Abdul Qadir

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Introduction

Onsite and offsite power supply

Description of Accidents

Effects of faults on plant’s components.

General Analysis

Event summery

Event scale

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The forsmark unit 1 is a 1010 MWe ,2928Mwth BWRplant, located in Sweden ,designed by ASEA Atom withtwo turbine generator and commissioned in 1980.

Status of the plant at site

Foresmark 1

Electrical fault occurred at this plant

Foresmark 2

This unit was Shut down for refueling and maintenanceat time of fault

Foresmark 3

This unit was operating at full load but not affected b/cit is connected with other switchyard.

The event at Forsmark nuclear station revealed severaldesign and operational oversights that could lead tosignificant challenge to nuclear safety.

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Electric power system of Forsmark 1 is divided as follows

Offsite power supply

There are two independent electric power supply line .

400KV

70KV gas turbine

On site power supply

Four 6.6 KV trains with each sized for 50% ECCS.

Four 500 v EDG secured buses

Four 500V battery secured buses

Four 220 AC power system(UPS)

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On 25th July 2005 date plant was operated at full powerand at the time 13:20:20 A dis-connector in the400 kVswitchyard opens during maintenance , creating an arcand a two phase short circuit.

The short circuit at the switchyard caused a gridvoltage fluctuation resulting in momentarydepression of the system voltage in response to thegird voltage fluctuation the voltage regulator on themain generator responded to restore the systemvoltage.(No.NG-T-3.8 page 15) .

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At same time of transient The generator backupprotection system disconnected Both generator circuitbreakers trip on under voltage, i.e. disconnecting thestation from the 400 kV from the grid and the controlsystem aligned the full output of the main generator tosupply power to the house loads.

At the instant of switching to house load operation,the high voltage output of the generator resulted in abrief but significant overvoltage. This high voltagevariation fed through the transformer which supplythe local power system and some other safety systemin the plants.

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The event generated two transients on the power supply tothe unit.

The first was an over voltage transient that causes thefailure of two un-interruptiple power supply

The second was a low frequency transient that causesdisconnection of the off-site power to the safety relatedbus bar.

70KV gird and gas turbine :

The 70KV system remained available through out the event,however ,the back up gas turbine did not start requested asstandby. The cause of this was lack of adequatemaintenance.

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Reactor

Reactor output is reduced by a partial scram.Changeover to house load operation and dumping ofsteam to the condenser.

The unit went to house load operation for a shortperiod of time before signal were received for reactortrip, isolation of the primary containment and thestart of the reactor safety.

The reactor core remained cooled throughout theevent

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Generator

When short circuit happened generator circuit breakeropen at same time and disconnect the station from400 kV gird.

Both generator circuit breaker was trip on low power(up to 5MW) at time 36 and 43 second.

Turbine

One turbine tripped due to low governing oil pressureat o5 second

Second turbine stop due to the high pressure in theturbine condenser At 33 sec.

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UPS system

At the instant of switching to house load operation,the maximum voltage approximately 130 % of ratedvoltage was experienced by the onsite power systemuntil the voltage regulator reduced the voltage down toa normal rating.

The typical system and component of UPS was designto withstand from 85%-110% percent overvoltage andunder voltage.

This over voltage resulted in loss of power from the 2out of 4 UPS AC grid in trains A and B. where as trainC and D were unaffected by the transient.

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• The battery backed UPS220v AC system isintended to supplyequipment that is essentialfor safe shutdown of thereactor .

• At 02 second rectifier inthe UPS system s (A and Bsubs) trips on a controlfault, and thee inverter inthe same systems trip onunder voltage.

These 220V AC systems were necessary for theoperation of the emergency diesel generators (EDG)and the equipment that measure the water level andpressure in the reactor pressure vessel.

The loss of two 220v AC bus bars caused loss of severalisolation signal and also several information in themain control room.

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Emergency Diesel generator

One EDG is connected with each sub of 500 V

In he event of disturbance ,start command are issuedautomatically to the four diesel generator (one on eachsub).All diesel generator started automatically .

Connection of EDG electric out put to the subs isdependent on the availability of the UPS AC Power inthe respective sub, so two generator connected withsub A and B was failed to connect, where as two otherdiesel generator in sub C and D supplied power to theinternal network throughout the entire incident.

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Ac network supplies the equipment that measure thewater level and pressure in the reactor pressure vessel.As two of the four instrumentation systems were notworking . this resulted in an automatic scram of thereactor .

In the reactor scram ,all the control rod must beinserted into the core and indication is provided in thecontrol room when they fully inserted ,but in this casethe loss of power on sub A and B meant that there wasno indication for half of the control rods.

In this event two out of four trains in each safetysystem were operationally available (SAF, Core sprayand Containment spray system) 17

The reactor was then initially cooled by dumpingsteam from the reactor pressure vessel to thecondensation pool in the reactor enclosure ,and bypumping in water via the two auxiliary fee waterpumps supplied from subs C and D. This reduce thepressure from 70 bar to 6 bar within 30 minute ,andthe water level in the reactor vessel fell to a lowest levelof 1.9 m above the top of the core.

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Restoration of Electric supply

After 22 minute control room manually connected tosub A and B EDG to their buses with the result that

Supervisory facility in the control room were restored.

Motor powered insertion of the control rods wascompleted in sub A and B, accompanied by indicationthat all the rods were inserted.

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The Switchyard:

In relation with maintenance activity in the 400KVswitchyard, misjudged the need to interlock an earthfault protection. had the interlock been placed ,thecommon bus bar protection would have isolated theshort circuit after 0.1 second.

Operation of generator circuit breaker:

The under frequency protection of both turbinegenerator operated incorrectly. When the connectionwith the switchyard was interrupted ,and the turbineemergency stop was triggered , the under frequencyprotection circuit of the generator should have trippedthe circuit breaker when the frequency had fallen belowa certain level

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220 V AC UPS system

UPS system was installed over 10 years ago. The UPSsystem was designed to meet the voltage level from85% to 110% of nominal voltage. The voltage variationthat actually occurred was mush grater .

EDG

All EDG are started automatically ,but the Twosupplying power sub to A and B failed to connect totheir respective subs as ,to do this they requireauxiliary power from the UPS .

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According to the SwedishNuclear Power Inspectionauthority SKI, the incidentwas rated 2 on theInternational Nuclear EventScale.Initially it was rated 1 since twogenerators remained online.But once it was discovered thatall four generators could havefailed due to the same fault,the event was upgraded to 2. 23