Unique cutting process employed successfully in underwater dismantling operations at the MZFR
RWE NUKEM, consortium leader for the project involving the dismantling of the reactor tank and tank components at the Karlsruhe Multi-Purpose Research Reactor (MZFR), has implemented a new cutting process for underwater dismantling of the thermal shield at the site. With the aid of this method, the lower 650 mm of the solid steel thermal shield, the thickness of which increases from 70 to 130 mm from the lowest to the highest point, can be cut apart and removed at a depth of 7 meters below the water's surface. The contact arc metal cutting process (CAMC) developed by the Institute für Werkstoffkunde (Material Science Institute) of the University of Hanover was adapted to the remote-controlled tool-holder system by RWE NUKEM under the supervision of the Forschungszentrum Karlsruhe GmbH (Karlsruhe Research Center). The most significant innovation was the use of a circulation cooling system for the CAMC electrode. The cutting process is remote-controlled by the tool-gripping system control unit that has been used effectively in underwater cutting operations using the plasma-cutting method in previous projects. In contrast to other thermal cutting processes, this method produces no slag. Consequently, the CAMC method is superior to the plasma-cutting process wherever the gaps behind the material to be cut are narrow. Because it was essential to prevent the thermal shield from baking on to the lower segment of the reactor pressure tank during cutting, the CAMC process was chosen in view of the highly disproportional relationship between the thickness of the thermal shield and the gap behind it. The moderator tank and the rest of the thermal shield have already been successfully dismantled by RWE NUKEM using the plasma-cutting system designed specifically for this operation by the Institut für Werkstoffkunde of the University of Hanover and the remote-controlled five-axis die carrier developed by RWE NUKEM.