UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001

August 10, 2007

NRC INFORMATION NOTICE 2006-18, SUPPLEMENT 1: SIGNIFICANT LOSS OF

SAFETY-RELATED ELECTRICAL

POWER AT FORSMARK UNIT 1 IN

SWEDEN

ADDRESSEES

All holders of operating licenses for nuclear power reactors, except those who have

permanently ceased operations and have certified that fuel has been permanently removed

from the reactor vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to Information

Notice (IN) 2006-18, “Significant Loss of Safety-Related Electrical Power at Forsmark Unit 1 in

Sweden,” to provide additional information regarding the incident that occurred at the Forsmark

Nuclear Power Station, Unit 1 (Forsmark-1), involving the loss of several safety-related

electrical busses. It is expected that addressees will review the information for applicability to

their facilities and consider actions, as appropriate, to avoid similar problems. However,

suggestions contained in this IN are not NRC requirements; therefore, no specific action or

written response is required.

DESCRIPTION OF CIRCUMSTANCES

On July 25, 2006, while the plant was operating at full power, a significant event occurred at

Forsmark-1, a 1020 Megawatt electric boiling-water reactor designed by ASEA-Atom. The

event began in the offsite 400 kilovolts (kV) switchyard, which consisted of two main buses and

one transfer bus, during a bus transfer operation being conducted to support maintenance work. 

However, due to inadequate administrative procedures, the transfer and connection activities

were not properly implemented. A disconnect switch that was carrying load was opened which

resulted in arcing and a two-phase short circuit at the disconnect switch. The fault was isolated

after approximately 400 milliseconds by the remote circuit breakers on two of the outgoing

transmission lines. The high-voltage fast-acting bus protection was ineffective during the bus

transfer operation.

When the short circuit occurred in the switchyard, the generator bus voltage dipped to

approximately 50 percent of nominal voltage for approximately 300 milliseconds. The breakers

on the high-voltage side of the main transformers tripped on low voltage. This resulted in a loss

of load, and a generator bus voltage surge to approximately 120 percent of nominal voltage for

approximately one second. The loss of offsite electrical load caused an automatic partial scram

of the reactor, a transition to steam dumping, and house load operation by the turbine

generators.

ML071900368
 IN 2006-16, Sup 1

 Page 2 of 3

After disconnection from the 400 kV grid, the plant electrical system continued to be connected

to the generator buses (house load operation). The voltage surge at the generator buses

caused tripping of the inverters of the battery-backed uninterruptible power supply (UPS) Subs

A and B in the plant auxiliary electrical system. [Subs are similar to the trains/divisions in U.S.

nuclear plants]. As a consequence, the rectifier and inverter of Subs A and B were bypassed,

as designed, by the fast-acting electronic/static switch. The voltage transient at UPS buses due

to the electronic switch transfer resulted in tripping of the recirculation pumps of Sub B. Reactor

water level remained within acceptable limits. 

The UPS Subs C and D were not impacted by the generator bus voltage surge due to the

slightly lower voltage surge seen by the related rectifiers and inverters because of slight

difference in the impedance path from the generators to the rectifiers. Subsequent tests of the

UPS showed that the rectifier in the UPS could not reliably block steep voltage transients larger

than 25 percent within the 80-130 percent range of nominal voltage. During such a voltage

transient, the inverter voltage surge protection could trip before the rectifier protection if the

rectifier and inverter direct current voltage protection settings are very close to each other. As

result of this lesson learned, Forsmark revised the rectifier and the inverter voltage protection

settings.

One turbine tripped due to low oil pressure caused by 50 percent voltage drop in the switchyard,

the other turbine tripped due to high pressure in condensers; this lead to low frequency

operation by the generators. However, the low frequency protection for the generators did not

actuate due to an incorrect phase connection of frequency relays. With the turbines tripped, the

generators continued to supply house loads, and the voltage and frequency continued to drop. 

When the voltage at the UPS Subs A and B decreased to 90 percent, the UPS Buses A and B

automatically transferred to the alternate parallel path through the isolating transformer. During

this transfer, UPS Subs A and B lost power for approximately 2 seconds (as per design), which

caused the reactor to trip on two-out-of-four connection logic.

When the frequency at the 6 kV buses dropped below 47 hertz for 3 seconds, the feeders from

the 6 kV buses to the 500 volt diesel buses tripped due to the underfrequency protection

provided at the 6 kV buses. Also, the UPS buses of Subs A and B (fed through isolating

transformers) lost power. However, the UPS buses of Subs C and D continued to be powered

by battery-backed inverters.

