Japan's nuclear watchdog has decided that the nation's trouble-plagued next-generation prototype reactor won't be allowed to restart due to safety violations.

The Nuclear Regulation Authority's announcement Wednesday is another blow to the Monju fast-breeder reactor, which the pro-nuclear government hopes will be key to plans for atomic waste disposal. Monju uses plutonium fuel instead of conventional uranium and produces radioactive substances that can be reused as fuel. After nearly 50 years in the works, the reactor is still struggling to get online.

The watchdog said Monju's operator, the Japan Atomic Energy Agency, had repeatedly failed to conduct mandatory inspections on an array of equipment and come up with improvement plans.

The operator can appeal the decision, but an official suspension order is likely to come by the end of May.