IR 05000271/1997008
| ML20216D259 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 05/08/1998 |
| From: | Meyer G NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | Reid D VERMONT YANKEE NUCLEAR POWER CORP. |
| References | |
| 50-271-97-08, 50-271-97-8, NUDOCS 9805200328 | |
| Download: ML20216D259 (3) | |
Text
May 8, 1998
SUBJECT:
CRACKING OF EDG LUBE OIL PIPING AT VERMONT YANKEE
Dear Mr. Reid:
NRC inspection Report 50-271/97-08 dated November 28,1997, discussed the skid-mounted piping on the emergency diesel generators (EDGs) at Vermont Yankee and your l
replacernent of some piping that had cracked at some piping welds. The replaced piping had previously been weld repaired in 1995. Based on our concern regarding the piping welds NRC further evaluated the acceptability of the EDG piping, as documented in the attached Task Interface Agreement (TIA), dated February 27,1998. Subsequently, this document was placed in the Public Document Room on April 9,1998. The evaluation concluded that "there are no immediate safety concerns regarding the near-term operability of the EDGs," but issues remained regarding the long-term operability.
We understand from conversations with Mr. Gregory Maret, Plant Manager at Vermont
!
Yankee, that inspections of the piping during routine surveillances have been augmented to specifically inspect the piping for any leaks. We request that you confirm the augmented j
inspection in writing and provide the actions you plan to address the long term operability
'
concerns on the piping. We request that your written response be provided within 60 days.
Sincerely,
,
Glenn W. Meyer, Chief (
g j-Civil, Mechanical, and Maten,als Engineering Branch Division of Reactor Safety Docket.No. 50-271 Enclosure: TIA, dated February 27,1998
\\\\\\
i'01 4 4 9805200328 980508
/
PDR ADOCK 05000271 G
.
!
-%
.
- s
.
!
'
M. Donald co w/ encl:
R. McCullough, Operating Experience Coordinator - Vermont Yankee G Sen, Licensing Manager, Vermont Yankee Nuclear Power Corporation D L Rapaport, Director, Vermont Public Interest Research Group, Inc.
D. Tefft, Administrator, Bureau of Radiological Health, State of New Hampshire Chief, Safety Unit, Office of the Attorney General, Commonwealth of Massachusetts D. Lewis, Esquire G. Bisbee, Esquire J. Block, Esquire T. Rapone, Massachusetts Executive Office of Public Safety D. Katz, Citizens Awareness Network (CAN)
M. Daley, New England Coalition on Nuclear Pollution, Inc. (NECNP)
State of New Hampshire, SLO Designee State of Vermont, SLO Designee Commonwealth of Massachusetts, SLO Designee j
l i
i l'
'
..
r
..
'
.
.
.
Mr. Donald Distribution w/ encl:
Region I Docket Room (with concurrences)
PUBLIC Nuclear Safety information Center (NSIC)
NRC Resident inspector
,
D. Screnci, PAO l
C. Cowgill, DRP R. Summers, DRP C. O'Daniell, DRP J. Wiggins, DRS L. Nicholson, DRS G. Meyer, DRS S. Chaudhary, DRS G. Morris, DRS DRS File B. McCabe, OEDO C. Thomas, NRR (COT) '
R. Croteau, NRR R. Correia, NRR F. Talbot, NRR inspection Program Branch, NRR (IPAS)
DOCDESK l
i
i
_ - _ _,._.
. _ _
. _ _. _.
DOCUMENT NAME: G:\\CMMEB\\MEYER\\WELDLTR.VY To receive a copy of this document. inchcate in the box: "C" = Copy without attachment / enclosure T = Copy with atwchment/ enclosure
"N" = No W
I-)FFICE Rl/DRS Rl/DRS ggt l R1/J) @ rg v l
. al l
.
[NAME SChaudhary sw GMeyerM.,,/ Como 4.d/l
,
lDATE 04/22/98 04/zz/98 g e4)
/98 04/ /98 04/ /98
'"
OFFI LIAL SEP RD CO'Y
..
i
,
. APP-15-1998 09:07 US tRC/DIV RX PRC1J 3014152102 P.03 y -.mk UNITED STATES
-
- ' y
,
s j
NUCLEAR REGULATORY COMMISSION
' *.,
F WASHINGTON, D.C. SM8HOM
-
%, nmf
!
