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PostPosted: Tue Oct 31, 2017 10:37 pm 
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On July 25th 2006 reactor 1 at the Forsmark nuclear power plant suffered an incident which was later classified as a 2 on the INES scale.

A simplified summary:
The event began as an error during maintenence work at the 400kV switchyard by the reactor which caused a short circuit.
As it turned out the rapid voltage fluctuations tripped UPS protections fitted to the Essential Power Buses, two out of four tripped and it was just happenstance that the remaining two did not also.
This resulted in about half of the control room apparatus going dark, with the remaining on battery power, 12 different safety systems including emergency boron injection was lost, the containment weakened since soome valves were open and the cooling to the reactor was partially interrupted.

In continued "holes in the cheese are lining up"-mode the backup diesel generators were relying on these buses being active in order to replace the off site power supply that had been lost, so the two generators connected to the tripped buses merely idled. The two connected to the surviving buses were in theory able to deliver power, however the actual automatic connection to the buses failed and so they too idled, leading to a complete loss of 500v power. A tertiary power supply in the form of a gas turbine failed to start due to an unrelated fault.

In the control room the situation was regarded as stressful since the confirmation that the reactor scram had been successful with fully inserted control rods was absent, however neutron readings were consistent with those of a scram-ed reactor, which the operators correctly recognized from simulator training.
After 22 minutes of dropping reactor coolant levels control room operators identified that they were able to manually connect the surviving power buses with the operational backup diesel generators and power was restored to cooling and the control room.

The greatest problem was that during a 1990's refit the battery backed buses had been fitted with overcurrent protectors that were calibrated to safeguard the component rather than the function.

The actual events were a bit more complex as can be viewed belowed and at the links.

Event list:
Quote:
Time: 13:20:20 A disconnector in the
400 kV switchyard opens, creating an arc
and a two phase short circuit.
+ 0 sec Both generator circuit breakers
in Forsmark 1 trip on undervoltage, i.e.
disconnecting the station from the 400 kV
grid.
+ 0 sec Reactor output is reduced by a
partial scram. Changeover to houseload
operation and dumping of steam to
the condenser.
+ 2 sec Rectifiers in the UPS systems (A
and B subs) trip on a control fault, and the
inverters in the same systems (A and B
subs) trip on overvoltage.
+ 2 sec First Incident Response Checks
in accordance with the Emergency Operating
Procedures (EOP) initiated by the
shift manager in the control room.
+ 5 sec One turbine tripped (emergency
stop) due to low governing oil pressure.
+ 18 sec Changeover to direct supply of
the battery backed AC network (Sub A)
due to low voltage. The instrumentation
chains supplied by the Sub A 220 V
network were without power for two
seconds, resulting (among other effects)
in Channel A of the emergency stop chain
tripping.
+ 24 sec The normal supply circuitbreakers
to the 500 V diesel backed distribution
systems open in Sub A and
Sub C due to low frequency on the 500 V
busbars. Instrumentation chains supplied
from the Sub A network are again
without power.
+ 24 sec Diesel start and connection in
Sub C. Connection of Sub A fails.
+ 33 sec Emergency stop of the second
turbine due to high pressure in the turbine
condenser.
+ 35 sec Changeover to direct supply of
the Sub A network due to low voltage.
The instrumentation chains supplied by
the sub B network were without power
for two seconds, resulting (among other
effects) in Channel B of the emergency
stop chain tripping. As both the A and
B channels had now tripped, this automatically
resulted in a complete reactor
scram.
+ 36 sec One generator circuit breaker
trips on low power (less than 5 MW).
+ 36 sec Changeover to 70 kV supply to
subs A and C due to low voltage in the
6 kV switchyard.
+ 37 sec The normal supply circuit
breakers to the 500 V diesel backed distribution
systems open in sub B and sub D
due to low frequency, less than 47 Hz for
more than three seconds on the 500 V
busbars. Diesel start and connection to
Sub D successful. Connection of diesel
generator to Sub B fails.
+ 40 sec The shift manager calls for
additional resources from the plant specialists
and from the incoming afternoon
shift.
+ 43 sec The second generator circuit
breaker trips on low power.
+ 43 sec Changeover to 70 kV supply via
subs B and D due to undervoltage in the
6 kV system.
+ 45 sec The first checks in accordance
with the Emergency Operating Procedures
are carried out by the operators.
Using signals from the neutron detectors
in the core, reactor output power is found
to be as expected. However, there is no
indication of full insertion of the control
rods powered from Subs A and B.
This situation is regarded as stressful,
but is recognised from simulator
training of similar situations.
+ 5 min The shift manager starts systematic
checks in accordance with the
Emergency Operating Procedures. Falling
water level in the reactor pressure
vessel is noted.
+ 8 min Still no indication that all the
scram control rods are inserted into the
core. However, checking the readings
from the neutron detectors clearly shows
that the reactor is fully shut down. It is
therefore considered that all control rods
are actually inserted into the core.
+ 14 min Two out of four auxiliary feed
water system circuits are noted as being
in operation and providing sufficient
cooling water flow to the reactor.
+ 15 min Falling water level in the reactor
pressure vessel results in checking that
at least two circuits in the emergency
core cooling system are in operation.
+ 20 min First review in accordance with
the Emergency Operating Procedures is
completed. The shift manager calls the
other operators to a quick meeting.
+ 22 min Manual restoration of power to
the diesel backed 500 V Sub A busbar.
+ 22 min Manual restoration of power to
the diesel backed 500 V Sub B busbar.
+ 23 min Indication that all control rods
are inserted is obtained. Hot Shut Down
Reactor status is verified.
+ 24 min The shift manager starts the
Emergency Operating Procedures review
again, as conditions have now changed,
in that the diesel backed Subs A and B
busbars are now energised.
+ 26 min Drive nuts of all the control rods
are noted as being in the inserted position.
+ 27 min The water level in the reactor
pressure vessel is noted as exceeding
3.1 m.
+ 30 min The water level in the reactor
pressure vessel is noted as exceeding
4.7 m.
+ 45 min Second Emergency Operating
Procedures review completed. The shift
manager answers ”Yes” to the Procedures’
question ”Is the reactor safely
sub critical, and is operational condition
stable?”

