The Japan Atomic Energy Agency committed grave safety errors in managing the troubled Monju prototype fast-breeder reactor in Fukui Prefecture, and top officials at the Nuclear Regulation Authority said Monday they plan to make sure it stays closed.

The closure order to the government-linked JAEA will effectively dash any hope of trying to restart the reactor by year's end, dealing another setback to Japan's long-stalled plan to set up a nuclear fuel recycling system.

In September, the now-defunct Nuclear and Industrial Safety Agency carried out surprise inspections and determined that JAEA failed to regularly check key components of the experimental 280,000-kw reactor, as required by internal rules.