05000325/LER-2003-002
| Brunswick Steam Electric Plant (Bsep), Unit 1 | |
| Event date: | 10-05-2003 |
|---|---|
| Report date: | 11-26-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 40226 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 3252003002R00 - NRC Website | |
Energy Industry Identification System (EHS) codes are identified in the text as [XX].
INTRODUCTION
On October 5, 2003, Progress Energy Carolinas, Inc. (PEC) received notification from GE Nuclear Energy (GENE) of a reportable condition in accordance with 10 CFR 21.21(d) (i.e., SC03-20, "Stability Option III Period Based Detection Algorithm Allowable Settings," dated October 4, 2003). SC03-20 discusses a condition of the stability Option III Period Based Detection Algorithm (PBDA) which can result in numerous, unexpected confirmation count resets in the event of an instability condition.
At the time of the notification, Unit 1 was in Mode 1, operating at approximately 94 percent of rated thermal power (RTP) and Unit 2 was in Mode I, operating at approximately 96 percent of RTP. These are the maximum achievable power levels for each unit based on the current status of extended power uprate (EPU) implementation.
At 1100 Eastern Daylight Time (EDT) on October 5, 2003, PEC declared the Oscillation Power Range Monitor (OPRM) (10) Upscale trip (i.e., Function 2.f of Technical Specification (TS) Table 3.3.1.1-1) inoperable in accordance with TS 3.3.1.1, "Reactor Protection System (RPS) Instrumentation" PC]. An alternate method to detect and suppress thermal hydraulic instability oscillations was placed into effect as directed by Condition I of TS 3.3.1.1. Although considered inoperable, the OPRM Upscale trip remains armed and functional.
At 1254 EDT on October 5, 2003, the NRC was notified (i.e., Event Number 40226), in accordance with 10 CFR 50.72(b)(3)(v)(A), of this event. Based on the information contained in SC03-20, this was considered a condition that could have prevented the fulfillment of the safety function of a system that is needed to shut down the reactor and maintain it in a safe shutdown condition.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function of a system that is needed to mitigate the consequences of an accident. Additionally, the event is being reported in accordance with 10 CFR 50.73 (a)(2)(i)(B) as operation which was prohibited by the plant's TSs. The OPRM Upscale trip has been considered operable, with the potential for numerous, unexpected confirmation count resets, since Unit I startup from the spring, 2002, refueling outage and Unit 2 startup from the spring, 2003, refueling outage when the Stability Option III modifications were implemented.
EVENT DESCRIPTION
In support of EPU, PEC implemented modifications to revise the Brunswick Steam Electric Plant (BSEP) thermal-hydraulic stability long-term solution from the Boiling Water Reactor Owners' Group (BWROG) Enhanced Option 1-A to the BWROG Option III solution. Stability Option III was implemented on Unit 1 in spring, 2002, and on Unit 2 in spring, 2003.
NRC FORM 3136A (I
- 2001) FACILITY NAME (1) DOCKET (2) - LER NUMBER (6)
SEOUENTIAL REVISION
' NUMBER � NUMBER YEAR
- ! �
- Stability Option HI relies upon the OPRM Upscale trip function to automatically detect and suppress anticipated thermal-hydraulic power oscillations, thus providing protection from exceeding the fuel Minimum Critical Power Ratio (MCPR) Safety Limit. One of three algorithms (i.e., the algorithm credited by the safety analysis and governed by the TSs) used in generating the OPRM Upscale trip is the PBDA.
The PBDA is an algorithm which provides a scram if there are a sufficient number of neutron flux oscillations, in a given period of time.
The technical bases for the PBDA was defined by GENE and supplied to licensees as safety related documentation in licensing topical reports. Specifically, NED0-32465-A, Reactor Stability Detect and Suppress Solutions Licensing Basis Methodology for Reload Applications, August 1996, defines the PBDA period confirmation adjustable variables for the OPRM to be the period tolerance and the conditioning filter cutoff frequency. The period tolerance could be adjusted in the range of 100 to 300 msec, and the conditioning filter cutoff frequency could be adjusted in the range of 1.0 to 2.5 Hz. Subsequent plant- specific evaluations extended the period tolerance range, on the low end, to 50 msec and the cutoff frequency, on the high end, to 3.0 Hz.
On July 24, 2003, a slow growing core wide instability event occurred at Nine Mile Point (NMP) Unit 2.
As a result of investigation of this instability event it was determined that numerous, unexpected PBDA confirmation count resets had occurred. Based on their analyses, GENE could confirm that performance of the OPRM with settings other than period tolerance of 100 msec or higher and cutoff frequency of 1.0 Hz would prevent exceeding the MCPR Safety Limit for all anticipated instability events.
Upon receipt of SC03-20, PEC confirmed the following OPRM settings.
Period Tolerance 100 msec 50 msec Cutoff Frequency 3.0 Hz 3.0 Hz As a result, the OPRM Upscale trip function was declared inoperable on each unit.
EVENT CAUSE
Although GENE has not performed a formal root cause evaluation for this condition, based on available information, it is believed that the root cause is an incomplete analysis, performed by GENE, when establishing the PBDA.
CORRECTIVE ACTIONS
1. Engineering Changes (i.e., EC 46335 for Unit 1 and EC 46730 for Unit 2) are in progress to revise the Unit 1 and 2 OPRM variables to the settings recommended in SC03-20. These modifications will be implemented and the OPRM Upscale trip functions returned to operable status by December 31, 2003, for both Unit 1 and Unit 2.
2. Plant procedure OMST-OPRM21R, "OPRM Tuning," will be revised in accordance with the recommendations of SC03-20. This procedure revision will be implemented by December 31, 2003.
SAFETY ASSESSMENT
The safety significance of this occurrence is considered minimal.
As discussed in SC03-20, there is no threat of fuel failure as a result of this condition. In addition, although considered inoperable, the OPRM Upscale trip function was fully operational and would have continued to protect the plant from thermal-hydraulic instabilities throughout the operating range. Although considered inoperable, the OPRM Upscale trip remains armed and functional. If such protection was not provided, operators arc trained to recognize instabilities and to take appropriate actions should instability occur.
PREVIOUS SIMILAR EVENTS
function was declared inoperable due to a non-conservative setpoint associated with the PBDA. This condition was identified by GENE in SCO2-21, "Stability Option � OPRM Tmin Specification," dated November 22, 2002. The corrective actions associated with LER 1-2002-001 could not have reasonably been expected to prevent the condition reported in the LER.
COMMITMENTS
Those actions committed to by PEC in this document are identified below. Any other actions discussed in this submittal represent intended or planned actions by PEC. They are described for the NRC's information and are not regulatory commitments. Please notify the Manager - Support Services at BSEP of any questions regarding this document or any associated regulatory commitments.
1. Engineering Changes (i.e., EC 46335 for Unit 1 and EC 46730 for Unit 2) are in progress to revise the Unit 1 and 2 OPRM variables to the settings recommended in SC03-20. These modifications will be implemented and the OPRM Upscale trip functions returned to operable status by December 31, 2003, for both Unit 1 and Unit 2.
2. Plant procedure OMST-OPRM21R, "OPRM Tuning," will be revised in accordance with the recommendations of SC03-20. This procedure revision will be implemented by December 31, 2003.