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On the Nuclear Shutdowns in Sweden

The AP has flashed the following story concerning an incident at the Oskarshamn nuclear power plant in Sweden:
Swedish nuclear authorities held an emergency meeting Thursday after two reactors were shut down at a plant in the southeast of the country.

The plant in Oskarshamn, about 250 kilometers (150 miles) south of the capital, Stockholm, shut down two of its three reactors late Wednesday after the company running the plant reported that "safety there could not be guaranteed."

The decision followed an incident last week at another nuclear plant in Sweden, in Forsmark, where backup generators malfunctioned during a power outage, forcing a shutdown of one of its reactors, said Anders Bredfell, a spokesman for the Swedish nuclear authority, SKI.
Since then, the former plant manager at Forsmark has been claiming that the incident at the plant is the most dangerous in the nuclear industry since Chernobyl, and that there was a risk of a meltdown. Greenpeace, rather predictably, has called for the country to shut down all of its nuclear power plants.

Per Jander over at the World Nuclear Association Blog disagrees:
This is absolute nonsense. The unit is in perfectly fine condition, plant management has sent in a report to the Swedish regulator (SKI) and is now awaiting permission to restart. No equipment is damaged, and reactor safety was never compromised.

A short circuit in an external switchyard resulted in a powerful transient and the power plant was automatically disconnected from the grid. Initially the power plant switched to in-house power generation, but through a complicated chain of events the turbines were stopped and power supply of the safety system was switched to two of the four back-up diesel generators. Normally all four diesel generators should provide the systems with power, but this time only two went online.

Safety systems are divided into four identical subsystems, each with their own diesel generator and capacity to manage 50% of the plant needs. If all subsystems and all diesels are working properly, there are twice the required capacity available. In this particular case, when two diesel generators started automatically and worked well during the entire chain of events, there were always sufficient power to cool the reactor and keep other safety functions online.

The serious aspect of this event is that the automatic power supply of safety systems were partly compromised because of a fault in the external grid. Because of possibilities of similar design, further three reactors in Sweden have been taken offline, and will not be restarted until the risk of a similar event is removed. The remaining six reactors in Sweden have a different design that prevents these kinds of issues.
When looking at this story, I hope our readers keep the Swedish domestic political situation in mind. As we've written in recent months, Sweden is on the verge of reversing its policy of phasing out the nation's nuclear power plants -- a notion that's shared by both center-left and center-right forces in the nation's parliament.

Further, public support for the industry has never been higher in the nation. A majority of Swedish voters are against early shutdown of the plants. For more on these polls, click here and here.

For many European Green parties, their efforts to shut down nuclear power plants all over the continent were a major high water mark in their history. The possibility that this decision in Sweden (and elsewhere) could be reversed would be a serious setback -- hence the tenor of Greenpeace's latest statement.

One last point: Even though the Swedish authorities didn't request the shutdown of any units, the owners of the plants did so voluntarily in order to address the problem.

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Comments

Bryan McHugh said…
The worry about having only two of the four units going on-line is whether this was due to luck. All four units were sujbect to the same possible influences from the complex power supply system in the plant
Rumour has it that the rectifiers were of the same fabrication of a unit that failed once in a German plant.
Immediate SKI review at the Forsmark 1 unit



Summary



After short circuiting in a switching station and subsequent complex events at the Forsmark

unit 1 NPP, SKI carried out a review in order to obtain independent view of what had

happened, its own consequences and the actions taken. Based upon in part preliminary

information SKI considers that the utility (FKA) seems to have handled the critical situation

correctly.



SKI can furthermore state that FKA is working actively, based on the present

information, in order to clarify the cause of the event as well as to work out suggestions for

actions to be taken. The event resulted in vital safety related equipment being non-functional

due to a common cause failure.



SKI’s view is that FKA’s evaluation of the gravity of the

event is correct. This SKI report provides an assessment of what analyses should as a

minimum be part of FKA’s report for assessing the state of safety and decisions on restart as

well as an assessment of the robustness of the plant in a broad perspective.



Background



On July 25 at the Forsmark unit 1 NPP there was a short circuit in a 400kV outdoor switching

station. Due to this the plant scrammed in a way that included a number of subsequent events

in a complex scenario. SKI was very quickly after this informed about the scram. Further

information was submitted to SKI on the following day. Since the event seemed to be difficult

to assess SKI decided on July 26 to carry out a so called RASK-review (an immediate and

short review that is directly initiated by SKI and with SKI staff visiting at the site).



