ML20236N646

Safety Evaluation Re Permanent Elimination of Liquid Penetrant Exam of Feedwater Nozzles at Facility.Due to Lack of Reasonable Assurance That Ultrasonic Exam Can Totally Replace Penetrant Exam,Request Unacceptable

ML20236N646
Person / Time
Site:	Vermont Yankee
Issue date:	08/07/1987
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20236N634	List: ML20236N631 ML20236N646
References
NUDOCS 8708120046
Download: ML20236N646 (7)

Text

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- * *%,h, UNITED STATES c g NUCLEAR REGULATORY COMMISSION

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l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION PROPOSED CHANGE TO THE FEEDWATER N0ZZLE INSPECTION VERMONT YANKEE NUCLEAR POWER CORPORATION VERMONT YANKEE NUCLEAR POWER STATION DOCKET NO.: 50-271 1.0' INTRODUCTION By letter dated .lanuary 5,'1997, Vermont Yankee Nuclear Power Corporation 4

requested permanent elimination of the liquid penetrant (PT) examination of the feedwater nozzle at the Vermont Yankee Nuclear Power Station. In the 1970's, inspections at 22 of the 23 boiling water reactor plants in the U.S. revealed cracking in the feedwater nozzle inner radius at 18 pl ants. The cracking problem stemmed from the cold feedwater that leaked through the loosened thermal sleeve and impinged on the hot nozzle surface, which is heated by the reactor water. The temperature difference between the feedwater and reactor water could range from 100'F to 400*F, depending on the mode of operation. The temperature fluctuation at the nozzle resulted in metal fatigue with eventual crack initiation. Once initiated, the crack would grow under the influence' of

. additional temperature and pressure transient cycles associated with startups, shutdowns, and abnonnal events. The staff's evaluation of and regulatory requirements regarding the feedwater nozzle cracking issue were documented in NUREG-0619. The staff recommended several hardware changes: (1) thermal sleeve /sparger modification to eliminate the bypass ge e

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t leakage at.the nozzle, (2) stainless steel cladding removal from the feedwater nozzles' because' it is prone to crack initiation, (3) feedwater low-flow controller modification to reduce the temperature fluctuation, (4) reactor water cleanup system rerouting, and (5) installation of an on-line'1eakage monitoring system. The staff also recommended inspection intervals for UT, PT, and visual examinations of the feedwater nozzle inner radii.

i According.to NUREG-0619 guidance, based on Vermont Yankee's. thermal sleeve /sparger configuration, a PT examination is required every 2 fuel cycles, UT examination every fuel cycle, and a visual examination of the sparger' every 2 fuel cycles. The preferable nozzle configuration is the triple-sleeve sparger with two piston-ring seals, without the stainless steel cladding. The original thermal sleeve /sparger at Vermont Yankee was replaced'with an interference fit sparger rattar than a triple-sleeve sparger.

'2.0 EVALUATION The licensee requested elimination of the PT examination based on the following:

(1) The existing UT examination is adequate, (2) .The PT examination in 1977,1979, and 1981 showed that no cracks were found in the feedwater nozzles, j

'(3) The interference-fit spargers were installed in 1976 which eliminated the leakage problem, i

(4) The leakage monitoring system was installed, and based on the o

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.- surveillance results, there has been no leakage at the feedwater nozzles, and (5) The high anticipated radiation dose rates, very high contamination levels, and the potential for high levels of airborne activity cause difficulties in maintaining radiation exposure as low as reasonably achievable (ALARA). The licensee estimated 15 man-rem (Ref. 2) of exposure are incurred during the examination procedure.

The staff concentrated their evaluation on (1) the accuracy and adequacy of the existing UT examination.in detecting feedwater nozzle cracks at

. Vermont Yankee and (2) acceptability of the UT examination replacing the PT examination reports (Ref. 2, 3, 4) which include examination data, equipment calibration, and examiner's training and certification. The staff found the subject matter in the report to be acceptable relative to applicable ASME Code Section XI requirements. The staff also reviewed the UT procedure specifically for Vermont Yankee's feedwater nozzles.

The UT procedure complies with Section XI of the ASME Code 1974 edition.

The actual UT examinations are conducted by the licensee's contractor, Nuclear Energy Services; however, the licensee's own examiner supervises the overall examination. The examiners have either Level I or II certification and the supervisor Level III. All examiners are required to have training on the mock-up of Vermont Yankee's feedwater nozzle and must pass a qualification test. The mock-up nozzle contains artificial cracks that extend from the cladding to the base metal. The NRC regional inspector had witnessed the UT examinations on the mock-up by the examiners (Ref. 5, 7, 8) and concluded that the UT examinations were done according to the procedure, that the examination results were properly l

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e documented and that the examiners were qualified and certified according to applicable regulatory and ASME Code requirements.

