4kv switchgear susceptibility to main steam line break

4KV Switchgear Susceptibility to Main Steam Line Break

By:

Akbar Moarefy

Kyle Millenaar PE

OE – Discovery of New Harsh Environment Conditions

• 10/19/09 Monticello issued LER 2009-005-00 due to the failure of a HELB door fusible link with potential result of losing 4kV SWGR in case of HELB.

• 04/18/10 Prairie Island entered LCO 3.0.3 after discovering U1 turbine building truck Aisle rollup door security fence was closed. This fence was required to remain open to drain flood water resulting from HELB in the turbine building.

• 9/16/11 Callaway issued LER 2010-009-01 due to failure in implementation of HELB defense (HELB doors) challenging equipment operability in areas of the plant that were intended to be mild.

• 2/10/12 Cooper (Green NCV): Failure to translate design requirements associated with turbine building HELB into design information to demonstrate protection of safety related 4160V switchgear and EDG

• 2/10/12 Braidwood (Green NCV): Failure to report non-conservative assumption in actuation time for fusible links that protect ESF switchgear room from HELB.

• 05/08/12 Braidwood (Green NCV): EQ failed to consider temp/humidity changes in EDG, ESG and MEER rooms during a turbine building HELB

• 06/07/12 Millstone entered TS action statement when they determined that the practice of opening the switchgear room doors as compensatory measure when ventilation was OOS could allow steam from HELB enter the SWGR.

DCPP – Discovery of New Harsh Environment Conditions

• In August 2014, Diablo Canyon Power Plant entered Technical Specifications 3.0.3 due to non-conservative assumptions in a legacy calculation resulting in the Design Class 1E emergency power supply switchgear (4kV switchgear) being exposed to a high-temperature, high-humidity environment during a main steam line break (MSLB).

• Corrective action was implemented by closing the fire dampers isolating 4kV SWGR from turbine deck atmosphere. Issue became past operability assessment (29 years)

Background

• Unit 1: 5-7-85 Start up date • Unit 2: 3-13-86 Start up date

• Legacy calculation dated 1983 (before plant startup) assumed dead volume of air in the 4kV SWGR preventing the entry of steam following HELB. It also made wrong assumptions about fusible links closing the fire dampers, isolating SWGR from HELB environment.

• Reexamination of calculation of record proved the assumption to be wrong. Following a HELB in turbine building steam can enter the 4kV SWGR when the thermostat controlled HVAC fans are not running causing humidity reach 100%RH.

• 4kV SWGR components were not qualified for a 100%RH environment.

False Assumptions

• Room HVAC supply fans provide a positive pressure of 0.018 psi.

• Model shows steam pressure to be approximately 10x greater than supply fan

Background

First stage high pressure turbine

Turbine building ceiling curved

Background

4kV Exhaust duct on Turbine Deck

Background

4kV switchgear panel bottom vent

4kV switchgear panel top vent

Is it possible to retract entry into 3.0.0?

• DCPP EQ Group recommended testing all the 4kV SWGR components under the newly discovered adverse condition.

• Other groups including AREVA suggested to pursue parallel success paths to prove past operability.

Is it possible to retract entry into 3.0.0?

• AREVA was contracted to lead the project. Nutherm was subcontracted to perform qualification tests.

• Initial proposal was received by AREVA on 8/12

• The Test Spec was drafted by DCPP on 9/2/14

• Testing was completed on 9/12/14

• Final report was issued on 9/19 (5 weeks)

Paths to Prove Past Operability

• “ALPHA” Path: Elimination of 0.9 sq-ft longitudinal split break – Double Ended guillotine break results in a rapid

isolation of the steam line and does not create adverse environment in the 4kV SWGR.

– a 0.9 sq-ft longitudinal split break which is not sufficient in size to isolate the steam line but does expose the vented cabinets in the switchgear room to a 100% humidity environment.

• STATUS: This success path failed


• “BRAVO” Path: Remove conservatism from GOTHIC model using NRC Operability determination guidelines such as IM-9900. goal was to prove the environment in the 4kV SWGR would remain mild following a HELB on the turbine deck.



• “CHARLIE” Path: Using Computational Fluid Dynamics (CFD) Analysis for the 4kV SWGR. Use CFD models of the switchgear rooms to simulate the temperature, flow fields and humidity distribution inside the switchgear rooms. These simplified models included the air flow within the switchgear room from the floor to the exhaust duct/turbine building interface. Goal of the model was to demonstrate that the hot humid air entering the room would flow out the normal air inlet without approaching the vented cabinet vent locations. Thus concluding that the SWGR components would not have been subjected to an adverse environment.



• “DELTA” Path: Computational Fluid Dynamics Analysis of Vented Cabinets. The simplified CFD models created in the “CHARLIE” path were used to extend to model the air flow and humidity distribution within the vented cabinets. Goal of this success path was to demonstrate that the even with 100% humidity in the 4kV SWGR, the components within the cabinets will not be exposed to condensing environment.



• “ECHO” Path: Vented Cabinet Components Already Qualified for 100% humidity conditions. Compare DCPP’s component list and HELB qualification requirements to existing industry qualification reports to eliminate or reduce the need to test components in humid environment.

• STATUS: Only one component was eliminated


• “FOXTROT” Path – Conduct Qualification Testing on the 4kV SWGR components.

• STATUS: 35 components representing 756 components used in the 4kV SWGR cabinets were successfully tested. Leakage current on a terminal block with broken barrier wall in a 100%RH condensing environment was measured to be minimal with no adverse impact. Potential for arc flash was eliminated by test on high voltage components.

TEST CABINET TEST CHAMBER

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4kv switchgear susceptibility to main steam line break

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