ML20013C086
| ML20013C086 | |
| Person / Time | |
|---|---|
| Site: | Millstone, Palisades, Saint Lucie, Arkansas Nuclear, Waterford  |
| Issue date: | 05/21/1997 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20013C077 | List: |
| References | |
| NUDOCS 9706240192 | |
| Download: ML20013C086 (6) | |
Text
.
9 ENCLOSURE t
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION TOPICAL REPORT ON RELAXATION OF REACTOR COOLANT PUMP FLYWHEEL INSPECTION REOUIREMENTS ARKANSAS NUCLEAR ONE 1 & 2 MATERIALS AND CHEMICAL ENGINEERING BRANCH DIVISION OF ENGINEERING
1.0 INTRODUCTION
On April 4,1995, Entergy Operations, Inc. (Entergy), the licensee for Arkansas Nuclear One (ANO) -l & -2, submitted a topical report SIR-94-080 [1],
i
" Relaxation of Reactor Coolant Pump Flywheel Inspection Requirements," for NRC review. This report, which provides an engineering analysis based on fracture mechanics, is intended to eliminate reactor coolant pump (RCP) flywheel 1
inservice inspection (ISI) requirements for five operating ABB Combustion Engineering Owners Group (CEOG) plants: ANO-1 & -2, Millstone 2, Palisades, St. Lucie 1 & 2, and Waterford 3.
Presently, AND-1 is the only plant with Technical Specifications (TS) requiring its RCP flywheel inspection be performed in 10-year intervals. All remaining plants have their flywheel inspections performed in accordance with their licensing commitments to Regulatory Guide (RG) 1.14 [2]. The regulatory position of RG 1.14 calls for an in-place ultrasonic volumetric examination of the areas of higher stress concentration at the bore and keyway at approximately 3-year intervals and a surface examination of all exposed surfaces and complete ultrasonic volumetric examination at approximately 10-year intervals. The flywheel inspection schedule is to coincide with the individual plant's ISI schedule as required by Section XI of the ASME Code.
2.0 BACKGROUND
The function of the RCP in the reactor coolant system (RCS) of a pressurized i
water reactor plant is to m;'ntain an adequate cooling flow rate by circulating a large volume of primary coolant water at high temperature and pressure through the RCS. A concern over overspeed of the RCP and the potential for failure of its flywheel led to the issuance of RG 1.14 in 1971.
Operating power plants have been inspecting their flywheels for over twenty years, and no flaws have been identified which affect flywheel integrity.
This inspection record, plus the licensee's concern over inspection costs and personnel radiation exposure, prompted it to submit this topical report to demonstrate through fracture mechanics analysis that flywheel inspections can be eliminated without impairing plant safety.
4 9706240192 970521 D OPRP CE CF
t 2
A similar topical report [3] for Westinghouse plants and for some Babcock and Wilcox plants was submitted on January 24, 1996, and a staff safety evaluation i
report (SER) [4] relaxing the RCP flywheel inspection interval from approximately 3 years to ten years was issued on September 12, 1996.
In that report, the RCP flywheels were evaluated to criteria specified in IWB-3612 of Section XI of the ASME Code, except that the crack initiation fracture I
toughness (K *) was used in the analyses for normal and upset conditions insteadoftbecrackarrestfracturetoughness(K acceptance criteria based on the applied stress inl)e.
IWB-3612 specifies i
nsity factor (applied K for analytical evaluation of flaws in ferritic steel components 4 inches and,)
greater in thickness.
IWB-3612 specifies that for applied K should be less than K, divided oy (10) ', normal conditions, i
i faulted conditions, the applied K should be less than K divided by (2) i ThemodifiedIWB-3612criteriawereacceptedbythestafY[4].
3.0 EVALUATION AND VERIFICATION The primary regulatory position of RG 1.14 regarding flywheel design concerns three critical speeds: (a) the critical speed for ductile fracture, (b) the critical speed for nonductile fracture, and (c) the critical speed for excessive deformation of the flywheel. This regulatory position specifies, as a design criterion, that the normal speed of the flywheel should be less than one-half of the lowest of these three critical speeds.
