The government of Japan estimates that it would take at least eight years to restart the Monju fast neutron reactor, should the decision is made to do so. According to this estimate, the cost of operating the reactor until the end of its life would reach ¥540 billion (about $4.82 billion). The estimate comes from the Japan Atomic Industrial Forum report, released in November 2016 (in Japanese).
Most of the cost, ¥320 billion, would be associated with operation and maintenance of the reactor (¥20 billion a year, assuming that the reactor operates for 16 years). ¥20 billion is allocated to the design of modifications required to comply with new safety rules, and ¥130 billion - to construction work.
An earlier estimate suggested that operating the Monju reactor for ten years would cost ¥600 billion. The cost of decommissioning the reactor was estimated to be ¥300 billion.
The reactor was shut down after a refueling accident in August 2010.