Management strategy of radioactive waste in the fusion DEMO reactor

Someya, Yoji; Tobita, Kenji; Kondo, Masatoshi*; Yanagihara, Satoshi*; Uto, Hiroyasu; Sakamoto, Yoshiteru; Asakura, Nobuyuki; Hoshino, Kazuo; Nakamura, Makoto; Tokunaga, Shinsuke

We are considering the reduction of radioactive waste which is generated in every replacement of an in-vessel component such as a blanket segment and divertor cassette for fusion DEMO reactor. Main parameters of DEMO reactor are 8.2 m of major radius and 1.35 GW of fusion output. Maintenance scheme is to replace the blanket segment and divertor cassette independently, as the lifetime of them is different. The blanket segment consists of some blanket modules mounted to back-plate. A conducting shell for plasma positional stability and high beta access is installed in between the blanket modules and back-plate. Total weight is estimated to amount to about 6,648 ton (1,575 ton of blanket module, 3,777 ton of back-plate, 372 ton of conducting shell and 924 ton of divertor cassette). The lifetimes of blanket segment and divertor cassette are assumed to be 4 years and 1 year, respectively, 9,420 ton wastes is generated in 4 years. Therefore, there is a concern that a contamination controlled area for the radioactive waste may increase because much the waste is generated in every replacement. In a hot cell, the blanket module is removed from the back-plate by a remote handing. A tritium, radioactive dust and decay heat are inherent in the in-vessel component. The hot cell environment for the maintenance is cooled by natural air convection to prevent diffusion of the tritium and radioactive dust. Since the hot cell made a concrete, on the other hand, the temperature of the hot cell must be maintained below 65C for preventing water evaporation. As a result of thermal analysis, the maintenance of blanket segment is started after 6 years of storage until the temperature required for the concrete in the hot cell. 6 years after shutdown, a dominant contact dose rate of the component was 100 Sv/hour.