The Japanese Atomic Energy Agency (JAEA) disclosed details of an accident that happened on August 26, 2010 at the Monju fast breeder reactor (here is the original JAEA letter). As described in the press, the accident involved a 3-tonne "fuel-replacement device" falling into the reactor vessel when being removed after a scheduled fuel replacement operation. According to JAEA, the accident may result in a delay in bringing the reactor back into operation, since the device may have damaged the reactor vessel wall.
The Monju reactor was restarted on May 6, 2010, after a 15-year shutdown that followed a major sodium leak and fire. As IPFM reported at the time, the decision to restart the reactor was quite controversial, with citizen groups and newspapers opposing the move.