Taking apart Greifswald, two ways26 June 2013
Experience dismantling Germany’s Greifswald nuclear power plant has shown that both the conventional cutting strategy and large component management strategy can be successful. However, the latter option is often favoured when considering economic and radiological factors. By Ralf Borchardt
Experience dismantling Germany's Greifswald nuclear power plant has shown that both the conventional cutting strategy and large component management strategy can be successful. However, the latter option is often favoured when considering economic and radiological factors. By Ralf Borchardt
Eight Russian-designed VVER-440 pressurized water reactors were originally planned for the Greifswald Nuclear Power Plant (KGR) on Germany's northeastern Baltic Sea Coast. The first unit started operation in 1973, with the second a year later; and units 3&4 followed in 1978 and 1979. In 1990, after the reunification of Germany the operating units Greifswald 1-4 along with the 70 MW Soviet-designed Rheinsberg Nuclear Power Plant (KKR), about 140 km south, were shut down. The commissioning of Greifswald unit 5 as well as the construction work at units 6 to 8 was also stopped.
Today Energiewerke Nord GmbH is responsible for the decommissioning and dismantling of both Greifswald and Rheinsberg, along with a number of other nuclear facilities. EWN is the legal successor of the former 'Kombinat Kernkraftwerke,' and has been solely owned by the Federal Ministry of Finance since 2000.
Overall licences for the decommissioning and dismantling of Rheinsberg and Greifswald were both issued in 1995. The Greifswald licence, granted by the responsible Ministries of the Federal State of Mecklenburg/Western-Pomerania on 30 June 1995, covers the dismantling of reactor units 1-5 with part licences for the dismantling of the reactor components. EWN has since applied for additional licences needed for further dismantling of the plant.
Immediately after shutdown, dismantling strategies for reactors 1 to 5 on the Greifswald site (Figure 1) were investigated. It was quickly concluded two strategies should be used: a cutting strategy and a large component strategy.
The cutting strategy (Figure 2) was initially selected for Greifswald units 1-4 (and also Rheinsberg) due to the length of time they had been in operation (12-24 years). It involved cutting the reactor components into segments and packaging them into containers for subsequent interim and decay storage. The large component strategy, which involved transporting the whole reactor pressure vessel with appropriate shielding to an on-site Interim Storage North Facility (ISN), was selected for Greifswald 5 .
Three cutting areas were installed for the remote dismantling of the reactor components from Greifswald 1-4: a dry cutting area, a wet cutting area, and a cutting area in the reactor pit.
The dry and wet cutting areas were located in the former steam generator rooms of units 2 and 4.
The dry cutting area was to be used to cut the reactor pressure vessel (RPV) and the low-activated parts of the reactor internals such as the reactor cavity and protective tube unit. The dry cutting area had two cutting places. In the first cutting place, rings were cut with horizontally, and the reactor components were cut from bottom to top. To balance the weight and for lowering the reactor components after cutting, a flexible wire hoist was used. After the horizontal cut, the ring was taken by a transport vehicle with turntable to the post-cutting place where it was cut vertically into segments. Finally, a power manipulator was used to handle the segments and package them into the storage/transport container.
The wet cutting area was to be used for the cutting of the core basket, reactor cavity bottom, as well as higher-activity parts of the reactor cavity and the protected tube unit. This area consists basically of a cutting pool with cutting devices, different transport and handling devices, a water cleaning system and a packing station. RPV components were cut from top to bottom. During the cutting, the components were fixed on the turntable in the cutting pool and covered with water (see Figure 4).
The cutting area in the reactor pit was prepared for in-situ cutting of the corresponding annular water tanks (biological shield) and the RPV insulation.
The remote dismantling and packing of the RPV and internals was tested and optimized for the use with activated reactors in a long model-dismantling phase with non-activated reactor components from units 7 and 8 that ran from October 1999 to July 2003 . The experience gained during planning and implementation of the model dismantling at Greifswald was also expected to form the basis for the remote dismantling of the Rheinsberg reactor.
