ML19319D119
| ML19319D119 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 04/02/1979 |
| From: | NRC OFFICE OF INSPECTION & ENFORCEMENT (IE) |
| To: | |
| Shared Package | |
| ML19319D120 | List: |
| References | |
| NUDOCS 8003130260 | |
| Download: ML19319D119 (2) | |
Text
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COMMON MODE FAILURE IN HIGH PRESSURE INJECTION SYSTEM Date and Place On March 12, 1979, the Toledo Edison Company reported an event at the Davis-Besse.luclear Power Station Unit 1, a pressurized water nuclear
. plant, located in Ottawa County, Ohio.
Nature and Probable Consecuences The event occurred on' January 3, 1979, while the reactor was operating at-approximately 96 percent power.
During a monthly surveillance test of the High Pressure Injection (HPI) pumps, there was no flow through the recirculation line from the pump dischcrge to the Borated Water Storage Tank (BWST).
Investigation revealed that a portion of the line exposed to the outside weather was frozen. This portion of the line to the BWST is corrion to both HPI pumps. The line has redundant heat tracing (a heated wire to prevent freezing); it is insulated; and it has a low-temperature alarm system to warn of freezing conditions. The line froze through; apparently because of prolonged sub-freezing temper-atures and a defect in the insulation.
There was no apparent malfunction of the heat tracing or temperature alarm circuits, although the temperature sensing elements are located in an area not exposed to the coldest temper-ature conditions.
This recirculation line serves two purposes: One is to provide a flow path for surveillance testing, and the other is to provide a minimum flow path to prevent possible damage to the pump in the event it is operated against a closed discharge valve or a reactor coolant system pressure greater than the maximum discharge pressure of the pump (approximately 1600 psig).
In its initial review of the event on January 3, 1979, the licensee concluded that the pumps were still operable with the recirculation line frozen. Therefore, no immediate action was taken to thaw the frozen section of the line. On January 5, 1.979, the line was thawed and the surveillance test of the pumps was successfully completed.
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_During a review of the event on March 12, 1979, the licensee concluded that there were special. conditions in which reactor coolant system pressure could decrease slowly enough to cause the HPI pumps to i
operate for a significant period of time at maximum discharge pressure with no water flow through the pump (shutoff head) if the recirculation line were frozen or otherwise blocked. Depending upon the length of
. time '.he HPI pumps operated at " shutoff head", the internal pump tempu.sture rise could damage the pumps and make them inoperable.
This type of event represents a common mode failure to both parallel trains _of the HPI system.
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-2 Cause or Causes This event is attributed to an inadequate design. The heat tracing provided on this recirculation line was unable to prevent line freeze-up
- when the line was exposed to sub-freezing temperatures for a prolonged period of time. The temperature sensors which control the heat tracing and which trigger the low-temperature alarm were located in an area not exposed to the coldest temperatures to which the piping is subjected.
Actions Taken to Prevent Recurrence Licensee As corrective action, the thermostatic temperature setting for the heat trace installed on the recirculation line was temporarily increased and the line was blown free as the frozen section was thawed. A temporary enclosure was built around the line and an additional heat trace was added. Surveillance testing to verify pump operability was performed following the thawing of the recirculation line. An engineering eval-uation is being conducted by the licensee to determine long-term corrective action.
NRC The Office of Inspection and Enforcement verified that'the above identified licensee actions were taken. Appropriate enforcement &ction will be taken.
The NRC Staff is reviewing this item for possible generic application since it appears that other plants may have similar piping arrangements.
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APR 21979 t IE File M ntral File i
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!O:07.A! CUM FOR: Oudley Tho:::pson, Executive Officer for Operations Support. IE FTA.'-::
Samuel E. Bryan, Assistant Directer for Field Ccordination, DP.01, IE SUEJECT:
POTEtiTIAL A8tiO2 MAL GCCURRENCE i.e have deterr.:f r.ed that the cccurren:c at March 12, 1979, at the Davis-Gesse Fouer Station, Unit Eo, 1, teets the critaria f::r :eter-mination of Acnorral Occurrences (A0s). This determination was
. based ca the precise that the occurrence represents a cesign Gficier.cy that cculd have resulted in a loss of plant caN5flity to perform an essential safety function.
A description of the potential A0, entitled, "Coc en i4cde Failure in High Pressure Injection Systen" is encicscd.
de request'that ycu transmit the precared text to NG C as scen as pcssible.
Samuel E. Bryan, Assistant Director for Field Coordination Division of Raacter Operations Inspection, IE Encic:ure:
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