05000254/LER-2016-002

LER-2016-002, Secondary Containment Differential Pressure Momentarily Lost Due to Air Line Failure (RWCU Pump Rm)

Quad Cities Nuclear Power Station Unit 1
Event date:	01-15-2016
Report date:	03-15-2016
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
Initial Reporting
ENS 51659	10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material
2542016002R00 - NRC Website

LER 16-002-00 for Quad Cities Units 1 and 2, Regarding Secondary Containment Different Pressure Momentarily Lost due to Air Failure (RWCU pump Rm)

ML16089A290
Person / Time
Site:	Quad Cities
Issue date:	03/14/2016
From:	Darin S Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
SVP-16-014 LER 16-002-00
Download: ML16089A290 (6)

PLANT AND SYSTEM IDENTIFICATION

General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

EVENT IDENTIFICATION

Momentary Loss of Secondary Containment Differential Pressure due to High Cycle Fatigue Failure of an air line in the Unit 1 Reactor Building Exhaust fan plenum.

A. CONDITION PRIOR TO EVENT

Unit: 1 / 2 Reactor Mode: 1 / 1 Event Date: January 15, 2016 Event Time: 20:38 hours Mode Name: Power Operation / Power Operation Power Level: 100% / 100%

B. DESCRIPTION OF EVENT

On January 15, 2016, at 20:38 hours, unexpected Main Control Room Reactor Building (RB) low differential pressure (DP) alarms were received due to loss of negative DP in the RB. At approximately the same time, Operations had entered the Unit 2 RWCU Pump room.

Reactor building DP was observed rising to 0 inches water column (WC) and peaked at +0.2 inch WC. Technical Specification (TS) 3.6.4.1 Condition A, and QGA 300, Secondary Containment Control was entered for both Units Secondary Containment being inoperable. With no operator action, a negative reactor building DP was restored at approximately 20:40 hours. TS 3.6.4.1 Condition A and QGA 300 were exited at 20:42 hours.

On January 16, 2016 at 01:42 hours, ENS #51659 was made to the NRC under 10 CFR 50.72 (b)(3)(v)(C), to report this event as an event or condition that could have prevented the fulfillment of a safety function of structures or systems that are needed to control the release of radioactive material.

An investigation of this event was performed which determined that a sheared air line to the 1C RB exhaust fan isolation damper depressurized a common air header to all three Unit 1 RB exhaust fan isolation dampers, causing them to fail open. This air line was internal to the exhaust ductwork, therefore it could not be identified through normal Operator rounds. With a normal fan lineup of two supply and two exhaust fans operating per unit, the isolation damper for the standby Unit 1 exhaust fan remained open. This created a recirculation path which was compensated by the RB DP control dampers throttling nearly full open. This significantly degraded the margin available for the system to respond to changes in building DP. When an entry was made into the Unit 2 RWCU HX room, the area DP control system responded as expected by throttling closed all area DP control dampers which decreased total building exhaust flow. With reduced margin available in the building DP control, this caused a positive pressure condition in the RB.

C. CAUSE OF EVENT

The cause was a sheared air line inside the Unit 1 RB ventilation exhaust plenum which depressurized the air header supplying operating air to all three Unit 1 RB exhaust fan isolation dampers, causing the dampers to fail open, including the one on the standby fan.

The cause of the sheared air line was high cycle fatigue failure as a result of an inadequate piping support for the vibration environment. The specific routing and support of this line is not detailed in design specifications and therefore was attributed to deficient worker practices during original installation. The failed component is original plant equipment and therefore this is considered a historical cause.

The building and area DP control systems are not interconnected, but can have substantial effects on each other.

Specifically, when a RWCU area room is opened, the sensed area pressure increases. This causes the area DP control system to reposition the area DP control dampers in the closed direction in an attempt to restore area DP.

This reduces building exhaust flow, which will cause building pressure to increase and the DP control dampers to further open to restore building DP.

D. SAFETY ANALYSIS

System Design The function of the secondary containment is to contain, dilute, and hold up fission products that may leak from primary containment following a Design Basis Accident (DBA). In conjunction with operation of the Standby Gas Treatment System (SBGTS) and closure of certain valves [V] whose lines penetrate the secondary containment, the secondary containment is designed to reduce the activity level of the fission products prior to release to the environment, and to isolate and contain fission products that are released during certain operations that take place inside primary containment, when primary containment is not required to be operable, or that take place outside primary containment.

Updated Final Safety Analysis Report (UFSAR) Section 6.2.3.1 provides that the safety objective of the secondary containment system, in conjunction with other engineered safeguards and nuclear safety systems, is to limit the release of radioactive materials so that offsite doses resulting from a postulated DBA will remain below 10 CFR 100 guideline values.