Objective



The objective was to make clear:

- what happened,

- the consequences, the importance to safety of the event,

- the root causes,

- areas in need of improvement in order to avoid the event happening again (including

improvements in the way to work),

- the licensee’s actions due to the event, and whether these actions would be sufficient

for continued operation.



Method



This RASK review was carried out by three SKI staff. The visit at the plant took place on July

27. SKI first received information from the FQ-department, the utility’s internal safety

department, on how the event was assessed by the utility. SKI then participated in two

meetings at Forsmark unit 1. SKI also interviewed the head shift engineer of the shift that had

been on duty during the event. SKI furthermore had separate meetings with the managers of

Forsmark unit 2 and 3.



It should be noted that work following the event continues at FKA and the information is

therefore preliminary and incomplete.



The course of events



A preliminary course of events has been submitted to SKI. SKI has received no information

on what happened in the reactor part of the unit. The information provided during the visit has

given the following picture of the course of events.



After disconnecting the unit from the grid due to the short circuit there was a partial scram

and both turbines for a short while transferred to house load operation. After the turbine trip

the reactor scrammed.



A number of conditions in the safety trains (in system 516, the reactor

protection system) tripped: several scram conditions, I-isolation and N-chain.



The reactor scram could be seen through WRNM even though the indication for control rod positions

was unclear due to the unit partly having lost its power supply. Water was pumped in using two of

the lines in system 327, the auxiliary feedwater system (2x22,5 kg/s). Four of the eight reactor

recirculation pumps were in operation. Pressure relief of steam from the reactor to the

condensation pool was done through two pressure relief valves in system 314, the automatic

depressurization system (about 2x50 kg/s) that had been opened via the N-chain. Reactor

pressure and water level in the reactor went down.



The display of the reactor level was ambiguous since some actuators were not active due to loss of power.

The water level was down 2 m and the pressure went down to 12 bar after about 20 minutes.

The emergency cooling system which had already started on isolation signals pumped water into the reactor

vessel for a short while when pressure had been reduced. Sprinkling was activated in the

containment. The shift team checked the level in the reactor vessel in order to be prepared to

activate the automatic depressurization system if the level were to be reduced to 1.1 m, in

accordance with the Emergency Operating Procedures. After 23 minutes the shift team

realized that there was a possibility to manually restart the two diesels that had stopped, and

after this the situation was quite quickly stabilized. The 6 kV bus bars were then already

operational. The decision could be taken to restart isolation signals and to stop sprinkling in

the containment.



The reactor was then at hot stand by.



After the shift handover to the ongoing shift the leaving head shift engineer had a debriefing

with her team.



SKI concludes that the event badly affected important redundant components, namely the

DC/AC inverters for feeding of the battery secured 500 kV-bus bar from a UPS

(Uninterrupted Power Supply). This means that this is a common cause failure event.



If the other two subs had been knocked out as well this would have led to a total loss of power,

including the battery secured net. This is a more severe case than was anticipated in the Safety

analysis report. During the visit there was no obvious direct cause for two subs being knocked

out, whereas the other two were not.



SKI furthermore states that:

- The work on assessing the course of the event seems to be well described in spite of

the difficulties obtaining the information about it. The events in the reactor part,

however, were not well described.



- In spite of a very unclear signal display, knocked out computer screens as well as the

loud speakers being out of order, the control room personnel seems to have done their

job according to their instructions. The control room also received valuable help from

the control room personnel at units 2 and 3.



The head shift engineer also summoned the next shift about an hour prior to the scheduled time.

The motive for this was to make sure that they were informed of the event well in advance, and the head shift

engineer also judged that it was uncertain whether her shift team could complete the

whole shift.



Possible causes for the event and contributing conditions



The initiating event occurred in connection with maintenance work done by SVK (The

company that administers and runs the national electrical grid in Sweden), and this was done

while unit 2 was out of operation due to its refuelling outage. SVK had written a work order

and had informed FKA about it. FKA would have had the right to react on the maintenance

being done exactly at this moment (and has done so in other cases), however this time there

was no need to react and ask for the maintenance work to be postponed. The reason for the

short circuit in the switching station has not been ascertained and SVK has still to submit a

report on the disturbance.



