Výběr zpráv ze sítě NucNet - 33. týden 2006

Preliminary Report Tracks Events Leading To Scram At Sweden's Forsmark-1

A preliminary report into the causes and consequences of an
incident which led to a reactor scram at Sweden’s Forsmark-1 nuclear reactor
unit says the operators of the plant “appear to have handled the situation in
a correct manner”.

The report, published yesterday in English by the Swedish Nuclear Power
Inspectorate, SKI, said investigations continue at Forsmark, where a short
circuit in a 400 kilovolt outdoor switching station on 25 July 2006 triggered
a chain of events “in a complex scenario”.

SKI said its initial findings were preliminary and incomplete because the
incident is still under investigation, but that “it is essential that a
complete picture of the steps in the event be put together and confirmed”.

The Forsmark-1 scram has been given a provisional level two ‘incident’
rating on the International Nuclear Event Scale (INES). The INES report sent
by Sweden to the International Atomic Energy Agency on 26 July said the scram
was successful, all control rods were inserted as expected and the emergency
core cooling system in two out of four trains “was more than sufficient” as
there was no loss of coolant.

The level two rating was because of the common cause failure of two out of
the four emergency power diesel generators.

The SKI report follows a visit by three of the organisation’s staff to take
part in an assessment of the incident on 27 July. The report said it is not
yet known why there was a short circuit in the switching station and that the
company that runs Sweden’s national grid, SVK, has not yet submitted a

However, SKI said preliminary analysis indicated that part of Forsmark’s
uninterruptible power supply (UPS) did not function properly at the time of
the incident, because of what is thought to be an “incorrect design”.

The UPS, installed around 1993-1994, should have automatically provided
enough power to essential back-up systems when the grid supply was

SKI’s report said unconfirmed information received by Forsmark suggests the
design fault was known because the supplier of the UPS had claimed experience
of a similar problem in the past and that the supplier “had taken measures to
prevent this error reoccurring”. SKI said: “This implies routines and
practices connected to experience feedback need to be checked.”

Forsmark has already said it will take action to have the design fault
corrected and that it assumes corrective action will also be needed at
Forsmark-2, where a UPS of the same design is installed. A review of whether
similar measures are needed at Forsmark-3 is under way.

Forsmark told SKI that it believes the design fault could be generic and has
informed other Swedish nuclear power plants, including Oskarshamn, where
yesterday plant management temporarily took off line Oskharshamn-1 and -2 for
precautionary checks into the back-up power supply systems. Forsmark has also
informed the Teollisuuden Voima Oy utility in neighbouring Finland, which
Forsmark said operates the same type of boiling water reactors as Forsmark-1
and -2.

Zdroj: NucNet

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