REACTOR DISMANTLING

Dismantling of sodium cooled reactors

In 2001 PROTEM was selected to dismantle the KNK II reactor in Germany. The stipulations of the project were to remotely dismantle the vessel and internal components of the sodium cooled reactor. PROTEM was chosen for this job because of their renowned reputation as an industrial partner to the major companies in the global nuclear industry. PROTEM participated in the first European dismantling operations performed in Germany and Belgium more than 20 years ago. Since then, PROTEM has manufactured completely automated, remotely operated equipment to complete the dismantling operations of nuclear reactor vessels and their internal components, which was a world premiere.

For the KNK II reactor, PROTEM was asked to dismantle the internal components and their insulating components within the biological shield and the upper wall. 

Due to the presence of residual sodium (used during the lifetime of the reactor as coolant) it would not be possible to introduce equipment that would lead to the creation or presence of humidity. The temperature could not go over 50°C and there could be no flying sparks. Indeed, sodium is highly explosive in the presence of water or temperatures over 50°C. To prevent any contact between sodium and humid air, the dismantling had to be performed in a dry air and nitrogen atmosphere. These were the main constraint which lead the design of the equipment: cold cutting (no sparks or temperature over 50°C), no lubrication (no water and humidity), ionizing radiation area and dry air/nitrogen atmosphere.

The configuration of the vessel was as follows: 

               Diameter of the Vessel: 1868 mm (73.54’’)

Height of the Vessel: 10,500 mm (34.4’)

Wall Thickness to be cut: Up to 170mm (6.7”)

Accessibility:  From the top and only from the inside

When PROTEM arrived on the project, 11 pieces of equipment were scheduled to cover all the cutting requirements. PROTEM took another look at the dismantling scenario and focused special attention on the cutting of the internal components and the volume of waste to be produced. A cold and dry cutting process was chosen due to the presence of sodium in the vessel. The cutting techniques utilized were, milling with high speed steel tool bits, sawing with a circular metal saw blade and shearing with hydraulic shears. The sequence of operations involved the dismantling of the internal components (from top to bottom), the dismantling of the bottom piping and then the dismantling of the vessel (from bottom to top). 

PROTEM created one multi-purpose piece of equipment that would be operated remotely. An airtight cell under a nitrogen atmosphere was created and erected on-site, so as to avoid sodium ignition during the dismantling operation. An enclosure with heavily shielded walls and remote controlled systems was set up above the reactor tank. All dismantling activities in the reactor area were executed from this enclosure with the tool being lowered into the reactor tank by the cell crane. All handling operations; the extraction and storage of the cut pieces, replacement of the tools, etc. were done by a robotic arm with no human intervention in the cell.

To perform the settings on its equipment, PROTEM created a four meter (13’) deep pool in its workshop to be able to install the partial mock-up of the vessel and its internal components. PROTEM engineers and technicians validated the operating procedures, finalized the procedures for dealing with degraded modules, ensured reliable handling and optimized the cutting and waste processing results. 

During the onsite operations, the reactor tank and its internal components were cut into pieces that fit into a waste drum. The dismantled parts were then loaded remotely into shielded transport containers. In total, about 43 metric tons of dismantled material waste (1300 components) was cut and transported to the waste processing center, HDB at Karlsruhe Research Center for further treatment. The weight of the heaviest piece was 350 kg (772 lbs.).  Sections ranging in size from 6 mm (.236”) in the vessel and protection liner to larger pieces of the reflector (170 mm (6.693”) were cut. Around three metric tons of chips were generated. 150 (150 liter) drums with pieces from the reactor and another 50 drums of chips were removed. The scenario and process implemented by PROTEM that enabled the compaction of waste from the cutting operations reduced the number of required drums by more than 5%. This is an even more significant savings when considering that the cost of manufacturing, implementation and management is around 200,000 euros ($226,458.25) per drum. 

There was a major issue with large quantities of residual sodium. 30 liters (8 gallons) were expected, but more was observed during the dismantling operation. Hardness was also much higher than expected due to the radiation levels, 300N/mm2 => 800N/mm2.  This slowed the overall cutting speed up to 50%. Another issue arose in that the chips tended to stick together. PROTEM devised a rotating head to break up the chips and allow for proper vacuuming. 

The main activities in the reactor building included many other operation beyond the dismantling of the reactor tank with its internal components performed with the PROTEM equipment. The removal of the thermal insulation was completed in 2011. Other stakeholders are currently working on the dismantling of the primary shield (started in 2014) and will continue with the disassembly of the activated biological shield and auxiliary systems and the decontamination of the residual structures.

THE KNKII was just one reactor of many which PROTEM has worked on. In fact, PROTEM equipment from the smallest standard machine to the largest custom designed equipment has been utilized successfully on many reactors all over the world. In Germany, alone, other projects have been completed in Greifswald, Niederaichbach, Philippsburg, Karlstein, Karlsruhe Eggenstein... 

Other PROTEM machines for nuclear projects include severing and weld preparation in a single operation, clamshell design for piping section maintenance, orbital cold cutting from the inside of the pipe using the standard clamshell machine design, internal cutting of heat exchanger tubes behind the tube sheet, beveling and/or facing of tubes and pipes for weld preparation, construction and maintenance of power plants, cutting of internal components with varying shapes, cold cutting with chips containment, under water cutting using orbital milling, under water dismantling of the internal components of the BR3 reactor vessel, cutting with a circular saw blade, CNC Flange facing and boring machines, multi-purpose machining and cladding equipment VVER 1000 and VVER 400.