05000263/LER-2001-008
| Docket Number | |
| Event date: | 03-12-2001 |
|---|---|
| Report date: | 05-11-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2632001008R00 - NRC Website | |
Description On March 12, 2001 with the reactor at cold shutdown, plant personnel identified an unanalyzed condition related to structural adequacy of two High Energy Line Break (HELB) barrier walls located on the 931ft elevation of the Turbine Building'. The HELB barriers separate two redundant divisions of essential 480V Motor Control Centers2 (MCC), and are required to withstand differential pressure forces due to a postulated HELB. A postulated HELB of a feedwater3 pump4 discharge line or a main steams line in the Turbine Building and a consequent failure of these walls could damage both divisions of 480V MCCs.
The two identified HELB barrier walls were originally installed as fire barriers and they are credited in the Environmental Qualification (EQ) analyses as HELB barriers. Modifications were installed to ensure that the HELB barriers would withstand the postulated HELB pressure forces as assumed in the EQ analysis.
Event Analysis
Analysis of Reportability This report is being submitted in accordance with 10 CFR 50.73(a)(2)(ii)(B) as conditions that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety. In the case of unanalyzed HELB barriers, both divisions of essential 480V MCCs could have been degraded.
Safety Significance
A feedwater or a main steam line break is an unlikely event. The feedwater and main steam piping has been analyzed seismically to be able to withstand the design basis earthquake. Also, this piping is included in the formal program for monitoring flow induced erosion/corrosion in piping systems and has been monitored for wall thinning. The thinning rates are within normally expected values. No piping replacement has been required in the feedwater or main steam piping. Failure due to either of these mechanisms is unlikely.
1 EIIS System Code = NM 2 EIIS System Code = SB 3 EIIS System Code = SJ 4 EIIS System Code = P 5 EIIS System Code = MCC A risk assessment performed by Monticello Probabilistic Risk Assessment (PRA) Group concluded that the failure of the HELB barriers described above has a low safety significance. This conclusion was based on the following considerations:
1. The frequency of a HELB in the feedwater or main steam lines.
2. The probability of failure of Division I MCCs.
3. The probability of failure of the HELB barriers.
4. The probability of failure of Division II MCCs.
5. Availability of both divisions of 4kV swithgearl equipment, Emergency Diesel Generators (EDG)8, RHR6, Core Spray', RCIC2, HPCI3, SRVs4, RHRSW5 and Hard Pipe Vent systems. Operator action would be required to manually open the injection valves' for the RHR and Core Spray systems.
Additional analyses could be performed that, if credited, would further lower the calculated safety significance of these HELB barrier failures.
Cause
Personnel error has been determined to be the cause of this event. Significant enhancements have been made to the thermal-hydraulic model of the Turbine Building using many more subcompartments than the previous analyses. These detailed models contain additional compartments in the building and have numerous subdivisions of large areas. In addition, flow paths representing openings in the rooms were included. The additional enhancements to the original model resulted in higher peak transient pressure forces for certain fire barrier walls credited as HELB barriers. These barriers were not evaluated for the higher pressure forces.
1 EIIS System Code = EB 2 EIIS System Code = BN 3 EIIS System Code = BJ 4 EIIS System Code = RV 5 EIIS System Code = BI 6 EIIS System Code = BO 7 EllS System Code = BM 8 EIIS System Code = EK 9 EIIS System Code = INV
Corrective Actions
Corrective Actions:
Modifications were installed to ensure that the HELB barriers would withstand the postulated HELB pressure forces. A plant walkdown of all fire barrier walls credited as HELB barriers was performed.
All identified HELB barriers were evaluated and found to be capable of performing their intended functions.
Preventive Actions:
Training and procedural improvements are being formulated to ensure proper identification and qualification of all HELB barriers.
Failed Component Identification None Similar Events A discrepancy in the High Energy Line Break analysis was reported in May 1996. This discrepancy was of a different type. The identified discrepancy involved an error in the licensing basis HELB analysis for the Turbine Building. The error resulted in impropery analyzing ambient temperatures in the vicinity of Division II MCC-142, Division II MCC-143 and the 4kV switchgear rooms for the limiting feedwater HELB. The re-analysis determined that these areas could become harsh environments; however, the licensing basis analysis did not predict these areas to be harsh environments.