Reportable event at the Krümmel NPP on June 28, 2007
The reportable event at the Krümmel nuclear power plant on June 28, 2007 consisted of a transformer fire , as a result of which there was an unintended reactor shutdown (RESA) and an unintended failure of the main feed pumps. These two system errors led to the classification as a reportable event. They fall in the lowest category (INES 0) on the International Scale for Nuclear Events . The emergency cooling systems functioned as designed, but further weaknesses in organization and technology became apparent in the course of the process (misunderstanding in the control room, penetration of fire smoke, failure of an archiving system). The event was widely reported in the media; the information policy of the operator has been heavily criticized in some cases.
Transformer fire
On June 28, 2007, a fire broke out on one of the two machine transformers on the site of the Krümmel nuclear power plant . The machine transformers transform the generator voltage of the power plant from 27 kV to the voltage level of the high-voltage network of 380 kV. The cause of the fire is a short circuit within the transformer, which led to the ignition of the transformer oil.
Fire fighting
The extinguishing work comprised the phases "fire fighting" and "post-extinguishing work". The fire fighting began 5 minutes after the fire broke out on June 28, 2007 at 3:07 p.m. with the arrival of the power plant's own fire brigade . At 3:15 p.m., the external fire brigade units from the Geesthacht and Grünhof-Tesperhude locations arrived at the power plant site. A total of 107 firefighters with a total of 16 emergency vehicles were on site during the fire fighting. At 22:00 on the same day, the fire fighting phase was completed with the initiation of the extinguishing work. The external fire brigade units requested for support had left the power plant site on June 28, 2007 at 11:55 p.m. With the end of the extinguishing work on June 30, 2007 at 3:45 p.m., the use of the power plant's own fire brigade was ended. There was no personal injury.
In parallel to the fire fighting, the disposal of the extinguishing water contaminated by transformer oil and foam-forming substances began on June 28, 2007 at 6:10 p.m. A total of 1600 m³ of contaminated fire fighting water was disposed of. By closing the system drainage, the environmentally harmful substances in extinguishing agents and transformer oil were retained in the power station's collecting facilities so that no water-polluting substances were discharged into the Elbe or could penetrate the ground.
Reactor shutdown
The Krümmel nuclear power plant transmits its electrical power to the 380 kV high-voltage network via two parallel-connected machine transformers. Due to the short circuit within one of the two machine transformers, which also triggered the ignition of the transformer oil and the ensuing fire, the power plant was disconnected from its 380 kV network in order to interrupt the flow of electricity from this direction. Since the generator was no longer able to dissipate the electrical energy it provided after being disconnected from the 380 kV network, the power plant was shut down immediately after this process. This happened by triggering the reactor emergency shutdown , RESA.
When the reactor emergency shutdown is triggered by the nuclear power plant's monitoring system, the reactor protection, there is an obligation to notify the nuclear regulatory authority. The basis for this is the criterion N 2.5.7 of the AtSMV “Requirement of safety devices (this includes the reactor emergency shutdown system) by the reactor protection system”. The operator complied with this by notifying the MSGF (Ministry of Social Affairs, Health, Family, Youth and Seniors of the State of Schleswig-Holstein) in Kiel on July 3, 2007 in writing of the reportable event N01 / 07.
The complete switchover of the power plant's internal power supply to the 110 kV external network by opening the second 380 kV circuit breaker is due to the sensitivity of the electrical protective devices. After checking these protective devices, the operator comes to the conclusion in his main report that the sensitivity of these protective devices will not be reduced.
Since the power plant continues to require electrical energy even after it has been switched off, it is switched to a reserve network, the 110 kV external network. This switching process is associated with a voltage interruption of 1.5 s. The “power failure” cited variously in the reporting relates to this technically necessary voltage interruption. The control room of the power plant is not affected by this voltage interruption, as the displays located there are supplied by batteries within the voltage break of 1.5 s and are available without interruption.
Reactor feed pump
After the switchover of the internal consumption (that is the electricity that the power plant itself needs to operate its systems) to the 110 kV external network, the reactor feed pumps did not switch on again automatically. The control of the reactor feed pumps was not designed for such an event sequence as it was after the short circuit inside the transformer.
The reactor feed pumps ensure the water supply to the reactor during normal power operation. Since feeding the reactor is one of the most important tasks in ensuring nuclear plant safety, every nuclear power plant has a large number of safety systems , usually pumps that can easily feed the reactor with coolant in every conceivable condition. The reactor feed pumps do not belong to these safety systems, because due to their high power consumption they also do not meet the requirements made of safety systems. If the reactor feed pumps fail, however, one of the safety systems must always be used to continue feeding the reactor.