February' 27, 1998 MEMORANDUM TO: Cecil O. Thomas, Jr., Director Project Directorate 1-3 Division of Resctor Projects 1/11 FROM:
O Richard H. Wessman, Chief
[VMechanical Engineering Branch Division of Engineering SUBJECT:
CRACKING OF EDG LUBE OIL PIPING AT VERMONT YANKEE (TAC NO MA0106)
By letter dated November 10,1997, Region I requested NRR's assistance in reviewing the structural integrity of the skid-mounted piping associated with the emergency diesel generators (EDGs) at Vermont Yankee (VY). VY recently completed an operability determination for the EDG in order to address a degraded condition in the skid mounted support piping associated with the EDGs. The welds in the skid mounted piping were found to be less than full-penetration welds as assumed in the original design basis. The EDGs are required to remain operable during a design basis earthquake.
Several plants, including Crystal River, Millstone, and Vermont Yankee, have experienced problems with degradation of these welds during normal operation. Subsequent evaluation showed significant lack of penetration and general lack of quality in the welds. In the VY case, the licensee has replaced a section of lube oil piping, added some additional supports, and performed metallurgical examination and destructive load testing of sample weldt removed from the system. Based on the examination and testing of the piping, the licensee completed an operability determination which concluded that the EDGs remain operable for both normal operational loads and seist.ile loads that would be experienced in a design basis earthquake.
The Mechanical Engineering Branoh and the Materials and Chemical Engineering Branch have completed a review of the licensee's submittals. Based on its review, the staff finds that there are no immediate safety concoms regarding the near-terra operability of the EDGs. However, the staff has identified a number of concerns in the structural analyses and metallurgical examinations pedormed by the licensee. Pending resolution of these concems, and further actions by the licensee to demonstrate long term operability, the staff is recommending augmented inspection of the EDGs durin;, testing. Our evaluation is attached.
Attachment: As stated CONTACT: J. Rajan,NRRIEMEB 415-2788 Attachment 8 8 0 3 m o m --
.
'
APR-15-1998 09:09 US PRC/DIU RX PROJ 3014152102 P.04
)
J
,
j
.,
,
i
.,
EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGutATION
'
PERTAINING TO THE CRACKING OF EDG LUBE OlL PIPING AT VERMONT YANKFF
INTRODUCTION Vermont Yankee (W) recently completed an operability determination for the emergency diesel generators (EDGs) in order to address a degraded condition in the associated skid mounted support piping. The welds in the skid mounted piping were found to be less than full-penetration welds as assumed in the original design basis. The EDGs are required to remain operable during a design basis earthquake.
Several plants, including Crystal River, Millstone, and Vennont Yankee, have experienced problems with degradation of these welds during normal operation. Subsequent evaluation showed significant lack of penetration and ganeral lack of quality in the welds. The Owners'
Group is pursuing this matter and the three licensees have taken differing corrective actions. In i
W's case, the licenses has replaced a section of the lobe oil piping, added some additional supports, and performed meta'lurgical examination and destructive load testing of sample welds removed from the system. Based on the examinatior; and testing of the piping, the licenses completed an operability determination which concluded that the EDGs, sithough degraded, remain operable for both normal operational loads and seismic loads that would be experienced in a design basis earthquake. In a TIA dated November 10,1997. Region I requested NRR's assistance to review and evaluate the adequacy of the licensee's operability determination.
BACKGROUND At the present time, the identified deficiency involves a degraded condition related to welds on vendor-supplied skid-mounted carbon steel piping associated with EDGs, DG 1 1A and DG 1-1B at W. Of specific concem are the welded joints in the lobe oil and jacket cooling lines which
potentially have less than full penetration weld configurations. This condition is contrary to the vendor's fabrication drawings which indicate full penetration welds. The Owners' Group disseminated information regarding a lube oil piping welded joint failure that occurred on one of the EDGs at Mil lstene Unit 2. The failure mode, at Mitistone, was determined to be due to vibration-induced fatigue during diesel operation. Although the failure was not catastrophic, it resulted in a through wall crack in a weld joint resulting in a lube oil leak.
The main piping systems on the EDG skid consist of lube oil, water cooling and after cooling.
