .
Charpy V-Notch Tensile Yield Strength Elongation in Reduction Notched-Bar Plane-Strain Energy Absorption Specimen ~ Strength, (0.2% Offset), 2 Inches, in Area, Tensile Strength, b) Fracture Toughness,g) at Room Temperature,
'Oriintation kai, kai percent percent ksi (K7 ) , kai E. ft-lb Radini 304.1 291.4 6 23.9 273.4 53.7 9
- Tang ntial .-
56.6 8 (c) Th3'. tensile properties of the steel were determined with standard specimens with a'O.500-inch diameter. reduced section; tha values reported are the average of two determinations.
- (b) Strsss-concentration factor, Kt = %5.
(c) Bend specimen loaded in three-point bending according to the procedures set forth in ASTM A-399-72.
as ~umaa.