|Duane Arnold Energy Center|
|Reporting criterion:||10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications|
|LER closed by|
|IR 05000331/2017003 (6 November 2017)|
|3312016003R00 - NRC Website|
|Person / Time|
|From:||Vehec T A|
NextEra Energy Duane Arnold
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML16342C569 (5)|
Reported lessons learned are incorporated into the licensing process and fed back to industry.
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I. Description of Event:
On October 18, 2016, with the unit shutdown for a planned refueling outage (Mode 5, Refueling, 0% power), an evaluation of data from the scheduled Main Steam Line Isolation Valve (MSIV) (System Code SB) and Main Steam Line Drain valve penetration Local Leak Rate Testing (LLRT) determined the 'as found' maximum pathway leakage for the 'B' Inboard MSIV, CV-4415, and the Outboard Main Steam Line Drain valve, MO-4424, in excess of the Technical Specification (TS) 220.127.116.11 leakage limit of 5 100 scfh for a single MSIV and 5 200 scfh for combined pathway leakage.
II. Assessment of Safety Consequences:
Reportability Evaluation CV-4415 'as found' leakage was measured at 253.8 scfh. TS Surveillance (SR) 18.104.22.168.9 requires verification of leakage rate through each MSIV is 5_ 100 scfh and that the combined maximum pathway leakage rate for all four main steam lines is 5 200 scfh when tested at ?. 24 psig. Failure to meet this SR requires entry into LCO 22.214.171.124.9, Condition D, restore leakage to within limits within8 hours
, or be in MODE 3 within12 hours
. The as found results exceeded the TS SR limit for individual and combined MSIV leakage and is reportable as a condition prohibited by TS and requires a Licensed Event Report (LER) to be submitted under 10CFR50.73(a)(2)(i)(B).
MO-4424 'as found' leakage was measured at 710 scfh. As stated in Updated Final Safety Analysis Report (UFSAR) Section 6.7.1, the total main steam pathway, which includes the four main steam lines and the inboard MSIV Drain Line, is limited to 200 scfh (i.e., the main steam line drains are included in the total leakage limit for all MSIVs). Therefore, the Main Steam Line Drains valve LLRT failures are included in this LER.
The Main Steam system conducts steam from the reactor vessel through the primary containment to the main turbine. In performing this function, it serves as part of the primary boundary to prevent radioactive release to the surrounding environment.
Duane Arnold Energy Center 05000-331 The MSIV Leakage Treatment Path is designed to mitigate the release of fission products following a Loss of Coolant Accident (LOCA). This is accomplished by directing any leakage past closed MSIVs and closed Main Steam drain line isolation valves to the main condenser via the Main Steam drain line. The volume and surface area of the condenser provides holdup time and plate-out surface for fission products.
Pathways to other systems connected to the Main Steam system are isolated to ensure leakage is processed through this path.
The NRC has previously assessed the consequences of BWR main steam line releases. In a paper published in 2002 (PRAB-02-01, "Assessment of BWR Main Steam Line Release Consequences"), the NRC determined an MSIV leakage rate that would be considered risk-significant and would warrant further investigation. This assessment is conservative since benefits of holdup time, plate-out, and gravitational settling are ignored. Given a greater than 20 factor of conservatism, the report concludes there should be no regulatory concern if the leakage past the best sealing valve in any single main steam line is less than 10,000 scfh. This result is reflected in the NRC's Inspection Manual Chapter 0609, Appendix H where a simple probabilistic risk framework is provided for use in identifying potentially significant findings related to containment integrity.
The as-found leakage rate in both the B' Main Steam line and in the Main Steam Line Drain line is well below the 10,000 scfh criterion. Therefore, this event was of low safety significance and had no impact on public health or safety.
This event did not result in a SSFF.
III. Cause of Event:
A Root Cause Evaluation (RCE) was completed. The RCE determined that the cause for the `13' MSIV failure was failure to perform periodic internal inspections of the MSIVs and the cause for the outboard MSL Drain valve was a non-optimal valve design for steam line drain application.
IV. Corrective Actions:
Immediate Corrective Action CV-4415 internals were re-machined to restore its leakage to below TS limits.
Corrective Actions for Cause of Event The frequency of internal inspections of MSIVs will be increased to once every 10 refueling cycles.
Actions are planned to replace the MSL Drain valves with a new valve design.
V. Additional Information:
Previous Similar Occurrences:
A review of NextEra Energy Duane Arnold LERs from the previous 10 years found no other instances of events related to Main Steam Lines failing "as found" LLRT testing.
ElIS System and Component Codes:
SB — Main Steam System Reporting Requirements:
This activity is being reported pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).