Power plant

Closing down nuclear plants in Germany, safely

PreussenElektra GmbH headquartered in Hanover is a wholly-owned subsidiary of E.ON SE (Essen) and operates all E.ON SE activities in the field of nuclear power. Managing Aging Plants’ Melina Schnaudt talked with Almut Zyweck (external communication) and Dr. Wolfgang Mayinger (Component Technology) about the maintenance of nuclear power plants and the effects of the German nuclear phaseout.
 
How many nuclear power plants does PreussenElektra currently operate?

Zyweck: We are in charge of the operational management of eight nuclear power plants. Three of them are in power operation, three are switched off , and two are currently being dismantled. The plants Brokdorf, Grohnde and Isar 2 are in use and have a combined installed capacity of 4.1 gigawatts. The plants Unterweser, Isar 1 and Grafenrheinfeld have been shut down. Unterweser and Isar 1 were decommissioned ad hoc after the nuclear accident in Fukushima in 2011. The Unterweser and Grafenrheinfeld plants are currently in the process of receiving approved for decommissioning and dismantling. Approval for dismantling Isar 1 has now been granted and nuclear decommissioning started in April 2017. At the nuclear power plants Stade and Würgassen the process of dismantling is already well advanced; In the case of Würgassen, nuclear decommissioning was already completed in 2014, so that only the buildings remain. However, these cannot be demolished yet, since radioactive waste is still stored in part of a building, which is connected to the former power station. In addition to the eight plants PreussenElektra is in charge of the operational management, there are five nuclear power plants in which PreussenElektra is involved.

Can you describe the maintenance process, in the case of a running plant, a factory that has been dismantled, and a plant that has been shut down?

Maintenance (IH) is a fixed process in our plants, our plants, also for plants in operation and plants in the dismantling process. It is carried out permanently and targeted in a so-called IH control loop during the entire life cycle. This IH control loop describes the phases of maintenance that are to be repeated: Specification of the maintenance methods, specification of the maintenance procedures, planning of maintenance measures, preparation of the maintenance documentation, implementation of the maintenance measures, and finally the evaluation of the maintenance measures so that the three main concept phases are continuously monitored: maintenance strategy, maintenance planning and control, as well as operational handling of measures.

For plants in operation all necessary requirements from the Atomic Energy Act must be complied with. In addition, the regulations and requirements of the rules, subordinate regulations, provisions, notices of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Bundesministeriums für Umwelt, Naturschutz, Bau und Reaktorsicherheit; BMUB), safety requirements and guidelines, as well as nuclear codes and standards and in case of non-nuclear parts of the conventional regulations must be stringently followed. In the case of plants in non-service or after-service, a large number of conditions also relevant to aging management are changing with regard to the reduction of potential hazards, the number and extent of the systems required to comply with the safety goals (for example, the removal of heat from the fuel elements), the reduction of the relevant loads, and the reduction of the relevant damage mechanisms.

With the decommissioning of many systems, which are no longer needed, the scope of the systems and components for which aging management and maintenance measures are to be implemented decreases too. The basis for the reduction of the scope of assessment for aging management is an assessment of the safety status of the plant for the determination of the systems and components still required in the respective phase of the dismantling.

Another difference from the plants in operation is of course that the relevant stresses and operating environmental conditions relevant for aging processes are significantly lower. While pressures of approx. 160 bar and 300–350°C prevail in the case of a pressurized water reactor, there are normally only systems with pressures around 10 bar and temperatures below 100°C. This also affects the mechanisms to be considered and thus the scope of the maintenance can be reduced according to the assessment and criterion. This has to be decided depending on every single system.

A map showing the locations of PreussenElektra GmbH, a wholly-owned subsidiary of E.ON SE (Essen), and the nuclear plants for which it is responsible for the operational management.
A map showing the locations of PreussenElektra GmbH, a wholly-owned subsidiary of E.ON SE (Essen), and the nuclear plants for which it is responsible for the operational management.

How has the maintenance of nuclear power plants changed in recent years?

Mayinger: In recent years, requirements have increased e.g. from new regulations in nuclear and conventional regulations as well as from operational safety regulation. In our continuous IH process, we always have a look at the experience feedback from the aging management and the maintenance measures from all our operating plants, but also from other nuclear plants in Germany, and around the world. We evaluate all the information on malfunction messages and reportable events in nuclear plants. The results are included in the maintenance measures. It can therefore be said that the scope of the maintenance measures has been increased mainly by legal and regulatory requirements but also by feedback from our own and other plants.

How often are the plants serviced?

