05000331/LER-2015-002

LER-2015-002, Unanalyzed Condition Due to Degraded Primary Containment Suppression Pool Coating

Duane Arnold Energy Center
Event date:	3-31-2015
Report date:	1-28-2016
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
Initial Reporting
ENS 50943	10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition
3312015002R01 - NRC Website

LER 15-002-01 for Duane Arnold Regarding Unanalyzed Condition Due to Degraded Primary Containment Suppression Pool Coating

ML16034A217
Person / Time
Site:	Duane Arnold
Issue date:	01/28/2016
From:	Vehec T A NextEra Energy Duane Arnold
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NG-16-0022 LER 15-002-01
Download: ML16034A217 (6)

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I. Description of Event:

The torus interior was recoated in Refueling Outage (RFO) 23 to address age management issues regarding the primary containment suppression pool. Existing coating systems, such as inorganic zinc, required frequent repairs each refueling outage and a more robust coating system was sought. Extensive testing concluded that Carboguard 6250N was the optimal coating system despite being a first-of-a-kind application for a torus recoat. The coating was designed to be a single layer application with a thickness of 45 ± 5 mils. However, as documented in Condition Report (CR) 1821308, dated November 8, 2012, widespread areas failed to meet this thickness criterion. The torus recoat project team decided to apply a secondary application to build-up thin areas to meet design specifications. After consultation with the coating vendor, Carboline, the team determined the secondary coating was within the recoat window specified in the Carboguard 6250N Product Data Sheet (PDS). After completion of the second application and verification of proper coating thickness by Dry Film Testing (DFT), the coating was forced cured in accordance with the PDS specifications and the torus was re-filled.

On October 9, 2014, an initial walkdown of the torus interior during RFO 24 discovered areas of coating delamination at the water line in Bays 1, 12, and 15, as identified in CR 1997546. Furthermore, the coating appeared to be sagging on several downcomers. The areas of coating delamination were coincident with areas that were subject to a secondary recoat during the torus recoat project in RFO 23. Subsequent walkdowns and underwater inspections identified additional areas of coating delamination in Bays 1, 2, 3, 5, 6, 7, 9, and 10, as identified in CR 1999648. These areas were also limited to the second coating application that was applied during the RFO 23. The inspections did not identify any primary coating delamination.

The torus coating protects the internal torus metal shell and supports from corrosion. The torus shell coating is a qualified coating designed to adhere to the torus shell following a Design Bases Accident (DBA). Any coating that does not adhere could result in additional debris burden to the Emergency Core Coolant System (ECCS) suction strainers. A loss of coating material does not lead to a rapid degradation of the torus metal. However, the delaminating coating has the potential to impact the ECCS suction strainers during a DBA.

This system is a Critical 1, Quality Level 1, Service Level 1 system. The severity of the event was classified as a Significance Level 1 event.

A Failure Investigation Process (FIP) Team was assembled and tasked with the responsibility to determine the apparent cause of the delamination and to restore the torus coating. Loosely adhered coating was removed through a combination of mechanical Duane Arnold Energy Center 05000-331 scraping and pressure washing. A total of approximately 5300 square feet of coating was removed, which represented about 94% of the secondary coating. As documented in Prompt Operability Determination (POD) 1997546-01, the as-left coating was determined to meet full qualification by inspection and adhesion testing.

II. Assessment of Safety Consequences:

The coating delamination did not affect the integrity of the torus as a pressure retaining component (there was no identified corrosion of the torus due to the delamination).

The coating delamination resulted in a significant increase in debris loading of the ECCS suction strainers and therefore, had the potential to impact Emergency Core Cooling Systems (ECCS) that interface with the torus. In particular, those ECCS systems that are designed to draw suction from the torus in various operating modes.

These ECCS systems include Residual Heat Removal (RHR), Core Spray (CS), and High Pressure Coolant Injection (HPCI). In addition, the Reactor Core Isolation Cooling (RCIC) system can be aligned to the torus as a suction source. The plugging of system strainers with delaminated coating could render some or all of the associated systems ineffective to fulfill their safety functions as specified in the Current Licensing Basis (CLB) for some or all of Cycle 24.

A Past Operability Review (POR) 1999648-01 performed an evaluation of the safety significance of this event. Specifically, the POR evaluated the impact on the Net Positive Suction Head (NPSH) of the Emergency Core Cooling Systems (ECCS) and Reactor Core Isolation Cooling (RCIC) due to torus secondary coating delamination.

The POR concluded that the ECCS and RCIC pumps were capable of performing their safety functions as described in the current licensing basis but were degraded over the time period between RF023 and RF024. However, as discussed in NRC Inspection Report 2014-011, there were several uncertainties in the evaluation such as the fact that the original ECCS suction strainer loading analysis assumed the debris mass was generated in the drywell and transported to the torus and strainers.

A LOCA under this assumption could lead to the ECCS suction strainer surface areas being blocked by the delaminated coating prior to the arrival of the drywell debris. An additional uncertainty was coating debris size characteristics during LOCA conditions.

Specifically, Carbogaurd 6250N characteristics have not been studied under LOCA conditions. Therefore it was determined that the event had a delta core damage frequency (CDF) between 1.0E-5/yr (upper bounding significance) and 1.0E-6/yr (lower bounding significance).

Based on the level of uncertainties in the POR, it was concluded that an unanalyzed condition existed. This event was reported to the NRC on March 31, 2015 under EN#50943 pursuant to 10CFR50.72(b)(3)(ii). In addition to being reported as an unanalyzed condition, 10CFR50.73(a)(2)(ii)(B), this event is being reported as a condition prohibited by Technical Specifications, 10CFR50.73(a)(2)(i)(B), an event or condition that alone could have prevented a loss of safety function, 10CFR50.73(a)(2)(v)(B) and (D), and common cause inoperability of independent trains or channels, 10CFR50.73(a)(2)(vii), due to the impact on the ECCS pumps and RCIC.

III. Cause of Event:

A Root Cause Evaluation (RCE) was completed. The RCE identified the following Direct, Root, and Contributing Causes.

DC1: Conditions for applying the second layer of torus coating were not met RC1: Less than Adequate Coating Application Specification and Work Instructions RC2: Less than Adequate Project Oversight and Control CC1: Less than Adequate Guidance for Curing Schedule CC2: Less than Adequate Contingency Planning for Areas of Low Millage

IV. Corrective Actions:

Immediate Corrective Action The degraded condition of the torus shell coating was inspected and repaired prior to plant startup from RF024. POD 1997546-01 determined that the remaining coating would not adversely impact the function of the ECCS suction strainers.

Corrective Actions to Prevent Recurrence

Administrative Control Procedure (ACP) 1602, Specification for Protective Coatings in Areas Outside the Primary Containment, and ACP 1603, Protective Coatings for Service Level 1 Applications Inside the Reactor Containment, were revised to require Wet Film Thickness measurements be taken during coating application.

Nuclear Fleet Administrative Procedure, PR-AA-100-1001, Project Management, was revised to implement the Enterprise Risk process to provide adequate project oversight for projects that meet the definition of Enterprise Risk.

V. Additional Information:

Previous Similar Occurrences:

A review of NextEra Energy Duane Arnold Licensee Event Reports found no other instances of torus coating delamination events.

EIIS System and Component Codes:

N/A Reporting Requirements:

This activity is being reported per 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition, a condition prohibited by Technical Specifications, 10CFR50.73(a)(2)(i)(B), an event or condition that alone could have prevented a loss of safety function, 10CFR50.73(a)(2)(v)(B) and (D), and common cause inoperability of independent trains or channels, 10CFR50.73(a)(2)(vii).