ML20207K791

Safety Evaluation Supporting Operation in Cycle 6 W/O Removing Flaws in Cracked HPI Nozzle

ML20207K791
Person / Time
Site:	Davis Besse
Issue date:	10/04/1988
From:	Office of Nuclear Reactor Regulation
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ML20207K781	List: ML20207K778 ML20207K791
References
NUDOCS 8810170023
Download: ML20207K791 (2)

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el WASHINGTON, D. C. 20555 SAFETY EVALVATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION TOLLUU LD150h LUMPANY DAVIS-BE5SE NUCLEAR POWER 51 ATION DOCKET NO. 50-346 During a combined ASME Section XI and pre-fueling visual inspection several "looseparts"werediscoveredintheDavis-BesseNuclearPowerStatIon(DBNFS)

Unit I reactor vessel. Two pieces were determined to be from a high pressure injection / makeup (HPI) nozzle thermal sleeve.

In a letter dated September 14, 1988, the licensee reported the results of field inspections, laboratory metal-lographic analysis, and fracture mechanics analyses of the cracked thermal sleeve and of the adjacent nozzle at the connection of the HPI piping to the reactor coolant system (RCS) cold leg.

The metalleraphic analysis of the failed thermal sleeve indicates that the cracks propaga+. by fatigue, with high cycle thermal stresses being the primary cause of crack.uwth, and flow induced vibratory stresses a secondary cause. The licensee concluded that the high cycle thermal fa'igue stresses resulted from thermal mixing of hot reactor coolant and relatively cc.d makeup flow at the outlet end of the sleeve during periods of low makeup flow. The licensee has replaceo the failed thermal sleeve with a design which will facilitate installation and reduce the effect of potential flow-induced vibration.

The licensee has committed: (a) to evaluate and, as practical, increase minimum bypass flow; (b) to provide for neans of accurately setting the minimum bypass flow; and (c) to provide procedures to clearly establish administrative control of bypass flow.

Dye penetrant examination of the nozzle adjacent to the failed thermal sleeve revealed ru1tiple indications between the RCS weld buttons and the nozzle knuckle region with some extending over the knuckle, into the cold leg piping. Grinding away of approximately one-sixteenth inch of clad material did not remove these indications.

It is believed that these indications are cracks resulting from high cycle thermal fatigue.

The fracture mechanics analysis indicates that high cycle thermal fatigue cracks are self limiting to a depth substantially less than the depth permitted by ASME Code when a defect is evaluated for brittle fracture considerations. The licensee's brittle fracture analysis met the margins rr. quired for continued operation in Subarticle IWB-3600 of Section XI of the ASME Code (1977 Edition through Sumer 1977 Addenda). The analysis indicates that high cycle thermally induced cracks in the HP!

nozzle with thermal sleeves intact will not grow to a size which will affect the integrity of the nozzle during the remaining life of DBNPS.

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2-s Ultrasonic examination of the nozzle revealed no reportable indications.

The licensee c intends that, based on the detection capability of the ultrasonic exami-nation, these examination results indicate that the cracks in the nozzle are shallow (less than 0.3 inches in depth) and have not penetrated through the clad into the base metal.

CONCLUSIONS:

Based on the results of the licensee's fracture mechanics analyses, the ultrasonic examination, the revised thermal sleeve design, and the licensee's commitments related to evaluation and monitoring of bypass flow, the staff concludes that DBNPS may be operated for Cycle 6 without removing the cracks in the HPI nozzle.

Principal contributor: Barry Elliot, ENTB Dated: October 4, 1988 I