The RB Ventilation is designed to maintain the RB (RB is common to both Units 1 and 2) at a negative pressure during normal operation.

Safety Impact Since both Units 1 and 2 share a common RB (secondary containment), the momentary loss of secondary containment differential pressure effects both units.

TS 3.6.4.1, Condition A, requires restoration of secondary containment to operable status within four hours. This four hour completion time provides a period of time to correct the problem that is commensurate with the importance of maintaining secondary containment during Modes 1, 2, and 3, since the probability of an accident occurring during this short period where secondary containment is inoperable is minimal.

The primary purpose of the secondary containment is to minimize the ground level release of airborne radioactive materials and to provide a controlled, elevated release of the building atmosphere under accident conditions. An engineering analysis was performed to demonstrate that during the time that secondary containment differential pressure increased to positive for approximately two (2) minutes, there would be a negligible effect on the resulting dose calculations. Secondary containment would have sufficiently contained radioactive materials during a LOCA such that all current dose limits would remain met. Secondary containment would have been able to perform its safety function. Therefore, the dose consequence from postulated releases from the RB during this short duration would be bounded by the existing design basis LOCA analysis. The safety significance of this event was minimal.

The engineering analysis that was performed demonstrated this event did not constitute a Safety System Functional Failure (SSFF). (Reference NEI 99-02, Revision 7, Regulatory Assessment Performance Indicator Guideline, Section 2.2, Mitigating Systems Cornerstone, Safety System Functional Failures, Clarifying Notes, Engineering analyses.) As such, this event will not be reported in the NRC Performance Indicator (PI) for SSFF since an engineering analysis was performed which determined that the system was capable of performing its safety function during this event when the secondary containment differential pressure increased to positive for approximately two (2) minutes.

Risk Insights The plant Probabilistic Risk Assessment (PRA) model gives no credit to secondary containment or RB Ventilation and does not include it in the model, hence the as-found conditions did not contribute to an increase in risk. In addition, overall RB negative DP was restored in approximately two minutes, so the physical integrity of the secondary containment structure was never compromised and the primary containment function was never lost.

In conclusion, the overall safety significance and impact on risk of this event were minimal.

E. CORRECTIVE ACTIONS

Immediate:

1. The sheared air line was replaced and a full system post maintenance test (PMT) was performed to verify that RWCU pump room.

Follow-up:

1. Establish additional trending for RB ventilation DP control dampers to identify potential decreases in system margin.

and 2 RB ventilation exhaust fan dampers.

F. PREVIOUS OCCURRENCES

The station events database, LERs, and INPO Consolidated Event System (ICES) were reviewed for similar events at the Quad Cities Nuclear Power Station. This event was attributed to an inadequate support installation of an air line during original plant construction. Based on the nature of this failure, the events listed below, although similar in topic, are not considered significant station experience that would have directly contributed to preventing this event.

• Station Issue Report (IR) 1412296: Low RB to Atmosphere DP Condition During Fan Starts, September 13, 2012 — Numerous issues were identified in 2012 with maintaining RB DP during fan starts and entries to RWCU rooms. The apparent cause that was identified during this troubleshooting was exhaust fan isolation dampers and area DP control dampers not operating properly due to numerous small air leaks. The air leaks were cuased by aged rubber gaskets and not vibration induced like the current event. System DP control improved significantly following these repairs.

• LER 254/2014-001-00, 5/5/14, Secondary Containment Differential Pressure Momentarily Lost Due to Fuel Pool Monitor Radiation Monitor Spike (3/4/14) — A Fuel Pool Channel 1B High Radiation spiked high causing the Unit 1 and Unit 2 RB Ventilation and Control Room Ventilation to isolate as designed. The SBGTS was already in operation for a scheduled surveillance. The previous event caused a momentary positive differential pressure until the RB Ventilation isolation dampers closed within 60 seconds and draw down the building within a 2-3 minute period. This event was caused while operating the RB Ventilation system and caused by a sheared air line on the exhaust fan isolation damper system.

• LER 254/2016-001-00, 3/11/6, Secondary Containment Differential Pressure Momentarily Lost Due to Air Line Failure (RWCU Hx Rm) (1/12/16) — This LER has the same cause and corrective actions as the current event.

This event and the current event occurred three (3) days apart. Troubleshooting was still in progress from the 1/12/16 event when the current 1/15/16 event occurred. Ongoing troubleshooting activities had not yet identified and corrected the deficient condition before the second event occurred.

G. COMPONENT FAILURE DATA

Failed Equipment: Male threaded brass pipe fitting Component Manufacturer: Unknown Component Model Number: N/A Component Part Number: N/A This event has been reported to ICES under Failure report No. 321170.