ML20083C868
| ML20083C868 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 12/13/1983 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20083C867 | List: |
| References | |
| IEB-83-02, IEB-83-2, NUDOCS 8312270079 | |
| Download: ML20083C868 (6) | |
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o'g UNITED STATES I
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g NUCLEAR REGULATORY COMMISSION
.p WASHINGTON. D. C. 20555 N.....f SAFETY EVALUATION BY TPE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO INTERGRANULAR STRESS CODR0SION CRACKING CAROLINA POWER & LIGHT COMPANY BRUNSWICK STEAM ELECTRIC PLANT, UNIT 2 DOCKET NO. 50-324 4
1.0 Introduction Brunswick Unit 2 was shut down on November 2, 1983 in accordance with the order issued on August 26, 1983 to inspect all ASME Class 1 austenitic cracking (IGSCC) piping that are susceptible to intergranular stress corrosion stainless steel in the Recirculation, Residual Heat Removal (RHR), Core Spray and Reactor Water Clean-up (RWCU) piping ~ systems.
Carolina Power & Light Company (the licensee) reported the results of the inspection in a meeting with the NRC staff on November 23,1983 -(see meeting sumary dated November 25, 1983 - reference 1) and in a subsequent letter dated November 28, 1983 (reference 2).
In addition, two letters-dated November 30,1983-(Serial number: LAP-83-549, reference 3, and ' serial ' number:~ LAP-83-554, reference 4)-
- included information regarding weld defects and residual stresses, respectively.
During this shutdown period, ultrasonic examinations were performed on 131 nonconforming welds. Of these,102 welds.were in the Recirculation system, 5 welds were in the RHR system and 24 welds were in the RWCU system. The Core l
Spray system piping is made of carbon steel which is resistent to IGSCC. The licensee indicated that all Class-1 welds susceptible to IGSCC in the above mentioned piping systems were ultrasonically examined, i
Background
Personnel from General Electric (Atlanta) and Lambert, McGill, and Thomas (LMT)-
performed the ultrasonic testing (UT) for the licensee.
Region II of NRC has determined that their'UT procedures, calibration standards, equipment and t
4 8312270079 831213 PDR ADOCK 05000324 P
PDR p
. IGSCC detection capabilities were satisfactorily demonstrated in accordance with I&E Bulletin 8?-02, and the same procedures and techniques were used in the UT examination. Region II also indicated that all their UT personnel I
conducting these inspections have received appropriate training in IGSCC inspection using cracked thick-wall pipe specimens. The " amplitude-drop" method was used for crack depth measurements, but additional crack depth measurements using crack tip diffraction was performed on all defective welds that were not repaired. The worst of the two crack depth measurements was used in the flaw evaluation of the unrepaired welds. The results of the UT examinations indicated that a total of 19 welds showed reportable linear i
indications, of which eight are 28" Recirculat. ion welds, two are 22" Recircu-i lation Manifold welds, five are 12" Recirculation Riser welds, one is 20" RHR weld and three are 6" RWCU welds.
All reported UT indications were short and shallow, and were in the weld heat-affect-zone (HAZ). Short axial cracks with depths not over 20% of the wall thickness were reported in two 28" Recirculatin welds and one 20" RHR weld. The deepest circumferential crack, which had a depth of 22% of the wall thickness, was reported in a 28" Recirculation weld (2-B32-28"-B5).
Except for weld 2-B32-22"-AM), the reported crack lengths in all defective welds did not exceed 2.375".
Weld 2-832-22-AM-5 was reported to have a total crack length of 11.5" (about 17% of the circumference) and a maximum crack depth of 20% of the wall thickness.
NUTECH performed flaw evaluations on all defective welds for the licensee.
