05000237/LER-2012-001
| Docket Number Sequential Fevmonth Day Year Year Month Day Year N/A 05000Number Nc). | |
| Event date: | 02-19-2012 |
|---|---|
| Report date: | 12-06-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 2372012001R01 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Dresden Nuclear Power Station (DNPS) Units 2 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 02 Event Date: 02-19-2012 Event Time: 0700 hours0.0081 days <br />0.194 hours <br />0.00116 weeks <br />2.6635e-4 months <br /> CST Reactor Mode: 1 Mode Name: Power Operation Power Level: 73 percent
B. Description of Event:
On February 18, 2012, a planned reactor down power was commenced in order to perform a control rod sequence exchange, feedwater regulating valve maintenance, and scram time testing.
The power reduction began at approximately 2200 hours0.0255 days <br />0.611 hours <br />0.00364 weeks <br />8.371e-4 months <br /> and was stabilized at approximately 73 percent rated thermal power on February 19, 2012, at approximately 0200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br />. During the subsequent planned activities average power range monitors (APRMs) [JC] gain alarms were received which indicated that the gains were not within the specified tolerances. With the APRM functions inoperable due to gain settings being out of tolerance, Technical Specification 3.3.1.1 allows delay into the associated condition and required actions for up to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> if the settings are non-conservative. Therefore the night shift control room operating crew did not immediately perform an adjustment of the gain settings. The operating crew completed their shift and performed turnover to the oncoming day shift.
On February 19, 2012, the oncoming day shift control room operating crew prepared to perform daily control room surveillances. The Unit 2 licensed operator performed a computer run which monitors calculated and indicated reactor power. The Unit 2 licensed operator observed that APRMs 4, 5, and 6 were indicating lower than actual calculated power. When assessed, it was determined that the three APRMs were indicating 5.7, 5.3 and 6.3 percent below actual power, respectively.
The set points for the APRM Fixed Neutron-High were set at 118 percent RTP. Therefore, the gain setting could be as much as four percent lower than the actual power and still meet the Technical Specification Allowable Value of 122 percent RTP. However in the identified condition, the Division 2 APRM system would not have generated a Fixed Neutron Flux-High trip signal prior to exceeding the Allowable Value specified in the plant's Technical Specifications. Once identified, the APRM gains were adjusted within the required tolerance at approximately 0749 hours0.00867 days <br />0.208 hours <br />0.00124 weeks <br />2.849945e-4 months <br /> in accordance with plant procedures.
Subsequent to the gains being adjusted, operations personnel from the day shift operating crew began to review the historical trends of the down power with regard to the APRM gain settings. It was determined that the gain tolerances were exceeded prior to the oncoming operating crew assuming the shift.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(A), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shut down the reactor and maintain it is a safe shutdown condition. Although the channels did not meet the Technical Specifications surveillance requirement, it did not result in a condition that was prohibited by the plant's Technical Specifications.
C. Cause of Event:
During a planned down power, the night shift operating crew reduced reactor power to approximately seventy-three percent. As expected, the down power maneuvers affected the gain setting for the APRMs. However, the magnitude of the change to the gain settings was not anticipated. This resulted in all the gains on one of the two Divisions experiencing an out of tolerance concurrently during the evolution. The movement of control rods, which are adjacent (Groups 9 and 10) to Local Power Range Monitors (LPRM), have a definite affect on the power inputs being received by the APRMs. These local power inputs resulted in the Divisions 1 and 2 APRMs indicating conservatively and non-conservatively, respectively. This occurrence is inherent to boiling water reactors.
During the control rod sequence exchange, the Division 2 APRMs went non-conservative. The LPRMs that inputted into the Division 2 APRMs were located adjacent to control rods (Grp 10) that were inserted during the sequence exchange, which results in a non-conservative indication. The LPRMs that inputted to the Division 1 APRMs were adjacent to control rods (Grp 9) that were withdrawn. This resulted in the Division 1 APRMs indicating conservatively. This is the expected divisional APRM response based on control rod sequence exchanges using Grp 9 and Grp 10 rods during the existing fuel cycle.
D. Safety Analysis:
The safety significance of this condition is low. The Flow Biased Neutron Flux-High trip function was conservatively set and would have generated a trip signal at approximately 104 percent rated thermal power which is well below the specified Technical Specification Allowable Value of 122 percent rated thermal power. Due to the Flow Biased Neutron Flux scram signal being available, the minimum critical power safety limit was not adversely impacted. Although the Fixed Neutron Flux- High trip would have exceeded the Technical Specification Allowable Value, the trip would have occurred prior to the value that is assumed in the plant's fuel cycle transient analysis. Therefore the health and safety of the public and plant employees was not compromised as a result of this condition.
E. Corrective Actions:
The operations rounds were revised to include an hourly check of APRM gain settings when unit load maneuvers are anticipated or planned to occur during the shift. This would ensure that out of tolerance gain settings are corrected within the two-hour technical specification allowance.
Additionally, new computer alarms which annunciate on sustained non-conservative indications for an APRM have been implemented on both operating units.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not reveal any similar reportable conditions.
G. Component Failure Data:
Not Applicable