05000483/LER-2003-004
| Callaway Plant Unit 1 | |
| Event date: | 4-11-2003 |
|---|---|
| Report date: | 6-9-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4832003004R00 - NRC Website | |
I. DESCRIPTION OF THE REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event has been determined to be reportable per 10CFR50.73(aX2XiiXB), as an unanalyzed condition, and 10CFR50.73(a)(2XvXD1 as a condition that could have prevented the fulfillment of a safety function to mitigate the consequences of an accident.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
Mode 1 at 100 percent power.
C. STATUS OF STRUCTURES, SYSTEMS OR COMPONENTS THAT WERE INOPERABLE AT THE START
OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
There were no components inoperable that contributed to this event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
On 4/11/03, while at 100 percent power, it was discovered that a note contained in Technical Specification (T/S) 3.3.9 for the Boron Dilution Mitigation System (BDMS), had been inappropriately applied during past reactor startups. This note states: "The boron dilution flux multiplication signal may be blocked in Modes 2 (below P-6 (Intermediate Range Neutron Flux) interlock) and 3 during reactor startup." This had been interpreted to allow blocking BDMS while withdrawing the Shutdown Banks in Mode 3. This action is not allowed in Mode 3 per Final Safety Analysis Report (FSAR) accident analysis.
The Callaway Mode 3 Accident Of Record (AOR) is discussed in FSAR Section 15.4.6.2. The acceptance criterion for a Mode 3 boron dilution event is that there is sufficient time available for automatic mitigation prior to the complete loss of shutdown margin. The BDMS generates a neutron flux-multiplication alarm that indicates an inadvertent boron dilution is in progress. The BDMS also initiates signals to automatically open valves BN-LCV-112D/E (Centrifugal Charging Pump Suction from Refueling Water Storage Tank) to initiate boration and then to close valves BG-LCV-I 12B/C (Volume Control Tank outlet isolation valve) to terminate the dilution. Re-boration occurs within 3.14 minutes after reaching the flux-multiplication condition. This occurs prior to the loss of shutdown margin at 6.22 minutes after reaching the flux-multiplication condition, had the condition gone unmitigated.
Since the Callaway Mode 3 Boron Dilution Event AOR has insufficient time available to credit operator actions to terminate this event prior to the loss of shutdown margin, the BDMS must be credited to mitigate the consequences of an inadvertent dilution in Mode 3. Therefore, the T/S 3.3.9 note which allows blocking the boron dilution flux multiplication signal in Mode 3 only applies to blocking BDMS just prior to withdrawing Control Bank rods. BDMS cannot be blocked while withdrawing Shutdown (S/D) Bank rods without blocking all potential dilution sources within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> specified by T/S 3.3.9. Withdrawing shutdown banks with BDMS blocked is not consistent with FSAR Section 15.4.6.2 Mode 3 analysis.
This problem was discovered while researching a revision for plant operating procedure OTG-ZZ-0001A, SHUTDOWN BANK WITHDRAWAL. While researching the requested procedure revision, it was identified that T/S 3.3.9 and T/S 3.3.9 Bases were not explicit in providing guidance for implementing the FSAR Accident Analysis for a boron dilution event. Safety Analysis engineers reviewed the wording and concluded that BDMS could not be blocked while withdrawing Shutdown Bank rods in Mode 3.
A historical review of reactor startups within the last 3 years indicated that BDMS was inappropriately blocked while withdrawing S/D Bank rods in Mode 3, on three separate startups. The first occurred on 11t24/02, the second on 12/17/03, and the third on 4/2/03.
An immediate corrective action was to issue OTG-ZZ-0001A revision 6. This revision removed a step which stated that BDMS could be blocked.
E. METHOD OF DISCOVERY OF EACH COMPONENT, SYSTEM FAILURE, OR PROCEDURAL ERROR
This problem was discovered while researching a revision for plant operating procedure OTG-ZZ-0001A, SHUTDOWN BANK WITHDRAWAL.
II. EVENT DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
Not applicable at the time of discovery on 4/11/03.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
Not applicable.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.
The source range or intermediate range reactor trip functions are available in Modes 2 and 3. Licensed operators are trained to immediately enter Emergency Operating Procedures (EOP) following any indication of a reactor trip.
"Subcriticality" is one of the critical safety functions continuously monitored throughout the EOPs to ensure reactor power does not exceed 5 percent. These procedures require the licensed operators to immediately borate the reactor coolant system if neutron flux is not decreasing. These procedural controls would ensure that there is no resulting fuel damage or radiological release from the plant. Therefore, this concern had low safety significance with minimal impact on the health and safety of the public.
III. CAUSE OF THE EVENT
Wording of T/S 3.3.9 and T/S 3.3.9 Bases did not provide clear guidance as to what constitutes "reactor startup". The Bases indicate BDMS could be blocked prior to withdrawing "rods" for startup. These words do not delineate between control banks and shutdown banks. Based on this unclear guidance, procedure OTG-ZZ-0001A was incorrectly revised allowing the blocking of BDMS prior to withdrawing shutdown banks.
IV. CORRECTIVE ACTIONS
T/S 3.3.9 Bases will be revised to clearly specify that BDMS can be blocked prior to withdrawing control rods for reactor startup.
An immediate corrective action was to issue OTG-ZZ-0001A revision 6. This revision removed a step, which stated that BDMS could be blocked, plus added steps to monitor source range counts and halt S/D Bank withdrawal if approaching a BDMS setpoint.
V. PREVIOUS SIMILAR EVENTS
Request (CAR) 200302704. A 3-year historical review of the CAR the three events reported in this LER.
confirmed that there were no past LERs involving BDMS.
are from the IEEE Standard 805-1984 and IEEE Standard 803A-1984 This event is documented in Corrective Action system identified no other related events beyond A review of Callaway LERs from 2000 to present
VI. ADDITIONAL INFORMATION
The system and component codes listed below respectively.
System: Not applicable.
Component: Not applicable.