The operational control of the reactor feed pumps has been expanded so that in the event of failures in the area of the internal supply, the automatic reconnection of the reactor feed pumps is guaranteed with the time sequence found.
Since the feeding of the reactor is of great importance, the failure of the operational reactor feed pumps is the second criterion, which is the subject of the reportable event N01 / 07. Criterion N 2.5.5 requires the power plant operator to report to the supervisory authority if more than one “main feed water pump” (reactor feed pump) fails.
On the international scale for measuring the severity of incidents in nuclear power plants, INES , the transformer fire and the subsequent abnormalities are not taken into account. Reportable events of this type are characterized by the fact that they are classified as “zero” according to INES. This classification means that the event is assigned little or no safety significance.
Reactor pressure, reactor level
After the failure of the operational reactor feed pumps, the reactor was fed by the high pressure feed pump of the safety system provided for this case. The automatic start of this high-pressure feed pump is triggered when the reactor level falls below 11.92 m. The reactor fill level acts as a "switch" which switches the high-pressure feed pump on at a value of 11.92 m and switches it off again when the fill level reaches 14.07 m. At the same time as the start of the high-pressure feed system, the live steam lines leaving the reactor are closed. When the main steam lines are closed, steam can no longer flow out in the direction of the turbine or turbine condenser, so that the reactor pressure increases. To limit the pressure, when a reactor pressure of 74 bar is reached, a safety and relief valve opens automatically and reduces the reactor pressure to a value of 69 bar before it closes again automatically. This process is repeated without the intervention of a reactor operator, the time intervals between the closing and opening of the safety and relief valves increasing with the decrease in the decay power.
The safety systems of the nuclear power plant fulfilled their function as intended. There were no deviations or abnormalities.
Depressurization of the reactor
In the further course of the event, a reactor driver opened two safety and relief valves for about four minutes and reduced the pressure in the reactor from 65 bar to 20 bar. Since the reactor can also be fed by further low-pressure pumps by lowering the pressure, this measure is harmless for the safety of the plant. At the same time, the temperature of the cooling water in the reactor is reduced from approx. 280 ° C to 220 ° C with this measure. This relatively quick “cooling down process” must be avoided under defined operating conditions of the system. The pressure drop from 65 bar to 20 bar, appropriate for the operating condition, takes place over a period of approx. The misunderstanding between the shift supervisor and the reactor operator about the type of pressure reduction is due to these two modes of operation.
The circumstances that led to the misunderstanding were examined from an occupational psychological point of view as part of an MTO analysis (people, technology, organization). A measure derived from this analysis is the introduction of a command language "3-way communication", which is binding for the shift personnel when switching operations.
Fire gas in the control room
The installation site of the machine transformers is approx. 20 m below the air inlet openings of the switchgear building. Since the supply air for the power plant control room is also provided by the ventilation system of the switchgear building, fire gases penetrated the control room at the beginning of the event until the control room ventilation was switched to recirculation mode. The shift supervisor then ordered the self-contained breathing apparatus, compressed air breathing apparatus, kept in the waiting area to be kept ready in the waiting area. After the necessary measures for fault control had been taken, a reactor driver temporarily put on one of the breathing apparatus. The other people working in the control room performed their work without breathing apparatus for the entire period.
Data acquisition, process computer
Within the first minute after the short-circuit in the machine transformer, the monitoring devices of the various systems generated more than 6500 messages that were archived in temporary computer memories. Before the messages are saved in the final archives, they are given a time stamp and sorted according to the time stamp. This is necessary because the order in which messages are received can differ from the actual history of the event sequence due to different signal transit times. During normal operation, logs are created from the incoming messages, which compile information about certain system areas. After the occurrence of a malfunction, the device for generating reports was overloaded due to previous extensions to the system. A work report comes to the conclusion, among other things, that the plant expansions exceeded the manufacturer's 2001 specifications . Due to the higher priority of the protocol device compared to the archiving device, the archiving device was temporarily no longer allocated any resources, which ultimately led to a computer switchover to the replacement system that was not actually necessary. When switching computers, those messages that had already been read from the temporary memory but not yet stored in the final archive were lost.
The error in the process computer system was eliminated by lowering the priority of the logging device over the archiving device.
Representation of the transformer fire in the media
The transformer fire in the Krümmel nuclear power plant aroused broad media and political interest.
As part of the reporting on the event, a large number of assessments and evaluations by various organizations or people were given. Depending on the interests of the cited group, the assessments with regard to the threat posed by the events vary.
