D&D TECHNIQUES APPLIED DURINGDECOMMISSIONING OF WASTE STORES AT NPP RHEINSBERG

Detlef Schmidt and Bernhard G. Christ
DETEC GmbH
D-63755
Alzenau, Germany
Phone: 49 6023 91 1349

ABSTRACT

NPP Rheinsberg (KKR) was the first nuclear power plant of the former German Democratic Republic and started operation in 1966 after nine years of construction. Based on the former safety philosophy, it was expected that the final storage of low and medium active waste would take place on the KKR site. A special purpose storage facility for solid and liquid radioactive wastes (ALfR, "Lager für radioaktive Abfälle") was used. The so-called ALfR consists of:

The overall decommissioning concept of KKR includes:

Due to the potential risk to the environment of the wastes stored, the ALfR-part of KKR is to be decommissioned first. Wastes shall be removed as soon as possible. The facilities and buildings of the ALfR are scheduled to be treated subsequently.

The removal of solidified radioactive evaporator concentrates from the Beton-Monolith is now complete. The decontamination of the building is to a point where only some contamination remains in the deeper layers of concrete floor. Wastes have been sent for final storage at the Morsleben repository (ERAM).

The removal of ALfR-solid is currently completed and the wastes generated are placed in Morsleben.

Liquid radioactive wastes of ALfR-liquid are separated into solid and liquid parts. The solid wastes will also be sent to Morsleben. The liquids will be stored in other KKR tanks for further treatment with NPP standard equipment.

The techniques applied and experience gained are described below.

INTRODUCTION NPP

Rheinsberg (KKR) is situated 80 km north of Berlin in a region of lakes and forest on the border of the Stechlin Nature park. KKR is equipped with a prototype PWR of the Soviet WWER-II design with a thermal power of 265 MW.

KKR was commissioned in 1966 and shut down in 1990 for safety reasons. Today, the plant is in a post-operating condition. In the framework of this post-operating phase, solid and liquid waste stored will be emptied and removed.

In December 1992 the owner of KKR, Energiewerke Nord GmbH (EWN), placed its first order to decommission the two solid waste storage facilities. In September 1994 a second order for emptying the liquid waste storage tanks (ALfR-liquid) was placed.

The work comprises of planning, preparation of licensing documents, decommissioning of the so-called "Beton-Monolith", emptying caverns of active waste, (ALfR-solid), with subsequent decontamination of the caverns, as well as the installation of mobile equipment for removing liquid wastes and slurries from storage tanks.

In a separate step, the dismantling of equipment, piping and the demolishing of buildings will occur.

LICENSING

All operations for decommissioning and decontamination work are carried out in accordance with and executed under the still valid operating license of KKR. Detailed reports for the decommissioning operations and for the protection of personnel and surroundings, have to be prepared and accepted by the appropriate authorities (Ministerium für Umwelt, Natur and Raumordnung; TÜV Berlin-Brandenburg; for final storage of wastes additional authorities and organizations are involved).

For each of the three projects, a separate licensing procedure was required. As a result, the field work with the Beton-Monolith could start independently in January 1994 while licensing of the other projects was still underway.

The license for emptying ALfR-solid was granted in September 1994, the license for ALfR-liquid was in early 1996.

TECHNICAL CONCEPTS AND RESULTS

Decommissioning of the "Beton-Monolith"

The so-called "Beton-Monolith" is situated outside of the direct active area. It consists of a 12m by 12m by 6m concrete building covered by a wooden roof without an active ventilation system. The inner concrete surfaces of the building were protected by PVC-film to prevent penetration of activity into the concrete during filling of the radioactive mortar. The Beton-Monolith contained approximately 110 m3 of medium activity solidified evaporator concentrates with a total beta/gamma activity of approximately 5 x 1011 Bq and a surface dose rate up to 680 mSv/h.

From the technical concept, it was expected to use the existing building structure (but improved) as a contamination barrier and to install as many auxiliary systems and technical equipment outside the building as possible. Furthermore, most of the equipment should be reused for emptying the caverns of ALfR-solid in the second step.

Therefore, access to the Beton-Monolith had to be established first (road and asphalt area). Next, the building was made air tight using a special tarpaulin covering the roof. A 20' container building was placed in front of the Beton-Monolith for personnel access, for health physics and for remote control of the decommissioning work inside the building. A second 20' container, divided into three contamination areas and bearing the drum filling and handling equipment was tightly connected to the first container and to one building wall. A third 20' container with the active ventilation system (air through-put of 5000 m3/h) completed the auxiliaries. Inside the Beton-Monolith building, a foil "ceiling" was installed as contamination protection for the inner side of the roof.

At the front end of the second container, a 2m by 2m hole was cut into the 25cm concrete wall giving access for the remotely operated vehicles equipped with a hydraulic hammer. Using TV surveillance, the active concrete material was loosened. After changing the tool hammer to a grab end, its waste was remotely placed into 180 L-drums. These drums were put into 200 L-standard drums and sent to the final repository of Morsleben (ERAM). A total of some 1300 drums, including material from a thin concrete protection layer on the floor was produced. During this project, less than 20 man-mSv were received by the personnel due to remote operation and extensive training of operators.

The emptied Beton-Monolith was decontaminated inside using a wet sandblasting technique. Due to penetration of liquids through gaps in the floor and corners (probably not well sealed by PVCfoil) activity was found in deeper regions of these areas. Static reinforcement of the building prior to further decontamination for final release from the atomic act and subsequent demolishing by conventional methods is required.

