05000249/LER-2008-003
| Docket Numbersequential Revmonth Day Year Year Month Day Year N/A N/Anumber No. | |
| Event date: | 11-03-2008 |
|---|---|
| Report date: | 12-31-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| 2492008003R00 - NRC Website | |
Dresden Nuclear Power Station (DNPS) Unit 3 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: 03 � Event Date: 11/3/2008 Reactor Mode: 4� Mode Name: Cold Shutdown� Power Level: 0 percent Reactor Coolant System Pressure: 0 psig
B. Description of Event:
On November 2, 2008, the DNPS Work Execution Center (WEC) Operations Field Supervisor performed a Pre-Job Brief (PJB) of a D3R20 Outage Activity, "Discharge CRD HCU ACCUM - Water Side Only," per procedure DOP 0500-05, "Discharging of CRD Accumulators with Mode Switch in Shutdown or Refuel." The PJB included the direction to perform conditional procedure steps for the Non-Licensed Operators (NLOs) to isolate all control rod drive (CRD) [AA] hydraulic control units (HCUs) [HCU] by closing 177 HCU insert valves (i.e., valve 3-0305-101) [V] and 177 HCU withdraw valves (i.e., valve 3-0305-102) with an operating CRD pump. On November 3, 2008, the NLOs began performing HCU valve closures for the outage activity.
On November 3, 2008, at approximately 1036 hours0.012 days <br />0.288 hours <br />0.00171 weeks <br />3.94198e-4 months <br /> (CST), with Unit 3 in a refuel outage, DNPS main control room (MCR) personnel observed an unplanned withdrawal of control rod D-7. The control rod withdrawal stopped at position 06 with no actions taken by MCR personnel. An unplanned withdrawal of control rods E-6 to position 18 and E-7 to position 16 also occurred and stopped with no actions taken by MCR personnel.
On November 3, 2008, at approximately 1156 hours0.0134 days <br />0.321 hours <br />0.00191 weeks <br />4.39858e-4 months <br /> (CST), all control rods were re-inserted per procedure to the full-in position by manually opening the associated HCU insert valve.
An Event Notification System call was made on November 18, 2007, at 1608 hours0.0186 days <br />0.447 hours <br />0.00266 weeks <br />6.11844e-4 months <br /> (CST) for the above-described event. The assigned ENS event number was 44665.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
C. Cause of Event:
The root cause of the unplanned control rod withdrawals is attributed to latent procedure deficiencies in DOP 0500-05, "Discharging CRD Accumulators with Mode Switch in Shutdown or Refuel," Revision 4 that were not identified during an Operating Experience Review of the Significant Event Notification (SEN) 264, "Unplanned BWR Control Rod Withdrawals While Shutdown," per procedure LS-AA-115, "Operating Experience Procedure.
� A Unit 2 start-up from an unscheduled mid-cycle forced outage in 2005 was delayed due to excessive nitrogen gas accumulation in HCU piping. This event prompted the development and issuance of Revision 4 to DOP 0500-05, which permitted isolation of any or all HCU accumulators by closing the HCU insert, and HCU withdraw valves to limit migration of nitrogen gas into HCU piping. The intended purpose was to shorten subsequent venting of drives after system restoration. The procedure did not contain any precautions, prerequisites, selection criteria or limitations for the quantity of HCUs to be isolated with an operating CRD pump. The root cause evaluation determined that the procedure lacked sufficient guidance for the intended use.
In 2007, the Institute of Nuclear Power Operations (INPO) issued SEN 264 that provided information based on recently reported historical events at several Boiling Water Reactors (BWRs) in Japan during outages, which occurred between 1978 and 2000. In each event, single or multiple control rods unexpectedly withdrew from the core without a deliberate command withdrawal signal. The SEN 264 stated in part:
"The unexpected rod withdrawals occurred during either isolation or restoration of multiple HCUs. With a CRD pump running and the majority of the HCUs isolated, CRD system pressures had increased sufficiently for some control rods to withdraw from the core when the associated HCU isolation valves were manipulated in a specific sequence.
"These events involved a scenario which was not well known within the industry, and other BWRs are potentially vulnerable if operated in a similar fashion without compensatory actions taken to address the anomalous operating conditions.
DNPS operations and engineering personnel reviewed the applicability of SEN 264 and concluded that although unlikely, the vulnerability exists at DNPS. Based on this conclusion, a procedure review was conducted in accordance with the requirements in procedure LS-AA-115, "Operating Experience Procedure," to identify procedures requiring revision to address the SEN 264 issue. The review identified procedures revisions were required to DOP 0300-08, "Control Rod Drive System Hydraulic Control Unit Isolation / Pump Isolation," DOP 0400-01, "Reactor Manual Control System Operation" and DGP 03-04, "Control Rod Movements." The procedure revisions made were considered adequate to address the SEN 264 issue and were completed in October 2007.
The RCR evaluation reviewed the cause of procedure DOP 0500-05 not being revised to address the SEN 264 issue and identified that the procedural requirements in LS-AA-115 to perform the procedure review provided insufficient guidance. LS-AA-115 did not require adequate technical rigor during the evaluation for effected procedures, did not require adequate documentation of the effected procedure evaluation, and lacked sufficient departmental, inter-departmental and cross-discipline reviews to ensure the issue under review is completely addressed.
D.S Safety Analysis:
The safety significance of the event is minimal. A detailed risk assessment was performed for this event and concluded that the risk of core damage was judged to be negligible due to the reactor _ remaining subcritical, no boiling in the core and redundant heat removal methods being available.
Reactor engineering calculations determined the core remained sub-critical by approximately 4.5 percent. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. Corrective Actions:
All control rods were re-inserted per procedure to the full-in position by manually opening the associated HCU insert valve.
Procedure DOP 0500-05 was revised to provide administrative barriers to prevent unplanned control rod withdrawal as described in SEN 264.
Procedure LS-AA-115 will be revised to require sufficient rigor, departmental, inter-departmental and cross-discipline reviews for high significance / risk OPEX items.
Procedure OP-DR-108-101-1002, "Operations Department Standards and Expectations," will be revised to require if a knowledge-based procedure conditional statement shall be executed, in a non emergency condition at the discretion of the supervisor, then a peer check by a second licensed operator shall be obtained.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not identified any LERs associated with unplanned control rod withdrawal.
G. Component Failure Data:
N/A �