3.1 MATERIAL INFORMATION l
The reference temperature RT 1, along with the operating temperature of the flywheels, determines the fracture toughness K, needed in the fracture g
mechanics analyses. Therefore, all licensees seeking application of this report to their plants need to verify the reference temperature RT for 7
Section XI of the ASME Code was develope,d for vesseYn) curve in Ap their RCP flywheels. Also, since the K v.s. (T-RT i
materials such as SA 533 B and SA 508, licensaes wita flywheels made of different materials need to justify their use of this curve to derive the K,directly or indirectly related i
values.
In both cases, if i
there are plant-specific test results which are to fracture toughness of the RCP flywheel material, they should be reported.
i 3.2 ANALYSIS FOR CRITICAL CRACK DEPTHS BASED ON NONDUCTILE FRACTURE 3.2.1 LICENSEES' EVALUATION RG 1.14 addresses both ductile and nonductile fracture of the flywheel.
However, the topical report only provided the critical crack depths based on linear elastic fracture mechanics (LEFM) analysis to address nonductile f;acture.
i The LEFM analysis in the topical report used a radial full-depth crack 4
emanating from the bore of a rotating disk to calculate the applied K, for the larger bore flywheels (ANO-1, Palisades, and St. Lucie 1 & 2).
For the smaller bore flywheels (ANO-2, Millstone-2, and Waterford 3), a model consisting of a crack emanating from a hole in an infinite plate was used.
In
4.
3 a response [5] to the staff's request for additional information (RAI), the i
licensee demonstrated that the former approach is conservative and the latter is slightly less conservative in the applied K calculation. This point will be considered by the staff in the subsequent ev,aluation. The fracture I
resistance for the flywheels was obtained from the lower bound K, curve of l
2 Sec fon XI of the ASME Code. Use of K wassuggestedbyRG1.1/. The load i
used in calculating the applied K, for,,ach flywheel was from the normal e
speed, which is 900 rpm for ANO-2, Millstone 2, Palisades, and St. Lucie 1 & 2 1
and 1200 rps for ANO-1 and Waterford 3.
The bounding value of 150% of the i
j normal speed was defined as the accident speed for each plant.
Since RG 1.14 does not contain fracture mechanics criteria for evaluating the period of time between inspections, the licensees used the same criteria as that in the previously approved Westinghouse topical report [6] in the fracture mechanics analysis for their RCP flywheels. The licensees used 3.0 for normal conditions and 1.4 for emergency and faulted conditions.
i In the original submittal, the effect due to centrifugal stresses and shrink-3 t
fit stresses were considered separately. The argument was that the maximum i
l stress due to centrifugal forces occurred at the highest rpm, but the maximum
~
j stress due to shrink-fit occurred at zero rpm.
In reality, the combined j
stress at the normal speed may exceed either of the two maximum stresses due to the centrifugal and the shrink-fit effect even though each individual a
contributor of the combined stresses is less than the maximum of its respective stresses. To respond to this concern, the licensee revised Figures 4
}
6-3 to 6-8 [1] by combining the shrink-fit stresses and the centrifugal i
stresses, and presented the updated applied K and fracture toughness v.s.
. crack depth for all flywheels in Figures 7 to,12 [5].
In combining the stresses, a margin of 1.0 was applied to the shrink-fit stresses, and a margin of 3.0 was applied to the centrifugal stresses. The resulting critical crack depths were summarized in Table 3 of Reference 5, and it showed that the j
smallest allowable flaw size is 0.43 inches,for Waterford 3 flywheels.
Fatigue crack growth was determined from the growth rate formula in Appendix A of Section XI. The topical report assumed an initial postulated crack depth l
of 0.25 inches. This size crack represents the maximum flaw size that could i
have been missed during ultrasonic testing (UT) inspections, and was based upon CEOG's judgement. This report assumed 4000 cycles of RCP startups and shutdowns, about eight times the design cycles for plant life. The crack i
growth after 4000 cycles is tabulated in Table 6-3 of SIR-94-080 for various i
flywheels. The largest growth is for Waterford 3 with a crack growth of j
0.0186 inch.
i Based on the small fatigue crack growth in flywheels, and the finding that the
{
smallest allowable flaw size from the LEFM analysis is greater than can be detected by ISI examinations, the licensees concluded that all flywheels meet i
the modified criteria of IWB-3612.
