IE Circular 76-06, Stress Corrosion Cracks in Stagnant, Low Pressure Stainless Piping Containing Boric Acid Solution at PWR's
CR78006 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 NOV 22 1976 J. P. O'Reilly, Director, Region I N. C. Moseley, Director, Region II J. G. Keppler, Director, Region III E. M. Howard, Director, Region IV R. H. Engelken, Director, Region V IE CIRCULAR 76-06, STRESS CORROSION CRACKS IN STAGNANT, LOW PRESSURE STAINLESS PIPING CONTAINING BORIC ACID SOLUTION AT PWR's The subject document is transmitted for issuance by close of business three days after date of this letter. The Circular should be issued to all holders of OL's for PWRs for action. Also enclosed is a draft copy of the transmitted letter. Dudley Thompson, Acting Director Division of Field Operations Office of Inspection and Enforcement Enclosures: 1. Circular 76-06 2. Draft transmittal letter . Transmittal letter for Circular 76-06 to each holder of a NRC Operating License for a PWR. Addressee: The enclosed Circular 76-06 is forwarded to you for action. If there are any questions related to your understanding of the actions required, please contact this office. Signature (Regional Director) Enclosure: IE Circular 76-06 . IE Circular 76-06 Date: STRESS CORROSION CRACKS IN STAGNANT, LOW PRESSURE STAINLESS PIPING CONTAINING BORIC ACID SOLUTION AT PWR's DESCRIPTION OF CIRCUMSTANCES: During the period November 7, 1974 to November 1, 1975 several incidents of through-wall cracking have occurred in the 10-inch, schedule 10 type 304 stainless steel piping of the Reactor Building Spray and Borated Water Make-up Systems at Arkansas Nuclear Plant No. 1. On October 7, 1976 Virginia Electric and Power also reported through-wall cracking in the 10-inch schedule 40 type 304 stainless discharge piping of the "A" recirculation spray heat exchanger at Surry Unit No. 2. A recent inspection of Unit 1 Containment Recirculation Spray Piping revealed cracking similar to Unit 2. On October 8, 1976 another incident of similar cracking in 8-inch schedule 10 type 304 stainless piping of the Safety Injection Pump Suction Line at the Ginna facility was reported by the licensee. Information received on the metallurgical analysis conducted to date indicates that the failures were the result of intergranular stress corrosion cracking that initiated on the inside of the piping. A commonality of factors observed associated with the corrosion mechanism were: 1. The cracks were adjacent to and propagated along weld zones of the thin-walled low pressure piping, not part of the reactor coolant system. 2. Cracking occurred in piping containing relatively stagnant boric acid solution not required for normal operating conditions. 3. Analysis of surface products at this time indicate a chloride ion interaction with oxide formation in the relatively stagnant boric acid solution as the probable corrodant, with the state of stress probably due to welding and/or fabrication. . -2- The source of the chloride ion is not definitely known. However, at ANO-1 the chlorides and sulfide level observed in the surface tarnish film near welds is believed to have been introduced into the piping during testing of the sodium thiosulfate discharge valves, or valve leakage. Similarly, at Ginna the chlorides and potential oxygen availability were assumed to have been present since original construction of the borated water storage tank which is vented to atmosphere. Corrosion attack at Surry is attributed to in-leakage of chlorides through recirculation spray heat exchange tubing, allowing buildup of contaminated water in an otherwise normally dry spray piping. ACTION TO BE TAKEN BY LICENSEE: 1. Provide a description of your program for assuring continued integrity of those safety-related piping systems which are not frequently flushed, or which contain nonflowing liquids. This program should include consideration of hydrostatic testing in accordance with ASME Code Section XI rules (1974 Edition) for all active systems required for safety injection and containment spray, including their recirculation modes, from source of water supply up to the second isolation valve. Similar tests should be considered for other safety-related piping systems. 2. Your program should also consider volumetric examination of a representative number of circumferential pipe welds by nondestructive examination techniques. Such examinations should be performed generally in accordance with Appendix I of Section XI of the ASME Code, except that the examined area should cover a distance of approximately six (6) times the pipe wall thickness (but not less than 2 inches and need not exceed 8 inches) on each side of the weld Supplementary examination techniques, such as radiography, should be used where necessary for evaluation or confirmation of ultrasonic indications resulting from such examination. 3. A report describing your program and schedule for these inspections should be submitted within 30 days after receipt of this Circular. . -3- 4. The NRC Regional Office should be informed within 24 hours, of any adverse findings resulting during nondestructive evaluation of the accessible piping welds identified above. 5. A summary report of the examinations and evaluation of results should be submitted within 60 days from the date of completion of proposed testing and examinations. This summary report should also include a brief description of plant conditions, operating procedures or other activities which provide assurance that the effluent chemistry will maintain low levels of potential corrodants in such relatively stagnant regions within the piping. Your responses should be submitted to the Director of this office, with a copy to the NRC Office of Inspection and Enforcement, Division of Reactor Inspection Programs, Washington, D.C. 20555. Approval of NRC requirements for reports concerning possible generic problems has been obtained under 44 U.S.C 3152 from the U.S. General Accounting Office. (GAO Approval B-180255 (R0062), expires 7/31/77).
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