ML20215G087
| ML20215G087 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 12/19/1986 |
| From: | Cintula T NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20215G085 | List: |
| References | |
| TASK-AE, TASK-T611 AEOD-T611, NUDOCS 8612240273 | |
| Download: ML20215G087 (3) | |
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AE0D TECHNICAL REVIEW REPORT
- UNIT: Limerick-1 TR REPORT NO.: AE0D/T611 DOCKET NO.: 50-352 DATE:
December 19, 1986 LICENSEE: Philadelphia Electric Co.
EVALUATOR / CONTACT:
T. Cintula
SUBJECT:
DENIED ACCESS DUE TO NEGATIVE ROOM PRESSURE EVENT DATE: December 26, 1985
SUMMARY
A report of denied personnel entry to several areas due to a negative room pressure differential was investigated. This negative pressure differential was caused by the inadvertent tripping of the ventilation supply fan while the exhaust fan continued to operate. The regulatory concern was that timely access necessary for operator action might not be available into areas containing safety-related equipment in a plant emergency with a similar failure. The licensee manually tripped the area exhaust fan to reduce the negative room pressure. However, it was noted that automatic tripping of the exhaust fan upon failure of the supply fan wculd have avoided this event. A search for delayed access events due to a negative room pressure differential did not find any similar events. Therefore, this event appeared to be an isolated case without significant generic safety concern.
INTRODUCTION A Licensee Event Report (LER 85-100) for Unit 1 of the Limerick Generating Station reported that a fire watch patrol had been delayed because they were unable to open certain airlock doors. The doors did not open because a negative pressure had been created in the enclosure and the airlock doors opened outward. Consequently, the fire watch was unable to physically cpen the airlock doors. The negative room pressure was caused by the inadvertent tripping of the enclosure ventilation supply fan while the exhaust fan continued to operate.
This event was investigated because denied personnel access due to a negative room pressure could be a potentially significant problem if access to a vital area during a plant emergency is postulated.
In this scenario, the inability of an operator to reach, operate and control essential equipment during an emergency, or preventing an operator from taking action necessary to compensate for an equipment malfunction could be assumed.
DISCUSSION On December 26, 1985 with Unit No.1 of the Limerick Generating Station at 98 percent power, the fire watch patrol was unable to enter four areas in the
- This document supports ongoing AE0D and NRC activities and does not represent the position or requirements of the responsible NRC program office.
8612240273 861219 PDR ADOCK 05000352 S
reactor enclosure within one hour of the previous inspection. The areas that could not be inspected were three fire doors and one fire damper at three different elevations in the reactor enclosure. These four enclosures separated designated safe shutdown fire areas or separate portions of redundant systems important to safe shutdown of the plant.
The four fire watch inspections exceeded the one hour time limit because the fire watch was prevented from entering these areas due to a ventilation equipment failure.
The reactor enclosure supply fans had tripped, while the exhaust fans continued to operate, and this resulted in a reactor enclosure negative pressure.
Consequently, the fire watch was unable to physically open the airlock doors and enter-the posted areas. About a half hour later, the enclosure exhaust fans were manually tripped from service to reduce the negative pressure. About four minutes later, room pressures had increased sufficiently for the fire watch to enter the reactor enclosure.
Operations personnel were familiar with the design and operation of the ventilation equipment. However, they were involved in returning other reportable equipment to service and made the cognitive decision to complete the task-at-hand in lieu of assisting the fire watch. When they became available to address the differential pressure problem, their actions were expedient and direct in reducing the negative room pressure. Thus, most of the time delay could be attributed to a delay due to other priorities rather than in difficulties in gaining entry to the reactor enclosure.
The immediate corrective action for this event was to manually trip the exhaust fan. However, it was subsequently learned that the licensee's long term corrective actions included:
(a)
Installation of remote status lights and alarms for indication of the operability of each fan.
(b) Automatic tripping of the exhaust fan after a preset time delay if the supply fan fails to operate.
The possible consequences of the event were minimal because smoke detectors in each of the four areas were operable and would have provided early detection of a fire in these areas.
In addition, one area was partially protected by a pre-action sprinkler system.
A search for similar events (from about 1980) of denied or delayed personnel entry due to a negative room pressure was undertaken. The search used the SCSS (Sequence Coding and Search System) Licensee Event Report data base, a limited review of NRC regional inspection reports, and other industry reports relating to plant operating experiences.
No similar events of denied access due to a negative room pressure were found.
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FINDINGS AND CONCLUSIONS A negative pressure was created in the enclosure because the supply fan tripped while the exhaust fan continued to operate. Because the access doors opened outward, personnel were unable to force the doors open. Personnel were able to enter the enclosure after the problem was recognized and the exhaust fan was manually tripped.
The dominant reason for the delayed access to the area was the conscious decision of operations personnel to address other concurrent problems at the plant. When operating personnel became available to address the negative pressure problem, their actions were direct and expedient in reducing the negative room pressure.
No evidence was obtained during this study to' indicate a generic problem associated with delayed access of operations personnel to rooms containing safety equipment due to negative room pressure.
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