ML19249E141
| ML19249E141 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 08/25/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19249E140 | List: |
| References | |
| TAC-11793, TAC-43142, NUDOCS 7909260453 | |
| Download: ML19249E141 (4) | |
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UNITED STATES
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NUCLEAR REGULATORY COMMISSION
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h SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATICN REGARDING FEEDWATER LINE CRACK INDICATIONS NORTHEAST NUCLEAR ENERGY COMPANY, ET AL.
MILLSTONE, UNIT NO. 2 FACILITY OPERATING LICENSE NO. OPR-65 00CKET NO. 50-336 As a result of radiographic inscections perfomed to ccmply with NRC Inspection and Enforcement Bulletin 79-13, Northeast Nuclear Energy Company (NNECO or the licensee) found linear circumferential indica-tions in the base metal in the vicinity of pipe to safe-end welds AC-G-1 and BC-G-1 and pipe to elbcw welds AC-G-2 and BC-G-2 at Millstone Unit No. 2 (Millstone 2). NNECO expanded the scope of the examination to include all welds to the first piping support or snubber and all high stress points in the containment on the feedwater lines. The remainder of the welds were found free of linear indications. NNECO obtained the services of Westinghouse Electric Corporation to evaluate the radiography and perform ultrasonic examinations to characterize the indications. Westinghouse developed an ultrasonic test procedure based on the experience gained in sizing cracks in similar ccaponents from other plants where the-actual crack depths were measured destructively.
The ultrasonic measurements of the cracks have been in very close agreement with the actual physical measurements. Using the same ultrasonic technicue, the depth of the indications in Millstone Unit 2 were found to be less than 105 of the wall thickness, with the exception of the indications in the vicinity of pipe to safe-end weld 3C-G-1.
Section XI, Table IWB-3515-2 of the ASME Code states that a depth to thickness percentage of 105 is acceptable for a clanar flaw of infinite length in ferritic steel (less than 50 ksi yield, I inch thick). The maximum depth for the flaws located near weld 3C-G-1 were recorted to be 0.110 inches.
These flaws are located at the top of the oi;:e and have a ccabined length of accroximately 9 inches. The minimum wall thickness in the vicinity was measured as 0.760 inches.
The licensee ::erfomed a visual examination of the feedwater picing supcorts and snubbers and verified ocerability and confomance to cesign.
The licensee reported that no ancmalies were found in the code requirec stress analyses for the feedwater piping and that margin exists frca the code allowable stresses.
7 90 920d46 1029 059
. Westinghouse performed a thermal fatigue analysis using temperature and frequency assumptions based on experience gained from the results of test programs at other facilities that have experienced feedwater line cracking. From a review of operating history and the results of the thermal analysis, NNECO states that cracks of the depth found at Millstone 2 can be explained by the temperature fluctuations in the feedwater. Westinghouse states that the thermal analysis also can explain cracks of a similar nature found at seven other Westinghouse plants.
Westinghouse addressed the piping integrity and compared the cbserved flaws with the size of flaws which could cause failure of the feedwater piping. Because of the toughness of the material and minimum operating temperature range, Westinghouse concludes that piping fracture will be plastic fracture and not governed by linear elastic fracture. The NRC Staff concurs with this position. Westinghouse performed a plastic instability analysis using the design loads including seismic for Millstone 2.
The results show that a through wall crack longer than 26 inches or a part through crack 70% of the wall thickness all the way around the pipe would be required to cause pipe rupture. The NRC Staff has performed independent !aad limit analyses for other plants with similar materials and has come to similar conclusions. The licensee points out that the analyses, the location of the cracks, and field experience with other feedwater piping cracks denonstrate that leaks would develop long before failure in the unlikely event the cracks did propagate significantly. The licensee further states that the indications near weld SC-G-1 are within the ductile failure limits of ASME XI, Paragraph I'.iB-3 600.
NNEC0 states that existing leak detection used to identify leakage from the primary system has adequate sensitivity to identify leakage from the feedwater system in a timely manner.
The licensee states that the plant design has been mcdified to include J tubes on the feedwater spargors to reduce the potential for water hammer, and that License Amendment No. 32 inc!udes a license condition that precludes the addition of auxiliary feecwater outside of the bouncs of test concitions performed to demonstrate water hammer did not cccur.
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.. The licensee tras committed to:
1.
Install monitoring equipment for temperature and stress measurements on the steam generator no. 2 feedwater line.
2.
Establish action points frcm on-site and off-site acceptance criteria as presented in the August 24, 1979 letter for initiating controlled plant shutdown if the limits are exceeded.
3.
Initiate a controlled shutdown to perform an ultrasonic examination of the most severe indications on the affected lines in the event of a feedwater flow instability (water-hamer);
4 By October 1,1979, submit a thorough program for repairing the affected welds. The pecgran will address the modifications required in the steam generator shield wall section.
5.
By October 31, 1979, ccmmence an approach to cold shutdown to perform ultrasonic examination of the four affected welds using the same methods, techniques and conditions as was used to previously characterize the indications, o.
Prior to startup, install local leak monitoring equipment capable of detecting a 0.5 gal / minute leak on both steam generator locos in the vicinity of the cracks.
7.
Initiate a controlled shutdown and inspect the affected welds should combined dynamic stress greater than 40 ksi be measured by the on-site monitoring equipment.
The NRC Staff has considered the safety significance of the feedwater pioing cracks at Millstone 2 and has concluded, based on the available information, that the feedwater oicing would be unlikely to rupture or leak as the result of design based loads including earthquakes in the interim period. The staff considers it corceivable, in the unlikely event of a severe water hamer, that failure of the feedwater line could 1029 061
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occur..However, it is considered unlikely that a severe water hammer event would occur in more than one feedwater line simultaneously. Thus, the worst reasonable consequence in the facility would be rupture of a single feedwater line. Because this event has been considered as a design base accident the facility is designed and has a procedure to cope with this occurrence.
Based on the foregoing considerations, the staff concludes that Millstone Unit No. 2 can safely operate without repair to the feedwater lines until October 31, 1979.
Environmental Consideration We have detemined that this evaluation does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact. Having made this detemination, we have further concluded that this action is insignificant from the standpoint of environmental impact and pursuant to 10 CFR
@51.5(d)(4) that an enviromental impact statement or negative declaration and environmental impact appraisal need not be prepared.
Conclusion We have concluded, based on the considerations discussed above, that:
(1) because the evaluation does not involve a significant increase in the probability or consequences of accidents previously considered and does not involve a significant decrease in a safety margin, the action does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in ccmpliance with the Commission's regulations and the action will not be inimical to the ccmmon defsnse and security or to the health and safety of the public.
Date: August 25, 1979 1029 062
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