Due to the loss of two trains of UPS, some instrumentation and control equipment was

impacted. The major impacts were:

• The network between the automatic processors and the operator stations, and the

process computer, was lost.

• The “control rod in” indication in the control room’s core map for all control rods

belonging to Subs A and B was lost.

• The average power range monitor and wide range neutron monitor indications that were

powered from Subs A and B were not available.
 IN 2006-16, Sup 1

 Page 3 of 3

The 500 volt safety buses lost power due to the tripping of feeder breakers between 6 kV buses

and 500 volt safety buses. Each emergency diesel generator (EDG) of Subs A, B, C, and D

received its startup signal due to low voltage. EDGs C and D successfully connected to the

safety buses. However, EDGs A and B tripped due to a “long startup time” since the EDG

speed measurement signal, powered from its respective UPS bus, was not available. 

The delayed tripping of main generator breakers occurred (approximately 38-45 seconds) when

the power supplied by each generator became less than 5 Megawatts in conjunction with the

previous turbine trip signal. Due to this, the 6 kV buses lost power and the automatic

switchover (dead bus transfer with 2 second delay) to the 70 kV alternate offsite occurred per

design. After approximately 22 minutes, voltage was restored to 500 volt diesel buses A and B

by manually closing the 6 kV ties between the 6 kV buses and the 500 volt diesel buses.

DISCUSSION

The lessons learned from the Forsmark-1 event that can be applicable to U.S. plants are as

follows:

(1) The voltage transient (due to the delayed fault clearing in the switchyard) can impact the

plant electrical system up to the UPS buses. 

(2) The protection relays, if dependent on proper phase connections, can malfunction if not

properly connected.

(3) The failure of UPS can impact important equipment/systems such as the reactor

recirculation pump control system and important control room indications.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation project manager.

/RA by TQuay for/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contacts: Vijay Goel, NRR/DE Brett Rini, NRR/DIRS

301-415-3730 301-415-3931

E-mail: vkg@nrc.gov E-mail: bar3@nrc.gov

Note: NRC generic communications may be found on the NRC public Website,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.
IN 2006-16, Sup 1

Page 3 of 3

The 500 volt safety buses lost power due to the tripping of feeder breakers between 6 kV buses

and 500 volt safety buses. Each emergency diesel generator (EDG) of Subs A, B, C, and D

received its startup signal due to low voltage. EDGs C and D successfully connected to the

safety buses. However, EDGs A and B tripped due to a “long startup time” since the EDG

speed measurement signal, powered from its respective UPS bus, was not available. 

The delayed tripping of main generator breakers occurred (approximately 38-45 seconds) when

the power supplied by each generator became less than 5 Megawatts in conjunction with the

previous turbine trip signal. Due to this, the 6 kV buses lost power and the automatic

switchover (dead bus transfer with 2 second delay) to the 70 kV alternate offsite occurred per

design. After approximately 22 minutes, voltage was restored to 500 volt diesel buses A and B

by manually closing the 6 kV ties between the 6 kV buses and the 500 volt diesel buses.

DISCUSSION

The lessons learned from the Forsmark-1 event that can be applicable to U.S. plants are as

follows:

(1) The voltage transient (due to the delayed fault clearing in the switchyard) can impact the

plant electrical system up to the UPS buses. 

(2) The protection relays, if dependent on proper phase connections, can malfunction if not

properly connected.

(3) The failure of UPS can impact important equipment/systems such as the reactor

recirculation pump control system and important control room indications.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation project manager.

/RA by TQuay for/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contacts: Vijay Goel, NRR/DE Brett Rini, NRR/DIRS

301-415-3730 301-415-3931

E-mail: vkg@nrc.gov E-mail: bar3@nrc.gov

Note: NRC generic communications may be found on the NRC public Website,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.

Distribution: IN Reading File

ADAMS Accession Number: ML071900368 

OFFICE DIRS:IOEB TECH EDITOR EEEB:DE BC:IOEB:DIRS BC:EEEB:DE

NAME BRini LCulp by e-mail VGoel MJRoss-Lee GW ilson

DATE 7/10/07 7/2/07 7/10/07 07/25/07 7/12/07

OFFICE D:DE LA:PGCB PGCB:DPR BC:PGCB:DPR D:PGCB:DPR

NAME PHiland CHawes DBeaulieu MMurphy TQuay for MCase

DATE 07/25/07 08/01/2007 08/07/2007 08/09/2007 08/10/2007

OFFICIAL RECORD COPY