ML16342A234
| ML16342A234 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 09/02/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML16342A235 | List: |
| References | |
| NUDOCS 9309140387 | |
| Download: ML16342A234 (6) | |
Text
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UNITEDSTATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 ENCLOSURE 1
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REAC OR REGULATION OF INDICATIONS IN THE SAFETY INJECTION ACCUMULATOR TANKS DIABLO CANYON UNITS 1
AND 2 PACIFIC GAS AND ELECTRIC COMPANY DOCKET NOS.
50-275 AND 50-323 I.
INTRODUCTION 1.1
~Per ose To determine whether Pacific Gas and Electric Co.
(PG&E), the licensee, properly dispositioned surface cracks in the cladding of the safety injection accumulator tanks according to the requirements of the ASHE Code.
1.2
~kk 2
During the fifth refueling outage for Diablo Canyon Unit 2, indications were found in the accumulator tanks 2-2 and 2-3 roll-bonded cladding that lines the inside of the tanks.
The cladding is Type 304 stainless
- steel, and the tanks are of SA516 Grade 70 carbon steel.
The cladding flaws were discovered during dye penetrant inspections of the seam welds and the heat affected zones.
The flaws initiate at the toe of the clad-to-clad or seam welds and extend into the cladding.
Host of the flaws identified by dye penetrant were examined by eddy current techniques from inside the tank and representative flaws examined by ultrasonic techniques from inside and outside the tank.
Data from both these methods confirmed that some of the cladding flaws extend through the cladding to the steel base metal interface and stop there.
The eddy current data indicate that some of the flaws are as deep as 150 mils.
The ultrasonic data for these same flaws correlate with the eddy current readings.
The ultrasonic test data show the deepest flaw is about 150 mils, which would extend up to the base metal interface but not into the base metal.
The seam welds in accumulators 2-1 and 2-4 were also examined by dye penetrant.
Indications were found in these tanks but they were mechanical discontinuities near the weld seams and not associated with the flaws in the cladding found in accumulators 2-2 and 2-3.
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During the fifth refueling outage for Unit 1 (which occurred before that for Unit 2) Accumulator 1-4 was examined.
The results of the inspection were that one rounded indication was found on a seam weld and was buffed clean.
The other three Unit 1 accumulators were not inspected.
Accumulator 1-4 was chosen as a representative tank based on the manufacturer's material test reports for the cladding.
Tank 1-4 has Type 304 stainless steel
- cladding, whereas the other three tanks have Type 304L.
Type 304L is considered to be significantly more resistant to intergranular stress corrosion cracking than Type 304.
The stresses in all eight tanks are identical.
Since the one Type 304 clad tank showed no evidence of cracking, the licensee did not expect cracking to have occurred in any of the three Type 304L clad tanks in Unit l.
Any cracking that may exist in these tanks would not be expected to extend beyond the cladding to base metal interface for the same reasons that the cracks found in the Unit 2 tanks did not.
In its submittal of April 19,
- 1993, PGLE sent an evaluation done by Westinghouse.
The evaluation found the cracks acceptable without repair.
II.
EVALUATION 2.1 ASME Section XI Code Re uirement The tanks were designed to ASNE Section III, 1968 edition.
The most recent edition (1992) of Section XI was used for the evaluation.
The analytical criteria for acceptance of indications have stayed the same since their original publication in 1974.
Paragraph IWB-3600 states that flaw indications that exceed the standards for allowable indications of IWB-5000 may be evaluated by analytical procedures, such as described in Appendix A of the Code, to determine the critical flaw parameters with respect to the location of the detected flaw.
The evaluation procedures and the acceptability criteria for these parameters are the responsibility of the Owner and subject to approval by the regulatory authority having jurisdiction at the plant site.
Paragraph IWB-3610 states that a flaw that lies entirely in the clad need not be evaluated.
The flaws found in the clad can be considered acceptable as is.
However, since the tanks were designed to include the cladding thickness as part of the design thickness, the cracks were evaluated.
The licensee did a fracture mechanics evaluation using the flaw acceptance criteria of Section XI and the analytical procedures of Appendix A of the Code.
There are no criteria in Section XI for these Class 2 components, so the evaluation used Class 1 criteria.
Applying the stricter criteria adds conservatism to the results.
2.2 Flaw Evaluation The surface flaws were postulated to be oriented normal to the maximum principal stress in the tank, the hoop stress, and have a range of lengths and
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depths.
Acceptability was determined according to criteria based on the stress intensity factor and on primary stress limits.
The loads used for the analysis are pressure loads.
Thermal stresses and seismic loads are negligible.
. The most likely mechanism for crack growth is fatigue.
Crack growth from fatigue was determined to be small enough to be neglected.
To determine the largest acceptable flaw sizes for the tanks two sets of calculations were carried out, one for the cylindrical shell and a second for the hemispherical head.
Results showed that any indication with a depth equal to the clad thickness would be acceptable, regardless of its length.
The evaluation also showed that the wall thickness of the steel tank, not including the cladding, met Code requirements with a significant margin of safety.
Some of the cracks in the stainless steel cladding extended to the carbon steel base metal, exposing the steel to the borated water environment.
Service experience has shown that high concentrations of boron can corrode carbon steel.
There is no mechanism for the borated water to become concentrated in the tank so its chemistry is the same as the primary coolant at the beginning of each fuel cycle.
The licensee cites cases in which carbon steel has been exposed to primary coolant environments for long periods with no detrimental effects.
- Horeover, any corrosion would occur slowly at the ambient temperatures at which the tanks operate.
Static load cracking and galvanic attack are also not a concern because a nitrogen blanket controls the oxygen to very low levels.
The licensee considered compliance with the Code with respect to localized thinning.
The design of the tanks included the thickness of the roll-bonded cladding as part of the structure to satisfy the thickness requirements, but analysis showed that the full thickness of the cladding is not required.
The required minimum thickness for local regions is about equal to the original design thickness of the carbon steel, both in the head and cylindrical shell.
2.3
~Monitorin The licensee will monitor the cracks in accordance with the ASHE Code, Paragraph IWC-2420.
The continued monitoring is necessary to ensure that no unexpected crack growth or corrosion had occurred.
III.
CONCLUSION Based on a review of the licensee's analysis the staff concludes that the existing indications in the stainless steel cladding of the safety injection accumulator tanks can be left as is without grinding or repair.
The staff concludes that the cracks were properly dispositioned in accordance with the ASHE Code.
Principal Contributor:
H. Banic Dated:
September 2,
1993
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