05000278/LER-2009-006
| Peach Bottom Atomic Power Station Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2782009006R00 - NRC Website | |
Unit Conditions Prior to the Event was in the process of shutting down for its 17th Refueling Outage (P3R17). The Rod Worth Minimizer was being considered inoperable at this time resulting in the need for additional verifications of control rod position during the control rod insertion sequence. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On 9/13/09, at approximately 2244 hours0.026 days <br />0.623 hours <br />0.00371 weeks <br />8.53842e-4 months <br />, Operations personnel (utility, licensed) manually scrammed the Unit 3 reactor (EIIS: RCT) as a result of an observed shortening reactor period.
The reactor was manually scrammed by placing the mode switch in the shutdown position. A reactor period of less than 50 seconds had been observed, which is below the procedurally controlled threshold of 50 seconds to manually scram the reactor during the shutdown process.
At the time of this event, Unit 3 was being shut down in preparation for its 17th Refueling Outage (P3R17). A 'soft' shutdown was in progress and it had been intended to insert all control rods (EIIS: AA) without performing a scram.
The scram function operated as designed and there were no complications as a result of the manual scram.
This report is being submitted pursuant to:
10CFR 50.73(a)(2)(iv)(A) — Manual Reactor Protection System (RPS) Actuation — This occurrence is reportable under this criterion since an unplanned manual scram was performed as a result of shortening reactor period.
As a result of the manual scram, Unit 3 entered Mode 3 (shutdown) at 2244 hours0.026 days <br />0.623 hours <br />0.00371 weeks <br />8.53842e-4 months <br />.
Analysis of the Event
There were no actual safety consequences associated with this event. The scram function operated as designed and there were no complications as a result of the manual scram. The remaining withdrawn control rods inserted as a result of the manual scram.
The manual reactor scram was performed as a result of a shortening reactor period. This manual scram is an anticipatory action to prevent an automatic RPS actuation due to short period. The normal shutdown procedure contains a conservative reactor, period of 50 seconds as the threshold to perform the manual scram.
� Analysis of the Event, continued Reactor period is defined as that amount of time in seconds required for neutron flux (power) to change by a factor of e, or 2.718.
In Mode 2 (Startup / Hot Standby) and Mode 5 (Refueling), the Wide Range Neutron Monitor (WRNM) system provides short-period trips to RPS. The WRNM system provides inputs into the RPS circuitry to ensure a reactor scram occurs in the event that core reactivity increase (shortening period) exceeds a predetermined reference rate. The Technical Specification (TS) allowable value for a WRNM short period RPS actuation is > 13 seconds.
The safety objective of the WRNM system (EllS: IG) is to detect conditions in the reactor core that could potentially threaten the overall integrity of the fuel barrier. In Mode 2, the most significant source of reactivity change is due to control rod withdrawal. The WRNM provides mitigation of the neutron flux excursion by monitoring the reactor period and providing an automatic scram when the TS allowable value is reached.
At the time of the scram, a soft shutdown (i.e., insertion of all control rods manually on a one-at a-time basis) was being performed. The plant was operating at a very low power level of approximately 0.2% power. The turbine — generator was already removed from service and control rods were being inserted in accordance with pre-established sequences. At the time of the scram, the reactor pressure was decreasing and was approximately at 840 psig. Reactor level was being maintained using the C Reactor Feed Pump using the startup level feedwater controller. At this very low power condition, the reactor is very sensitive to any sources of positive reactivity insertion such as feedwater injection. A small injection of feedwater results in a positive reactivity insertion. At the time of the event, the feedwater automatic controller was providing small batch feeds to the reactor. The final batch feed prior to the scram resulted in a power increase from approximately 0.1% to 0.2% power and had an associated reactor period of less than 50 seconds.
Had a manual scram not been performed, the automatic scram function was operable and would have provided automatic RPS protection, if the reactor period would have decreased to below the TS allowable value setting.
A similar event would not be expected during startup activities at low power levels because of the inherent negative reactivity insertions during startup conditions due to the creation of voids in the reactor coolant.
The event was considered not to be risk significant.
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Cause of the Event
The cause of the manual scram was due to a shortening reactor period during a planned soft shutdown using control rods. The manual scram was required due to the sensitivity of the reactor to small positive reactivity insertions at very low power levels.
Underlying causes include:
- Delays in control rod insertion as a result of procedurally required control rod position verifications that were required due to an unplanned unavailability of the Rod Worth Minimizer
- Fluctuations in feed flow through the feedwater startup control valve
- Decreasing reactor pressure due to the use of steam for the main turbine steam seals and other steam users
Corrective Actions
Plant shutdown procedures for Units 2 and 3 will be revised to provide additional control of operational activities to minimize the likelihood of a short period when performing a soft shutdown.
The learnings from this event will be shared with licensed operations personnel.
Appropriate additional actions are being taken as a result of the root cause investigation performed for this event.
Previous Similar Occurrences There were no previous LERs identified relating to unplanned manual reactor scrams due to a shortening reactor period.
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