The evaluations were based on the methodology provided in the new ASME Code Section XI IWB-3600. The new Code IWB-3600 provides flaw acceptance criteria for the austenitic stainless steel piping based on a limit load approach which was approved by the ASME Main Committee in May 1983 and is expected to be published later this year. The results of NUTECH's flaw evaluations (Reference 1), including crack growth calculations, indicated that 11 (eight 28" Recirculation welds, two 22" Recirculation Manifold welds, one 20" RHR weld) of the 19 defective welds did not require weld overlay repair because the calculated flew sizes of those 11 welds at the end of a 6-month period did not even exceed two-thirds of the new Code allowable limit.
NUTECH also performed weld overlay design and repairs for the licensee.
Eight of the 19 defective welds were weld overlay repaired. The over-lay thickness was designed to meet the new IWB-3600 limits. The overlay applied to the eight defective welds has a minimum thickness of 0.2 inch. The minimum lenghs of the overlay varied from 3.8 inches to 5 inches, and were selected to reinforce the weld structure and minimi7e i
the end effects.
Region II of NRC Has confirmed that the weld overlay l
repairs were perforned in accordance with qualified and approved pro-cedures consistent with ASME Code requirements.
s
. The licensee reported that the as-measured axial shrinkages from the eight overlay repair welds were in the range of 0.22 inch to 0.37 inch.
The stresses caused by this shrinkage on the 11 unrepaired defective welds were calculated to be very small. The largest value was reported to be 664 psi.
In the crack growth calculation, the small shrinkage stresses due to weld overlay were not considered.
During the repair process there were three defects noted (see reference 3).
One of the defects was a " blowout", with an underlying pinhole, one was a pinhole and one was a quarter-inch long axial crack indication.
These weld defects were adequately repaired.
Evaluation We reviewed the licensee's submittals, including the analysis of the weld overlay designs, and the calculation of IGSCC crack growth to support the continuing service for a 5-month period (approximately 3600 hours0.0417 days <br />1 hours <br />0.00595 weeks <br />0.00137 months <br />). Our review included the nine overlay repaired welds and the 11 unrepaired defective weids (eight 28" Recirculation welds, two Recirculation manifold welds and one 20" RHR weld). The licensee indicated that Brunswick Unit 2 will enter the refueling operational condition approximately March 15, 1983, but no later than April 30, 1984.
In the IGSCC crack growth calculations, which bounded the crack growth in all the unrepaired defective welds, the stress intensity factor was calculated by conservatively assuming the crack at the reported crack depth (22% of the wall thickness) to be 360 in circumference.
The highest sustained stress calculated for any weld was 9680 psi. This bounding value was used for all the crack growth calculations. The results of the calculations indicated that the worst calculated flaw size at the end of a five month period did not exceed 30% of the wall thickness. This is well within the staff's criterion of two-thirds of the new Code allowable limits.
We have reviewed the IGSCC crack growth calculations and agree with their conclusion that the continued operation for a period of five months with the 11 defective welds in as-is condition is acceptable. Our conclusion is based on the following considerations:
(1) Code des,ign safety margin We performed an independent crack growth calculation to ensure that the Code design safety margin in the 11 unrepaired defective welds in mairtained.
The crack growth in the 11 unrepaired defective welds was bounded by t.his calculation.
A sustained stress of 10 Ksi, including the largest shrinkage stress of 664 psi, was used in this
calculation. The stress intensity factor (K ) was calculated based 7
on a cylindrical model of 28 inches diameter pipe assuming a complete 360 circumferential crack at a depth of 22% throughwall. The crack growth rate curve used in our calculation is more conservative than that used by NUTECH, and is an upper bound of GE ard EPRI's crack growth data in furnace sensitized material and tested in 0.2 ppm 02 water. Our calculations showed that the initial crack depth of 22, would grow to a depth of about 33% at the end of a 5 month period as the crack is relatively short (17% of the circumference).
Even if the reported initial crack depth is doubled to 44% of wall thickness, the final crack size at the end of a 5 month period is calculated to be only about 52% of.the wall thickness which is still well within the new Code allowable limit (75% of wall thickness). Therefore, we conclude the Code design safety margin will be maintained in the 11 defective unrepaired welds during the continued operation for a period of 5 months.