Change of strategy
For Greifswald unit 5, EWN planned to transport the RPV as a whole component to the Interim Storage North Facility (ISN), on the same site. The four reactor internals were to be packed into shielded transport and storage devices and transported to the ISN for decay storage. In December 2003 the RPV of unit 5 was lifted and stored, without cutting, in the ISN.
Following an evaluation of its experience at unit 5, EWN considered a change in the disposal strategy for reactors 1-4 at Greifswald and the reactor at Rheinsberg (see also Table 1). Ultimately, it decided to continue with the cutting and packing of the reactor internals of Greifswald 1 and 2, which had started in August 2004, and also at Rheinsberg where dismantling activities started in September 2006 .
In parallel EWN also investigated a new decommissioning strategy (Figure 5):
- The RPVs of units 1 to 4 and the reactor internals of units 3, 4 and 5 of Greifswald, as well as the Rheinsberg reactor pressure vessel to be intermediately stored as shielded large components in the ISN
- The reactor internals (core basket and protective tube unit) of units 3 and 4 to be packed into shielded transport and storage devices for decay storage
- The dismantling of the annular water tank and the RPV insulation is to be continued in the reactor pit.
Dismantling the RPV internals: Units 1&2
Before hot operation could begin at Greifswald 1&2, a dismantling report had to be checked and approved by the authorities. The equipment used for the model dismantling in unit 5 also had to be installed in unit 2.
First, a milling machine was used to cut the nozzles (main coolant pipe connections) of the primary circulation pipe of the RPV. Dismantling of all 12 RPV nozzles was successfully completed in just 29.4 hours, around 2.5 hours per nozzle.
In parallel, the RPV internals were cut and packed within the wet and dry cutting areas. This work started in August 2004 and was successfully finished in July 2007. Overall, eight RPV internals with a total mass of 217.7 tons and a total activity of 2.15 x10^16 Bq were remotely cut and packed into a total of 135 containers, six storage and transport casks and 20 waste drums. The applied collective dose was approximately 110 mSv, 40% below the expected collective dose .
The dismantling of the RPV internals at Rheinsberg was carried between September 2006 and September 2008, taking into account the experience from Greifswald. The wet cutting area was installed in a former spent fuel pond. Four RPV internals with a total mass of 41.22 tons and total activity 3.56 x10^15 Bq were cut underwater and remotely packaged.
Dismantling the RPV internals: Units 3-5
The packing of the four RPV internals for each units 3, 4 and 5 at Greifswald into a shielding and transport container and their subsequent transport to and storage in the ISN was carried out from April until June 2006.
Based on the experience gained from previous large component transports (RPVs from units 1, 2 and 5 and the 4 reactor internals of unit 5 in shielding and transport containers) it became clear that the disposal of the reactor components from units 3 and 4 should continue to follow the strategy for large components storage. At the same time, possibilities for further optimization were examined.
EWN decided to pack the reactor cavity/cavity bottom together with the RPV and to transport it into the ISN. For the reactor internals' protective tube unit and core basket, the disposal in shielding and transport containers was preferred. However, since the reactors of units 3 and 4 operated for much longer than unit 5, these components were highly activated and new considerations had to be taken into account to ensure their safe handling and storage.
The complete process was therefore fully tested with the non-activated reserve core basket of plant II (units 3&4). During this cold test all process steps, especially the remote ones, could be sufficiently tested and the personnel could be comprehensively trained.
After only four weeks for realization, the shielded transport and storage device with the core basket of unit 4 arrived at the ISN, as the last of the four shielding and transport devices from units 3 and 4.
Dismantling of the RPV: Units 1&2
The preparation of the transport of the reactors 1 to 4 started in 2005. At units 1 and 2 the transport of the RPVs inside the reactor building had to be carried out with the reactor hall crane. For this reason the processes had to be optimized and the amount of shielding minimized so that the maximum lift capacity of 250-ton of the reactor hall crane was not exceeded. This involved carrying out precise activation calculations of the RPV material, based on radiological measurements and samples taken in 2004, and conducting static investigations to find out the areas of the building (in particular the area of the rail corridor) that required reinforcement.
