Monju Nuclear Power Plant

Monju (もんじゅ) was a Japanese sodium-cooled fast reactor, located near the Tsuruga Nuclear Power Plant, Fukui Prefecture. Its name is a reference to Manjusri. Construction started in 1986 and the reactor achieved criticality for the first time in April 1994. The reactor has been inoperative for most of the time since it was originally built. It was last operated in 2010 and is now closed.

Monju was a sodium cooled, MOX-fueled, loop-type reactor with three primary coolant loops, designed to produce 280 MWe from 714 MWt. It had a breeding ratio of approximately 1.2. The plant is located on a site that spans 1.08 km² (267 acres), the buildings occupy 28,678 m² (7 acres), and it has 104,680 m² of floor space.

An accident in December 1995, in which a sodium leak caused a major fire, forced a shutdown. A subsequent scandal involving a cover-up of the scope of the accident delayed its restart until May 6, 2010, with renewed criticality reached on May 8, 2010. In August 2010 another accident, involving dropped machinery, shut down the reactor again. As of June 2011, the reactor had only generated electricity for one hour since its first testing two decades prior. As of the end of 2010, total funds spent on the reactor amounted to ¥1.08 trillion. An estimated ¥160–170 billion would be needed to continue to operate the reactor for another 10 years. As of 2014, the plant had cost ¥1 trillion ($9.8 billion).

A final decision on the project (e.g. to decommission or extend funding) was due by end 2016, and a decision to close the facility was made in December 2016. In December 2017 the Japan Atomic Energy Agency applied for approval of its decommissioning plan by the Nuclear Regulation Authority. Decommissioning and dismantling are planned to be completed by 2047 and is expected to cost ¥375 billion.