From: http://analys.se/wp-content/uploads/201 ... 2007-1.pdf
More detailed information at that link and at this lessons learned text:
https://www.nrc.gov/docs/ML1020/ML102070360.pdf
Slideshow detailing changes at Forsmark in response to this incident:
http://www.vtt.fi/files/projects/HFS/hf ... jurman.pdf¨

It has been hotly debated how serious the incident was, some claim it came close to a meltdown, others disagree.
However a Forsmark employee, Pär Lansåker, calculated that - with the (now rectified faults) present - the risk of a meltdown was 1 in 52 per year. He also calculated that the likelihood that all four buses would have tripped during this particular incident was 12%.
Beware of the holes in the cheese...

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PostPosted: Wed Nov 01, 2017 8:50 am 
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I hope they gave the 'Duty Electrical Team' due kudos for their prompt cross-connection...

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PostPosted: Wed Nov 01, 2017 4:00 pm 
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Nik_SpeakerToCats wrote:
I hope they gave the 'Duty Electrical Team' due kudos for their prompt cross-connection...

The operator that figured it out was mentioned by name in the press later on, though he seemed to be mostly enbarassed by it.
He was on duty in the control of one of the other reactors at the plant and arrived at control room 1 to support eight minutes into the event.
The computers was down as previously mentioned so the electrical status was uncertain, a control board light that should have been lit if a bus was available was also dark but he guessed that this was incorrect and suggested to the duty manager that they should attempt to connect it. At 22 minutes he got the go ahead and that resolved the most important issues.

The duty manager recounted events in an article later, especially this following passage I though was interesting. During the first few seconds they thought it was a fairly normal scram but then all hell broke loose:

”This time many things happened at once: the instruments in the reactor control room indicated that the coolant level was too high, that it was too low, that the pressure in the reactor was too high, that there was a leak both inside and outside of the containment. It simply seemed like a whole lot of everything bad that can happen in a nuclear power plant happened all at once.
’I just stood there, mouth wide open for a second or two, just stood there and watched. More and mpre thibgs happened, I heard the emergency ventilation kick in, the fire alarm... My head was spinning and I said ”what the hell!” Then it was all focus and ”let’s go”.’”

Not an enviable situation to be in.

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