05000313/LER-2014-001
| Docket Number | |
| Event date: | |
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| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3132014001R00 - NRC Website | |
A. Background
The design basis flood protection elevation for Arkansas Nuclear One, Units 1 and 2 (ANO-1 and ANO- 2) safety-related components and structures is 361 ft. Mean Sea Level (MSL). The design basis flood is a combination of the probable maximum flood (PMF) (358 ft. MSL), which is based on a 10,000 year probability, and the assumed coincident failure of the upstream Ozark Dam (+3 ft.).
The ANO-1 and 2 Safety Analysis Reports (SARs) state that Seismic Class/Category 1 structures are designed for the PMF of 361 ft. MSL and Seismic Class/Category 1 systems and equipment are either located on floors above 361 ft. MSL or protected by walls, waterproof doors, hatches, etc. Additionally, Startup Transformer #2 (SU2) [FK][XFMR] and its associated bus work are one of the required alternating current sources and path for off-site power. The feed from this transformer is expected to be protected against flooding up to elevation 361 ft. MSL.
B. Plant Status At the time of the condition, ANO-1 and ANO-2 were in Mode 1 (100% power).
C. Condition Description Over the past several months ANO has conducted walk downs as follow-up activities in response to Near-Term Task Force (NTTF) Recommendation 2.3, Flooding Walk down of the NTTF Review of Insights from the Fukushima Dai-ichi Accident. During these walk downs plant personnel documented multiple instances of ANO-1 and ANO-2 design features that may not have provided adequate external flood protection for safety-related equipment located below the design basis flood elevation of 361 ft. MSL. Examples of the identified deficient design features and procedures include:
· Missing or deficient internal conduit seals · Missing or deficient seals · Unsealed removable closure plates · Missing gaskets · Roof leaks · Ground water intrusion through penetrations, building joints and conduits · Degraded hatch gaskets · Floor drain system cross connected between flood protected areas and non-flood areas Due to the lack of designed isolation features on these pathways, the potential existed for flood waters to migrate into the Auxiliary Building where pumps and equipment required for safe shutdown are located. Some of these deficient barriers existed between the non-flood protected Turbine Building and the Auxiliary Building. Flooding of the Turbine Building from internal pipe breaks is an additional vulnerability that existed due to the degraded flood barriers that existed between these two buildings.
It has been determined that a small segment of the SU2 4160V non-segregated bus is less than five inches below the maximum design basis flood elevation of 361 ft. MSL. It is assumed that flood water at the maximum predicted flood elevation could enter the bus duct causing a fault and loss of offsite power for both ANO-1 and ANO-2.
Additionally, the Natural Emergencies procedures lack detail or omit flood barriers that require operator action to verify component position that, in some cases, are identified in design basis documents.
D. Root Causes A root cause evaluation identified the following two root causes:
(1) There was a lack of configuration control of external flood barriers. This included controlled design drawings that do not properly display or define flood barriers (penetration seals, conduits, etc.). Additionally, operating procedures were identified that did not contain the detail necessary to ensure manual actions would be implemented resulting in the potential for ineffective response to external flooding.
(2) There was a lack of robust design of external flood protection barriers. Several instances were identified where the floor drain system was cross-connected between non-flood protected areas and flood protected areas with no method of isolation. Historically and recently, silicone foam was used as a penetration sealant. This product is not watertight and there have been instances of degradation over time. Additionally, unscheduled embedded conduits that penetrate flood barriers were not properly sealed.
The following contributing causes were also identified:
· Inadequate flood penetration barrier walk downs · Breakdown in the Operating Experience (OE) process · Lack of understanding of risk significant flood barriers
E. Corrective Actions
Corrective actions include:
· Where deficiencies have been identified, including the SU2 non-segregated bus, reestablishment of qualified flood barriers · A process for notifying operations personnel when a work order breaches a flood barrier was developed · An OE was posted to alert the industry on findings related to flood barrier walk downs · Engineering documents are being revised to address flood barrier configuration control · Flooding walk down reports are being verified for completeness and accuracy · As deficiencies in the natural emergency procedures are identified, they are being revised, as applicable, to ensure operator actions for flooding mitigation are addressed · Flood barriers will be labeled to provide in-field awareness of flood protection features · Additional review of documentation to identify credible flood paths · Oversight of supplemental employee performing follow-up walk downs · Training will be developed and provided to Entergy employees that include the risk significance and importance of maintaining flood barriers.
Additional corrective actions are located in the ANO Corrective Action Program.
F. Safety Significance Evaluation Arkansas Nuclear One is required to be protected from flooding within the station's design and licensing basis. The safety-related equipment required to mitigate the consequences of an accident were potentially affected by this condition. There are several factors which could mitigate the impact of these deficiencies depending on the specific conditions encountered during an actual event. These mitigating factors include variations in the actual flood levels and the significant warning time (days) prior to the onset of the event which would allow the application of addition resources for implementation of mitigating actions including condition identification, sealing of pathways, and water removal actions. Should mitigating strategies be delayed, the open pathways could have jeopardized the ability of the safety-related equipment to perform the design basis function during an accident. There were no actual consequences.
G. Basis for Reportabililty 10 CFR 50.73(a)(2)(ii)(B):
Any event or condition that resulted in:
(B) The nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.” Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to:
(B) Remove residual heat:
(ANO-1 and ANO-2 Decay Heat Removal) H. Previous Events 10 CFR 50.73(b)(5) states that this report shall contain reference to “any previous similar events at the same plant that are known to the licensee.” NUREG 1022 reporting guidance states that term “previous occurrences” should include previous events or conditions that involved the same underlying concern or reason as this event, such as the same root cause, failure, or sequence of events.
A review of the ANO corrective action program revealed numerous identified conditions that are both historical and present day deficiencies in the flood barrier program. The previously identified conditions contain the same underlying concerns and causes that are identified in this LER.
There have been no previous LERs submitted to identify this condition.
Energy Industry Identification System (EIIS) codes and component codes are identified in the text of this report as [XX].