In 2005, the manufacturing and installation of the new facilities and equipment, as well as the heavy load transport from plant I (units 1 and 2) to the ISN was commissioned. Construction preparation work began at plant I and at the ISN in February and was finished end of May 2007. In August 2007, EWN received the licence for the strategy change. Transports of the RPVs of units 1 and 2 were carried out, on schedule, between 8 November and 23 November 2007.
Dismantling of the RPV: Units 3&4
Once the unit 3&4 core baskets and protecting tube units had been transported to the ISN, EWN started intensive preparation works for the transport of their RPVs (plus inserted reactor cavity/cavity bottoms). Extensive safety analyses were of major importance during the preparation phase.
The static reinforcements required for units 3&4 were much more extensive than for units 1&2, as the total mass of the reactor with shielding was approx. 360 Mg and a force of 5270 MN was acting on the construction during the loading of the reactor cavity/cavity bottom into the RPV.
The following dismantling strategy was developed for units 3&4 on the basis of the experience made during the previous reactor transports:
- Dismantling of the cavity with cavity bottom from the RPV with the help of the reactor protection containers, as well as transport and parking in the maintenance pit
- Unshielded transport of the RPV from the installation position with the reactor hall crane and remote control to the rail corridor
- Placement of the RPV in a shielding cylinder (in vertical position)
- Transport of the cavity/cavity bottom with the help of the reactor protection container to the RPV in the rail corridor and placement in the RPV
- Closing of the RPV flange with a shielding plate with transport traverse
- Tilting of the reactor (using a carrying portal with wire hoist) into horizontal position in a transport unit.
- Moving the reactor in the rail corridor out of the reactor building
- Reloading of the reactor onto a heavy load transport unit
- Transport to the ISN (internal transport)
- Reloading of the reactor back onto a heavy transport unit
- Moving the reactor to the parking position in hall 7 of the ISN.
To distribute the load during the tilting of the shielded RPV, a support was installed in the cable cellar below the rail corridor. For the transport of the reactor into the ISN, the reactor had to be tilted into the horizontal position. For this purpose a tilting device had to be installed below the northern rail corridor hatch. This tilting device, already used for the transport of the RPV units 1, 2, 5 and 8, was located above the supports in the cable cellar on a load distributing double-tracked slide way.
The reactor support and the working platform was mounted above the tilting device, the shielding cylinder was fixed by mounting supports. The southern half of the hatch could be closed at any time. To move the tilted reactor in the rail corridor, a transport unit was installed. This device consists of an approx. 65m-long double-tracked slide way built on a load distributing sub-construction, and two pairs of skidshoes which each are connected to a saddle and have an integrated hydraulic jack.
First, while the units 3 and 4 RPVs were in the installation position in the reactor pit, the reactor internals were removed and the RPV was emptied of residual water. The fixing elements of the RPV were also removed; all 12 reactor coolant pipes and instrumentation lines were disconnected, closed and shielded. The flange opening of the RPV was closed.
For the transport of the RPV from the installation position to the rail corridor, the same equipment was used as for the transport of the RPV units 1, 2 and 5, without shielding because of the crane's weight restrictions. Due to the maximum load capacity of the reactor hall (250 t), the RPV was transported without the shielding cylinder for the core area, because the overall weight of RPV with shielding plate and RPV-traverse was already 225 Mg.
The crane was operated from behind a radiological shielding, using remote control equipment and video cameras. This prevented exposing the personnel involved to additional radiation. During the transport, access to the area was restricted to those involved in the project.
After the proper positioning of the reactor above the shielding cylinder, the reactor was lowered by the reactor hall crane and inserted into the shielding cylinder. The process was continuously monitored using video equipment. After placing the reactor in vertical position on the tilting device in the area of the hatch opening, the reactor was secured by means of a support and dismounted from the reactor hall crane.
Once the shielding cylinder had been fixed the RPV, the reactor cavity with cavity bottom was inserted in the RPV. This was carried out with the help of a reactor protection container. Due to the weight of the reactor (RPV + reactor cavity with cavity bottom + shielding) now about 360 Mg, it was not possible to reload and tilt the reactor using the reactor-building crane. Therefore, a carrying portal with wire hoist was used for this task.
