05000389/LER-2009-001
| Docket Number | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(I)(B) |
| 3892009001R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LERNUMBER (6) PAGE (3)
Description of the Event
A review was completed of a Seabrook Station Operating Experience Report involving the affects of low ambient air temperatures on the operation of the main feedwater isolation valve (MFIV) stroke times. Seabrook's OE Report 26387,"Adverse Effect On Feed Water Isolation Valve Stroke Test Due To Unusual Conditions Encountered During A Forced Outage," identified conditions that ambient air temperatures at or below 60°F would potentially increase stroke times for MFIVs [EIS:SJ] that have actuators that are hydraulic/pneumatic. St Lucie Unit 2 MFIVs are hydraulic/pneumatic actuated valves similar to those at Seabrook using the same hydraulic fluid. An investigation by the vendor has shown that an increase in viscosity of the hydraulic fluid in the actuators at temperatures of 60 degrees or below may create an environment within the valve actuator that increases the valve closure time above the technical specification limit of 5.15 seconds. St Lucie Unit 1 MFIVs use a nitrogen medium for actuation of the valves and are not affected by low ambient temperature conditions.
Cause of the Event
An evaluation of the event concluded the increased hydraulic fluid viscosity caused by low ambient temperatures could have resulted in slower than required stroke time of the MFIVs on Unit 2. Contributing factors included; lack of a minimum operating temperature for the MFIVs in design specifications allowing the vendor to select 60°F as the minimum design temperature; inadequate operating procedures allowing the MFIVs to operate at temperatures below 60°F; inadequate temperature monitoring; and latent design weaknesses.
Analysis of the Event
Based on a review of available vendor data and documents it was concluded that at temperatures below 60°F the Unit 2 MFIVs could potentially be considered inoperable as a result of the slower than permitted stroke times by plant Technical specification (TS) of 5.15 seconds and therefore reportable in accordance with 10CFR50.73 (a) (2) (i) (B), as a condition prohibited by Unit 2 technical specifications.
A review of past ambient temperatures has identified several instances in which the ambient temperature at the MFIVs has been below 60°F. Low ambient air temperatures below 60 degrees could result in slower stroke times than those permitted by plant TS of 5.15 seconds. The slower stroke times would not meet current TS requirements to isolate the "Not Nuclear Safety Related" feedwater system from the "safety related" portion of the feedwater piping during a main steamline break (by way of MSIS) or a main feedwater break line break (by way of AFAS).
Further review by Engineering of temperature data (hydraulic fluid temperature vs.
ambient temperature) from February 12, 2005 for Unit 2 MFIVs and hydraulic fluid temperature vs. ambient temperature identified no meaningful correlations since temperatures were found to be at 60°F or above. Reviews of available PSL MFIV documents, Chemistry MET temperature and plant data retrieval system (PI), reached similar conclusions. The conclusion reached from these reviews and analysis was the MFIVS could have been inoperable longer than the required TS allowable times and therefore is being reported in accordance with 10CFR50.73(a)(2)(I)(B), a condition prohibited by Unit 2 technical specifications.
FAMLITYNAME(7) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) This condition could have been applicable to any or all of the four feedwater isolation valves on Unit 2. Documented stroke times for all four MFIVs have been less than the 5.15 seconds criteria during past testing. There are no other hydraulic/pneumatic valves in the steam trestle. Unit 1 MFIVs are nitrogen medium action valves and are not affected by this condition. This is a legacy issue applicable to Unit 2 MFIVs and therefore is not considered a repeat event. .
Analysis of Safety Significance St. Lucie Unit 2 TS 3.7.1.6 requires four MFIVs to be Operable in Modes 1, 2, and 3 except when the MFIV is closed and deactivated. The loss of all MFIVs would result in entry into TS 3.0.3 for failure to isolate any flow path and require action within one hour for Unit 2 to be placed in a Mode in which TS 3.7.1.6 does not apply (Modes 4, 5, or 6).
Review of records over the last three years show the worst case condition (loss of all MFIVs) for ambient temperatures less than 60°F existed for a 22-hour period, with 39 °F being the lowest temperature reached. During this period, Unit 2 was in a Mode in which TS 3.7.1.6 applied. Based on vendor manual information and Engineering judgment, the TS MFIV stroke time of 5.15 seconds could have been exceeded if automatic closure had been required due to a steamline break or main feedwater line break.
Due to the complexity of the MFIVs a precise closure time could not be calculated, however, the containment analysis used for the installation of replacement steam generators for Unit 2 was revisited to assess the affects on containment, containment pressure, and off-site doses. The new containment analysis used an improved estimate computer code (Gothic) using replacement steam generator conditions, which are conservative with respect to the old steam generators. Results of this review concluded peak containment pressure would have remained below the design pressure limit of 44 psig and containment integrity would not be challenged until an internal containment pressure of approximately 95 psig was reached. Given the worst-case ambient temperature identified 39°F, and assuming the MFIVs fail to stop full main feedwater flow, significant "margin to containment failure" still remained. Site boundary dose was also reviewed and determined not to be impacted for MFIV closure times in excess of 5.15 seconds.
Based on this evaluation, the potential safety consequences of the event are judged to be low and there would have been no significant impact on the health and safety of the public.
Corrective Actions
The corrective actions and supporting actions are entered into the site corrective action program. Any changes to the proposed actions will be managed under the corrective action program.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Corrective Actions Taken 1. Temporarily installed temperature indicators in the steam trestle on MFIVs, on both headers, to monitor MFIV temperatures to ensure they do not go below 60°F.
2. Reviewed MFIVs and trestle design for other covers in lieu of the tarp.
3. Reviewed vendor and design documents of main turbine for applicability.
4. Covered MFIVs with tarps and placed space heaters to maintain ambient temperatures above 60°F 5. Revised procedure ADM-04.03, "Cold Weather Preparation." to include MFIVs on Unit 2 and guidance for maintaining temperature above 60°F 6.Added MFIV actuator temperature to electronic shift operations management systems (ESOMS) data logger to check and log ambient temperatures each shift to ensure actions are taken; if temperature falls below 65°F.
Long Term Corrective Actions
- Replace MFIV actuators to operate at all temperatures at the site.
Similar Events A search of the corrective action database for St. Lucie was performed to identify events related to a Main Feedwater Isolation Valves and none were found. This event is not considered a repeat event.
Failed Components
Main Feedwater Isolation Valves, Anchor Darling Valve Co., Model 20"-W7920697