1 The PT examination is preferable for detecting surface cracks which, depending on the crack orientation, the UT examination may not be able to detect accurate'v. Nonnally, the examiner moves a transducer along the outside surface of the vessel shell and nozzle barrel to detect nozzle cracks. The transducer emits a sound wave which passes through the nozzle (about 6 inches thick) and shell (abcut 5 inches thick) and it will receive a reflecting wave signal if a crack is present. The reflecting signal depends on the material properties of the metal, distance from the transducer, crack size and orientation, and type of transducer. Due to the presence of stainless steel cladding, the sound wave has to travel through two media with different material properties which cause the speed of emitting and reflecting waves to change. The change of the wave speed could give a unrealistic crack length, or at worst, no crack detection. The staff is concerned about the small surface cracks that may not be detected. Also, the licensee's UT examination procedure did not discuss a special procedure to compensate for the presence of cladding in the crack detection. Based on these concerns, the staff does not have reasonable confidence that the UT examination could replace the PT examination in detecting minor cracks.

The licensee stated that past PT examinations have shown no cracks in the feedwater nozzles. This is not a valid basis for eliminating future PT  !

examinations because as the reactor vessel ages, the cracks are more )

likely to occur. However, the staff shares the licensee's, concern about the ALARA issue. Ai. alternative to eliminating PT examinations is to lengthen the interval between the PT examinations.

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In NUREG-0619, the staff recommended that utilities install on-line 1 ,

leakage monitoring systems at the feedwater nozzles. Once the I

effectiveness of the leakage monitoring system has been demonstrated, the staff will modify its PT examination requirement to give credit for the

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leakage monitoring system. The licensee's on-line leakage monitoring system consists of 16 thermocouple attached to the outside surface of the feedwater nozzles. The NRC resident inspector periodically reviews i

the temperature curve <, from the leakage monitoring system. The results have shown little or no leakage at.the feedwater nozzles. The present favorable results do not guarantee that the nozzles will be leakage-free in the future. It is possible that the interference fit may eventually loosen due to temperature fluctuation or materials aging processes so that leakage will occur in the future. However, the advantage of this system is that it provides an early warning to the licensee so that corrective actions can be taken.

CONCLUSION The staff believes permanent elimination of the PT examination is inappropriate for the specific nozzle configuration at Vermont Yankee because of the following: (1) the staff does not have reasonable assurance and confidence that the UT examination will replace the PT examination in accurately detecting minor surface cracks; (2) the stainless steel cladding may lead to crack initiation in the future due to normal aging processes and adverse environment between two dissimilar materials. The periodic PT examinations provide a redundant and defense-in-depth assurance for cracks that would not otherwise be detected by the UT examination; (3) as the reactor vessel ages, the interference fit between the nozzle and sparger may develop

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. gaps and result in feedwater leakage, which will eventually cause cracks; and (4) the feedwater nozzle is an integral part of the reactor vessel and pressure boundary. It is important to the public safety to have a redundant PT examination of the nozzle.

The staff finds that the PT examination interval at Vermont Yankee may be extended from every 2 fuel cycles to every 6 fuel cycles. As described in NUREG-0619, the 6-fuel-cycle inspection interval is for a nozzle configuration that has no cladding and a sparger that is welded to the safe end of the feedwater nozzle. Although the feedwater nozzl.e at Vermont Yankee is not the same as the above nozzle configuration, the staff judges that a 6-fuel-cycle interval is appropriate considering (1) that the licensee's UT examination is acceptable relative to the ASME Code requirements; (2) the high personnel exposure required to perfona the PT examination (ALARA considerations) and (3) the effectiveness of the leakage monitoring system.

l The first fuel cycle in this revised PT examination schedule starts with the 1987 fuel cycle. Should cracks be detected in the future, the PT examination interval should revert to every 2 fuel cycles. The extension of the PT examination interval does not apply to the UT examination nor visual l examination. The licensee must continue to perform the UT examination of the feedwater nozzles every fuel cycle and visual examination of the sparger every 2 fuel cycles.

Date:

Principal Contributor: J. Tsao l

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• Reference (1) Snaider, R., "BWR Feedwater' Nozzle and Control Rod Drive Return Line Nozzle Cracking," NUREG-0619, USNRC, November,1980.

(2) Letter, VYNPC to USNRC, dated March 28, 1986.

(3) . Letter, VYNPC to USNRC, dated May 26, 1983.

-(4) Letter, VYNPC. to USNRC, dated January 5,1987.

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-(5) USNRC, Inspection Report No. 50-271/87-06, dated April 23, 1987 l (6) USNRC, Inspection Report No. 50-271/84-13, dated August 1, 1984.

.(7) USNRC, Inspection Report No. 50-271/83-32, dated November 12, 1985.

(8) USNRC, Inspection Report No. 50-271/83-02, dated June 11, 1983.

(9) Letter, VYNRC to.NSNRC, dated June 2,1987 l

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