The lube oil discharge piping between the lube oil pump discharge and the oil filter is of special concem since this portion of the piping experiences the most vibration. Oilleaks occurred in a cracked weld in this piping section at Millstone and W. It consists of a 4.5 inch diameter,0.25-inch wall thickness carbon steel (ASTM A513) material. It has two elbows and a 5-inch diameter coupling which fits over the 4.5-inch diameter piping via rubber ferrule-type gaskets on both ends to allow thermal growth and vibration isolation. This sect:on of piping and connecting welds have been replaced both at Millstone and W and all welds in this section are !n compliance with ASME Code requirements.
.
.
i Attachment
-
l
-_
\\
APR-15-1998 09:09 US NRC/DIV RX PROJ 3014152102 P.05
.
'
'
-
.
'
.
2,
.
.
DJSCUSSION The licensee perfonned a root cause investigation with assistance from the vendar, Fairbanks Morse (FM) in an Pfort to more clearly understand the original seismic qualification design basis for the skid mounted piping and the condition of welds on this piping (References 1 and 2). The original Ebasco EDG procurement specifications, WNP-VI-IV-G-1, Revision 2 dated March 27, 1969, required the vendor to del'ver a seismically qualified package which met W specific requirements. The specnications required that the equipment be capable of performing its intended safety function during a seismic event. The seismic loads, discussed in the specifications, were static acmieration values which were consistent with the technology employed M the 1960s.
.
A walkdown and visual examination of the potentisily affected skid mounted piping was performed at W on September 12,1g97 by the licensee. Based on the visual inspection of the welds, it appears that the weids were most likely performed by employing a single pass seal
bead with two additional passes over the single pass resulting in a double crown,
,
The licensee notes that reference to a specific piping code (i.e., ANSI B31.1) for the design of the skid mounted piping did not exist within specification WNP-VI IV-G-1, Revision 2. This was intentionally deleted in the specifications because FM took exception to the B31.1 code and utilized their intomal procedures / processes for the fabrication of the piping. FM stated that these intemal procedures / processes were equivalent to commercial standards and good industry practice and were deemed acceptable based on many years of successfulinservice operation of
. their equipment. FM further stated that the seismic qualification methods utilized for the engines supplied to W (and also in general during the late ig60s and earfy ig70s) were to analyze the engine block itself and all other major skid-mounted components, such as heat exchangers, strainers, filters, etc. concentrating on anchorage requirements. The interconnected piping was not specifically analyzed. As years progressed, the development of more specific seismic qualification standarde (IEEE 344 71 and -75) became available and a selected generic analysis of bounding piping geometry was performed for subsequent nuclear industry cuctomers. As prevjously noted, this was not the case for plants of W's vintagei therefore, no quantitative analysis exists. However, FM stated that the analysis would apply to W and plants of Ws vintage as the same design concepts were employed utilizing the same intamal procedures / processes and equivaient selsmic loading. The specifications also required a hydrostatic test of the piping systs,ms to 1.5 times the design pressure.
The diesels are within the scope of W's USl A46 program. As part of the A46 program resolution, a walkdown of the diesels was conducted in response to Generic Letter 87-02,
" Seismic Qualmcation of Equipment in Operating Plants," utilizing guidance contained in the Seismic Qualification Utility Group's (SQUG) Generic implementation Procedure (GIP). It is noted that the procedural guidance contained in the GIP was developed from compilation of experience data from the performance of diesel engines similar to W's which have actually experienced earthquakes of magnitude in excess of W's seismic design basis. The walkdowns, qualitative in nature and performed by trained seismic engineers, included consiceration of the skid mounted piping and focused on the existence of potential seismic-induced vulnerabilities including interaction effects related to proximity with other equipment, structural failure of non-
-
APR-15-1998 09:09 US HPC/DIV RX PROJ 3014152102 P.25
-
\\
.
.
.
)
'.
seismic components (commonly referred to as seismic il/I), and overall flexibility of lines. The licensee acknowledges that these walkdowns could not have foreseen or discovered the issue
-
of weld penetration but they do provide supporting evidence that known seismic vulnerabilities related to the performance of the piping under seismic loading are not present.