Despite the reports published by the nuclear power plant on the events, there are no representations in the media that evaluate the technical processes in more detail. The concerns about nuclear safety expressed in newspapers and news broadcasts are largely based on the information policy of the operating group Vattenfall. The partly incomplete, partly contradicting or provisional information provided by the group in relation to the events of June 28, 2007, opened up a lot of space for speculation, which was used by the various organizations to underpin their respective positions with regard to nuclear energy.
As an example, the question regarding the influence of the fire on the reactor building should be mentioned at this point, which was the subject of many discussions following the assessment of the fire. The operator's statement that the reactor building was not affected by the fire refers to the immediate effects of the fire, such as the spread of flames or the penetration of smoke gases through adjacent room areas, the ventilation system or cable or pipe ducts. The representation that the reactor building was affected as a result of the fire relates to the behavior of systems within the reactor building that were activated as a result of the fire. For example, the reactor was shut down after the short circuit within the transformers and disconnection from the 380 kV network. Since the installation site of the reactor and the emergency shutdown system is inside the reactor building, this was also affected by the fire according to this argument.
A journalistic review of the events, in which, among other things, an attempt was made to explain the technical background in more detail, was carried out by WDR television with the assistance of Vattenfall. As part of the reporting, various sequences were filmed in the Krümmel and Brunsbüttel nuclear power plants and in the headquarters of Vattenfall Germany in Berlin. In addition, interviews were held with those in charge of the Vattenfall Group and an engineer from Sweden who was named as an expert on nuclear power plants. The report was broadcast on November 19, 2007 on WDR television.
The report gave rise to different reactions.
In its issue of November 20, 2007, the Kölner Stadtanzeiger acknowledged the outstanding journalistic quality and objectivity of the show. The Society for Plant and Reactor Safety (GRS), on the other hand, comes to the conclusion that the report contains “a large number of technically incorrect facts and contexts”. The GRS is the scientific and technical expert organization for reactor safety of the Federal Environment Ministry. Due to the blatant contradiction between the WDR's representations and the GRS's assessment of the events of June 28, 2007, from the point of view of GRS, GRS felt compelled to write a public statement on the WDR broadcast.
The comparison of the documents from WDR, Kölner Stadtanzeiger and GRS illustrates the basis on which the discussions about the events of the transformer fire in Krümmel are often conducted.
The following is a list of further presentations in the daily press that deal with individual aspects of the transformer fire.
The daily newspaper (taz) reported in its edition of July 7th, 2007 about "Problems with the emergency shutdown", because the reactor operator lowered the pressure of the reactor unnecessarily quickly with two valves from 65 bar to 20 bar.
The Netzeitung has in its edition July 4, 2007 indicated that the incident "was harder than expected." In addition, the falling reactor fill level is discussed. In addition, the article quotes a report by the IPPNW , International Doctors for the Prevention of Nuclear War, Doctors in Social Responsibility e. V., who sees an increased risk of fire in older German nuclear power plants, since the oil inventories of the main coolant pumps are located within the containments. This assertion has to be relativized insofar as, according to the article, the boiling water reactors affected by the “dangerous conceptual faulty design” do not have the main coolant pumps typical for pressurized water reactors .
The film “ Residual Risk ”, shot in 2010, is based on this reportable event.
swell
- ↑ Main part of the interim report prepared by Vattenfall on the event of June 28, 2007
- ↑ Tagesspiegel from June 29, 2007
- ^ Report on fire fighting in the interim report prepared by Vattenfall on the event of June 28, 2007
- ^ Report on environmental pollution in the interim report prepared by Vattenfall on the event of June 28, 2007
- ↑ Nuclear Safety Officer and Reporting Ordinance
- ↑ Notification form of the reportable event on the transformer fire
- ↑ Report on the process computer system of the interim report created by Vattenfall on the event of June 28, 2007 ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ↑ Netzeitung of July 3, 2007 ( Memento of July 5, 2007 in the Internet Archive )
- ↑ Spiegel online from July 4, 2007
- ^ "The incident - What really happened in the Vattenfall nuclear power plant?", Broadcast on WDR television on November 19, 2007 ( Memento from November 9, 2007 in the Internet Archive )
- ↑ Critique of the Kölner Stadtanzeiger on the film "The Incident - What Really Happened in the Vattenfall Nuclear Power Plant ?" ( Memento of the original from November 21, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ GRS statement on the film "The incident - What really happened in the Vattenfall nuclear power plant?"
- ↑ daily newspaper of 7 July 2007
- ↑ Netzzeitung für Deutschland from July 4, 2007
- ↑ Film review of "Residual Risk" from January 21, 2011
