05000328/LER-2015-002
| F | |
| Event date: | 11-10-2015 |
|---|---|
| Report date: | 01-06-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3282015002R00 - NRC Website | |
| ML16008A969 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah, F  |
| Issue date: | 01/06/2016 |
| From: | Schwarz C J Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 15-002-00 | |
| Download: ML16008A969 (8) | |
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I. Plant Operating Conditions Before the Event
At the time of the event, Sequoyah Nuclear Plant (SQN) Unit 1 reactor was operating at 100 percent rated thermal power (RTP) and SQN Unit 2 reactor was operating at 81 percent RTP. The condition described in this LER did not impact SQN Unit 1.
II. Description of Events
A. Event:
On November 10, 2015, at 1502 Eastern Standard Time (EST), the SQN Unit 2 main control room was notified by workers in containment that two cold weather suits had been dropped into the equipment pit portion of the reactor cavity. Operators declared the two containment recirculation drains to be inoperable and entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.6.15, "Containment Recirculation Drains," and TS LCO 3.0.3. The first suit was removed from the equipment pit at 1553. At that time, only one of the drains remained inoperable and LCO 3.0.3 was exited. The remaining suit was removed from the equipment pit at 1556, and LCO 3.6.15 was exited. Plant conditions were restored to normal within the allowed LCO times and no plant shutdown was required. The two cold weather suits in the Unit 2 reactor cavity area created the potential for obstructing the flow path for containment recirculation.
Obstruction of the drains could adversely affect the safety function of the Containment Spray and Emergency Core Cooling Systems that are needed to mitigate the consequences of a design basis accident. The effect of this condition resulted in an unanalyzed condition that significantly degraded plant safety, and a condition that could have prevented the fulfillment of the safety function that is needed to remove residual heat and mitigate the consequences of an accident.
B. Status of structures, components, or systems that were inoperable at the start of the event and contributed to the event:
There were no structures, components or systems that were inoperable at the start of the event.
C. Dates and approximate times of occurrences:
'The event occurred at 1502 on November 10, 2015. Maintenance Services personnel had refueling outage. Five cold weather suits were tied together by rope and were being lowered by hand from the ice deck to the refueling floor. When the spotter signaled to stop the load, the rope jerked and the five suits slid out from the rope. Two of the cold weather suits fell under the hand rail and into the equipment pit portion of the reactor cavity.
C. Dates and approximate times of occurrences (continued):
Dates and Times Description November 10, 2015, 1502 EST Five cold weather suits were dropped onto refuel floor; two of the suits fell into equipment pit portion of the reactor cavity.
November 10, 2015, 1502 EST Main control room was notified.
November 10, 2015, 1502 EST TS LCO 3.6.15 and LCO 3.0.3 were entered.
November 10, 2015, 1553 EST First suit retrieved from Unit 2 equipment pit.
November 10, 2015, 1553 EST LCO 3.0.3 exited.
November 10, 2015, 1556 EST Second suit retrieved from Unit 2 equipment pit.
November 10, 2015, 1556 EST LCO 3.6.15 exited.
D. Manufacturer and model number of each component that failed during the event:
There were no components that failed during the event. The cold weather suits are full body suits manufactured by "Iron-Tuff.
E. Other systems or secondary functions affected:
There were no other systems or functions affected by this event.
F. Method of discovery of each component or system failure or procedural error:
The event was observed by the maintenance services personnel.
G. The failure mode, mechanism, and effect of each failed component, if known:
There were no failed components for this event. The event was due to human error.
H. Operator actions:
Following notification that two suits had fallen into the Unit 2 equipment pit, operators in the main control room declared the two containment recirculation drains inoperable and entered TS LCO 3.6.15, "Containment Recirculation Drains." There are no LCO actions in 3.6.15 that address the loss of both drains, therefore the operators also entered LCO 3.0.3. Operators exited LCO 3.0.3 after the first suit had been retrieved, and exited LCO 3.6.15 after the second suit was retrieved.
I. Automatically and manually initiated safety system responses:
There were no automatic or manually initiated safety systems in response to this event.
III. Cause of the event
A. The cause of each component or system failure or personnel error, if known:
The cause of the condition is personnel error. It was determined that the Maintenance Services personnel failed to identify and mitigate potential hazards and risks during the pre-job briefs, 2- minute rule and walk downs.
