ML20091R035
| ML20091R035 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 06/08/1984 |
| From: | Dente T BWR OWNERS GROUP |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| RRG-8403, NUDOCS 8406140236 | |
| Download: ML20091R035 (3) | |
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T.1 Dente, Chairman P.O. Box 270 e Hortford, Connecticut 06101 e (203) 666-6911 X 5489 June 8, 1984 RRG-8403 Mr. Harold R. Denton Director of Nuclear Reactor Regulation U.S. Nucicar Regulatory Commission Washington, D.C.
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Dear Mr. Denton:
SUBJECT:
Regulatory Response Group (RRG) Response to Inconel 182 Wold Butter Crack Indications REFERENCE (1): RRG Meeting with NRC on Tuesday, June 5,1984 to discuss indications identified at the Boston Edison Pilgrim Nucicar Power Station In Reference (1), the RRG and the BWR Pipe Crack Owners Group met with you and your staf f in order to provide the NRC with a reasoned response to the indications observed in the Inconel 182 weld butter in the RPV nozzle to safe end weld at the Pilgrim Nucicar Power Station. This letter serves to document the conclusions and reasoned response of that meeting and reiterates the basis for the continued safe operation of BWR Nucicar Units.
In early May 1984, during replacement of recirculation piping at Pilgrim, liquid penetrant indications were observed in Inconel 182 weld butter in the RPV nozzle to safe end weld. Based on recent laboratory tests, GE had concluded that Inconel 182 was susceptible to IGSCC and had recommended inspection of such welds during piping replacement programs. Inspections to date at Pilgrim have found indications in three out of ten recirculation inlet nozzle welds and one out of two recirculation outlet nozzle welds.
The cracks were axially oriented and initiated in the Inconel 182 weld metal on the ID surface.
Boat samples taken from the RPV recirculation l
outlet nozzle weld region confirmed intergranular (interdendritic) stress corrosion cracking.
The safety implication of the cracking'in the Inconel 182 weld butter has been evaluated. The significant results from the evaluation are described here:
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o y o Cracking observed to date in the inconal 182 weld butter has been predominantly axial with length approximately inch.
Such short axial cracks do not pose any safety concerns. The length of the axial cracks in the weld butter is inherently limited by the adjoining
' materials which are more resistant to ICSCC (Inconal 82 root pass and the unsensitized stainless steel safe and or low alloy secol nozzle).
Even if the observed cracking were through-wall, the code intended safety margins are maintained.
In fact, the critical crack size for a through-wall crack in the low alloy steel nozzio is in excess of 22 inches for the 12-inch diameter recirculation inlet nozzle and 27 inches for a 28-inch diameter outlet nozzio. The corresponding critical size for cracks in the stainless stool safe end is even larger. Clearly, the observed cracking in well below
'the critical size. Therefore, even if the crack propagates to be through-wall, leak-before-break will be maintained and the crack would be detected by the normal Icak monitoring systems, o ICSCC crack initiation time and crack growth rate in Inconel 182 are less severe than those in wold sensitized stainless steel.
Therefore, the extent of cracking in the Inconal weld butter is expected to be similar to or less severe than that observed in
'304 stainless steel piping welds. Thus there is time for orderly implementation of remedial measures.
In any case, the observed cracking does not pose safety concerns.
o Inspection of Inconel 182 weld metal on RPV recirculation nozzle / safe end weld has been conducted at Duane Arnold as part of the safe and replacement program. No similar indications were found during this insp'ection.
In addition Type 308 stainless steel wolds between the RPV nozzles and safe ends at Monticello showed no similar indications following inspection. The fabrication history of the Pilgrim RPV nozzle / safe end welds is currently being evaluated to determine whether the observed cracking could be due to metallurgical or residual stress factors unique to Pilgrim.
o Axial cracks in the Inconel weld metal which extend beyond the weld crown into the safe end or nozzle base material can readily be detected.
o While UT inspection of weld butter is more dif ficult, it can be done, especially with proper qualification on mockups with actual weldment geomet ry. Circumferential cracks which are of greater fracture concern can be detected with greater reliability.
o Leak-before-break margins remain valid and structural integrity is
. maintained.
Based on the above, it is believed that the observed cracking in the Inconel 182 weld butter does not pose any safety concerns, and is no worse than IQSCC previously discovered in BWR stainless stool piping and can be mitigated with an orderly remedial program.
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h, During the course of discussions in Reference (1), an inquiry was made as to the interpretation of the I&E Bulletin 82-03 and I&E Bulletin 83-02 l
inspection' scope. This inspection would, if performed on the safe end Heat Affected Eome (MAE) in the safe end to nosate weld in the recirculation system, provide added information regarding propagation of cracking into the safe end. The industry participant in Reference (1) representing the Hatch 2 Unit committed to an inspection of six (6) of the reactor recirculation system safe end-to-nousle welds and will report the results to the NRC.
The BWR pipe Crack Ouners Group will be responding to the NRC further on this issue in approximately one month. During that period of time, both the Technical Advisory Committee and the Senior Representatives of the Owners Group will be meeting to define program and resource requirements for further investigating inspection, integrity and remedy application issues applicable to Inconel weld butter cracking.
Very truly yours, f.L- / Sk Thomas J. Dente TJD/ gap 4
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