At Forsmark Multiple Safety Layers were Effective
in protecting the reactor from the consequences of multiple failures
in some electronic circuits.
by Frigyes Reisch, Sweden
International Electrotechnical Commission (IEC),
Lyon 2006 Meeting
“Much ado about nothing” these famous
words of Shakespeare fit well the excitement that was generated
over some trivial failures that took place at Forsmark. A word
from an angry person about the distant possibility of core meltdown
got the Swedish and international press - and of course the anti-nukes
in a frenzy - ,
even though nothing dramatic actually happened at the plant.
On July 25, in the high voltage (400 kV) switch
yard during maintenance work a short circuit between two phases
occurred because an energized disconnector was opened and thereby
an arc arose (see the Simplified single line wiring diagram).
The following transient (see the Voltage perturbation diagram)
revealed some inaccurate adjustments in a couple of electrical
circuits. The reactor was automatically shut down. That worked
The “uninterruptible” power supply
systems (UPS) are configured with an AC/DC converter that fed
the batteries and a DC/AC converter that support the uninterrupted
power supply bus bar (see the simplified principal block diagram).
The diesel generators’ speed controls that are necessary
for diesel start are supplied from the UPS fed bus bars. The perturbation
on the ordinary power supply penetrated through the system. The
internal protection system disconnected the DC/AC converter prior
to the AC/DC converter’s protection, for two of the four
The main generator’s low frequency protection
was installed with erroneous phase order and therefore did not
disconnect the 20 kV generator breakers in time. That disconnection
would have connected in
an alternative (70 kV) power supply. The alternative power supply,
like the diesels were connected in to the system manually some
twenty minutes later.
Now all three Forsmark reactors are in operation
The main improvements made are as follows; the battery secured
network was made to endure severe disturbances, DC power supply
to the diesel generators is assured, faulty indications in the
control room at the loss of power supply are corrected, the gas
turbines at the plant can be started from the control room.
With hindsight there are a few observations that
we can make: the robust rotating DC/AC converters were replaced
with new electronic boxes and so were the AC/DC converters, also
the low frequency actuation protection of the generator breakers
was replaced with new ones. All these new electronic boxes were
tested individually, no integrated tests were performed. Previously,
tests were required to disconnect the grid and to follow how the
on-site power supply fulfilled its function. This insight had
been lost in recent years.