B. The cause(s) and circumstances for each human performance related root cause:
The causes and circumstances for the human performance deficiencies include the following:
1) Organization and Programmatic Weaknesses including omission of information/actions that resulted in inadequate preparation and review. Maintenance Services personnel failed to identify and mitigate risks prior to starting work. Removing the upper ice vent curtain on-line is an outage time/duration saving opportunity that should have been risk evaluated in depth prior to execution.
2) Overconfidence of the Maintenance Services personnel who underestimated the task complexity, scope, or depth resulting in a lack of adequate contingency planning.
3) Habit intrusion of the Maintenance Services personnel who were performing a task that was based on past experience without fully understanding the current situation.
Each of the above causes are fully documented in condition report (CR) 1103003
IV. Analysis of the event:
SQN Unit 2 was coasting down in power to begin a scheduled refueling outage. Outage preparations were in progress and included work activities inside upper containment. The maintenance services personnel had completed removal of the Unit 2 upper ice vent curtain and were lowering cold weather suits by rope from the ice condenser deck (elevation 802) to the refuel floor (elevation 734). Five cold weather suits that were tied together were being lowered by hand to the refueling floor. The spotter signaled to stop the load when it reached elevation 738. As the rope reached elevation 738, the rope was stopped quickly that caused five suits to slide out from the rope. Two of the cold weather suits fell under the hand rail and into the reactor cavity area. The maintenance services personnel notified the main control room operators immediately following the event.
The cold weather suits are full body suits and are capable of blocking the refueling canal drains which are 14-inches in diameter. The safety functions of the drains are to return any containment spray water from upper containment to the active sump in lower containment during a design basis accident. The drains are a feature addressed by TS 3.6.15, "Containment Recirculation Drains," and require the refuel canal drains to be OPERABLE in MODES 1, 2, 3 and 4. The TS ACTION condition only addresses one inoperable drain with a required action to restore the drain to operable status within one hour or be in MODE 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and MODE 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
The condition of having two drains inoperable places the plant into TS LCO 3.0.3. Operators in the main control room entered TS LCO 3.6.15 and TS LCO 3.0.3.
Maintenance services personnel removed the.first suit from the equipment pit which allowed the plant to exit TS LCO 3.0.3. The remaining suit was removed from the equipment pit and TS LCO 3.6.15 was exited. Plant conditions were restored to normal within the allowed LCO times and no plant shutdown was required.
V. Assessment of Safety Consequences
There were no actual safety consequences as a result of the event. Plant safety systems were not required to function and no complications were experienced. No TS LCO limits were exceeded and the UFSAR analyses of the event remained bounding.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:
There were no other components that could have performed the same function as the refueling canal drains.
B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident:
This event did not occur while the reactor was shut down. The recovery time for restoring the plant from the unanalyzed condition was accomplished in less than an hour (within the TS LCO action time). The safety-related systems that were needed to remove residual heat or mitigate the consequences of an accident potentially were not available during this event.
C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from discovery of the failure until the train was returned to service:
The elapsed time from discovery until the drains were returned to service was 54 minutes.
VI.
Corrective Actions Corrective Actions are being managed by TVA's corrective action program under CR 1103003
A. Immediate Corrective Actions:
The condition was corrected immediately by removing the cold weather suits from the equipment pit. No other issues were identified.
B. Corrective Actions to Prevent Recurrence or to reduce probability of similar events occurring in the future:
The corrective actions include the addition of risk mitigation strategies to the containment access control procedure, 0-PI-OPS-000-011.0, "Containment Access Control During Modes 1-4.
VII: Additional Information A. Previous similar events at the same plant:
A review of the previous reportable events for the past 3 years at SQN found the following two similar events caused by human error.
1) LER 1-2013-001, Latent Design Input Inconsistencies Adversely Affect Probable Maximum Flood Analysis, identified two human performance related causes: organizational behavior rooted in over-confidence resulted in the input errors (latent computer modeling inconsistencies), and management failure to provide oversight and conservative decision- making involving the impact of changes to the river system on the calculated PMF.
2) LER 2-2014-001, Misalignment of Containment Purge Radiation Monitors Results in Condition Prohibited by Technical Specifications, identified one human performance related cause: Operations staff involved in the event demonstrated less than adequate standards and responsibility for procedure use and adherence.
B. Additional Information:
None
C. Safety System Functional Failure Consideration:
This event resulted in a safety system functional failure in accordance with 10 CFR 50.73(a)(2)(v)(B) and (D).
D. Scrams with Complications Consideration:
This event did not result in an unplanned scram with complications.
VIII. Commitments:
None.