ML062960428

Public Meeting Summary for Turkey Point Nuclear Plant Unit 3, Loss of Decay Heat Removal Event

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: ML062960408 ML062960428
References
Download: ML062960428 (22)

Text

Regulatory Conference NRC Region II Turkey Point Nuclear Plant Unit 3 Loss of Decay Heat Removal Event 1

Agenda

• Introductions

• Overview

• Topics of Discussion

- Event description

- Corrective actions

- Thermal-hydraulic analysis of event

- Mitigating actions

- SDP Analysis

• Closing Remarks 2

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 3

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 4

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 5

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 6

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) 7

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) 8

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 9

FPL Analysis of Loss of RHR Event

• 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

• Use results to develop FPL SDP event tree

• Using event tree and failure probabilities, calculate change in core damage frequency 10

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 level near reactor vessel flange

• 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 11

Initial Plant Conditions (contd)

• SG secondary side water levels average 84 %

wide range

• SG atmospheric steam dumps full open

• Both RWSTs with inventory ~295,000 gal per unit available for HHSI pump use while maintaining NPSH

• Equipment required to mitigate loss of RHR in service

• 2nd qualified Unit Supervisor supervising draindown 12

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 13

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 14

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 15

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 16

Key Factors for Additional NRC Consideration

• Base RHR restoration time on NPSH requirements (9 hr) rather than core boiling (21 min)

• Failure of PORVs to open for feed & bleed does not result in core damage

• Late restoration of RHR based on additional time provided by SG reflux cooling and feed &

bleed

• Additional RWST inventory management strategies to extend availability of HHSI suction source 17

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 18

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 19

Conclusions

• FPL agrees that it did not comply with requirements of 10 CFR 50.65(a)(4)

• Review of SDP analysis shows low safety significance with delta CDF < 1.0E-6/yr

• FPL has taken timely and aggressive corrective actions to prevent recurrence 20

Regulatory Conference Open Discussion Questions 21

Regulatory Conference Final Remarks 22