3.2.2 STAFF'S EVALUATION i
The staff agrees that if applied correctly, the LEFM is an acceptable methodology. - Further, the staff determined that performing an elastic-plastic j
fracture mechanics (EPFM) analysis is not necessary because LEFM analysis is
4 appropriate for the thickness of the flywheel and its operating temperature (about 100 F). The staff also agrees [4] that meeting the margin based on applied K,ified in RG 1.14 for nonductile fracture of the flywheel.from IWB-361 speed spec The staff disagrees with the licensee's approach of applying a margin of 1.0 to the shrink-fit stresses in its LEFM analyses because plastic col bpse is not the anticipated failure mechanism for the ferritic material at the flywheel operating temperature of 100*F.
ihe cited precedence for using the l
margin of 1.0 for piping expansi.on stress from Appendix C, "Evaluatien of flaws in Austenitic Piping" of the A3ME Code is not applicable here.
In the realm of LEFM, there is no distinction between the nature of the shrink-fit and centrifugal stresses in the applied K, Westinghouse topical report, the calculation, therefore, the same margin should be applied to both.
In the i
margin of 3.0 was applied to both the centrifugal and shrink-fit stresses.
Also, since the initial postulated crack depth of 0.25 inches has not been substantiated, the staff used the initial postulated crack depth of 0.33 inch and a 10-year crack growth of.013 inch from Reference 4 in this evaluation.
The initial postulated crack depth of 0.33 inch was based on industry experience with the inspection of ferritic components with short metal paths.
Reference 4 indicated that it is unlikely that any defect that could challenge flywheel integrity would be missed by the inspections.
Applying the margin of 3.0 to both the centrifugal and the shrink-fit stresses, the staff found that only ANO-1 and Palisades flywheels meet the requirements of IWB-3612. The margin for flywheels was 2.65 for St. Lucie 1 &
2, 2.82 for ANO-2, 2.57 for Millstone 2, and 2.68 for Waterford 3.
It was mentioned [5] that the infinite plate model of 60-2, Millstone 2, and Waterford 3 is slightly less conservative. The staff estimated from Figures 4, 5, and 6 of Reference 5 that the applied K for ANO-2, Millstone 2, and t
Waterford 3 flywheels might be 5% higher. This would reduce the margin for ANO-2 to 2.69 (from 2.82), the margin for Millstone 2 to 2.45 (from 2.57), and the margin for Watarford 3 to 2.55 (from 2.68).
Margin is used to account for uncertainties in the LEFM analyses, e.g. the applied loading, the stress analysis, the existence of undetected fabrication defects, the assumed flaw size, and unaccounted for stresses such as residual 4
j stresses. The staff determined that, except for the assumed flaw size, all key input parameters to the LEFM can be estimated with high certainty.
Therefore, a margin around 2.5 is adequate for this application. The staff determined that the flywheels will have adequate fracture toughness during the service period for RCP flywheels of all pldnts listed in this topical report, and a 10-year inspection period appears reasonable.
The analyrical results for emergency and faulted conditions were provided in Figures 6-9 to 6-14 [1].
From these figures and from considering the significant & crease of the shrink-fit stresses at a much higher speed, the staff determined that the normal and upset conditions are controlling for the flywheel s.
This is consistent with the finding in Reference 4 for the Westinghouse topical report on this issue.
L l-i t
5 e
. Finally, the staff wants to clarify that IWB-3612 is for cracks discovered c
during inservice inspections. Applying IWB-3612 to postulated cracks in the i
LEFM analysis for the RCP flywheels is conservative because so far no flaws which affect flywheel integrity have been reported from the industry.
Therefore, using the criteria from IWB-3612 but with a safety factor of 2.5 3
for the normal and upset conditions provides acceptable fracture toughness for flywheels.
4 i
j 3.3 COMPLIANCE WITH THE EXCESSIVE DEFORMATION FAILURE CRITERION 4
j The primary concern of RG 1.14 over excessive deformation is the enlargement of the bore that could cause a separation of the flywheel from the shaft or 4
could cause an imbalance of the flywheel leading to structural failure. The main concern is the loss of shrink-fit at high speed.
If a loss of shrink fit occurred, the keys on the flywheels may not be able to prevent relative displacement between the wheel and the shaft.
l Table 4 of Reference 5 summarized the remaining shrink-fit for accident conditions for all flywheels. This remaining shrink-fit was calculated by subtracting the centrifugal displacement at accident conditions from the initial shrink-fit. When the remaining shrink-fit is zero, a total los.: of shrink-fit occurs.
