ML19274E795
| ML19274E795 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 04/12/1979 |
| From: | Stallings C VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| 248-040579, 248-40579, NUDOCS 7904240422 | |
| Download: ML19274E795 (2) | |
Text
'
VIHUIN1A E x.ncr a C AND POWEH COMPANY H a c n M oxo.Vi noin A caust April 12, 1979 Mr. liarold R. Denton, Director Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Bethesda, MD 20014 Serial #248/r'.0579 Docket #50-338 Attention:
Mr. D. Vassallo
Dear Mr. Denton:
During a telephone conversation on April 5, 1979 with Mr. J.P. O'Reilly of Region II, we reached an agreement that North Anna Unit I would not be returned to service until we provided technical justification to the Office of Nuclear Reactor Regulation that the flow splitter plates installed on this unit are structurally sound. Today we met in Bethesda with representatives of NRR and presented the following information in support of continued operation of North Anna Unit 1.
I During a routine cleaning of the reactor coolant loop crossover leg
)
pipes in North Anna Unit II, cracks were discovered in splitter plate 2-C installed in the reactor coolant system pipe elbow adjacent to the reactor coolant pump. As a result of the discovery, an examination was conducted by Vepco on the other splitter plates on Unit 2 to determine their condition.
The splitter plate is not a structural member and neither it nor the observed cracks affect the reactor coolant system pressure boundary.
S Subsequent to this observation, a thorough examination was performed on North Anna Unit 1 to assure the integrity of the flow splitter plates in tha t unit.
Design Purpose i
The design and fabrication of the reactor coolant pump suction elbows installed at North Anna Units 1&2 includes a splitter plate to enhance flow distribution at the pump impeller inlet and to increase uniformity in velocity distribution, which results in some degree of improved pump performance.
The splitters are of multiple-piece construction, fabricated by welding three sections of curved stainless steel plates to the inner vall of a 90 degree cast stainless steel elbow. The elbows themselves are designed and inspected to the requirements c' ANSI B31.7, 1969 and ASME Section III, 1971 edition.
Examination on Unit 1 i
Subsequent to the examination of the reactor coolant system splitter plates on Unit 2, an ultrasonic examination (UT) of each sp]ttter plate on Unit 1 was performed from the outside diameter of the pipe c.tbow to determine the integrity of the splitter plate and weld.
It was clear from these
(.
0 7904240 $ F I
p.,
examination results that the flow splitters of Unit 1 are structurally sound.
_ Justification of Unit 1 Splitter Plate Integrity
'1he S-N diagram for austinitic stainless steel as shown in Section III of the ASME B&PV code, shows that ac approximately 106 cycles, the failure curve becomes relatively flat. Therefore, if failure from high cycle fatigue does not occur prior to approximately 106 cycles, the component would not be expected to f ail during the remainder of the plant life.
Since the resonant frequency of the splitter plate in water is on the order of 150hz, the number of cycles of vigration during the 10 day hot functional test was approximately 1.3 x 10 cycles.
This indicates that if fatigue of the splitter plate was going to occur due to high cycle fatigue, it would have occurred prior to completion of the hot functional test.
Failure of the splitter plate in Unit 2 was due to high cycle fatigue as confirmed by metallurgical examination and did occur sometime during the hot functional test.
SinceUnit 1 has been operational for approxinately one year, the accumulated number of cycles is on the order of 5 x 10, which is well beyond 10 cycles 9
6 where failure should have occurred.
In summary, the U-T examina tion performed on Unit 1 clearly demonstrates that the splitter is structurally sound and can perform for the remainder of the plant life.
Having demonstrated the integrity of North Anna Unit 1 flow splitters, we urgently request your immediate approval to allow the return to service of this very vital part of Vepco's generating capacity, and your prompt j
notification to Region II of this approval.
Very truly yours, 6.?D7. Jia e q C.M. Stallings Vice President Power Supply and Production Operations cc:
Mr. J.P. O'Reilly I