05000331/LER-2010-006
| Duane Arnold Energy Center | |
| Event date: | |
|---|---|
| Report date: | |
| Initial Reporting | |
| ENS 46372 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor, 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3312010006R00 - NRC Website | |
I. Description of Event:
On October 29, 2010 with the reactor shutdown for Refuel Outage (RFO) 22, a Condition Report (CR) was written requesting a review for applicability of NRC Event Notification 46372. The event, which occurred at LaSalle County Station, involved the seismic qualification of their SBLC Test Tank. Based on a question from the Nuclear Regulatory Commission (NRC) during a Component Design Basis Inspection (CDBI), LaSalle had determined that their SBLC Test Tanks were not seismically qualified while filled with water and could fail during a seismic event, potentially causing a failure of safety-related, seismically qualified SBLC equipment located near-by.
On November 10, 2010, engineering analysis determined that when the Test Tank was 3/4 full of water, as directed by Surveillance Test Procedures (STP), it could not withstand Design Bases Earthquake (DBE) seismic loads and would possibly have caused a failure of the safety-related, seismically qualified SBLC equipment during a DBE seismic event.
II. Assessment of Safety Consequences:
The safety significance of this event is minimal. The safety function of the SBLC system is to provide a backup method, independent of the control rods, to initiate a reactor shutdown and maintain the reactor subcritical as the nuclear system cools. The probability of a DBE seismic event concurrent with an Anticipated Transient Without Scram (ATWS) condition is very low. Additionally, in the event that boron injection is required but cannot be injected using the SBLC system, procedures provide guidance for utilizing the Reactor Water Cleanup (RWCU) system as an alternative means of injecting boron solution into the reactor vessel to shut down the reactor.
This event is being reported as an event or condition that at the time of discovery could have prevented fulfillment of a safety function under 10 CFR 50.73 (a)(2)(v)(A)/(D), a condition prohibited by Technical Specifications (TS) under 10 CFR 50.73 (a)(2)(i)(B), and a common-cause inoperability of independent trains or channels under 10 CFR 50.73 (a)(2)(vii).
This event did result in a Safety System Functional Failure.
III. Cause of Event:
Apparent Cause:
A cause evaluation was completed for this event. The evaluation determined that the cause of this event was from a latent design issue that occurred during design and construction of the plant.
Before the plant was licensed, design rules changed when NRC Safety Guide #29 was issued in June of 1972.
At the time of implementing the Safety Guide, the site failed to properly implement the requirements for the SBLC system. The SBLC Test Tank was not re-designed to include the requirements of the Safety Guide (i.e. 2 over 1 seismic requirement) prior to plant start-up.
Contributing Causes:
1. The failure to originally design (prior to the implementation of Safety Guide #29) the SBLC Test Tank to withstand the effects of a DBE.
2. The evolving regulatory requirements during plant design and construction. The Safety Guide was implemented between the time that the SBLC Test Tank was designed and the plant started up. This led to installing the test tank without ensuring that it would not cause the failure of the SBLC system during a DBE.
3. The requirement to maintain the SBLC Test Tank full of water following completion of routine STPs.
IV.� Corrective Actions:
Corrective Actions Taken to Address Apparent Cause:
The SBLC procedures were revised to require that the Test Tank be empty when the SBLC system is required to be operable.
Corrective Actions Needed to Address Contributing Causes:
1. Develop design basis calculation qualifying the SBLC Test Tank as seismically adequate when empty.
2. Review Operating procedures and ensure that leaving the Test Tank empty is an acceptable long term solution.
Corrective Actions Needed to Address Extent of Condition:
The site will perform a Seismic Qualification Utility Group (SQUG) type walk down to assure seismic ruggedness of other non safety-related mechanical test equipment used for testing safety related systems that are not included on the Safe Shutdown Equipment List. This walk down will verify that this equipment cannot cause a failure of the safety-related system during a DBE.
V.A Additional Information:
Previous Similar Occurrences:
A review of Licensed Event Reports (LERs) from the past 5 years did not identify any previous similar occurrences.
EIIS System and Component Codes:
BR — Standby Liquid Control System (BWR) Reporting Requirements:
This event is being reported as an event or condition that at the time of discovery could have prevented fulfillment of a safety function under 10 CFR 50.73 (a)(2)(v)(A)/(D), a condition prohibited by Technical Specifications (TS) under 10 CFR 50.73 (a)(2)(i)(B), and a common-cause inoperability of independent trains or channels under 10 CFR 50.73 (a)(2)(vii).