ML20215E913
| ML20215E913 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 12/11/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20215E891 | List: |
| References | |
| NUDOCS 8612230165 | |
| Download: ML20215E913 (9) | |
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. o UNITED STATES 8^ o NUCLEAR REGULATORY COMMISSION e WASHINGTON, D. C. 20666 3
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR PEGULATION RELATING TO THE EVALUATION OF THE FLAW INDICATIONS DETECTED IN THE REACTOR COOLANT PUMP CASING ARKANSAS POWER AND LIGHT COMPANY ARKANSAS NUCLEAR ONE, UNIT NO. 1 DOCKET NO. 50-313
1.0 BACKGROUND
During the current 1986 refueling outage, the licensee, Arkansas Power and Light Company (AP&L), conducted an inservice inspection (ISI) of the Arkansas Nuclear One, Unit 1 (ANO-1), "A" reactor coolant pump (RCP) casing weld using conventional radiography (RT). This examination was performed in conjunction with the complete removal of the RCP rotating assembly for other reasons. The RT indicated the presence of a flaw which exceeded the standards of ASPE Section XI IWB-3000 for allowable flaw size. The licensee describes the indication as a series of slag inclusions having an effective length (per ASME Section XI criteria) of 5.66 inches. It is located in the vertical weld which connects the upper and lower scroll welds of the pump casing. Radiographic pardllax techniques indicate that the top of the indication is at least 1.5 inches below the outside surface of the weld. The weld is approximately 2.6 inches thick in this area. The licensee performed supplemental ultrasonic testing (UT) in the region of the flaw indication.
The licensee reviewed the original radiographs taken at the time of initial fabrication to determine whether the flaw existed prior to service. The licensee used two Level III Inspectors who found five small inclusions that are part of the current flaw indication. However, these five inclusions were considered acceptable per the ASME Code during the preservice exami-nations. Due to the quality of preservice radiographs, equipnent was brought onsite to perform computer enhancement of the radiographs.
The original construction radiographs for the remaining three pumps were also reviewed (by a Level III Inspector), searching for any preservice flaw indications or weak areas in film density. These areas were computer-enhanced (by another Level III Inspector) in an attempt to identify any unacceptable flaws that were not identified previously.
Portions of approximately 20% of all preservice radiographs were computer-enhanced. From this review "C" and "0" pump casings were determined to have no unacceptable preservice flaw indications. However, the computer enhancement of "B" pump revealed an unacceptable flaw indication in the same general weld area as the "AY pump.
8612230165 861211 PDR ADOCK 05000313 O PDR
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Using the computer enhancement process, the flaw indication on "B" pump was determined as 1.5" in length. The original construction radiograph of this area shows a flaw of 0.625" in length which was acceptable per ASME Code requirements at that time. The wall thickness in the area of the flaw indication is 3.1". Ultrasonic techniques (UT) were used in an attempt to better characterize the flaw. Due to the material of the pump casing (coarse grained statically cast stainless) and the small size of i the indication, UT was not able to characterize the flaw with sufficient specificity. From these examinations, the flaw size was conservatively determined to be no larger than 1.5" long by 1.5" deep from the inside surface. However, for the fracture mechanics evaluation, the licensee has assumed a size of 4.65" by 2.325".
The licensee performed a fracture mechanics evaluation of the pump casing weld flaws in accordance with ASME Code Section XI and stress analysis of the pump casing with flaws in accordance with ASME Code Section III. The analyses were performed with bounding flaw sizes in both pumps.
2.0 TECHNICAL REVIEW CONSIDERATIONS The Technical Specifications (TSs) for the ANO-1 require inpervice examination of ASME Code Class 1, 2, and 3 components shall be performed in accordance with Section XI of the ASME Code as required by 10 CFR 50.55a(g)(4) except where specific written relief has been granted by the Commission. Some plants were designed in conformance to early editions of this ASME Code Section, consequently certain requirements of later editions and addenda of Section XI are impractical to perform because of the plants' design, component geometry, and material of construction.
