ML19296D813
| ML19296D813 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 02/06/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19296D812 | List: |
| References | |
| IEB-79-13, NUDOCS 8003130267 | |
| Download: ML19296D813 (2) | |
Text
k,,
pa arog'o UNITED STATES
'g 8
-.,, c g
NUCLEAR REGULATORY COMMISSION
- '"_p WASHINGTON, D. C. 20555
%.... /
SAFETY EVALUATION BY THE li
.E OF NUCLEAR REACTOR REGULATION REGARDING ACTIONS TAKEN IN RESPONSE TO BULLETIN 79-13 "CRACKI_NG IN FEEDWATER SYSTEM PIPING" NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT UNIT NOS. 1 AND 2 DOCKET N05. 50-282 AND 50-306 Introduction On June 19 and July 16, 1979 the Northern States hwer Company (NSP) responded to our May 25, 1979 request for in?.rmation regarding possible cracking in auxiliary feedwater lines at the vrairie Island Nuclear Generating Plant Unit Nos. I and 2.
In the responses all information was supplied except the inspection results from the January 180 refueling outage for Unit No. 2.
Background
On May 20, 1979, Indiana and Michigan Power Company notified the NRC' of cracking in two feedwater lines at their D.C. Cook Unit 2 facility.
The cracking was discovered following a shutdown on May 19 to investigate leakage inside con-tainment.
Leaking circumferential cracks were identified in the 16-inch feedwater elbows adjacent to two steam generator nozzle to elbow welds.
Sub-sequent radiograchic examinations revealed cracks in all eight steam generator feedwater lines at this location on both Units 1 and 2.
On May 25, 1979, a letter was sent to all PWR licensees by the Office of Nuclear Reactor Regulation which informed licensees of the D.C. Cook failures and requested specific information on feedwater systm design, fabrication, inspection and operating historic To further explore the generic nature of the cracking proolem, the Office of :nspection and Enforcerent reouested licen-sees of P'4R plants in current outages to imediately conduct volumetric examination of certain feedwater pioing welds.
As a result of these actions several other licensees reported cracking in the steam generator nozzle to feedwater piping weld vicinity.
On June 25, 1979, IE Bulletin 79-13 was issued.
The Bulletin required inspection of the steam generator nozzle-to-pice welds and adjacent areas within 90 days and reinspection of these welds, the feedwater piping welds to the first support, the feedwater pioing to containment penetra-tion and the auxiliary feedwater to main feedwater piping connection at the next refueling outage.
3003\\30264
Le
, Evalua tion In conformance with the Bulletin the Prairie Island Plant Unit No.1 initial radiographic and visual examinations has been completed and no evidence of cracking in the feedwater piping has been found.
From the results of instrumentation installed at several plants which have experienced feedwater piping cracks and other modeling and analyses by a utility sponsored Owners Group, significant cyclic stresses occur in the feedwater piping in the vicinity of the steam generator nozzle from mixing and stratifica-tion of cold auxiliary feedwater with hot water from the stean generator during low flow conditions. fietallurgical analyses of the cracked feedwater piping has identified the mode of failure as fatigue assisted by corrosion.
The Owners Group is expected to complete their investications and make recommenda-tions for changes in design and operating procedures by February 1980.
In addition, the NRC has instituted a Pice Crack 5tudy Group to review thi? and other pice cracking problems in PWR's.
It is anticipated that the Pipe Crack Study Grouc will complete its work by June 1980 and provide recommendations to be implemented by Staff as new criteria for operating plants, if recuired.
Although cracking has not been identified through the inspections performed to date for the Prairie Island Plant, the staff feels that the cyclic stress induced by the thermal transients present when cold auxiliary feedwater at low rates is injected into the main feedwater may result in cracking at these facilities. The staff and the Owners Group both have performed independent analyses and have determined that flawed piping could withstand challenges from operating and faulted loads in-cluding seismic and limited water hammer loads without loss of piping integrity. Pipe breaks have occurred in the past in feecwater piping as the result of water hammer loads. However, mea ures such as "J" tubes have been instituted and operational char.ges have occurred to minimize the possibility of water hammer.
In the unlikely event of a feedwater pipe break from a severe water hammer, the consequences have been analyzed as a design base accident and measures have been established to deal with the event.
Conclusion We conclude that the nondestructive inspections performed and scheduled and the analyses perferred for flawed pioing ensure that the ciping intecrity until the recorrendations of the Owners Grouc and the Pice Crack Study Grouc have been evaluated.
Should the Sta#f determine that further actions are recuired af ter evaluation of the Owners Group and Pipe Crack Study recorrendation, the licensees will be notified at that tine.
Date: February 6,1980
-