The Japan Atomic Energy Agency has begun decommissioning the Monju fast breeder reactor in Tsuruga, a process expected to take 30 years.
Monju reached criticality for the first time in 1994, with a series of problems following this before the reactor was decommissioned in 2016.
Nuclear Regulation Authority Chairman Toyoshi Fuketa says they want to proceed with the work with a sense of tension, with top priority placed on safety over schedule.