Paragraph 10 CFR 50.55a(g)(6)(i) authorizes the Commission to grant relief from those requirements upon making the necessary findings.
Based on detection of the flaw indication in the "A" RCP casing weld during inservice inspection, the applicable edition of Section XI Code for ANO-1 provides the necessary procedures for disposition of the detected indica-tion and specifies additional examination of other RCPs during this outage.
In a letter dated November 24, 1986, the licensee identified specific ASME Code requirements regarding additional and augmented inspections that the .
licensee determined to be impractical to perform at ANO-1 and requested l relief from these requirements.
3.0 RELIEF REQUESTED The licensee requested relief from specific ISI requirements and provided supporting technical information. The NRC staff reviewed this information as related to the design, geometry and materials of construction of the components. The licensee's ISI Program is based on the requirements of ASME,Section XI, 1980 Edition including Addenda through Winter 1981, and remains in effect until December 19, 1994.
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3.1 Code Requirements 3.1.1 Paragraph IWB-2430 Additional Examinations states:
"a. Examinations performed during any inspection that reveal indications exceeding the allowable indica-tions standards of IWB-3000 shall be extended to include an additional number of components (or areas) examined initially during the inspection."
"b. If the additional examinations reveal further indica-tions exceeding the allowable standards, the remaining number of similar components (or are.as) within the same examination category shall be examined to the extent specified in Table IWB-2500-1."
3.1.2 Subparagraph IWB-3122.4 Acceptance by Evaluation states:
"a. Where the acceptance criteria of IWB-3600 are satisfied, the area containing the indication shall be subsequently reexamined in accordance with IWB-2420(b) and (c)." ,
3.1. 3 Paragraph IWB-2420 Successive Inspection states:
"b. If flaw indications are evaluated in accordance with IWB-3122.4 and the component qualifies as acceptable for continued service, the areas containing such flaw indication shall be reexamined during the next three inspection periods listed in the schedules of the inspection programs of IWB-2410."
"c. If the reexamination required by (b) above reveal that the flaw indications remain essentially unchanged for three successive inspection periods, the component examination schedule may revert to the original schedule of successive inspections."
3.2 Code Relief Requested
- 3. 2.1. lhe licensee requests relief from the provisions of paragraph IWB-2430. The licensee considers impractical the disassembly of RCPs "B", "C" or "0" to perform additional inservice radiographic examinations.
3.2.2. The licensee requests relief from the provisions of paragraph IWB-2420. The licensee considers impractical the disassembly of RCP "A" for the sole purpose of performing successive inservice radiographic examinations.
. 8 3.3' Licensee's Basis For Relief The licensee has based his request on nondestructive examination (NDE) results, a fracture mechanics evaluation and a stress analysis.
In addition, the licensee proposes alternatives to the specific NDE requirements proposed by the Code.
3.3.1 Nondestructive Examination Results On "A" RCP the licensee performed a volumetric examination-of the casing weld in accordance with ASME Code Section XI using radiography. The RT. indicated a flaw that exceeded the acceptance standards of ASME Code Sec. tion XI IWB-3000.
Computer enhancement of the construction radiographs deter-mined that a similar flaw indication was present at'the time of fabrication. UT was utilized to conservatively determine a flaw size for the fracture mechanics evalua-tion for "B" RCP.
3.3.2 Fracture Mechanics Evaluation The fracture mechanics analysis was performeIf by Babcock &
Wilcox (B&W) and submitted by the licensee for review by the NRC staff in the letter dated November 24, 1986. The B&W analytical evaluation of the casings for both RCPs.