The 70 kV-net was probably instable. This is to be confirmed by SVK. The instable voltage in

the 70 kV-net led to the 6 kV-net also being unstable. When automatic switch tried to connect

the 500 V-net the 6 kV-net was too unstable, and automatic switch then tried to feed the 500

V-net from the diesels. It is essential that a complete picture of the steps in the event be put

together and confirmed.



The reason for two of the battery secured bus bars being knocked out is, according to the

primary analysis, that the voltage transient tripped the rectifier as well as the inverter, which

according to the utility FKA is due to incorrect design. The inverter should have been in

operation to make the batteries feed the 500 kV-net. The tuning of the protective devices

should be done in such a way that these trip selectively, so that the DC/AC-converter for

battery voltage to the 500 kV-net is protected.



The UPS (AEG delivered) were installed in about 1993-1994 as an improvement of the

former rotating transformers. Information from AEG to the utility FKA, but not confirmed,

claims that a similar event occurred in an NPP in Germany, and that AEG was aware of the

problem and had taken measures to prevent this error reoccurring. This implies routines and

practices connected to experience feedback need to be checked.



One problem was that the list of events was far from complete. Many events were registered,

however with no time recorded, and probably some events were missing altogether. This has

meant that detective work is needed to investigate the course of events.



The licensee’s judgment of the importance to safety and immediate, as well as planned,

actions



The licensee FKA judges that the event is a category 1 event in accordance with SKI

regulations which means that an SKI decision is required for restart.



As mentioned above the design of the voltage transient leading to knocking out the inverter,

as well as the rectifier being knocked out, is judged by FKA to be due to an incorrect design.

FKA will remedy this.



Since the same component is installed at unit 2 the utility FKA assumes that the same actions

have to be taken at unit 2. A review is going on at unit 3 to find out whether there are similar

problems there. When synchronizing the diesels unit 3 states that this unit has another solution

that would have led to all diesels being synchronized in a similar situation.



The utility FKA judges that the problem might be generic and has therefore informed the

other Swedish licensees as well as the Finnish utility TVO (that has the same kind of BWR as

F1 and F2) about the event.



SKI’s judgement as to whether the licensee’s actions are sufficient for the short term



FKA:s work is intended to provide as complete a description of the course of events as

possible, and to prepare and carry out plant modifications in order to make sure that battery

supply will not be lost in the event of loss of grid.



It is essential that the licensee FKA in its report accounts for the entire course of events in all

vital aspects, and moreover of how various parts such as the turbine system and reactor

system were affected.



In this report the complex effect on the reactor protection system (the

516-system) shall be included.

All the scram conditions tripped, and what does this mean?



Have the protective systems functioned in the way they should have?



Systems functioning the expected manner, is this good?

The above mentioned action to prevent the battery secured net from being knocked out seems

currently to be essential.



The event is exceptional and has led to major pressure for the personnel at Forsmark. It is not

obvious to SKI that the people involved have received sufficient debriefing.



SKI’s judgment and suggestions for further actions



SKI judges that the licensee FKA must submit analyses on at least the following areas in

order to provide material for



- an assessment of safety prior to restart

- an assessment of the robustness of the unit in a broader perspective:



o The course of events. An account of electric supply, for process systems and

for handling.



o The voltage transient. Connections to the preparations for restart of the unit,

possible dependences of power level. Verification of the transient registered.



o Account of dimensioning requirements for the unit equipment that can be

exposed to voltage fluctuation.



o Is the present design of UPS robust enough for protecting the battery supply?



o The issue of selectivity in protective equipment for electric systems in a broad

perspective.



o In what way are the observations and experiences of the operators taken into

account, in the short and long term?
Immediate SKI review at the Forsmark 1 unit



Summary



After short circuiting in a switching station and subsequent complex events at the Forsmark

unit 1 NPP, SKI carried out a review in order to obtain independent view of what had

happened, its own consequences and the actions taken. Based upon in part preliminary

information SKI considers that the utility (FKA) seems to have handled the critical situation

correctly.



SKI can furthermore state that FKA is working actively, based on the present

information, in order to clarify the cause of the event as well as to work out suggestions for

actions to be taken. The event resulted in vital safety related equipment being non-functional

due to a common cause failure.