The tilting of the RPV into the horizontal position was realized just before transporting the reactor to the ISN. After having connected the flexible wire hoist with the transport-traverse, the reactor support could be unfastened. By alternating movement of the carrying portal and lowering of the flexible wire hoist hook the reactor was brought into a slanted position. Because the reactor is taller than the hatch opening above the rail corridor it was necessary to move the tilting device twice during the tilting process. The most thrilling moment was lowering the reactor down on to the transport unit's two saddles. Here it was necessary to minimize cross-loading forces on the transport saddles. By precise definition of the tilting axis and use of the minimum possible crane hook speeds this was accomplished.
The shielded RPV was skidded outside to a hydraulic lifting unit with a lifting capacity of 1100 Mg and a lift height of 7.3 m was positioned. With the help of this lifting unit the complete reactor with shielding was lifted from the transport saddles and loaded on the heavy load transport vehicle from type SPMT (self propelled modular transporter). After load securing the reactor was transported to the ISN with a maximum speed of 3 km/h (see fig. 12).
The transport of the reactor of unit 4 was realized immediately after finishing the transport of the reactor from unit 1 in November 2007.
Biological shield/ RPV insulation
After the removal of the RPV out of the reactor pit, the annular water tank and the RPV insulation were moved from inside the reactor pit. First the shielding plates above and below the former core zone were mounted. Thus it was possible to manually dismantle the lower part of the RPV insulation. As the RPV insulation contained asbestos, the work had to be carried out according to special regulations. Once the lower RPV insulation had been removed, an inner shielding ring was installed. The gap between the annular water tank and the shielding ring was filled with concrete to reduce the dose rate. With the help of a wire saw the annular water tank including RPV insulation was cut into 11 segments. These segments were lifted into the reactor hall and tilted into a horizontal position. Then, the segments were put into special 20' containers with shielding and transported to the ISN for decay storage and future conditioning.
The dismantling of the reactors at Greifswald and Rheinsberg has now been completed. The project has shown that both the cutting and large component strategies can be successfully realized. The strategy chosen will depend on the priorities at a particular site. The conventional cutting strategy always seems to be reasonable when facilities for decay storage and future conditioning are not available. On the other hand if facilities are available, interim storage of large components is not only an alternative to cutting but a favourable option especially when considering economic and radiological aspects.
Following on from the Rheinsberg and Greifswald project, EWN has now started preparation work for dismantling the AVR, prototype pebble bed reactor at the Julich Research Center. This year, the reactor with a weight of about 2100 Mg will be lifted in a material lock, which was especially constructed for this purpose, by wire hoists, will be turned into a horizontal position and put on a self-propelled modular transporter (SPMT), for transportation to an interim storage facility, around 200 m away. Testing of the equipment to be used for this process is currently being manufactured and tested.
EWN has also been charged with the dismantling of the reactor of the Obrigheim Nuclear Power Plant. In 2005 this pressurized water reactor was shut down after more than 35 years of operation. The dismantling of the reactor is to be carried out by remote cutting and packing of the activated reactor components. Much of the equipment from the dismantling of the KGR and KKR will be used. The remote cutting of the RPV internal is carried out under water in the reactor room. The RPV insulation is situated directly at the RPV and thus has to be removed and packed with manipulators in the reactor room. The RPV will be reloaded into the fuel element pool and remotely cut with using water as shielding. At present, the equipment for cutting and packing of the RPV internals is mounted in the reactor building. The start of the hot works was planned for May 2013.
About the author
Ralf Borchardt ([email protected]), Energiewerke Nord GmbH, Latzower Straße 1, 17507 Rubenow, Germany.
This article is based on paper 13043 'Implemented Dismantling Strategies for the Reactors in the Greifswald and Rheinsberg Nuclear Power Plant,' presented at the Decommissioning Challenges - Industrial Reality and Prospects, 7-11 April 2013, Avignon, France.
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