"A" and "B" was performed in accordance with the fracture mechanics criteria of ASME Code Section IWB-3600. The applicable documents are B&W 32-1167118-00, "ANO-1 Pump Case-Crack Growth Due to Transients"; B&W 32-116579-00,
" Fracture Mechanics Analysis of ANO-1 'A' Pump Case Indica-tion"; B&W 32-1167147-00, " Fracture Mechanics Analysis of ANO-1 'B' Pump Case - A Surface Flaw"; and B&W 32-1165899-00,-
" Fracture Mechanics Analysis of ANO-1 'B' Pump Case Indica-tions".
3.3.3 Stresses in the Pump Casings (ASME Code,Section III)
B&W performed an analysis of the stresses in the "A" and "B" RCP casings in the region where the flaws are located.
B&W performed these analytical evaluations to determine
- whether the pump casings with flaws still meet ASME Code Section III criteria. The applicable documents are B&W 32-1165802-01, "ANO-1 Pump Case Stresses" and B&W
+ 32-1167147-00, " Fracture Mechanics Analysis of ANO-1 'B' ,
Pump Case-A Surfhce Flaws".
i 4.0 STAFF EVALUATION
{ The B&W analytical evaluation was performed in accordance with the fracture
! mechanics criteria in ASME Code Section XI IWB-3600. The ASME Code fracture I mechanics evaluation depends upon the size of the flaw, the location of the i
,. .. _ _ _ ~ _ _ - ._ _ _ - _ _ _ _ _ . ._ _ m _ . _ __ . . _
N flaw,=the' amount of the flaw growth, the fracture toughness of the material and the stress distribution during the limiting transient.
The transient conditions evaluated by B&W include the design transients and the corresponding number of design cycles as listed in Table 4-8 of the ANO-1 Safety Analysis Report. B&W concludes that growth of the flaw
- in the volute of the "A" RCP casing is due primarily to plant heatup and s !
cooldown cycles. Final flaw size is an accumulation of incremental amounts of growth due to all transients.- Flaw growth rate is predicted by the following equation where da/dn is.in inches / cycle and AK g is in ksi Jin. ,
da/dN = 1.8 x 1010 (A Ky )3.3 ,,
Using the "A"' pump as an example, the calculated crack growth for 240 heatup-and cooldown cycles is 39 mils, for 15 field hydrotests is 0.1
' mils and for all remaining transients is less than 0.1 mils. Thus, of ;
the calculated total crack growth, 99.5% is due to the 240 heatup and cooldown cycles. An analysis of the crack growth in the "B" pump casing
- i. would be expected to yield similar results. The NRC staff agrees that only the plant heatup and cooldown transients contribute significantly to '
- flaw growth.
l The flaws in both volutes of "A" and "B"'RCPs are in the vertical weld
, seam. .The pump case proper is fabricated from ASTM A351-69, type CF8M stainless steel. Material properties are available for type CF8A base
- metal which has slightly higher tensile strength than type CF8M, but
- has similar Charpy values at room temperature. B&W assumes that fracture
- toughness properties for types CF8M and CF8A weldments are equivalent. The ,
- CF8A' stainless steel weldsent specimens were made from a 2-3/8 inch thick
- compound-butt weld using the filler metal ER308L and E308L. The J yp of this material is 1171 in-lb/inz as documented in Reference 1. The Value of
~
K 7c used in the fracture mechanics analisis is 179 ksi Vin and is derived fr5m the value'of J I The NRC staff believes these are reasonable values
- for J and K iscussed below, there~is more than sufficient margin i
to' acdbuntvariance foJC.inAs b. of J IC*
the value 4
The applicable stresses at the locations of the flaws in the pump volutes are the same for both "A" and "B" RCPs. The maximum tensile stresses are:
Maximum Stresses at the Flaw Location ,
Normal,& Emergency & >
j Location Upset (ksi) Faulted (ksi) i Inside surface 15.9 17.5 l Midsurface 12.6 15.0 i Outside surface 9.4 -0.6 i
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For nornal and upset conditions $ the membrane stress,is 12.6 ksi and the i bendingistress is 3.3 ksi. For amergency'and faultbj conditions, the ,
membrane l steess is 15.0 ksi and th.e bending stress is 2.5 ksi.t ,
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The flaw in the volute of the "h" RCP is characterized as ic single, D 1 semi-elliptical inside surfacejflaw, 1.1 inches deep'and 5.66 1 inches long yielding an aspect' ratio of .0.19. At the location of the flaw, the pump casing is 2.6 inches thick which yields a flaw depth to ,4 -
thickness ratio of 0.42. Using ASME Code Section XI Appendix A, the !