SKI’s view is that FKA’s evaluation of the gravity of the

event is correct. This SKI report provides an assessment of what analyses should as a

minimum be part of FKA’s report for assessing the state of safety and decisions on restart as

well as an assessment of the robustness of the plant in a broad perspective.



Background



On July 25 at the Forsmark unit 1 NPP there was a short circuit in a 400kV outdoor switching

station. Due to this the plant scrammed in a way that included a number of subsequent events

in a complex scenario. SKI was very quickly after this informed about the scram. Further

information was submitted to SKI on the following day. Since the event seemed to be difficult

to assess SKI decided on July 26 to carry out a so called RASK-review (an immediate and

short review that is directly initiated by SKI and with SKI staff visiting at the site).



Objective



The objective was to make clear:

- what happened,

- the consequences, the importance to safety of the event,

- the root causes,

- areas in need of improvement in order to avoid the event happening again (including

improvements in the way to work),

- the licensee’s actions due to the event, and whether these actions would be sufficient

for continued operation.



Method



This RASK review was carried out by three SKI staff. The visit at the plant took place on July

27. SKI first received information from the FQ-department, the utility’s internal safety

department, on how the event was assessed by the utility. SKI then participated in two

meetings at Forsmark unit 1. SKI also interviewed the head shift engineer of the shift that had

been on duty during the event. SKI furthermore had separate meetings with the managers of

Forsmark unit 2 and 3.



It should be noted that work following the event continues at FKA and the information is

therefore preliminary and incomplete.



The course of events



A preliminary course of events has been submitted to SKI. SKI has received no information

on what happened in the reactor part of the unit. The information provided during the visit has

given the following picture of the course of events.



After disconnecting the unit from the grid due to the short circuit there was a partial scram

and both turbines for a short while transferred to house load operation. After the turbine trip

the reactor scrammed.



A number of conditions in the safety trains (in system 516, the reactor

protection system) tripped: several scram conditions, I-isolation and N-chain.



The reactor scram could be seen through WRNM even though the indication for control rod positions

was unclear due to the unit partly having lost its power supply. Water was pumped in using two of

the lines in system 327, the auxiliary feedwater system (2x22,5 kg/s). Four of the eight reactor

recirculation pumps were in operation. Pressure relief of steam from the reactor to the

condensation pool was done through two pressure relief valves in system 314, the automatic

depressurization system (about 2x50 kg/s) that had been opened via the N-chain. Reactor

pressure and water level in the reactor went down.



The display of the reactor level was ambiguous since some actuators were not active due to loss of power.

The water level was down 2 m and the pressure went down to 12 bar after about 20 minutes.

The emergency cooling system which had already started on isolation signals pumped water into the reactor

vessel for a short while when pressure had been reduced. Sprinkling was activated in the

containment. The shift team checked the level in the reactor vessel in order to be prepared to

activate the automatic depressurization system if the level were to be reduced to 1.1 m, in

accordance with the Emergency Operating Procedures. After 23 minutes the shift team

realized that there was a possibility to manually restart the two diesels that had stopped, and

after this the situation was quite quickly stabilized. The 6 kV bus bars were then already

operational. The decision could be taken to restart isolation signals and to stop sprinkling in

the containment.



The reactor was then at hot stand by.



After the shift handover to the ongoing shift the leaving head shift engineer had a debriefing

with her team.



SKI concludes that the event badly affected important redundant components, namely the

DC/AC inverters for feeding of the battery secured 500 kV-bus bar from a UPS

(Uninterrupted Power Supply). This means that this is a common cause failure event.



If the other two subs had been knocked out as well this would have led to a total loss of power,

including the battery secured net. This is a more severe case than was anticipated in the Safety

analysis report. During the visit there was no obvious direct cause for two subs being knocked

out, whereas the other two were not.



SKI furthermore states that:

- The work on assessing the course of the event seems to be well described in spite of

the difficulties obtaining the information about it. The events in the reactor part,

however, were not well described.



- In spite of a very unclear signal display, knocked out computer screens as well as the

loud speakers being out of order, the control room personnel seems to have done their

job according to their instructions. The control room also received valuable help from

the control room personnel at units 2 and 3.



The head shift engineer also summoned the next shift about an hour prior to the scheduled time.

