ML19345G154
| ML19345G154 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 05/27/1975 |
| From: | Hartley F CONNECTICUT YANKEE ATOMIC POWER CO. |
| To: | |
| References | |
| AO-50-213-75-02, AO-50-213-75-2, NUDOCS 8103090026 | |
| Download: ML19345G154 (2) | |
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ABNORMAL OCCURRENCE REPORT
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Report Number:
50-213/75-2 N'
O' ? /y y.f",Y Report Date:
5-27-75 Occurrence Date:
5-21-75 Facility:
Connecticut Yankee Atomic Power Company Haddam Neck Plant Identification of Occurrence:
Broken Long Holddown Bolts #3 & #4 Steam Generators Conditions Prior to Occurrence: Refueling Shutdown Description of Occurrence:
On May 21, 1975 during preparatory work associated with the installation of a new steam generator support and seismic restraint system, one of the original system long bolts (#55) on the #3 steam generator was found broken. A long bolt alarm should have been activated on the bolt position monitoring system, however, no alarm had been received. However, a "short bolt" alarm for #7 microswitch position was indicated but the associated short bolt was checked in the normal position.
The monitoring system was designed to alert the operators to a possible bolt problem and was intended to serve temporarily until a new steam generator hold-down system could be designed and installed.
Prior to the discovery of the broken bolt no "long bolt alarm" had been received, nor was one present at the time of the discovery.
Designation of Apparent Cause of Occurrence:
Installation Analysis of Occurrence:
When the possibility of a broken long bolt became known, progress towards removal of the bolt position monitoring system was temporarily suspended and an investigation initiated to determine why the alarm system had failed to identify the condition.
A check of the electrical circuitory disclosed that the microswitch giving the "short bolt" alarm was installed on the broken'long bolt and vice versa.
Further checking revealed that the idratification numbers for #7 (short bolt) and #8 (long bolt) microswitches were reversed. The satisfactory electrical continuity checks preformed during installation of the monitoring system therefore did not find the improperly marked loation numbers.
A complete investistion of the holddown support system was conducted and revealed two additional broken "logg"obolts on #4 steam generator (bolts #51 and #59). No alarm had been received to indicate that these bolts were broken and an examination of the alarm system indicated that the systet was functioning normally.
In fact all alarms were functionally tested and performed properly. The two affected microswitches were removed for shop testing to insure that the twe switches were
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ABNORMAL OCCURRENCE REPORT 50-213/75-2 not damaged by the bolts " breaking". Shop tests as well as functional tests inplace indicated that no malfunction of the microswitches existed; consequently it appears that upon breaking, the two bolts did not " rise" sufficiently to actuate the alarms.
In the original installation it was assumed that upon breaking the bolts would " pop up" a significant amount, based on observation of the original twenty four broken bolts. The alarm system microswitches were set to actuate with only several thousandths of an inch movement but.were not designed for great accuracy. Additionally, it is likely that the settings drifted during the seventeen months service. At any rate, tests indicated that slightly more movement would have actuated the alarms.
Plant records and normal reports will document any further findings of the investigation, however, since this system is being removed from service it does not appear to warrant further reporting at this time.
Corrective Action:
During the current refueling and maintenance outage, Plant Design Change #180 is expected to be completed. PDC #180 consists of a completely new, independant method of steam generator seismic restraint which has been reviewed and approved by the NRC.
Failure Data:.
- 1) Abnormal Occurrence Number 73-8, September 28, 1973 reported twenty three broken bolts (16 long, 7 short).
S.G. #1-1, S.G. #2-11, S.G. #3-3 & S.G. #4-8.
- 2) Abnormal Occurrence Number 73-12, December 7,1973 reported one broken bolt (long) S.G. #3-1.
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Plant Superintendent Page 2 of 2
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