ML20204C334
| ML20204C334 | |
| Person / Time | |
|---|---|
| Issue date: | 11/17/1978 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Haller N NRC OFFICE OF MANAGEMENT AND PROGRAM ANALYSIS (MPA) |
| Shared Package | |
| ML20204C338 | List: |
| References | |
| RTR-NUREG-0090, RTR-NUREG-90 NUDOCS 7811290185 | |
| Download: ML20204C334 (4) | |
Text
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C WASHINGTON, D. C. 20555 November 17, 1978 MEMORANDUM FOR:
Norman M. Haller, Director, Office of Management
& Program Analysis FROM:
Harold R. Denton, Director, Office of Nuclear Reactor Regulation
SUBJECT:
ABNORMAL OCCURRENCE REPORT TO CONGRESS FOR THIRD QUARTER CY 1978 NRR concurs with the draft Commission Paper forwarded by your memorandum dated October 26, 1978, subject to the inclusion of the attached revision to Item 75-5, Pipe Cracks at Boiling Water Reactors.
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Harold R. Denton, Director Office of Nuclear Reactor Regulation
Attachment:
Revision to Item 75-5 e
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781129 MSX
I PIPE CRACKS AT B01LfMG WA?ER REACTORS 1
Pipe Cracking has occurred in the heat affected zones of weldt in primary system pipino in Boilina Water Reactors since the mid lun't. These cracks have occurred mainly in Tyoe 304 stainless steet that is beine used in most operating BWRs. The major problem is recognized to be intergranular stress corrosio cracking (IGSCC) of austenitic stainless steel components that have been made susceptible to this failure mode by being " sensitized" either by post-weld heat treatment or by sensitization of a narrew heat affected zone near welds.
" Safe ends" (short transition pieces between vessel nozzles and the piping) that have been highly sensitized by furnace heat treatment while attached to vessels during fabrication were very early (late 1960s) found to be susceptible to IGSCC. Because they were susceptible j
to cracking
. the Atomic Energy Commission took the position in 1969, that furnace sensitized safe ends should not be used on new applications.
Most of the furnace sensitized safe ends in older plants have been removed or clad with a protective material, and there are only a few EWRs that still have furnace sensitized safe ends in use. Most of these, however, are in smaller diameter lines.
Earlier reported cracks (prior to 1975) occurred primarily in 4" diameter recirculation loop-by-pass lines and in 10" diameter core spray lines. Mofe recently cracks were discovered in recirculation riser piping (12" - 14")
in foreign plants.
Cracking is most often detected during Inservice Inspection using ultrasonic testing techniques. Some piping cracks have been discovered as a result of primary coolant leaks.
Because of these occurrences of BWR primary system cracking, there has been a wide variety f>f actions undertaken by the NRC. These actions included:
issuance of Regulatory Guide 1.44 on " Control of the Use of Sensitized Stainless Steel" issuance of Regulatory Guide 1.45 on " Reactor Coolant Boundry Leak Detection Systems" closely following the incidence of cracking in BWRs, including foreign experience encouraging replacement of furnace sensitized s.tfe ends
. a 2
requiring augmented inservice inspection (additional more frequent i
ultrasonic examination) of " service sensitive" lines, i.e., those that have experienced cracking requiring uporading of leak detection systems Pipe cracking and furnace sensitized safe end cracking has been recently reported in larger (24" diameter) lines in a GDdesigned BWR in Gemany with over 10 years of service. Because the safe ends on that facility had been furnace sensitized during fabrication. IGSCC was suspected. As a result of concerns regarding these furnace sensitized safe ends, a safe end was removed in order to perfom destructive examination. During laboratory examination of the removed safe end, including a small section of attached pipe, cracks were discovered at various locations in the safe end and in the weld heat affected zone of the pipe.
The cracks in the pipe weld area were very shallow with the maximum depth less than about 5 m (about 1/8"). Cracking in the furnace sensitized safe end was somewhat deeper. The German experience was the first known occurrence of IGSCC in pipes as large as 24" in diameter.
In June 1978, a through-wall crack was Oiscovered in an Inconel recirculation riser safe end (10" diameter) at the Duane Arnold facility.
The crack has been attributed to IGSCC although the material in this instance is different from the Type 304 stainless steel that has been historically found to crack. Subsequent ultransonic examination discovered indications in six of the other seven safe ends. Following their
- removal, cracking was discovered in all eight safe ends.
The cracking appeared to have originated in a tight crevice between the inside wall of the safe end and an internal thermal sleeve. Such crevices are known to enhance IGSCC. Differences in materials, geometry, stress levels and crevices appear to make the problem at Duane Arnold unique to a particular type of recirculation riser safe end (Type I). As a result of this e,ent, ultrasonic examination of the other Type I safe ends in US BWRs,1.e., at the Brunswick 1 ano 2 facility, was conducted. No significant indications wera found in i
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i Unit 2 and one indication, was identified at Unit 1.
Although this indication is relatively minor and is not " reportable" pursuant to the NRC Regulations, it is continuing to be evaluated. The ultrasonic indication which was found will be reevaluated at another plant shutdown' scheduled for later in 1978.
In addition to discussions with General Electric (the reactor vendor) j regarding recent pipe cracking experience, General Electric was asked to provide an in-depth report on the significance of recent events regarding current inspection, repair and replacew r.t programs. They were also asked to address any new safety concerns related to the occurrence of cracking in large
' main recirculation piping. Based on information presented by General Electric to date, and extensive staff evaluation, it was concluded that the recent occurrences did not constitute a basis for imediate concern about plant safety.
nor require any new imediate actions by licensees.
The staff briefed the Corrission on pipe cracking in BWRs on August 31, 1978, and on Se;tember 14, 1978, re-established an NRC Pipe Crack Study Group.
4 The Study Grou; will specifically address the following issues:
j the significance of the cracks discovered in large diameter pipes relative to the conclusions and recomendations set forth in the referenced report and in its implementation document NUREG-0313, resolution of concerns raised over the ability tc use ultrasonic techniques to detect cracks in austenitic stainless steel, the significance of the cracks found in large diameter sensitized safe ends,and any recorriendations regarding the currect NRC program for dealing with this mati.er, the potential for stress corrosion cracking in PWRs and the significance of the safe end cracking at Duane Arnold relative to similar material and design aspects at other facilities, a
The Study Group is scheduled to complete its evaluation and resort in January 1979. In addition to the study group effort, the NRC has underway generic technical review efforts regardino flaw detection which are aimed at improving piping inspection techniques and requirements.
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