The licensee's analysis Indicated that the section of piping between the lube oil pump discharge and the oil filter was the most highly stressed and most susceptible to the fatigue failure due to vibration. This section of piping was chosen for stress analysis and was considered as a i
bounding sample of all skid-mounted piping configurations. In addition, the licensee discovered J
that this section of piping at W experienced a leak on weld no. 2 in igg 5. According to plant records, the as-found condition was a slag inclusion of approximately 3/16 inches in the face of weld no. 2 with several arc striites. The corrective action for this leak at that time was to remove the am strikes, grind the weld and remove visible defects, and roweld using approved procedures and conduct a post weld liquid penetrant examination on this and other welds on this section of the piping. Although this section of piping was replaced in late 1997, concerns regarding the balance of the skid-mounted piping are the focus of this evaluation. Some of the staffs concems were partially resolved, others remain to be addressed. These are discussed later in the evaluation.
According to FM personnel, as far as they are awara of, the cracked wold discovered at Millstone was the first reported failure in the skid-mounted piping of a FM engine. Similar engines have successfully operated for many thousands of hours (some applications continuously) at many commercial, military, and maritime organir.ations. Many of these engines are routinely subjected to severe environmental conditions including temperature, vibration, and shock. Given this successful operating history. FM maintains it is reasonable to expect W's equipment will continue to support their safety function in the staffs view, qualitative Information from commercial and other facilities are not directly applicable for supporting long-term operability at W. However, the generally low magnitudes of seismic and vibratory load j
demands on the skid-mr>urited piping and the successful operating history cited above does
'
provide adequate confidence for continued operation in the near term.
STRUCTURAL EVALUATION
,
The licensee considers that the W EDGs, although degraded, are still operable. A major aspect I
of the baris for continued operation was an analysis performed by the licenses to demonstrate the structural integrity of the skid-mounted piping under normal operation and during a seismic event. As stated earlier, the section of the piping between the lube oil pump discharge and the filter was determined to be the most highly stressed and hence it was the primary focus of this analysis. Other vulnerable sections of the skid-mounted piping were also analyzed.
The ADLPIPE computer code was used by the licensee to evaluate the lube oil pump discharge
'
piping. The loading conditions considered were intemal pressure, deadweight, thermal, vibration
,
and seismic inertia. The piping system stresses were compared to the allowable values of the B31.1 Code and the ASME Standard OM 3 (References 3 and 4). Thiust loading from pressure was calculated manually and combined with the appropriate ADLPIPE wiress results and ASVE Code equations. In addition, vibration data monitored during actual engine operation was used as input into the piping model to address vibration effects.
-
l APR-15-1998 09210 US WC/DIU RX PROJ 3014152102 P. 0'?
,
.
e i
.
Two ADLPIPE models were constructed in order to address and bound specific issues
)
pertaining to the welded joints in the tubing sections. One model served to previde the baseline data. It consisted of as built piping gsometry, and used the nomhil wall thickness provided by the vendor. The second model used a well thickness of 0.1 inchea for the section of tubing from the lube oil pump connection up to the 5-inch diameter coupling. From the S mch diameter
,
coupling to the filter, a wall thickness of 0.029 inches was used. These wall thicknesses were l
based on preliminary measurements of two partial penetration weld depths, which the licensee i
considers to be bounding values.
The licensee performed dynamic response spectrum analysis for seismic looding. The amplified
.
response spectra for the floor above tbs EDGs la the turbine building, were utilized using the ASME Code Case N4,11 damping. The spectra are considered conservative by the licensee j
since the diesel and ths associated skid piping (i.e., lube oil dischsige tubing) are on grade at the bau mat elevation, which will only experience ground accelerations instead of the amplified
'
response of the building structure. Dynamic response spectrum analysis was also performed for the assessment of stresses due to vibration. Response spectra in the form of spectral accelstations were obtained from actual vibration data monitored at several locations on the Iube oil discharge tubing sections. The worst case response was applied to the ADLPIPE model.
The resulting calculated stress values due to vibration were assessed using the guidelines of ASME Standard OM 3.