The motive for this was to make sure that they were informed of the event well in advance, and the head shift

engineer also judged that it was uncertain whether her shift team could complete the

whole shift.



Possible causes for the event and contributing conditions



The initiating event occurred in connection with maintenance work done by SVK (The

company that administers and runs the national electrical grid in Sweden), and this was done

while unit 2 was out of operation due to its refuelling outage. SVK had written a work order

and had informed FKA about it. FKA would have had the right to react on the maintenance

being done exactly at this moment (and has done so in other cases), however this time there

was no need to react and ask for the maintenance work to be postponed. The reason for the

short circuit in the switching station has not been ascertained and SVK has still to submit a

report on the disturbance.



The 70 kV-net was probably instable. This is to be confirmed by SVK. The instable voltage in

the 70 kV-net led to the 6 kV-net also being unstable. When automatic switch tried to connect

the 500 V-net the 6 kV-net was too unstable, and automatic switch then tried to feed the 500

V-net from the diesels. It is essential that a complete picture of the steps in the event be put

together and confirmed.



The reason for two of the battery secured bus bars being knocked out is, according to the

primary analysis, that the voltage transient tripped the rectifier as well as the inverter, which

according to the utility FKA is due to incorrect design. The inverter should have been in

operation to make the batteries feed the 500 kV-net. The tuning of the protective devices

should be done in such a way that these trip selectively, so that the DC/AC-converter for

battery voltage to the 500 kV-net is protected.



The UPS (AEG delivered) were installed in about 1993-1994 as an improvement of the

former rotating transformers. Information from AEG to the utility FKA, but not confirmed,

claims that a similar event occurred in an NPP in Germany, and that AEG was aware of the

problem and had taken measures to prevent this error reoccurring. This implies routines and

practices connected to experience feedback need to be checked.



One problem was that the list of events was far from complete. Many events were registered,

however with no time recorded, and probably some events were missing altogether. This has

meant that detective work is needed to investigate the course of events.



The licensee’s judgment of the importance to safety and immediate, as well as planned,

actions



The licensee FKA judges that the event is a category 1 event in accordance with SKI

regulations which means that an SKI decision is required for restart.



As mentioned above the design of the voltage transient leading to knocking out the inverter,

as well as the rectifier being knocked out, is judged by FKA to be due to an incorrect design.

FKA will remedy this.



Since the same component is installed at unit 2 the utility FKA assumes that the same actions

have to be taken at unit 2. A review is going on at unit 3 to find out whether there are similar

problems there. When synchronizing the diesels unit 3 states that this unit has another solution

that would have led to all diesels being synchronized in a similar situation.



The utility FKA judges that the problem might be generic and has therefore informed the

other Swedish licensees as well as the Finnish utility TVO (that has the same kind of BWR as

F1 and F2) about the event.



SKI’s judgement as to whether the licensee’s actions are sufficient for the short term



FKA:s work is intended to provide as complete a description of the course of events as

possible, and to prepare and carry out plant modifications in order to make sure that battery

supply will not be lost in the event of loss of grid.



It is essential that the licensee FKA in its report accounts for the entire course of events in all

vital aspects, and moreover of how various parts such as the turbine system and reactor

system were affected.



In this report the complex effect on the reactor protection system (the

516-system) shall be included.

All the scram conditions tripped, and what does this mean?



Have the protective systems functioned in the way they should have?



Systems functioning the expected manner, is this good?

The above mentioned action to prevent the battery secured net from being knocked out seems

currently to be essential.



The event is exceptional and has led to major pressure for the personnel at Forsmark. It is not

obvious to SKI that the people involved have received sufficient debriefing.



SKI’s judgment and suggestions for further actions



SKI judges that the licensee FKA must submit analyses on at least the following areas in

order to provide material for



- an assessment of safety prior to restart

- an assessment of the robustness of the unit in a broader perspective:



o The course of events. An account of electric supply, for process systems and

for handling.



o The voltage transient. Connections to the preparations for restart of the unit,

possible dependences of power level. Verification of the transient registered.



o Account of dimensioning requirements for the unit equipment that can be

exposed to voltage fluctuation.



o Is the present design of UPS robust enough for protecting the battery supply?



o The issue of selectivity in protective equipment for electric systems in a broad

perspective.



o In what way are the observations and experiences of the operators taken into

account, in the short and long term?

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