05000410/LER-2002-001
| Docket Number03 21 2002 2002 - 001 - 00 5 20 2002 | |
| Event date: | 03-21-2002 |
|---|---|
| Report date: | 5-20-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4102002001R00 - NRC Website | |
I. Description of Event
shut down for refueling outage 8 (RFO8), and in mode 5, a reactor scram signal was generated due to high level in the Scram Discharge Volume (SDV). No control rod motion occurred as a result of the scram. The high level in the SDV occurred when air from the Service Air System (SAS) supplying the Scram Air Header (SAH) was lost and the scram valves failed open, allowing water to enter the SDV. Service Air, which is supplied by the Instrument Air System (IAS), was inadvertently lost during the planned isolation of the IAS.
The isolation of the IAS was being done in accordance with a new procedure, N2-PM-@075, Instrument Air Outage — RFO8. The procedure allowed the IAS to be isolated without ensuring that an alternate air supply to the SAS was in service. (NOTE: The SAH is normally maintained pressurized by the IAS. In this particular event, the IAS was being taken out of service and depressurized to accomplish planned repairs and modifications to this system. The plan involved having the SAS in service and pressurized throughout the IAS outage and maintaining the SAH pressurized from the SAS.) After identifying IAS isolation as the cause of loss of the SAS, the necessary valves were opened and the SAS returned to service.
The IAS outage, and associated repairs and modifications, had been planned to occur during RF08. This work could not have been done with the IAS in service or the plant on line. The IAS outage was significant because of its scope and the complexity of system interactions. The project had been planned for RFO8 even though a final commitment to proceed was not made until late December, 2001.
After the need for a new procedure to control and sequence the IAS outage was identified, procedure development was initiated, and the final draft of N2-PM-@075, Instrument Air Outage — RFO8, was completed in early February 2002. The validation and technical review of N2-PM-@075 were then completed, and the procedure was issued on March 16, 2002.
The implementation of the procedure was treated as a special evolution with the requirements of general administrative procedure GAP-SAT-03, Control of Special Evolutions, applicable. Special Evolution briefings for N2-PM-@075 were held as required by GAP-SAT-03.
II. Cause of Event
The cause of the automatic reactor scram was a high level signal in the SDV. The high level in the SDV was a consequence of the failure of the procedure's author and reviewers to ensure that the procedure for isolating the IAS, N2-PM-@075, included steps for maintaining air supply to the SAS. Contributing causes include evolving changes in the work scope leading to late involvement of the reviewers, and insufficient time allotted for procedure validation and review.
III.Analysis of Event
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), because it was a valid automatic actuation of the Reactor Protection System (RPS) due to a high level in the SDV.
Prior to receipt of the scram signal, the reactor had been shut down and was in mode 5. No rod motion occurred as a result of the scram signal. Additionally, procedure N2-PM-@075 required the plant to be in mode 5 prior to commencing isolation of the IAS, which led to the scram signal.
From a probabilistic risk perspective, the event is not considered risk significant. This determination is based on a qualitative assessment, as the plant was shut down with all control rods inserted at the time of the event. In addition, the partial isolation of the IAS while shut down is judged to be not risk significant.
Based on the above, the automatic scram resulting from high level in the SDV did not pose a threat to the health and safety of the public or the plant personnel.
IV.Corrective Actions
1. The performance of the procedure, N2-PM-@075, for isolating Instrument Air was stopped and Service Air was restored.
2. Procedure N2-PM-@075 was revised to ensure that an alternate supply to Service Air is in service prior to isolating Instrument Air.
3. Procedure GAP-SAT-03, Control of Special Evolutions, will be revised to include more specific direction regarding preparation for special evolutions.
4. The guidance in the validation and verification checklists that are used during review of new or revised procedures will be enhanced.
5. The event summary will be reviewed by the Operations Department procedure authors and reviewers.
6. A summary of this event, and the lessons learned, have been shared with Branch Managers at NMPNS Unit 1 and Unit 2.
V . Additional Information
A. Failed Components:
none
B. Previous similar events:
Prior to 1999, NMPNS Unit 2 had numerous instances where inadequate procedure preparation or review caused surveillances to be either missed or inadequately performed. In Licensee Event Report (LER) 99-06, "Inadequate Surveillance of Automatic Depressurization Nitrogen Supply System Isolation Valves," the procedural deficiency was identified as the result of a lack of knowledge, not inadequate review. LER 99-13, "Relays in Multiple Systems Were Not Correctly Tested as Required by Technical Specifications," identified the cause as an inadequate procedure due to a lack of understanding of the system test boundaries by the procedure developers and reviewers. LER 99-21, "Reactor Vessel Pressure Relays were not Correctly Tested as Required by Technical Specification Due to an Inadequate Procedure," identified the cause as an inadequate procedure resulting from an omission in the development of the procedure and the failure of reviews to identify the omission. The last revision to the procedure had been made in 1993. The weakness in the procedure was discovered as a result of corrrective actions from LER 99-13. In LER 01-004, "Reactor Scram Due to Inadequate Main Steam Isolation Valve Surveillance Procedure," an inadequate procedure was identified as the cause. However, the inadequacy was not due to lack of verification and validation of correctness of the procedure.
C. Identification of components referred to in this Licensee Event Report:
Components IEEE 805 System ID�IEEE 803A Function Reactor Protection System� JC� N/A Main Turbine System� TA N/A Control Rod Drive System� AA� N/A Instrument Air System LD N/A Service Air System� LF� N/A Breathing Air System LH N/A Valve� LF� ISV Turbine TA TRB Rod� JC� ROD