05000338/LER-2010-003
| Document Numbersequential Revmonth Day Year Year Month Day Year North Anna Power Station 05000339Number No | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3382010003R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT During the Unit 2 Spring 2010 Refueling Outage (RFO) evidence of minor leakage at the cavity seal ring was identified. A boric acid inspection of the Reactor Pressure Vessel (RPV) nozzles (EllS System — AB, Component — NZL) was subsequently performed. The inspection identified the degraded condition of the supplemental neutron shielding saddle assemblies. The configuration of the supplemental neutron shielding saddle assemblies supplemental neutron shielding saddle assemblies, a decision was made to inspect the Unit 1 supplemental neutron shielding saddle assemblies during the Fall 2010 RFO.
On September 13, 2010, the inspection of the Unit 1 supplemental neutron shielding saddle assemblies determined it was not configured as indicated on plant drawings. The supplemental neutron shielding installed on top of the nozzles contained gaps preventing it from fully performing its intended function. The length of time the Units 1 and 2 supplemental neutron shielding saddle assemblies have been degraded is unknown.
Corrective actions were initiated to evaluate removal of the supplemental neutron shielding saddle assemblies.
During development of the Design Change to remove the supplemental neutron shielding saddle assemblies, Engineering review of the Design Change that installed the supplemental neutron shielding saddle material noted that the original Tempmat insulation on the Unit 1 RPV nozzles was replaced with thinner Microtherm insulation per Design Change (DC 79-S07). This conflicted with the GSI-191 vendor walk-down. A review of GSM 91 walk-down reports, material specifications, design changes, and drawings was initiated. As a result of this review, on September 24, 2010 subsequent inspections revealed Microtherm insulation on the Intermediate RCS Loop which is within the North Anna 1 containment GSI-191 Zone of Influence. The corresponding Unit 2 RCS Intermediate loop insulation had been mitigated. In addition, on September 28, 2010, Microtherm was visually verified to exist on the Unit 1 Reactor Coolant Pumps (RCPs) (EMS Component — P) bowls, which is within the Zone of Influence (Z01). On September 28, 2010, management decided to voluntarily shutdown Unit 2 based on inconsistent documentation to perform insulation inspections in equivalent areas. The visual inspection confirmed the existence of Microtherm in Unit 2 containment ZOI. In subsequent investigations, Microtherm was discovered to be installed in Units 1 and 2 on the RCPs, the RCP outlet nozzles, the Intermediate Leg elbows (on Unit 1 only), and other locations.
In addition, Calcium Silicate (Cal-Sil) insulation was determined to be installed in both Units containment ZOls. Based on hand-over-hand walk-downs that were performed there were containment ZOI. Hand-over-hand walk-downs for the Unit 2 containment ZOI identified approximately 62 cubic feet of Microtherm and 9 cubic feet of Cal-Sil.
The debris source walk-downs conducted in 2003 and 2004 for the original GSI-191 project identified the existence of Microtherm insulation in Unit 2 containment but not in Unit 1.
0NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER Design Change (DCP 07-004) was implemented to replace Microtherm and most Cal-Sil (in the ZOI) in Unit 2 with approved insulation. The Design Change for Unit 1 (DCP 07 129) addressed only Cal-Sil. The existence of encapsulated Microtherm on the RPV nozzles was unknown to the original GSI-191 project team and it was only identified during the investigation of the supplemental neutron shielding. The presence of Microtherm and Cal-Sil in the Unit 1 and 2 containment ZOls was not bounded by debris load testing of the containment sump strainers (EIIS System — BP, Component — STR). The North Anna Power Station (NAPS) Supplemental Response to NRC Generic Letter 2004-02, Serial No.
08-0019 dated February 29, 2008 stated that Microtherm insulation had been removed containment.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event posed no significant safety implications since no Design Basis Accident (DBA) occurred on either Unit during the time the unacceptable insulation remained within the Unit 1 and 2 containment ZOls. The unacceptable insulation in the Unit 1 and 2 containment ZOls was not bounded by debris load testing of the containment sump strainers. Dominion preliminary PRA calculations show a Conditional Core Damage Frequency (CCDF) of 5.04E-07 due to this event. This is below the regulatory threshold of 1.0E-06. The likely-hood of a Loss of Coolant Accident event is very small and the operations staff has procedures to mitigate such a condition in addition to receiving training to recognize sump blockage conditions. Therefore, the health and safety of the public were not affected by this event.
There is no ongoing vulnerability as a result of the comprehensive actions taken as described in section 4.0 below. The hand-over-hand walk-downs identified the extent of condition and ensured that unacceptable insulation does not exist within Unit 1 and 2 containment ZOls.
On September 29, 2010, at 1415 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.384075e-4 months <br />, an Information Only notification was made to the NRC Operations Center to identify the Unit 2 shutdown to allow inspections for unacceptable insulation. The visual inspections confirmed the existence of Microtherm in Unit 2 Containment ZOI. This event is reportable pursuant to 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.
3.0 CAUSE The preliminary root cause evaluation of the event determined a flawed methodology was used to determine the types and quantities of insulation during the GSI-191 containment walkdowns, resulting in unacceptable insulation not being included in the containment debris inventory and subsequently remediated. Contributors to this event were an incomplete set of insulation drawings to perform plant walk-downs, ineffective oversight � and over-reliance on vendors, and ineffective project management during inventory walk downs and subsequent remediation.
4.0 IMMEDIATE CORRECTIVE ACTION(S) ETE-NA-2010-0008, Disposition of Microtherm Insulation on North Anna Unit 1, and ETE NA-2010-0009, Final Disposition of Microtherm Insulation on North Anna Unit 2, governed the actions necessary to identify and remediate unacceptable insulation within the ZOls for NAPS Units 1 and 2. The information for these ETEs was determined by a complete review of the insulation vendor drawings, Stone and Webster design changes for specifications associated with insulation, insulation specification changes, and design changes which could have impacted insulation. These document reviews provided the bases for engineering hand-over-hand walk-down of piping and components to determine the installed insulation type. Unacceptable insulation (Cal-Sil and/or Microtherm) was fully remediated when identified during walk-downs within the ZOls for Units 1 and 2.
5.0 ADDITIONAL CORRECTIVE ACTIONS The root cause evaluation team evaluated other aspects of the NAPS GSI-191 sump modification project—specifically coating program health, debris source classification, long term chemical effects, and sump strainer design configuration. The Team determined that initial and subsequent actions associated with the aspects of the NAPS GSI-191 sump project were, and continue to be effectively governed by station policies and procedures.
Additional corrective actions from the final root cause evaluation will be documented in the Corrective Action Program and tracked to completion.
6.0 ACTIONS TO PREVENT RECURRENCE Actions to prevent recurrence from the final root cause evaluation will be documented in the Corrective Action Program and tracked to completion.
7.0 SIMILAR EVENTS None.
8.0 ADDITIONAL INFORMATION None.