The AP has flashed the following story concerning an incident at the Oskarshamn nuclear power plant in Sweden:
Per Jander over at the World Nuclear Association Blog disagrees:
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.
Technorati tags: Nuclear Energy, Nuclear Power, Environment, Energy, Politics, Sweden, Electricity
Swedish nuclear authorities held an emergency meeting Thursday after two reactors were shut down at a plant in the southeast of the country.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.
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.
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.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.
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.
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.
Technorati tags: Nuclear Energy, Nuclear Power, Environment, Energy, Politics, Sweden, Electricity
Comments
Rumour has it that the rectifiers were of the same fabrication of a unit that failed once in a German plant.
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?
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?