The remainder of the skid-mounted EDG piping was reviewed by the licenses for potential structura! weak links in the system. The review was accomplished by visual field observation and manual calculations where required. The potential weak links were Indicated either by a lack
'
of physical structural support, or at weld joints that could be affected by the behavior of mechanical couplings..As stated earlio!, the piping wall thickness was uniformly decreased in the ADLPIPE model to account for the depth of partial penetration weld joints in the system. The 4.5 inch x 0.25 inch piping wall was decreased to 0,1 inches (including the elbow wall thickness)
based on preliminary metallurgical measurernents of the welded joints. No credit was taken for the additional strength contributed tr/ the weld crowns, j
A vibration-induced failure mode characterized by fatigue is considered the most likely potential failure mode by the licenses. Such a faiwre would occur over a long period of time due to propagation of an existing flaw during engine operation re,sulting in a through wall crack with subsequent leakage. This, in the licensee's view, is not characterized as an abrupt or catastrophic failure. According to the licensee, given the successful cperational history of W's diesels, as well as those at other FM supplied units, it is unlikely that imminent initure resulting in complete severance of the piping will occur. In addition, the licensee argues that should fa!!ure occur, it would likely be characterized as a leak which would be detectable through alarm functions or operator surveillance. With the backup dissal as well as the available VY tie li-), a safe plant shutdown could be aw.omplished if required. The staff concurs with the licensee if it can be established that the weld geometry assumed in the analytical model is indeed bounding.
However, in the staffs view, this has not been fully established yet.
The licensee has also performed load tests on the removed section of the piping from the W EDG which was replaced. Weld numbers 4,6 and 8 on tube oil pump discharge piping were selected for testing. Both axial and bending loads were applied. The results of the load tests
'
nPR-15-1998 09:11 US NRC/DIV RX PPM 3014152102 P.08
.
.
..
'.
g
.
,
..
demonstrated that the as-installed welds will withstand loads greater than those which cause stress in the piping to exceed the yield strength of the piping material. Also, the tests demonstrate that these weld geometries have significant strength with respect to the design loads and the B31.1 Power Piping Code allowable stresses. However, these results are applicable to the three repocimens tested and there is no assurance that these are bounding geometries. h is the staff's understanding that additional tests are being performed by the Owners' Group on piping replaced in FM diesels at other nuclear plants.
)
Based on a review of the licensee's analyses, the staff noted that the piping stresses are
-
generally low for the applied loads and assumed weld geometries with a significant margin to B31.1 Code allowables. The notable exceptions are the elbow locations where thrust loadings
~
produce incicased bending stresses. At these locations, while the stresses are high, they are within ellowable limits. The maximum stresses occur at the weld joint No. 2 on the elboiv located just outboard of the tube oil pump discharge.
!
The staff notad that all loadings satisfy Code requirements at the maximum operatinD Praasure of 36 psi with the reduced wall of 0.1 inches. At design precsure, the reduced tubing wall of 0.1 inches does not meet the allowable stress values of the ASME Code Section III, subsection NB Equation 11 (stress due to sustained loads). However, when seismic loading is considered, the tubing stresses do satisfy the higher allowable stress value of Code Equation 12 (stress due to occasional loads). Since both Equations 11 and 12 of the Code need to be satisfied, the staff identified this as a concem.
In response to staff concem regarding the noncompilance with the ASME Code requirements,
,
addit onst calculations for this load case were performed by using as-found data at the critical i
location. Weld joint No.2, which is considered a criticellocation, was shown to correlate to a j
mean value of 0.15 inches for the "B" diese! lube oil piping. This is considered less conservative i
but more representative of actual conditions by the licensee. The stress results for this load case j
were shown to meet ASME Code Section til Equation 11 allowables considering a maximum l
upper bound pressure equivalent to the relief valve setting at 80 psi. The staff finds the assumed value of 0.15 inches in this calculation questionable, particularly in light of concoms l
raised in the metallurgical evaluation. Hence, non-conformance with the ASME Code j
requirements remains a staff concem.
METALLURGl CAL EVALUATION i
The replaced piping is made of carbon steel (ASTM A513). The licensee performed a metallurgical examination of six weld joints (#2, #3, M, #6, #7 and #8) located in the piping section removed from the emergency diesel generator "B' (EDG-1 B). The licensee's stress analysis has shown that the replaced piping section is most susceptible to cracking because the
engine operating stress in this piping section is higher than in any other piping section. The l
i highest stresses are located at weld joint #2. Florescent Liquid Penetrant examination was performed on the OD surface of the subject piping section. With the exception of an OD surface l
'
crack about S/8 inch in length that was found at raid joint #2, no other OD surface indication was detected. Each of the six weld joints was quarter sectioned for a total of 48 cross sections.
)
Each cross section was polished and etched for metallurgical examination and the measurements of weld penetration. All six weld joints were shown to be partial penetration
RPR-15-1998 09:12 In NRC/DIV RX PROJ 3014152102 P 09
.
