ML062960428
| ML062960428 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 10/23/2006 |
| From: | Florida Power & Light Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML062960400 | List: |
| References | |
| Download: ML062960428 (22) | |
Text
1 Regulatory Conference NRC Region II Turkey Point Nuclear Plant Unit 3 Loss of Decay Heat Removal Event
2 Agenda
- Introductions
- Overview
- Topics of Discussion
- Event description
- Corrective actions
- Thermal-hydraulic analysis of event
- Mitigating actions
- SDP Analysis
- Closing Remarks
3 Overview
- FPL agrees that it did not comply with requirements of 10 CFR 50.65(a)(4)
- FPL has learned from the loss of decay heat removal event and has taken actions to prevent recurrence
- FPL evaluation concludes that the change in core damage frequency is less than 1.0E-6/yr
4 Event Description
- Initial conditions
- Unit 3 in Mode 5
- Draindown in progress to support reactor head removal
- Sequence of events
- While restoring power to 3C 480V load center, spurious undervoltage signal sent to 3A load sequencer
- 3A load sequencer de-energized 3A 4kV bus, causing loss of running 3A RHR pump
- 3A EDG re-energized 3A 4kV bus
- 3A load sequencer does not automatically re-start the 3A RHR pump after loss of offsite power
- Operator started 3B RHR pump and terminated the event in approximately 9 minutes
5 Causes
- Insufficient defense in depth to prevent the event
- The outage risk assessment procedure was insufficient
- Experience in maneuvering plant was low with significant shutdown maintenance in progress
- Vendor human error in the configuration of auxiliary switch contacts on a 480V load center breaker that went undetected
6 Immediate Corrective Actions Taken
- Senior management team augmented by fleet after event for additional oversight
- Additional reviews of remaining outage schedule performed
- Additional controls of protected plant and switchyard equipment implemented
- Outage schedule changes subject to more rigorous review and approval process
7 Long Term Corrective Actions
- Outage risk assessment and control procedure upgraded
- Responsibility for procedure transferred to Operations
- PNSC approval required for procedure changes
- Clearly identifies required protected in-service equipment for higher risk evolutions
- Provides logic ties for risk significant activities
- Use of dedicated and more experienced licensed operators for outage planning and risk assessment (complete)
8 Long Term Corrective Actions (contd)
- As-left auxiliary switch contact configuration to be verified by Nuclear Receipt Inspection for 4kV & 480V breakers (complete)
- Plant procedures for safety-related breakers revised to check auxiliary switch contact configuration on 4kV & 480V breakers (completed for procedures needed for Fall outage breaker work)
- Applicable plant procedure revised to defeat the sequencer during replacement of 480V load center breakers (complete)
9 Long Term Corrective Actions (contd)
- Fleet peer reviews of outage schedule (complete)
- Management challenge of outage schedule (prior to Fall outage)
- Enhanced operator and staff training on shutdown risk assessment (in-progress, complete prior to Fall outage)
- Outage risk management improvements (perform prior to RCS draindown)
- Pressurizer code safety removed
- At least two Core Exit Thermocouples available (until just prior to detensioning reactor vessel head)
- Containment closure ability confirmed
10
- Thermal-hydraulic simulation to determine effects of loss of RHR scenarios
- Case 1 - No operator actions
- Case 2 - HHSI feed only
- Case 3 - HHSI feed & PORV bleed
- Using event tree and failure probabilities, calculate change in core damage frequency FPL Analysis of Loss of RHR Event
11 Initial Plant Conditions
- 63 hours7.291667e-4 days <br />0.0175 hours <br />1.041667e-4 weeks <br />2.39715e-5 months <br /> 50 minutes after shutdown
- prior to shutdown reactor was at ~ 50% power for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />
- RCS being drained to support reactor vessel head lift
- RCS temperature ~115 oF
- RCS vented via:
- Reactor vessel head vent line with 0.219 diameter orifice
- Pressurizer vent line 0.742 diameter
- A-RHR in service
- B-RHR in standby
12 Initial Plant Conditions (contd)
- SG secondary side water levels average 84 %
wide range
- SG atmospheric steam dumps full open
- Equipment required to mitigate loss of RHR in service
- 2nd qualified Unit Supervisor supervising draindown
13 Case 1 - No Operator Action
Conclusion:
- With no operator action, RHR cooling will be restored simply by starting an RHR pump within approximately 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> after event initiation
- No core damage with RHR pump start anytime during first 9 hrs of event
14 Case 2 - HHSI Feed Only
Conclusion:
- Able to sustain steady state condition for at least 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> with single RWST
- No core damage for at least 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />
- Sufficient time available to implement RWST inventory management or SG secondary water makeup
15 Case 3 - HHSI Feed & PORVs Bleed
Conclusion:
- No core damage for at least 16 hrs using both RWSTs
- Sufficient time available to restore RHR or implement RWST inventory management
16 Thermal-hydraulic Analysis Conclusions
- SG reflux cooling will prevent core damage without operator action for at least 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />
- The minimum time to start a RHR pump is at least 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> (time to boil is overly conservative as the criterion for RHR pump start)
- Feed & bleed prevents core damage regardless of pressurizer PORVs position
- Managing RWST inventory is proceduralized with options to:
- Throttle HHSI flow
- Establish RWST makeup
- Use opposite unit RWST
17
- Failure of PORVs to open for feed & bleed does not result in core damage
bleed
- Additional RWST inventory management strategies to extend availability of HHSI suction source Key Factors for Additional NRC Consideration
18 Summary of SDP Results
- Based on a more detailed SDP analysis FPL estimated the total CDF increase for this event to be approximately 2.0E-7/yr
- CDF increase below risk significance threshold of 1.0E-6/yr
- FPL concluded this violation to be GREEN
19 ROP Cornerstone
- NRC ROP Cornerstone for this finding should be Initiating Events
- ROP Initiating Events Cornerstone objective: limit frequency of events that upset plant stability and challenge critical safety functions
- Definitions: NRC Manual Chapter 0308 - ROP Basis Document
- Initiating Events-such events include reactor trips due to turbine trips, loss of feedwater, loss of off-site power...
- Mitigating Systems-include those systems associated with safety injection, residual heat removal, and their support systems...
- Event attributable to the loss of 3A 4kV bus normal electrical power to the running 3A RHR pump, not involving a failure attributable to the RHR System
20 Conclusions
- FPL agrees that it did not comply with requirements of 10 CFR 50.65(a)(4)
- FPL has taken timely and aggressive corrective actions to prevent recurrence
21 Regulatory Conference Open Discussion Questions
22 Regulatory Conference Final Remarks