05000325/LER-2002-001
| Brunswick Steam Electric Plant (Bsep), Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| 3252002001R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
INTRODUCTION
On November 22, 2002, GE Nuclear Energy (GE) identified a reportable condition in accordance with 10 CFR 21.21(d) (i.e., SCO2-21, "Stability Option III: OPRM Tmin Specification"). SCO2-21 states that the Stability Option III period based detection algorithm (PBDA) range for minimum period of detection (i.e., Tmin) values specified in Licensing Topical Report NEDO-31960-A, "BWR Owner's Group Long- Term Stability Solutions Licensing Methodology (Supplement 1)," was non-conservative and recommended that the Oscillation Power Range Monitor (OPRM) [IG] be considered inoperable if the Tmin value is set greater than 1.2 seconds. At the time of the notification, Unit 1 was in Mode 1, operating at approximately 94 percent of rated thermal power (RTP), the maximum achievable power level based on the current status of extended power uprate (EPU) implementation.
Progress Energy Carolinas, Inc. confirmed that the Tmin value was set at 1.4 seconds for the Unit 1 OPRM channels and, at 1328 Eastern Standard Time (EST), declared the OPRM 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" [JC]. 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.
At 1629 EST on November 22, 2002, the NRC was notified (i.e., Event Number 39397), in accordance with 10 CFR 50.72(b)(3)(v)(A), of this event. Based on the information contained in SCO2-21, 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 non-conservative value for Tmin, since startup from the Spring, 2002, refueling outage when the Stability Option HI modification was implemented.
EVENT DESCRIPTION
In support of EPU, Progress Energy Carolinas, Inc. is implementing 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 I-A to the BWROG Option III solution. Stability Option DI was implemented on Unit 1 in Spring, 2002, but has not been implemented on Unit 2.
Stability Option III 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, with a period of oscillation between the Tmin and Tmax values.
Oscillations outside of the Tmin/Tmax range will not be evaluated by the PBDA. In accordance with NEDO-31960-A, the Unit 1 values for Tmin and Tmax were set at 1.4 seconds and 3.0 seconds, respectively.
SCO2-21 states that the Stability Option HI PBDA range for Tmin values, specified in NEDO-31960-A, was non-conservative and recommended that the OPRM be considered inoperable if the Tmin value is set greater than 1.2 seconds. This recommendation was based on a GE calculation which demonstrates that, at high core flow conditions (i.e., approximately 60 percent of rated flow), the expected neutron flux oscillation period for a thermal-hydraulic instability event can be less than 1.4 seconds. Therefore, with Tmin set to 1.4 seconds, the potential existed that the PBDA may not detect an expected oscillation.
Upon receipt of SCO2-21, Progress Energy Carolinas, Inc. confirmed that the Tmin value was set at 1.4 seconds for the Unit 1 OPRM channels. The OPRM Upscale trip function was declared inoperable and actions were initiated to change the Tmin value to 1.2 seconds. This modification was completed on November 27, 2002 and, at 0917 EST, the OPRM Upscale trip function was returned to operable status.
SCO2-21 also states that the OPRM is considered operable for values of Tmax set to 3.0 seconds. Upon receipt of SCO2-21, Progress Energy Carolinas, Inc. confirmed that the Tmax value was set at 3.0 seconds for the Unit 1 OPRM channels.
EVENT CAUSE
Although GE 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 GE, when establishing oscillation period ranges to be used for the PBDA.
CORRECTIVE ACTIONS
1. A modification was completed, on November 27, 2002, to revise the Unit 1 PBDA Tmin value from 1.4 seconds to 1.2 seconds.
2. The engineering change (i.e., EC 46730) for installing the Unit 2 Stability Option III modification has been revised to change the required Tmin value to 1.2 seconds. This modification will be implemented during the Unit 2 refueling outage in March 2003.
SAFETY ASSESSMENT
The safety significance of this occurrence is considered minimal.
The condition when the expected neutron flux oscillation period for a thermal-hydraulic instability event can be below 1.4 seconds exists only at high core flow conditions (i.e., approximately 60 percent of rated flow).
This is a region in which the plant typically does not operate for an extended time and for which the probability of thermal-hydraulic instabilities occurring is low. Even if an instability had occurred under these conditions, SCO2-21 states that the failure of automatic detection at these conditions is not expected to result in fuel failures but may lead to violation of the TS MCPR Safety Limit.
In addition, although considered inoperable with Tmin set greater than 1.2 seconds, the OPRM Upscale trip function was fully operational and would have continued to protect the plant from thermal-hydraulic instabilities throughout the operating range. If such protection was not provided, operators are trained to recognize instabilities and to take appropriate actions should an instability occur.
PREVIOUS SIMILAR EVENTS
A review of reportable events for the past three years has not identified any previous similar events.
COMMITMENTS
No commitments are being established in this report.