stress intensity fector for normal and upset canditions is 32.4 ksi/in. 4 These values of the' stress Antensity. tactor are based on a final crack '
t depth of 1.139 inchts. The acceptance criteria in IWB-3612 require for' \ $ .
normal conditions that1X '< K '
dtions^that K K /J)g Bed se $u//10 the material and for emergency is very ductile, 33% ar.d faulted con 4 elongation, K g 3 16 i 5.5 Emergencyandfaultd8,Kk /K > 1 4I4 ; 4*8 Ic I Thus,thefracturemechanicsevaluationofthbweldflawinthevoluteof ~
"A" RCP demonstrates that the flaw is acceptable because it meets the safetymarginsspecifiedinIWR-3612of[SectionXI'oftheASMECode.
The veld flaw in the volute of the "B" RCP is characterized as a single, semi-elliptical inside surface f1 w2.325 iaches deep and 4.65 inches long ,
yielding an aspect ratio of 0.5. At the location bf the flaw, the pump casing 1,s 3.1 inches thick which yirids a flaw tepti, to thickness ratio of 0.75. Using the ASME Code Section XI Appendix A, the stress intensity factor for normal and upset condi; ions is 26.76 ksidin and for emergency
, and faulted conditions is 31.3 ksilin. These values of stress inten:ity factor are based on a final crack depth of 2.346 inches. The acceptance criteria in IWB-3612 requireconditions for' normal conditions K //10 and for emergency and faulted that K K,, that
//2. Kde<caullthe
- material is very dutile, 33% elongation, g K ,g <is @ proximately equal to Ic at the operating temperature.
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s The ratios are: s Flaw in B" Reactor Coolant Pump Casing Acceptance Condition Criteria Calculated Normal and upset, K y > 3.16. 6.69 Emergencyandfaulte$grKyc(gy, > 1.414' 5.72 a ,
e Thus, the fracture mechanics evaluition of the wel'd flaw in the volute '
5 of "B" RCP demonstrates that the flaw is acceptable because it meets the safety margins specified in IWB-3612 of Section XI of the ASME Code.
The fracture mechanics analysis for both "A" and "B" RCP casings shows margin over that specified in the IWB-3612 acceptance criteria. The smallest margin is 74% above that specified,in the acceptance criteria. .
The margins in the acceptance criteriai are intended to compensate for un- [
certainty in such factors as crack size, crack growth, material properties r and calculated values of stress. Therefore, concerns regarding material 1 properties or residual stresses are adequately compensated for by the margins available. The NRC staff concludes that the subject flaw indications :'
are acceptable based on the fracture mechanics evaluations performed by the licensee.
The flaw in "A" RCP casing is treated by the ASME Code as an inservice discovered flaw despite any evidence to the contrary that the flaw existed preservice. Having found the flaw during this outage,'ASME CodeSectionXIIWB-2430requiresanotherRCPcasingbevolumetrically) examined during the outage. This requires disassembly of the pump.
Based on the re-review of the construction radiographs using computer enhancement, the "B" RCP is the logical choice for volumetric examination.