Emptying of the four Caverns with Active Solid Waste

ALfR-solid comprises a free standing 30m by 12m underground arrangement of 13 concrete caverns. These caverns were intended to take up all solid waste arising from NPP operation. Later, to prevent penetration of rain water into the caverns through the temporarily closed openings, the caverns were covered with a Dutch barn type construction.

Four large cube-shaped caverns were used as storage until the shut down of KKR. The caverns contain, in total, approximately 600 m3 of solid active waste from NPP operation and from hot cell operation (the other caverns are empty). Waste varies from spent clothes, foil, garbage, insulation material to pipes, tools, smaller components and in two of the caverns, several drums of higher activity waste. Dose rates measured at the plug openings of the 50 cm thick ceiling showed some 600 mSv/h and 2 Sv/h on drum surface (spots). Total beta/gamma activity of waste is calculated as approximately 1012 Bq.

Work prior to the evacuation of the caverns started with the enlarging and closing of the partially open covering. Inside this light weight construction, an airtight and moveable steel caisson (7m by 6m by 2m) was erected over the first cavern. The existing ventilation extract pipes of caverns are decoupled from the NPP's ventilation system and are connected to the 20' container with the active ventilation and filter system transferred from the Beton-Monolith decommissioning task. The corresponding 20' container for personnel access and health physics was attached to the hall also.

The ceiling (50 cm of reinforced concrete) of the first cavern was partly cut under the caisson using a diamond saw to give required access to the waste. By means of video surveillance and a remotely operated crane, waste was transferred into 180 L-drums and 200 L-drums coupled contamination free to the caisson. The drum filling equipment from the Beton-Monolith task was used. Single large waste components removed from the cavern were cut and wrapped under the caisson and are treated elsewhere in KKR. The remotely operated vehicle equipped with the grab end was placed on the caverns floor to remove the residual waste that was out of reach of the crane.

The cleared cavern was meanwhile decontaminated internally by high pressure water jet cleaning. The other three caverns will be emptied and decontaminated one after the other, under the moved caisson. This started in mid 1996 by the NPP's own staff using the above mentioned equipment and continues today.

The last two caverns contain seven self-made containers of different shape and size with higher active waste from the hot cell operation. They were dropped into the solid waste and covered. Removal of these containers is expected to occur by remote control using the caisson's equipment as well as the steel eyes that were fixed into the concrete bindings the waste pieces. Due to the limited bearing capacity of the ceilings, containers will be transported with a special lorry from the caisson to one end of the hall. By means of a separately installed crane they will be put into shielded containers ready for final disposal.

From the 600 m3 of waste approximately 3000 drums will be produced. Most of the drums have to undergo a drying step prior to transport to Morsleben. The dose commitment for the staff is calculated as approximately 100 man-mSv.

Emptying of Liquid Waste Storage Tanks

During the KKR operation, evaporator concentrates and resin/charcoal/salt sludges were generated from liquid waste treatment. Wastes were collected and stored in a special building in an underground stainless steel storage tanks each with a capacity of 500 m3 in volume and 13 m in diameter. Three of these tanks were still in use (here called, ALfR-liquid), one contained approximately 190 m3 evaporator concentrate (approximately 1 x 1012 Bq beta/gamma), two contained approximately 140m3 of a mixture of dewatered spent ion exchange resin, charcoal and mainly aluminum sulfate (approximately 4 x 1012 Bq beta/gamma) as well as a considerable amount of contaminated water. Wastes have to be withdrawn as soon as possible due to the potential danger of leakage.

Inside the above mentioned building, a automatically operated mobile drying unit, a decanter centrifuge of Alpha level type, was installed. By means of this rack-mounted installation, solid particles down to 1 mm diameter together with the main amount of undissolved activity will be separated from the liquid phase. The solids, with approximately 50% moisture, will be filled directly into drums of 200 L volume. For intermediate storage and for final storage at Morsleben repository, they have to be shielded by outside concrete shieldings.

Decantates containing dissolved salts and the remaining activity, will further be treated in existing NPP installations.

For mixing of sediments and removal from the tanks, a specially designed reusable process module is temporarily installed in either storage tank. The equipment consists of a submerged stirrer, a sludge pump, piping, as well as, rotating cleaning nozzles altogether mounted on a moveable support. By means of this equipment, one tank already was emptied, salt crusts and deposits of boric acid were loosened from walls and bottom. The decanter centrifuge was already tested with active sludge in a test program. License for permanent operation is expected immediately. The sludges will be then pumped into an existing 10 m3 KKR tank. Here the required solid content of waste for optimum centrifuge conditions can be adjusted if required. By means of flexible hoses, waste is then transferred to the centrifuge.

The operation of the decanter, emptying of tanks and final cleanup of the emptied tanks using a high pressure water jet will be performed by the NPP's own staff.

CONCLUSION

The continuous D&D of waste storage facilities at NPP Rheinsberg demonstrates that such projects can be performed with current licensing procedures and with appropriate and available technologies. With the installation of these technologies and the starting help by qualified contractors, the NPP's own staff can continue the tasks by themselves.

It also demonstrates that solidified evaporator concentrate waste from NPP and hot cell operation and liquid waste could safely be remedied from a proposed "final" storage site.