~
.,
'
-
.
welds, each weld geometry was characterized by poorjoint fit up and without any weld end j
preparation. The reported minimum weld penetration in six weld joints varied from 17.4% to 58% of the pipe wall thickness. During the metallurgical examination, a through wall crack was identified in weld joint #2, however, its length on the ID surface was not measured. The through wall crack did not leak during operation. Small e acks initiated from the we!d root ID surface with a depth of 0.057 inches and 0.00g Inches were found in weld joints #3 and #8, respectively. The licensee performed a failure analysis for weld joints #2, #3 and #8 by examining the fracturn i
'
surfaces with Scanning Electron Microscopy (SEM) and other techniques. Heavy oxides were observed on the surface of the short cracks at weld joints #2 and #3 and the irdtial portion of the
-
through well crack at weld joint #2. The licensee reported those fatigue striations associated with the cracks were found in weld joints #2 and #3. Each observed crack was reported to be connected to a slag inclusion at the weld root ID surface. The licensee stated that a weld repair was performed at weld joint #2 during a 1996 outage because leakage was found at this weld joint.
The licensee suggested that the observed small cracks at weld joints #3 and #6 and the initial portions (about 0.06 inches) of the through wall crack at weld joint #2 are fabrication defects, initiated by shrinkage stresses resulting from cooldown of the fabricated welds. From the mrtallurgical examinations it appears that the small cracks are not active since it is filled with heavy oxides. The licensee stated that the measured vibrational pip 5g stresses are small (below the threshold stress) and that the vibrational stresses alone will not be able to cause cyclic crack growth. Therefore, the licensee attributed the major driving force for the crack propagation at weld joint #2 to be the residual stresses resulting from the 1996 weld repair. 'Ihe staff aDrees with the licensee's assessment that the residual stresses play an important role in
promoting the crack growth.
The licensee performed an evaluation using Lineer Elastic Fracture Mechanice (LEFM) similar to the method described in GL 90-05 to determine the limiting flaw size at which the structural integrity of the cracked piping weld can be maintained. 'PC CRACK," a computer code developed by Structural Integrity Associates (SIA) was used for the evaluation. The limiting flaw size was determined using two separate LEFM crack models. One model assumes a through-wall circumferential crack in a cylinder under tension and bending and the other model assumes part through-wall circumferential Inner diameter (10) surface crack in a cylinder under tension.
The results of the evaluation have shown that the limiting flaw sizes in both models are significantly larger than the 0.625 inch circumferential crack found in wold joint #2. The licensee's evvustion is reasonable in that it confirms the structural integrity of the degraded weld
'
joint #2.
EVALUATION FINDlMGS Based on its review of the licensea's analysis, the staff has identified the following findings related to both structural and metallurgical evaluations:
.
w.
fGR-15-1999 09:12 US HRC/DIV RX PROJ 3014152102 P.10
,
)
-
.
.
-
.
'.
'
,
(1) Minimum Wald Penetration The metallurgical examination was performed on six weld joints in the removed section. The reported minimum weld penetration in those six welds varied from 17.4% to 58% of the pipe wall thickness. Due to the lack of proper weld end preparation and joint fit up, there is no assurance that the minimum weld penetration in the remaining tube oil piping will fall within that range.
(2) Weld Shrinkage Crack Shrinkage cracks were observed in 50% of the weld joints examined (three out of six). The depth of the observed shrinkage cracks varied from 0.00g inches to 0.060 inches. The initiation of the shrinkage crack is known to depend on the welding heat input, weld geometry, joint fit up and the' weld cooling rate. Since these conditions tend to vary from
weld to weld, it is difficutt to make a reasonable estimation regarding the limiting shrinkage i
crack size that will be initiated in the remaining skid mounted piping.
{
.
(3) Wald Residual $ tresses For the through well crack found in weld joint #2, the licensee attributed the main driving force for the crack growth to the presence of the residual stresses resulting from the weld repair performed in 1996. The licensee considered this weld to be an atypical weld because this is the only weld having a record of weld repair. The licenses did not discuss the root cause for the observed leakage at weld joint #2 because failure analysis was not performed on this weld prior to repair. Based on the fact that the weld joint #2 was not repaired when the leakage was observed in 1g96, it is apparent that the driving forces for the through-wall
.
cracking are the as-fabricated weld residual stresses and the stresses genereted under normal operating conditions. Therefore, the possibility exists that similar cracking may occur at other weld joints on the skid-mounted piping.