Relief from this requirement is granted for this outage for the following reasons. First, the NRC staff has reasonable assurance that the experience resulting from "A" RCP will be similar for "B" RCP; that is, no flaws not already indentified will be revealed. Second, the fracture mechani'cs analyses performed have shown small growth of fl us far'the life of the plant; thus, flaw growth during one refueling cycle would be extremely small. Third, disassembly of the "B" RCP at this outage would result in additional man rems of exposure withcut an expected gain in the informa-tion already available. Fourth, at lresent i the NRC staff'is not aware of any reliable, external, volumetric examination techniques viable for a RCP casing that could be used during this outage. However, the inspection is needed to confirm the calculation and conclusions of the fracture mechanics evaluation and to verify that there are no other flaws present.
Instead, the licensee proposes a 100% examination of the weld on "B" RCP at the next refueling outage. The pump at the next outage will not be disassembled for the sole purpose of performing an inservice radiographic examination. If the pump is disassembled for other reasons, then a radio-
r 4
graphic examination will be performed. Otherwise, the best available external surface examination technique will be used. The licensee will t
describe to the NRC staff the technique in detail at least three months prior to its use at'the next refueling outage.
5.0 CONCLUSION
S The licensee's request for relief from ASME Code Section XI IWB-2430, additional examination requirement, is granted. Specifically, the licensee does not have to volumetrically examine "B" RCP casing during this outage.
The bases for this conclusion are because the licensee has reviewed 100%
of computer enhanced "B" RCP casing construction radiographs using two level III inspectors, used UT to determine a bounding size for the flaw in the vertical weld, performed fracture mechanics analysis that indicates the flaw is acceptable according to ASME Code Section XI'IW8-3612, and performed a stress analysis that demonstrates the casing with the flaw still meets the ASME Code Section III stress criteria. In other words, flaw growth during the next operating cycle would be extremely small and, thus, would not compromise the integrity of "B" RCP. At the next refueling outage, the licensee will perform a 100% volumetric external surface examination of the "B" RCP casing weld using the best available technology. If the pump is disassembled for other reasons before the next outage, then a radiographic volumetric examination is to be performed at that time. If the pump is not disassembled, the licensee will use " state-of-the-art" external surface volumetric technology and procedures to perform the required examination.
The staff is to be notified of the " state-of-the-art" external surface volumetric technologies and procedures to be used at least three months prior to the examination. The results of this examination will determine what actions should be taken with regard to "C" and "D" RCP casings.
The licensee's request for relief from ASME Code Section XI IWB-2420 is granted. The licensee wil7 commence an augmented ISI program for the "A" RCP casing during the next 10 years. Specifically, the disassembly of "A" RCP for the sole purpose of performing inservice radiographic examinations is not required. Instead, the licensee will use " state-of-the-art" external surface volumetric technologies and procedures to perform a volumetric examination from the external surface unless the pump is disassembled for other reasons. As stated previously, the NRC staff is to be notified of the volumetric technologies and procedures to be used at least three months prior to_the examination.
The~ flaw in the "A" RCP casing is acceptable according to ASME Code Section XI criteria. The fracture mechanics analysis demonstrates that the flaw meets the IWB-3612 criteria. In addition, analysis of the pump casing with the flaw demonstratbs that the "A" RCP still meets the ASME
- Code Section III stress criteria and, thus, is acceptable for continued operation.
Dated: December ll,1986 Contributors: R. Wright, M. Hum I
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REFERENCES
- 1. M. G. Vassilaros et al., " Stainless Steel Compact Data, NRC Piping.
Material Program at David Taylor Naval Ship Research and Development Center, presented to ASME Section XI, Task Group on Pipe Flaw Evaluation, San Antonio, June 1984.
- 2. Letter and attachments from J. Ted Enos, Arkansas Power and Light Company, to John F. Stolz, U.S. Nuclear Regulatory Commission, ICAN118604, November 11, 1986.
- 3. Letter and attachments from J. Ted Enos, Arkansas Power and Light Company, to John F. Stolz, U.S. Nuclear Regulatory Commission, ICAN118610, November 24, 1986.
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