Decommissioning of the prototype fast breeder reactor KNK in Germany

Full text: The Compact Sodium-cooled Nuclear Reactor Facility (KNK) was an experimental nuclear power plant of 20 MW electric power erected on the premises of the Karlsruhe Research Center. The plant was initially run as KNK I with a thermal core between 1971 and 1974 and then, between 1977 and 1991, with a fast core as the KNK II fast breeder plant. The plant was shut down in August, 1991, after Germany drops out of the fast breeder technology for political reasons. Under the decommissioning concept, the plant is to be decommissioned completely to green field conditions in ten steps, i.e. under the corresponding ten decommissioning permits. To this day, nine decommissioning permits have been issued, the first one in 1993 and the most recent one, number nine, in 2001. The entire nuclear fuel and all movable core internals have already been disposed of. The sodium coolant has been removed except for some residues clinging to inner surfaces and in inaccessible locations. The tertiary systems (water-steam loop with the turbine) and the secondary sodium systems, including the associated auxiliary systems and buildings, have been taken out and demolished, respectively. The ventilation system, the electricity supply facilities, and the reactor entrance and exit lock have been adapted to the requirements of the decommissioning steps to follow. The primary system including the primary sodium dump tank and the fuel element store and the rotating reactor top shield of the reactor vessel were dismantled. The work conducted so far has been based on eight decommissioning permits. Under the 9th Decommissioning Permit, the reactor vessel with its internals, the primary shield, and the biological shield are to be dismantled. The residual sodium volume in the reactor vessel was estimated to amount to approx. 30 l. The maximum Co-60 activation is on the order of 107 - 108 Bq/g; the maximum dose rate in the middle of the vessel was measured in November 2002 to be 27 Sv/h. The difficulty involved especially in dismantling KNK, on the one hand, is posed by the residual sodium in the plant, which determines the choice of neither wet nor thermical techniques to be used in disassembly. Another difficulty is caused by the depth of activation by fast neutrons, as a result of which not only the reactor vessel proper, but also the entire primary shield (60 cm of grey cast iron) and large parts of the biological shield must be disassembled and disposed of under remote control. Under the 10th and last Decommissioning Permit, the remaining auxiliary systems (sodium washing plant, ventilation plant, liquid effluent system, gaseous effluent system, etc.) are to be dismantled and any buildings remaining are to be decontaminated, measured for clearance, and then demolished, if necessary. Then the site is to be recultivated. The work is to be finished probably at the end of 2005. The presentation should give a short overview of the dismantling work and the experiences over the last ten years and particularly of the future concept for remote controlled dismantling as well as of the concept for packaging and final disposal. (author)