NUTECH's overlay design for the eight defective welds (five 12" riser welds and three 6" RWCU welds) was based on the conservative assumption that all cracks were throughwall cracks. This assumption eliminates the concern regarding the uncertainties in the UT sizing of crack depth because crack depth need not be considered in the overlay design. The length of the reported cracks in the eight repaired welds were all very short which varied from 0.75 to 3.25 inches (about 4% to 9% in circunference). Because of the compressive residual stresses formed at the inner surface and extending approxi-mately half-way through the wall after weld overlay, crack growth in the circumferential direction is expected to be very limited.
Based on the limit load analysis, the assumed short throughwall cracks would not have significant inpact on the Code design safety nargin in the eight overlayed welds. Therefore, we conclude that NURECH's overlay repairs will provide added assurance of safe operation during the next 5 month period.
(2)
Short cracks All 11 unrepaired defective welds have relatively short cracks.
Except for one unrepaired weld which has a total crack length of 17%
of the circumference, the crack length in the other 10 unrepaired welds is less than 10% af the circumference. Based on limit load analysis, such short length flaws, even assumed throughwall, will not have a significant effect on the structural integrity of the weld
(3) Leak Testing A hydrostatic test in accordance with ASME Section XI will be per#ormed on joint 2-G31-6"-15 prior to start-up. The other joints cannot be isolated from the reactor; therefore, an in-service leak test will be
. performed at normal operating pressure and temperature during start-up.
These tests will give adequate assurance o# the integrity of the overlays.
(4) Short operating period The licensee indicated that Brunswick Unit 2 will be in refuel mode no later than April 30, 1984, therefore, the operating period prior to the next refuel. outage would not be more than 5 months.
Based on our recent experience with BWRs operating with confirmed IGSCC cracked piping welds, we do not expect that the reported short cracks in the 11 unrepaired welds would grow to the extent of com-promising the safety of the plant during the continued operation of the plant for a period of 5 months.
(5) Augmented Leak Detection Although the conservative calculations discussed above indicate that the cracks in the unreinforced welds will not progress to the point of leakage during the next 5 month period, and very wide margins are expected to be maintained over crack growth to the extent of compro-mising safety, uncertainties in crack sizing and growth rate still remain. Because of these uncertainties, it is prudent to tighten the reouirements for the monitoring of unidentified leakage.
The licensee has agreed to additiodal monitoring and tighter limits on unidentified leakage, which are summarized belcw:
1.
The reactor coolant system leakage will be limited to a 2 gpn increase in unidentified leakage within any ?4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> period (leakage shall be monitored and recnrded once every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />).
Should this leakage limit be exceeded, the unit will immediately start an orderly shutdown.
The unit will be placed in at least hot shutdown within the next I? hours and in cold shutdown within the following 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
2.
At least one primary containment sump collection and flow monitoring system will be operable. With the prinary containment sumo collection a,4 9 mi nn"" + r rire sveten increrable, restore the inoperable system to operable status within ?a bours or immediately initiate an orderly shutdown and be in at least hot shutdown within the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and in cold shutdown within the following 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
J
1+. We concluded that implementation of the above measures will provide adequate assurance that possible cracks in pipes will be detected
-before growing to a size that could compromise the safety of the plant.
Summary and Conclusions We have reviewed the licensee's submittals regarding the actions taken or to be taken during this confirming order outage on the inspection, analyses and repairs of Recirculation, RHR and RWCU piping systems in the Brunswick Unit 2 plant. This includes a description of the defects found, description of repairs, stress and fracture mechanics analysis.
We conclude that the Brunswick Unit 2 plant can he safely returned to power and operated in its present configuration at least for one 5-month period.
Nevertheless, we still have concern regarding the long term growth of small IGSCC cracks that may be present but not detected during this inspection. _ Therefore, we require that plans for inspection and/or modification of the recirculation and other RCPB piping systems during the next refueling outage be submitted for our review at least one month before the start of the next refueling outage.
Principal Contributors:
W. Koo G. Johnson S. Mackay M. Grotenhuis Date:
December 13, 1983
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