Forsmark NPP detail presentation

1. Abdul Qadir
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2.  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 BWR
plant, located in Sweden ,designed by ASEA Atom with
two turbine generator and commissioned in 1980.

4. Status of the plant at site
 Foresmark 1
Electrical fault occurred at this plant
 Foresmark 2
This unit was Shut down for refueling and maintenance
at time of fault
 Foresmark 3
This unit was operating at full load but not affected b/c
it is connected with other switchyard.
 The event at Forsmark nuclear station revealed several
design and operational oversights that could lead to
significant challenge to nuclear safety.
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5. 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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8.  On 25th July 2005 date plant was operated at full power
and at the time 13:20:20 A dis-connector in the400 kV
switchyard opens during maintenance , creating an arc
and a two phase short circuit.
 The short circuit at the switchyard caused a grid
voltage fluctuation resulting in momentary
depression of the system voltage in response to the
gird voltage fluctuation the voltage regulator on the
main generator responded to restore the system
voltage.(No.NG-T-3.8 page 15) .
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9.  At same time of transient The generator backup
protection system disconnected Both generator circuit
breakers trip on under voltage, i.e. disconnecting the
station from the 400 kV from the grid and the control
system aligned the full output of the main generator to
supply power to the house loads.
 At the instant of switching to house load operation,
the high voltage output of the generator resulted in a
brief but significant overvoltage. This high voltage
variation fed through the transformer which supply
the local power system and some other safety system
in the plants.
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10. The event generated two transients on the power supply to
the unit.
 The first was an over voltage transient that causes the
failure of two un-interruptiple power supply
 The second was a low frequency transient that causes
disconnection of the off-site power to the safety related
bus 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 as
standby. The cause of this was lack of adequate
maintenance.
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11.  Reactor
 Reactor output is reduced by a partial scram.
Changeover to house load operation and dumping of
steam to the condenser.
 The unit went to house load operation for a short
period of time before signal were received for reactor
trip, isolation of the primary containment and the
start of the reactor safety.
 The reactor core remained cooled throughout the
event
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12.  Generator
 When short circuit happened generator circuit breaker
open at same time and disconnect the station from
400 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 pressure
at o5 second
 Second turbine stop due to the high pressure in the
turbine condenser At 33 sec.
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13.  UPS system
 At the instant of switching to house load operation,
the maximum voltage approximately 130 % of rated
voltage was experienced by the onsite power system
until the voltage regulator reduced the voltage down to
a normal rating.
 The typical system and component of UPS was design
to withstand from 85%-110% percent overvoltage and
under voltage.
 This over voltage resulted in loss of power from the 2
out of 4 UPS AC grid in trains A and B. where as train
C and D were unaffected by the transient.
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• The battery backed UPS
220v AC system is
intended to supply
equipment that is essential
for safe shutdown of the
reactor .
• At 02 second rectifier in
the UPS system s (A and B
subs) trips on a control
fault, and thee inverter in
the same systems trip on
under voltage.

15.  These 220V AC systems were necessary for the
operation of the emergency diesel generators (EDG)
and the equipment that measure the water level and
pressure in the reactor pressure vessel.
 The loss of two 220v AC bus bars caused loss of several
isolation signal and also several information in the
main control room.
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16.  Emergency Diesel generator
 One EDG is connected with each sub of 500 V
 In he event of disturbance ,start command are issued
automatically to the four diesel generator (one on each
sub).All diesel generator started automatically .
 Connection of EDG electric out put to the subs is
dependent on the availability of the UPS AC Power in
the respective sub, so two generator connected with
sub A and B was failed to connect, where as two other
diesel generator in sub C and D supplied power to the
internal network throughout the entire incident.
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17.  Ac network supplies the equipment that measure the
water level and pressure in the reactor pressure vessel.
As two of the four instrumentation systems were not
working . this resulted in an automatic scram of the
reactor .
 In the reactor scram ,all the control rod must be
inserted into the core and indication is provided in the
control room when they fully inserted ,but in this case
the loss of power on sub A and B meant that there was
no indication for half of the control rods.
 In this event two out of four trains in each safety
system were operationally available (SAF, Core spray
and Containment spray system) 17

18.  The reactor was then initially cooled by dumping
steam from the reactor pressure vessel to the
condensation pool in the reactor enclosure ,and by
pumping in water via the two auxiliary fee water
pumps supplied from subs C and D. This reduce the
pressure from 70 bar to 6 bar within 30 minute ,and
the water level in the reactor vessel fell to a lowest level
of 1.9 m above the top of the core.
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19.  Restoration of Electric supply
 After 22 minute control room manually connected to
sub 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 was
completed in sub A and B, accompanied by indication
that all the rods were inserted.
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20.  The Switchyard:
 In relation with maintenance activity in the 400KV
switchyard, misjudged the need to interlock an earth
fault protection. had the interlock been placed ,the
common bus bar protection would have isolated the
short circuit after 0.1 second.
 Operation of generator circuit breaker:
 The under frequency protection of both turbine
generator operated incorrectly. When the connection
with the switchyard was interrupted ,and the turbine
emergency stop was triggered , the under frequency
protection circuit of the generator should have tripped
the circuit breaker when the frequency had fallen below
a certain level
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21.  220 V AC UPS system
 UPS system was installed over 10 years ago. The UPS
system was designed to meet the voltage level from
85% to 110% of nominal voltage. The voltage variation
that actually occurred was mush grater .
 EDG
 All EDG are started automatically ,but the Two
supplying power sub to A and B failed to connect to
their respective subs as ,to do this they require
auxiliary power from the UPS .
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23. According to the Swedish
Nuclear Power Inspection
authority SKI, the incident
was rated 2 on the
International Nuclear Event
Scale.
Initially it was rated 1 since two
generators remained online.
But once it was discovered that
all four generators could have
failed due to the same fault,
the event was upgraded to 2. 23