05000374/LER-2014-001
| Lasalle County Station, Unit 2 | |
| Event date: | 08-05-2014 |
|---|---|
| Report date: | 10-03-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3742014001R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and led back to industry.
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ICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
LaSalle County Station Unit 2 is a General Electric Company Boiling Water Reactor with 3546 Megawatts Rated Core Thermal Power.
A. CONDITION PRIOR TO EVENT:
Unit(s): 2 Reactor Mode(s): 1 Event Date: August 5, 2014 Event Time: 1734 CDT Mode(s) Name: Power Operation Power Level: 100%
B. DESCRIPTION OF EVENT:
On August 5, 2014, at approximately 1734 hours0.0201 days <br />0.482 hours <br />0.00287 weeks <br />6.59787e-4 months <br /> CDT, Unit 2 automatically scrammed from 100% power on high neutron flux, followed by a Group I containment isolation. Following the Group I isolation, the control room operators noted that the position indication for valve 2B21-F022C, the inboard 2C Main Steam Isolation Valve (MSIV)[SB], showed dual indication rather than full closed.
Troubleshooting of the 2C MSIV determined that the valve stem disk had separated from the stem, which allowed the main disk to drop into the main steam flow path. The resulting reactor pressure transient added positive reactivity, which caused the high neutron flux scram. Increased steam flow in the other three main steam lines resulted in a nearly simultaneous high main steam line flow Group I containment isolation.
This occurrence is reportable under 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in the automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B). An ENS report was made to the NRC (EN 50346) at 2120 CDT on August 5, 2014, pursuant to 10CFR 50.72(b)(2)(iv)(B) and 50.72(b)(3)(iv)(A).
This event constitutes an unplanned scram with complications in accordance with NEI 99-02, Revision 7.
C. CAUSE OF EVENT:
The cause of the stem-disk separation on the 2C MSIV was fretting wear attributable to marginal design. A formal root cause investigation was conducted, which determined that the root cause of the event was a legacy decision made in 2008.
The vulnerability of the Rockwell International MSIV to stem-disk separation was a known issue. In 1989, Rockwell developed an "MSIV Improvement Package" with a more robust stem-disk design configuration. The station initially planned to install this upgrade on all 16 MSIVs (two Units with four inboard and four outboard MSIVs each); however, based upon the results of inspections performed on several MSIVs, the upgraded design was installed on only seven before it was decided to defer the remaining nine installations until additional corrective maintenance work was required. This decision was made in 2008 using the Operational and Technical Decision Making (OTDM) process.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. A reactor scram with closure of the MSIVs is an analyzed event. Reactor pressure control was maintained using reactor core isolation cooling and the safety relief valves. Reactor level control was maintained with the feedwater system initially and then with use of the Low Pressure Core Spray (LPCS) system. High pressure core spray was operable throughout the event. The normal heat sink through the main condenser could have been re-established by resetting the Group I containment isolation signal and opening the MSIVs in one main steam line. The main turbine bypass valves could then be opened as necessary to transfer decay heat to the main condenser.
E. CORRECTIVE ACTIONS:
- The upgraded design was installed in the four remaining Unit 2 inboard MSIVs. This was completed in August 2014 during the forced outage following the event.
- The upgraded design will be installed in the five remaining Unit 1 MSIVs that still have the vulnerable stem- disk assembly.
- Previous decisions that used the OTDM process to defer installation of a configuration change intended to mitigate High and Medium consequence issues will be reviewed using the Nuclear Risk Management Process implemented on 7/9/14. The Nuclear Risk Management Process is a consistent process to evaluate and manage risk across a broad range of potential risks including the Operational Decision Making process (former OTDM process). This process addresses issues identified in the root cause investigation associated with the 2008 decision to defer the upgraded design for the MSIVs. Those issues included assessment of degradation rates and the review and verification that input data for decision making is complete and accurate.
F. PREVIOUS OCCURRENCES:
There have been no previous occurrences of an MSIV stem-disk separation at LaSalle.
G. COMPONENT FAILURE DATA:
Rockwell — Edward Flite Flow Balanced Stop Valve, 26 inch, Model 1612