ML20084R938
| ML20084R938 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 02/18/1976 |
| From: | Stephenson B COMMONWEALTH EDISON CO. |
| To: | James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| Shared Package | |
| ML20084R941 | List: |
| References | |
| 155-76, RO-50-249-74-29, NUDOCS 8306160076 | |
| Download: ML20084R938 (2) | |
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Comm e:lth Edison N. i Dresden ear Power St ti:n ib
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Morris, Illinois 60450 g% t7 Telephone 815/942-2920
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fG BBS Ltr. #155-76 February 18, 1976 Mr. James G. Keppler, Regional Director Directorate of Regulatory Operations-Region 111 U. S. Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, Illinois 60137
SUBJECT:
FOLLOW-UP REPORT TO LETTER DATED OCTOBER 2, 1974, ENTITLED "THROUGH-WALL CRACK ON TORUS /DRYWELL PURGE LINE, UNIT-3."
References:
1)
Report No. 50-249/1974-29 2)
Drawing No. H-356 Report Number: 50-249/1974-29A Report Date:
February 18, 1976 Occurrence Date:
September 24, 1974 Facility:
Dresden Nuclear Power Station, Morris, 111.
INTRODUCTION The cause of the through-wall crack on the Unit-3 torus /drywell purge line had not yet been determined at the time of the original abnormal occurrence report (50-249/1974-29, October 2, 1974).
Additional information has since come to light; accordingly, this follow-up report is being submitted per the station's commitment of October 2, 1974.
DESIGNATION OF APPARENT CAUSE OF OCCURRENCE The cracked section of the 18-inch line was sent to the company's Operational Analysis Department (OAD) for testing.
0AD determined that the piping 0
had a nil-ductility temperature (NDT) of 80 F.
0AD postulated that the pipe had been subjected to slight pre-loading during installation, resulting in a built-in strain on the piping.
8306160076 760218 PDR ADOCK 05000249 2046 S
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O Febru.<v is, is76 Mr. Jtmes G. Kcppler O
- It is conjectured that a temporary loss of vaporizer efficiency during drywell inerting caused liquid nitrogen to contact the stressed piping.
The crack probably originated in a nipple-to-pipe weld; the relatively high NDT of the piping, as well as the pre-load on the line, undoubtedly contributed to the propagation of the crack through the pipe wall.
Even a brief loss of heating steam from the boilers would seriously impair the efficiency of the vaporizer during the flow conditions of dryweII Inerting.
The resultant IIquid nitrogen would cause severe contraction of the piping, magnifying any abnormal stresses or imperfections.
CORRECTIVE ACTION As stated in the original report, immediate corrective action was to examine the pipe for other cracks, and to initiate a repair program.
No other Indications were found, and a new pipe segment was promptly welded into place.
Radiographic tests proved the new welds to be satisfactory.
Since that time, a thermocouple and strip chart have been installed on the vaporizer discharge line near the vaporizer. This modification will facilitate rapid isolation of the vaporizer in the event of low temperature indication.
Precautionary steps have been added to the inerting procedure which ensure close monitoring of the strip chart recorder and of the heating boilers during the inerting process.
This will enable the operator to regulate nitrogen flow so that the pipe temperature remains above a predetermined limit.
Finally, the company's Station Nuclear Engineering Department is presently evaluating a modification which would add a temperature control valve to the line. This modification would permit automatic isolation of the line in response to low temperature indication.
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D c B. B. S'tephenson Station Superintendent BBS:RJR:smp FIIe/NRC
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