05000387/LER-2014-003

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LER-2014-003, Loss of Secondary Containment during Technical Specification SR 3.6.4.1.5 Drawdown Testing
Susquehanna Steam Electric Station - Unit 1
Event date: 03-04-2014
Report date: 05-01-2014
Reporting criterion: 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
Initial Reporting
ENS 49867 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material
3872014003R00 - NRC Website
v • d • e

20555-0001, or by internet e-mail to Infocollects.Resource@ nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

Susquehanna Steam Electric Station 05000387

CONDITIONS PRIOR TO THE EVENT

Unit 1 — Mode 1, 100 percent Rated Thermal Power Unit 2 — Cold Shutdown, 0 percent Rated Thermal Power There were no systems, structures, or components that were inoperable prior to the start of the surveillance test that would have contributed to the event.

EVENT DESCRIPTION

On March 4, 2014 at 0025 EST, during Technical Specification (TS) surveillance SR 3.6.4.1.5, drawdown testing of Secondary Containment failed to meet acceptance criteria of SR 3.6.4.1.5 due to maximum flow rate exceeding the allowable value. The testing was performed with the Unit 1 Railroad Bay aligned as Zone III to Reactor Building (RB) HVAC [EIIS: VA] shutdown and a controlled Zone II Secondary Containment breech established. Actual in-leakage while in the untested alignment was 3301 cubic feet per minute (cfm), which is in excess of the TS SR 3.6.4.1.5 acceptance criteria of less than or equal to 2885 cfm.

Upon failure of the surveillance, Secondary Containment ventilation was realigned to a known previously successfully tested alignment for Zones I, II and III. This alignment consisted of all Reactor Building Zones in service and Zone III aligned to the Railroad Bay. On March 4, 2014 at 0128 EST, operability was restored and the Secondary Containment LCO 3.6.4.1 was exited. This event was determined to be reportable as an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> ENS (# 49867) notification in accordance with 10 CFR 50.72(b)(3)(v)(c) for a loss of safety function. There is no redundant Susquehanna Secondary Containment system.

In accordance with 10 CFR 50.73(a)(2)(v)(C), this LER is being submitted for an event or condition that at the time of discovery, could have prevented the fulfillment of the safety function of Secondary Containment to control the release of radioactive material.

CAUSE OF THE EVENT

The direct cause of this event was due to air in-leakage into Secondary Containment past Door-101 [EIIS: DR] and the Truck Bay Access Hatch Cover 2H24.

The apparent cause was due to inadequate margin for the untested configuration with Unit 2 Reactor Building HVAC shutdown and a controlled Zone II Secondary Containment breach established.

BACKGROUND

The Secondary Containment boundary for TS surveillance test SE-170-011 encompasses Zone I and Zone Ill Secondary Containments and will test the Zone I / II and Zone III / II boundaries with Zone II at atmospheric conditions (i.e. No-Zone). Previously, testing per SE-170-011 was performed with Zone II RB HVAC in service, which could influence the amount of in-leakage since Zone II was being maintained at negative 0.32 inch water BACKGROUND (cont.) column). Therefore, the boundary components include the following:

  • Door-101, Unit 1 Reactor Building (RB) Railroad Bay Access Door (Zone III to Outside - No-Zone)
  • 2H24, 818 Truck Bay Access Hatch, (Zone III to Zone II - No-Zone)

Historically, Secondary Containment testing configuration is two zone operation with Secondary Containment isolated on each zone (i.e., RB recirculation fan operating) and with the opposite Unit's normal HVAC system in service. In this configuration, the differential pressures across the recirculation isolation dampers are different than with the opposite Unit's normal HVAC system shutdown. Operating with the opposite Unit's HVAC in service can cause the Standby Gas Treatment System (SGTS) exhaust flow rate to be:

  • Artificially low (leakage through the opposite Unit's exhaust system) or
  • High (excessive in-leakage through the opposite Unit supply dampers).

Since the RB recirculation dampers are inspected, the physical leakage characteristics (such as small gaps) should be similar. The main driver of leakage is the differential pressures across the dampers. A review of start- up test data revealed that with RB HVAC in operation:

  • The recirculation supply and exhaust dampers have a differential pressure of approximately 1.5 in water gauge (w.g.)
  • The filtered exhaust system has a much larger differential pressure (approximately 8 in w.g.).

Due to the large differential pressure, the filtered exhaust dampers provide the largest amount of leakage.

Based on the above, the inspection history of the RB Recirculation dampers, the operational history of the RB (HD27601A/B, HD27602A/B, and HD27657A/B), and the new untested alignment; the maximum allowable in-leakage per TS Bases 3.6.4.1 may not be achievable.

ANALYSIS / SAFETY SIGNIFICANCE

Actual Consequences:

Failure of Door-101 and the Truck Bay Access Hatch Cover 2H24 to seal properly, along with testing in a previously untested alignment with the opposite Unit's RB HVAC shut down, resulted in excessive air in-leakage into Zone III Secondary Containment. This Secondary Containment testing was performed per TS surveillance procedure SE-170-011 with the Unit 1 Railroad Bay aligned as Zone III to verify integrity while performed in a previously untested alignment. The untested alignment is Unit 2 RB HVAC shutdown with a controlled Zone II Secondary Containment breech established.

Actual Consequences: (cont.) Upon failure of this surveillance, Secondary Containment was re-aligned to a previous successfully tested and operable alignment. This operable alignment is with jumpers installed per procedure TP-070-020 to electrically cross tie Zone II isolation signal with a Zone III isolation signal and a Zone I isolation signal with a Zone III isolation signal. Thus, if an isolation on Zone I / Ill or a Zone II / Ill were to occur, a Zone I / II / Ill isolation would be initiated.

Potential Consequences:

In-leakage through both Door-101 and the Turbine Building (elevation 818 feet) Truck Bay Access Hatch Cover 2H24, along with Unit 2 RB HVAC shut down, Secondary Containment could have unknowingly been inoperable when Zone II was relaxed from Secondary Containment. If a Zone I / III secondary containment isolation event were to have occurred while in this configuration, resultant accident dose could have exceeded that described in Susquehanna Steam Electric Station's (SSES's) current licensing basis.

In summary, there were no actual consequences to the health and safety of the public as a result of this event.

CORRECTIVE ACTIONS

Completed Actions:

  • Installed shielding on the Core Spray piping adjacent to the Control Structure to gain margin in SSES's analysis for Control Room Operator dose post-accident. In exchange for this gain, the allowable SGTS exhaust flow rate (i.e. Secondary Containment in-leakage) was increased to yield a negligible overall change in Control Room Operator dose. This change in dose strategy allowed the Unit 1 and Unit 2 TS SR 3.6.4.1.5 Secondary Containment in-leakage acceptance criteria to be increased from 2885 cfm to 4000 cfm.

Planned Actions:

  • Repair/replace the bottom seal plate and the top and bottom seals on Door-101.
  • Install caulk in the joints for the Truck Bay Access Hatch Cover.
  • Revise applicable procedures acceptance criteria to new increased SGTS Exhaust Flow Rate values.

PREVIOUS SIMILAR EVENTS

  • LER 50-387(388)/2014-003-00: Loss of Secondary Containment due to Failed Solenoid Valve in the Reactor Building Zone I Ventilation Exhaust System

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