.
(4) Crack Growth
<
Fatigue striations were iound on the fracture surface at weld joints #2 and #3. This observation tends to support the contention that the cyclic crack growth is the major mode of c:ack growth in the lube oil piping system. Mthough the vibrational stresses have shown to be small on the affected piping. the cyclic loading resulting from the on-off transient when
,
'
performing the testing of the diesel generators may provide an appreciable driving force for fatigue crack growth, in any event, the leakage found at weld joint #2 clearfy demonstrates that the initial shrinkage crack can become active and grow through-wall with the existing es-fabricated weld residual stresses under normal operating conditions. The observed crack
g owth rate at weld joint #2 does not appear to be very high since the through-wall portion of the crack is only about 5/8 inch in length after a period of plant operation close to one fuel cycle. However, it is still necessary to evaluate the potential crack growth in the remaining skid-mounted piping to ensure its long term structural integrity.
raPR-15-1998 09:13 US ISC/DIU RX PROJ 3014152102 P.11
,
..
.
'
'.
s
,
(5) Piping structuralintegrity
)
.
In the piping structural analysis discussed earlier, the assumed value of 0.16 inches for the pipe wall thickness appears to be highly uncertain in view of the concems related to the minimum weld penetration, weld residual stresses and crack growth. The results of the i
structural analysis do not meet the requirements of the ABME Code section til at the design pressure when a reduced wall thickness of 0.1 inches was used as a bounding wall thickness.
(6) Fatigue evaluation The fatigue evaluation is based on comparison of stresses due to steady state vibration and seismic inertia with the requirements of ASME Standard OM 3. The analysis does not take into consideration the cumulative effect of the vibrational loading resulting from transients during the monthly testing of the DGs. In the absence of quantitative data on the cumulative I
fatigue damage the degraded welds may have sustained to date, they remain vulnerable to failure during a seismic event.
-
(7) Hydrostatic test of the piping system The licensee's reliance on pressure testing of the replaced section of the piping as well as the initial qualification hydrostatic test of the piping system to 1.5 times the design pressure does not provide assurance against fatigue failure due to cyclic loadings experienced by the affected piping.
CONCLUSION Based on its review of the licensee's operability evaluation, the staff concludes that there is reasonable assurance regarding short term operability of the EDGs contingent upon compensatory measures that the licensee should establish The NRR staff recommends that the skid mounted piping should be inspected with visual examinations during and after each EDG testing. This conclusion regarding the adequacy of the short term operability is based on the following factors:
The replacement of the most highly-stressed portion of skid-mounted piping by the licensee
such that all weld joints are in compliance with the ASME Code requirements; The demonstration by the licensee via structural analysis, metallurgical examinations, data
.
monitoring and load testing that the stresses and vibrat'en levels in the remainder of the skid-mounted piping are generally low; The likelihood of a tailure modo characterized by a leak and the availability of a backup
.
diesel and W tie line,
,
Augmented inspection during EDG testing, and
.
.
AFR-15-1998 09:14 LE NRC/DIU RX PROJ 3014152102 P.12
_,
'
..,
.
,
.
g o
.
The observed crack growth at the most highly-stressed weld joint which indicated that
-
accolorated crack growth is not likely to occur in the near term.
In the evaluation findings, the staff has identified a number of concems in the structural arx!
metallurgical analyses performed by the hcensee. A credible crack growth evaluation cannot be performed without a quantitative knowledge of weld penetration and the shrinkage crack size.
Therefore, the long term structuralintegrity of the remaining skid mounted piping cannot be assured based on the information currently available. The NRR staff recommends that Region I review the conclusions of this evaluation and the identifkui concems regarding the long-term operability of the EDGs with the licensee to ascertain the licensee's intentions regarding its corrective action.
REFERENCES 1. Letter, S. Goodwin, Vermont Yankee, to K. Jabbour, NRC, dated November 4,1997, supporting operability of W EDGs.
2. Letter, Cottec Industries Fairbanks Morse to USNRC, dated December 18,1997, related to weldments on the FM EDGs.
3. ANSI B31,1-1977 Edition * Power Piping.'
4. ASME O&M 8tandard OM-3-1982, " Requirements for Preoperational and initial Start up Vibration Testing of Nuclear Power Plant Piping Systems."
PrincipalContributors:
J. Rajan W. Koo l
.
.
.
TOTnL P.1P