Reference 5 reported that the largest remaining shrink-fit j
is 0.006 inch (St. Lucie 1 & 2), and the smallest is 0.0 inch (Waterford 3).
j Based on these reported values, the staff concludes that, except for the 4
flywheels for Waterford 3, all flywheels satisfy the excessive deformation failure criterion. The staff will pursue the issue of the loss of shrink-fit l
of flywheels at the accident speed with Waterford 3 on a plant-specific basis.
1 i
l
4.0 CONCLUSION
S i
The Materials and Chemical Engineering Branch has completed its review.sf the j
licensee's submittals and has determined that the evaluation methodology in i
the reports is appropriate and the criteria meet the intent of the design j
criteria of RG 1.14.
i i
For the-RG criteria on the critical speeds which affect flywheel integrity, the staff concluded that (1) all flywheels meet the proposed nonductile i
fracture criteria, and will have adequate fracture toughness during their
{
service periods, and (2) all flywheels except those for Waterford 3 satisfy j
the excessive deformation criterion of RG 1.14.
4 This report requests complete elimination of flywheel inspections. The staff i
believes that even for flywheels meeting all the design criteria of RG 1.14, as modified in this SER, inspections should not be completely eliminated.
Inspections are performed in part to protect against events or degradation i
that is not anticipated and has not been considered in the analysis. This philosophy is consistent with the requirements in the'ASME Code for successive i
inspections for flaws evaluated to the Section XI acceptance criteria.
Therefore, the staff will not accept total elimination of flywheel inspection.
However, considering that flywheel inspections can be conducted when RCP motor maintenance is required (about every 8 years from a limited survey [6]), the staff concluded:
l i
i
~
i..
s (1) Licensees for ANO-2, Palisades, Millstone 2, Waterford 3, and St. 8.ucie 1
~
& 2 who plan to submit a plant-specific application of this topical report need to verify the reference temperature RT 'erials other than SA 533 8 and SA for their RCP flywheels. Also, if these licensees have flywheels made of mIt 508, they need to justify the use of the K v.s. (T-RT y of Section XI of the ASME Code to derive their respectfve) Kcurve in Appendix A l
values.
In both u
cases, they should report any existing plant-specific test results which are directly or indirectly related to fracture toughness of the RCP flywheel material.
l (2) Since ANO-1 already has a unique flywheel inspection program of 10-year intervals, this SER does not affect its status regarding flywheel inspections.
Lir:ensees meeting-(1) above should either conduct a qualified in-place UT examination over the volume from the inner bore of the flywheel to the circle i
of one-half the outer radius or conduct a surface examination (MT and/or PT) l of exposed surfaces defined by the volume of the disassembled flywheels once every 10 years. The staff considers this 10-year inspection requirement not i
burdensome when the flywheel inspection is conducted during scheduled ISI j
inspection or RCP motor maintenance. This would provide an appropriate level of defence in depth.
The staff will pursue the issue of the loss of shrink-fit of flywheels at the i
accident speed with Waterford 3 on a plant-specific basis.
j 1
5.0 REFERENCES
1.0 Entergy Operations, Inc., letter from J. W. Yelverton (Entergy) to USNRC Document Control Desk with enclosed report, SIR-94-080, " Relaxation of Reactor Coolant Pump Flywheel Inspection Requirements," April 4, 1995.
j l
2.0 USNRC, Regulatory Guide 1.14, " Reactor Coolant Pump Flywheel Integrity,"
j 1971; Revision 1, August 1975.
3.0 Duquesne Light Co., letter from George S. Thomas (DLC) to USNRC Document Control Desk with enclosed reprt, WCAP-14535, " Topical Report on Reactor Coolant Pump Flywheel Inspection Elimination," January 24, 1996.
4.0 USNRC, letter from Brian W. Sheron (USNRC) to Sushil C. Jain (DLC) with enclosed SER, " Acceptance for Referencing of Topical Report WCAP-14535, Topical Report on Reactor Coolant Pump Flywheel Inspection Elimination,
" September 12, 1996.
5.0 Entergy Operations, Inc., letter from Dwight C. Mims (Entergy) to USNRC Document Control Desk, " Response to Questions Related to Relaxation of Reactor Coolant Pump Flywheel Inspection Requirements," December 9, 1996.
6.0 Duquesne Light Co., letter from Sushil C. Jain (DLC) to USNRC Document Coatrol Desk, " Response to Request for Additional Information Concerning WCAP-14535; RAI Dated July 24, 1996* August, 2, 1996.
v