Mayinger: We have a major revision in our plants once a year. This takes about fifteen to twenty-five days depending on the extent of the planned measures. During this time the system stops operating. In addition, the plant is continuously maintained throughout the year. Zyweck: Of these recurring tests there are 3000–4000 per year. Depending on the security system, there are of course different test cycles, which are defined in the testing manual. There are examinations, which only have to take place every four years, other functional tests are carried out every four weeks. The safety systems are quadruple redundant in any system, so one can always be turned off to perform the required tests for an examination.

How many people are responsible for the maintenance of a plant?

Mayinger: 300–400 people are responsible for the maintenance and the reliable operation of a plant. During the large annual revision, there are also a lot of foreign personnel on site, so that during the revision, around 1500 people are working on the plant during peak periods. These are manufacturers, assembly companies, service providers, test personnel for non-destructive tests, who carry out the maintenance of the pumps and fittings, and the condition of the systems and components.

What should be paid particular attention to during maintenance?

Mayinger: Observation of the entire maintenance control circuit is important, not only the maintenance of the component to be maintained, but also the planning. Thus, a pre-job briefing is carried out with the colleagues who perform the maintenance. In this briefing the important points are discussed: what is to be inspected, what might happen, what safety precautions must be taken, etc. It is therefore the whole cycle from technical planning and scheduling, to work preparation, and work safety, for example, whether a scaffold is needed and how the scaffold needs to be secured in order to ensure the safety of the colleagues on site, also needs to be planned in detail. When carrying out the maintenance procedure, attention must be paid to work safety and, of course, to the proper performance of the work. The documentation of the work and the de-briefing are also very important for us, so that the maintenance measures can be integrated into a closed ‘knowledge management process’ and improvements can be made.

What factors determine how long a power plant runs and when it is shut down?

Zyweck: It is very important to distinguish between the technically possible running time and the special situation in Germany, that the government wants to get out of the nuclear power industry and prescribes remaining times for the nuclear power plants by law. Even with our two plants, regarding (the plants had no safety deficit) Unterweser and Isar 1, shut down after Fukushima ad hoc, there was at no time a safety deficit. Technically they could have continued operating for a long time. The date of commissioning plays a subordinate role, because we are constantly updating the plants to the latest state-of-the-art. It depends on the safety state of the power station. All systems on the grid must meet exactly the same safety requirements.

Mayinger: With our aging management program, operation monitoring and our maintenance measures we have a sound knowledge about the integrity and functional condition of the systems to ensure safe operation. We are able to detect damage mechanisms in time in order to be able to initiate the necessary remedial measures. In 2002, the amendment of the Atomic Energy Act restricted the amount of electricity to be produced for each plant, which in effect had a restriction of about thirty-two years of operation, regardless of the condition of the plant. The Atomic Law specifies a fixed shutdown date and a residual current quantity, which may still be produced. This determines when the power plant goes off the grid.

Zyweck: Isar 1 and Unterweser belonged to the facilities which were immediately shut down. At that time, the Federal Government decided that all plants that went online before 1980 plus the nuclear power plant ‘Krümmel’ should be shut down. It was a purely political decision. The sad thing is that after Fukushima extensive investigations have been carried out not only in Germany but also in Europe, which have all shown that our facilities are in no way comparable with the conditions in Fukushima. We had to stop our plants and the running times of the other nuclear power plants were shortened.

Mayinger: After Fukushima, the European Commission carried out an EU stress test to determine whether there are any design deficiencies. All German nuclear power plants have participated in this test and all have passed the EU test. The government of Germany has certified the power plant operators in the report presented on the 6th Review Meeting of the Nuclear Safety Convention from 24 March to 4 April 2014 in Vienna a high safety level and a high robustness of their installations.

Zyweck: Of our facilities, Isar 2 is the last one that needs to be shut down in 2022.

How is the dismantling of nuclear power plants carried out and how long does it take?

Zyweck: When the plant is switched off , the so-called non-service or after-service starts. Parallel to this, the decommissioning approval procedure is running. This entire approval procedure usually takes four-to-five years. During this time, the system is prepared for dismantling, removing the fuel elements from the reactor pressure vessel and placing them in the decay basin. They are then transferred to transport and storage containers and transferred to the temporary storage facility on the premises. As soon as we receive the approval from the supervisory authority the actual dismantling begins, which on average takes another twelve to fifteen years. Decommissioning begins in the heart of the plant, the reactor pressure vessel. Here, the fittings are removed, disassembled, decontaminated and packaged. Work is carried out from the inside to the outside, so that during dismantling nothing can be seen from the outside. In the end there are only the ‘empty’ buildings, which are cleaned then. After about two years, as soon as the plant is released from nuclear supervision, the buildings are demolished.

 

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